EP4627317A1 - System, method and device for measuring the frame compliance of a durometer - Google Patents

System, method and device for measuring the frame compliance of a durometer

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Publication number
EP4627317A1
EP4627317A1 EP23828474.9A EP23828474A EP4627317A1 EP 4627317 A1 EP4627317 A1 EP 4627317A1 EP 23828474 A EP23828474 A EP 23828474A EP 4627317 A1 EP4627317 A1 EP 4627317A1
Authority
EP
European Patent Office
Prior art keywords
indenter
internal body
durometer
displacement
external body
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP23828474.9A
Other languages
German (de)
French (fr)
Inventor
Giulio Barbato
Maurizio GALETTO
Giacomo MACULOTTI
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Politecnico di Torino
Original Assignee
Politecnico di Torino
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Politecnico di Torino filed Critical Politecnico di Torino
Publication of EP4627317A1 publication Critical patent/EP4627317A1/en
Pending legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/40—Investigating hardness or rebound hardness
    • G01N3/42—Investigating hardness or rebound hardness by performing impressions under a steady load by indentors, e.g. sphere, pyramid
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/0058—Kind of property studied
    • G01N2203/0076—Hardness, compressibility or resistance to crushing
    • G01N2203/0078—Hardness, compressibility or resistance to crushing using indentation
    • G01N2203/0082—Indentation characteristics measured during load
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/02—Details not specific for a particular testing method
    • G01N2203/0202—Control of the test
    • G01N2203/0212—Theories, calculations
    • G01N2203/0218—Calculations based on experimental data

Definitions

  • the present invention relates to a system for measuring the frame compliance of a durometer according to the preamble of claim 1. Furthermore, the present invention relates to an associated method and an associated device for measuring the frame compliance of a durometer.
  • the present invention especially applies to the field of durometers used for hardness scales based on the measurement of the depth of penetration.
  • a durometer is a measurement instrument designed for measuring the hardness of materials, and that many durometer types exist, each one intended for a specific application and having a particular measurement scale.
  • the indenter unavoidably has to act both as a tip position feeler and as a member applying the force to the tested material.
  • the point where the displacement must be measured is the indenter tip, but, in the best-case scenario, the displacement transducer will detect the position of its rear surface in abutment with the durometer.
  • the indenter must transmit the test force, which in the common tract produces at least an elastic deformation in the indenter adding up to the displacement of its tip, thus introducing an erroneous increase in the penetration reading.
  • Gali rock (Officine Galileo) is also known in the art, wherein, during calibration, the indenter is supported by a support that can be made to translate vertically to explore the displacement stroke while the test load is being applied to the indenter.
  • this device permits evaluating the frame compliance curve during a force application cycle, nor the deformability contribution given by the durometer's indenter.
  • Patent EP2075566B1 discloses a solution wherein a test machine is developed which permits an inherent characterization of frame compliance, and which is based on the measurement, by means of displacement sensors that are also used during the test, of the displacement of mechanical components of the test platform in a conditions of first contact between the indenter and the test surface and in a fullload condition, wherein the relative displacement and the applied force make it possible to estimate the frame compliance.
  • Figs. 1A and IB show, respectively, a perspective view and a plan view of a system and a device for measuring the frame compliance of a durometer according to the present invention
  • Fig. 2A shows a sectional side view along the half-planes A-A of the system and device of Fig. IB
  • Fig. 2B shows a sectional side view along the half-planes B-B of the device of Fig. IB.
  • reference numeral 100 designates as a whole a system for measuring the frame compliance (referred to as "Cf") of a durometer according to the present invention.
  • the durometer (the frame compliance Cf of which is to be measured by means of the system 100, the device 1 and the method according to the present invention) is not depicted in its entirety; in fact, the drawings only show an indenter 2, an optional zero reference ratchet 3 and a support surface (also called “anvil") 4 belonging to said durometer.
  • the anvil 4 is adapted to house at least one sample of a material to be tested.
  • the durometer further comprises actuating means for applying a force F to said indenter 2 (said actuating means are not shown in the accompanying drawings), and measuring means associated with processing means for obtaining, for each force F applied to the indenter 2, a measurement of at least one first displacement value hi indicating the displacement of the tip of said indenter 2.
  • actuating means for applying a force F to said indenter 2
  • measuring means associated with processing means for obtaining, for each force F applied to the indenter 2, a measurement of at least one first displacement value hi indicating the displacement of the tip of said indenter 2.
  • the measuring means nor the processing means of the durometer are shown in the accompanying drawings, and that they may respectively comprise at least one measurement sensor and a control unit of the durometer.
  • the zero reference ratchet 3 or the anvil 4 of the durometer is used as a reference to provide said first displacement value hi.
  • the system 100 according to the present invention further comprises a device (designated as a whole by reference numeral 1 in the accompanying
  • the device 1 comprises a plate 10 provided with a hole 11 positioned substantially in the centre of said plate 10.
  • the durometer to be tested when equipped with a zero reference ratchet 3, it lies on a top surface 10A of the reference plate 10, causing it to undergo elastic deformation and settling of the contact conditions.
  • the contact force is applied before the indentation starts, and that it is nominally constant, so that its variations during the test according to the present invention are substantially negligible and are not taken into account by the device 1 and by the method according to the present invention.
  • the plate 10 which, as aforementioned, constitutes the reference surface
  • the elastic deformation due to possible variations of the force applied by the zero reference ratchet 3 can be made certainly negligible.
  • any variations in the contact conditions between a zero reference ratchet 3 and its reference surface will also occur during the normal use of the durometer; therefore, the errors that such components may produce are included in the frame compliance Cf of the durometer.
  • the device 1 further comprises an external body 20 having a substantially tubular shape, i.e. a shape with a hole or cavity developing longitudinally in said external body 20, wherein said plate 10 is fixed to a top end 20A of said external body 20.
  • the assembly consisting of the plate 10 and the external body 20 is shaped substantially like a cup, in particular like an upsidedown cup (as particularly visible in Figures 1A, 2A and 2B) when the device 1 is associated with the durometer to measure its frame compliance Cf.
  • the plate 10 and the external body 20 are mutually coupled by fastening means 12 of a type known in the art.
  • said external body 20 has a tubular and cylindrical shape, since such a cylindrical shape can be easily made (e.g. by turning) and has no geometrical edges that might act as stress concentration points.
  • the plate 10 has a substantially circular shape in a plan view, i.e. a shape substantially corresponding to that of the top end 20A of the external body 20 to which said plate 10 is fixed. It is however clear that the shape of the external body 20 (and hence the shape of the plate 10) may also be different, e.g. prismatic and such as to have a hole or cavity along its longitudinal development.
  • the device 1 comprises an internal body 30 positioned within the external body 20, wherein said internal body 30 is coupled with the external body 20 by coupling means that allow the internal body 30 to make a linear and substantially coaxial movement relative to the external body 20, and wherein a top portion 31 of the internal body 30 is positioned at the hole 11 of the plate 10.
  • the internal body 30 has a substantially cylindrical shape, in particular said internal body 30 having a longitudinal axis substantially coinciding with a longitudinal axis of the external body 20 (said longitudinal axes are not shown in the accompanying drawings); in this context, the centre of the hole 11 of the plate preferably lies on the longitudinal axis of the external body 20 and of the internal body 30.
  • the device 1 further comprises a replaceable test button 50 having a top wall 51, in particular consisting of a flat surface, adapted to receive the tip (or contact zone) of the indenter 2, said test button 50 being positioned in the hole 11 of the plate 10, so that said top wall 51 is flush with the top surface 10A of the plate 10, and so that a bottom wall 52 of said test button 50 lies on the top portion 31 of the internal body 30.
  • a replaceable test button 50 having a top wall 51, in particular consisting of a flat surface, adapted to receive the tip (or contact zone) of the indenter 2, said test button 50 being positioned in the hole 11 of the plate 10, so that said top wall 51 is flush with the top surface 10A of the plate 10, and so that a bottom wall 52 of said test button 50 lies on the top portion 31 of the internal body 30.
  • the external body 20 of the device 1 comprises a bottom end 20B adapted to lie on a top plane of the support surface or anvil 4 of the durometer, in particular the external body 20 being positioned on said support surface or anvil 4 in such a way that the indenter 2 of the durometer is positioned at the test button 50 and/ or at the hole 11 of the plate 10 (or on top of such elements).
  • the interchangeability of the test buttons 50 implies that the force is transferred between two surfaces in mutual contact, i.e. between the bottom wall 52 of the test button 50 and the top portion 31 of the internal body 30; this may inevitably result in possible spurious displacements between said two surfaces in mutual contact, which may induce an error in the displacement reading.
  • the bottom wall 52 of the test button 50 and the top portion 31 of the internal body 30 are so designed as to create a conical coupling (e.g. ASME) or to create an isostatic abutment in order to reduce or eliminate said possible spurious displacements between their surfaces in mutual contact.
  • a conical coupling e.g. ASME
  • an isostatic abutment in order to reduce or eliminate said possible spurious displacements between their surfaces in mutual contact.
  • the test button 50 is made of high-hardness material, e.g. tungsten carbide, the compliance of which will prove negligible when measuring the frame compliance Cf of a durometer in accordance with the present invention.
  • high-hardness material e.g. tungsten carbide
  • the device 1 further comprises at least one measurement sensor associated with a control unit (not shown in the accompanying drawings) to provide, based on the measurement of the first displacement value hi taken by the durometer, a measurement of a second displacement value h e of the internal body 30 relative to the external body 20, produced by the force F applied to the indenter 2 (and hence to measure the displacement of the indenter 2 under actual load conditions, in particular for the purpose of performing a full calibration of a durometer that comprises said indenter 2).
  • a control unit not shown in the accompanying drawings
  • the designations of the displacement hi of the tip of the indenter 2 as the "first" displacement value and of the displacement h e of the internal body 30 relative to the external body 20 as the “second" displacement value should not be considered as time-related indications, in that such designations are solely useful to better identify such different displacement values.
  • said first displacement value hi represents the measurement of the displacement of the tip of the indenter 2
  • it may also be defined as “durometer reading hi”.
  • the second displacement value h e since it represents the measurement of the displacement of the internal body 30 relative to the external body 20 of the device 1, it may also be defined as “device reading h e ".
  • the frame compliance Cf of the durometer is determined by evaluating the ratio between:
  • the frame compliance Cf of the durometer can thus be expressed by the following formula:
  • the processing means of the durometer and the control unit of the device 1 have been described above as separate components; in such a case, they can be designed to be able to communicate with each other, whether via wired or wireless connection.
  • the device 1 should be considered as a calibration tool with no substantial connection with the instrument to be calibrated (i.e.
  • said processing means and said control unit are implemented as a single control unit (which may also be defined as “main control unit"), in particular associated with the actuating means and the measuring means of the durometer and also with at least one measurement sensor of the device 1 to control the application of the control forces F, acquire the durometer reading hi from the durometer, acquire the device reading h e from the device 1, and make the calculations of the frame compliance characteristic Cf.
  • control unit i.e. a control unit distinct from the processing means of the durometer and the control unit of the device 1, or
  • said coupling means comprise a plurality of laminae 40 extending from said internal body 30 and mating with an inner wall 21 of the external body 20.
  • the measurement sensor of the device 1 comprises at least one electric resistance strain gauge associated with at least one lamina 40 for measuring the displacement of said at least one lamina 40 (and hence for measuring the displacement of the internal body 30 relative to the external body 20); preferably, in particular in order to attain the necessary accuracy, the measurement sensor of the device 1 comprises at least one pair of strain gauges, wherein each strain gauge is associated with a respective lamina 40.
  • the device 1 according to the present invention comprises at least one complete strain gauge bridge, wherein each strain gauge is suitably associated with the different laminae 40.
  • each lamina 40 is a sort of elastic hinge which may perform the task of guiding the linear and substantially coaxial movement of the internal body 30 relative to the external body 20, and also the task of measuring the displacement of the internal body 30 relative to the external body 20.
  • such an embodiment also turns out to be particularly suitable for measuring the frame compliance Cf of a durometer with limited testing space, since it permits the construction of a device 1 which is short in height.
  • said coupling means may also be implemented otherwise, and that they may comprise, for example, a recirculating-ball sleeve, a ball-bearing guide or other similar systems.
  • the measurement sensor of the device 1 may be implemented in several ways as well, e.g. including at least one linear displacement transducer (also known as LVDT, "Linear Variable Displacement Transducer”), or optical-scale systems, or piezoelectric transducers, etc.) for measuring the displacement of the internal body 30 relative to the external body 20.
  • linear displacement transducer also known as LVDT, "Linear Variable Displacement Transducer”
  • optical-scale systems or piezoelectric transducers, etc.
  • the coupling means comprise a plurality of laminae 40, Fig. IB (wherein the plate 10 is represented as being substantially transparent, in order to make visible the components of the device 1 located within the external body 20) and Figs. 2A and 2B show that said plurality of laminae 40 preferably comprise two superimposed tiers of laminae 40 to ensure an axial translation of the internal body 30 relative to the external body 20, wherein each tier comprises three laminae 40 that are substantially equally spaced (i.e. mutually spaced by 120°), and wherein the laminae 40 of the first tier are offset by approximately 60° from the laminae 40 of the second tier.
  • This embodiment makes it possible to restrain the spurious movements of the internal cylinder 30, thus absorbing any spurious Force-Moment components; in addition, with such an architecture the device 1 according to the present invention can be manufactured by simple, low-cost machining on a machine tool.
  • the plate 10, in particular the top surface 10A of said plate 10, provides a reference surface whereon the zero reference ratchet 3 (if present) of a durometer rests.
  • the fixed part of the device 1 i.e. the external body 20
  • the anvil 4 of a durometer which acts as a reference in the absence of the zero reference ratchet 3
  • the movable part of the device 1 i.e. the internal body 30, the test button 50 lying on the internal body 30, and the laminae 40 that join the internal body 30 to the external body 20
  • the tip of the indenter 2 of the durometer is directly connected to the tip of the indenter 2 of the durometer.
  • the displacement of the movable part of the device 1 (that is, under settled penetration conditions and with a preload corresponding to the maximum value of the scale to be tested, the actual displacement of the tip of the indenter 2) is measured by means of at least one measurement sensor associated with a control unit of the device 1.
  • said at least one measurement sensor may comprise at least one electric resistance strain gauge associated with at least one lamina 40 (preferably, the measurement sensor comprises at least one pair of strain gauges, wherein each strain gauge is associated with a respective lamina 40, or the measurement sensor comprises at least one complete strain gauge bridge, wherein each strain gauge is suitably associated with the different laminae 40) for measuring the displacement of said at least one lamina 40, and hence for measuring also the displacement of the internal body 30 relative to the external body 20 (i.e. obtaining the device reading h e ); as an alternative, said at least one measurement sensor may comprise at least one linear displacement transducer for directly measuring the displacement of the internal body 30 relative to the external body 20 (i.e. the device reading h e ).
  • the device 1 permits decoupling the force transmission path from the displacement measurement path, so that the displacement of the tip of the indenter 2 can be measured while excluding any errors due to elastic deformation and settling of the mating areas between the parts transmitting the force by contact, which are always present in the durometers' mechanisms.
  • the device 1 is suitable for the implementation of adequate metrological characteristics, and also permits the characterization of Primary Sample Machines, providing actual referrability to the metrological chain.
  • the device 1 can also characterize, at least in terms of resolution, the frame compliance of micro-range and nano-range durometers, not only of macrorange durometers.
  • the device 1 further comprises a bottom 60 secured to the external body 20 under the internal body 30 and adapted to act as a stroke limiter for said internal body 30 (in particular, to prevent overloading the laminae 40 in the preferred embodiment shown in the accompanying drawings, i.e. to limit the displacement of the internal body 30 to the maximum value which is considered to be safe for the laminae 40).
  • the bottom 60 comprises a central portion 61 and a peripheral portion 62, in particular said central portion 61 being taller than the peripheral portion 62 and having substantially the same width (or diameter) as a bottom portion 32 of the internal cylinder 30 (as clearly shown in Figures 2 A and 2B).
  • the coupling between the external body 20 and the internal body 30 is effected in such a way as to leave a gap between a bottom portion 32 of the internal body 30 and the central portion 61 of the bottom 60, which keeps the displacement of the internal body 30, and hence the bending of the laminae 40, within safety limits.
  • the peripheral portion 62 of said bottom 60 and the inner wall 21 of the external body 20 are provided with respective threads allowing them to be coupled to each other.
  • the bottom 60 is coupled to the external body 20 in such a way that it is not at the same level as a bottom end 20B of the external body 20.
  • the bottom 60 is coupled to the external body 20 in such a way as to be positioned within said external body 20 at a given distance from the bottom end 20B of the external body 20 (as shown in Figures 2A and 2B).
  • the device 1 comprises two reference surfaces which can be used for measuring the second displacement value h e (or “device reading h e "), wherein a first reference surface consists of the top surface 10A of the plate 10, for calibrating durometers equipped with a zero reference ratchet 3, and wherein a second reference surface consists of the bottom end 20B of the external body 20, for calibrating durometers having, as a zeroing surface, the top plane of the anvil 4.
  • said top plane of the anvil 4 is normally used in known durometers (e.g. durometers that are not equipped with a zero reference ratchet) as a reference surface, in particular for measuring the first displacement value hi (or "durometer reading hi").
  • the laminae 40 which have been described above, and which are included in the preferred embodiment of the device 1 according to the present invention, are sized to undergo their maximum safety deformation when a given force F is applied.
  • the selected applied force F corresponds to the lowest full- scale value in the category of the durometers to be calibrated (the sizing of the laminae 40 must, of course, be different for the macro, micro and nano ranges). It is however known that durometers, although they have an almost constant displacement full-scale value, can operate with different force scales; as a consequence, in order to allow the device 1 according to the present invention to be used with loads higher than those of the lowest scale, it must be possible to modify the stiffness of the device 1.
  • the device 1 comprises an elastic element 70, which can be changed from time to time in accordance with the maximum load to be applied, interposed between the internal cylinder 30 and the bottom 60.
  • an elastic element 70 which can be changed from time to time in accordance with the maximum load to be applied, interposed between the internal cylinder 30 and the bottom 60.
  • suitable elastic elements 70 having different stiffness, to be placed between the internal cylinder 30 and the bottom 60, it is possible to adapt the device 1 to the different force scales that may be found in durometers.
  • said elastic element 70 comprises a spring, in particular a coil spring, preferably housed in a suitable centring seat 71 formed in the bottom 60 and/ or in the bottom portion 32 of the internal body 30.
  • the device 1 according to the present invention has been conceived for use with any hardness scale based on the measurement of the depth of penetration.
  • the method according to the present invention comprises the following steps: a) activating, in particular via processing means of a durometer, actuating means of said durometer in order to apply a force F (which may also be defined as "test force” F) to an indenter 2, suitable for obtaining at least one impression on the test button 50 of the device 1; b) measuring, in particular via measuring means associated with said processing means of the durometer, at least one first displacement value hi (which displacement may also be defined as “durometer reading hi”) indicating the displacement of a tip of said indenter 2 produced by said force F applied to the indenter 2; c) providing a measurement, in particular via said measurement sensor associated with the control unit of the device 1, of at least one second displacement value h e (which displacement may also be defined as “device reading h e ") of the internal body 30 relative to the external body 20 of said device 1, in particular said second displacement value h e being produced by said force F applied to the indenter 2; d) determining the frame compliance Cf of the dur
  • step d) the frame compliance Cf and the systematic error e s of the tested durometer are determined by means of the following formulae:
  • the particular provisions of the bottom 60 and the elastic element 70 make it possible to use the device 1 according to the present invention with different loads, in that they allow the device 1 to be adapted to different forces that may be applied by the durometers, while at the same time preventing the laminae 40 from being overloaded by limiting the displacement of the internal body 30 to a maximum value considered to be safe for the laminae 40.

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Abstract

The present invention relates to a system (100) for measuring the frame compliance (Cf) of a durometer, said system (100) comprising a durometer equipped with an indenter (2), actuating means for applying a force (F) to said indenter (2), and measuring means associated with processing means for obtaining, for each force (F) applied to the indenter (2), a measurement of at least one first displacement value (hi) indicating the displacement of a tip of said indenter (2). The main feature of the present invention lies in the fact that said system (100) comprises a device (1) comprising: - a plate (10) provided with a hole (11), in particular said hole (11) being positioned substantially in the centre of said plate (10); - an external body (20) having a substantially tubular shape, wherein said plate (10) is fixed to a top end (20A) of said external body (20); - an internal body (30) positioned within the external body (20), wherein said internal body (30) is coupled with the external body (20) by coupling means that allow the internal body (30) to make a linear and substantially coaxial movement relative to the external body (20), and wherein a top portion (31) of the internal body (30) is positioned at the hole (11) of the plate (10); - a replaceable test button (50) having a top wall (51), in particular consisting of a flat surface, adapted to receive a tip of the indenter (2) of the durometer, said test button (50) being positioned in the hole (11) of the plate (10), so that said top wall (51) is flush with a top surface (10A) of the plate (10), and so that a bottom wall (52) of said test button (50) lies on the top portion (31) of the internal body (30); - at least one measurement sensor associated with a control unit to provide a measurement of at least one second displacement value (he) of the internal body (30) with respect to the external body (20), in particular said second displacement value (he) being produced by said force (F) applied to the indenter (2), wherein the frame compliance (Cf) of the durometer is determined by evaluating the ratio between: - the difference between said first displacement value (hi) of the indenter (2), detected by the measuring means and the processing means of the durometer, and said second displacement value (he) of the internal body (30) relative to the external body (20), detected by the measurement sensor and the control unit of the device (1), and - the force (F) applied to the indenter (2).

Description

SYSTEM, METHOD AND DEVICE FOR MEASURING THE FRAME COMPLIANCE OF A DUROMETER
DESCRIPTION
The present invention relates to a system for measuring the frame compliance of a durometer according to the preamble of claim 1. Furthermore, the present invention relates to an associated method and an associated device for measuring the frame compliance of a durometer.
The present invention especially applies to the field of durometers used for hardness scales based on the measurement of the depth of penetration.
It is known in the art that a durometer is a measurement instrument designed for measuring the hardness of materials, and that many durometer types exist, each one intended for a specific application and having a particular measurement scale.
The principle of operation is nearly identical for all durometer types: a point (called indenter) is pushed under a known force against the material to be tested, thereby producing, by plastic deformation, a permanent impression, the size of which is correlated with a corresponding hardness, the latter being determined by measuring the impression. For some hardness scales a typical size of the impression on the material surface is measured, whereas other hardness scales require measuring the impression depth. In this latter case, which is very common, the measurement of the depth of penetration is affected by a systematic error due to the durometer's frame compliance. In fact, some parts of the durometer must simultaneously transfer the force to the indenter tip and the displacement thereof to the displacement transducer, which takes the measurement; under the applied force, such parts get deformed and their couplings settle, thus transferring to the transducer a displacement which differs from the one of the indenter's tip. For example, the indenter unavoidably has to act both as a tip position feeler and as a member applying the force to the tested material. The point where the displacement must be measured is the indenter tip, but, in the best-case scenario, the displacement transducer will detect the position of its rear surface in abutment with the durometer. At the same time, the indenter must transmit the test force, which in the common tract produces at least an elastic deformation in the indenter adding up to the displacement of its tip, thus introducing an erroneous increase in the penetration reading.
Moreover, this phenomenon is complicated even further by the presence of contact connections between the various parts transmitting the force, such as, for example, between the durometer7 s indenter holder and indenter, between the indenter7 s metallic body and diamond tip, etc. The frame compliance coefficient of the durometer, meaning the ratio between the spurious, elastic and contact displacements (as previously exemplified herein) and the applied force, is not constant for all possible applied loads, but may vary due to the non-linear behaviours of the contact conditions under different loads. It is therefore necessary to evaluate the frame compliance value throughout the load range.
Several methods are currently known in the art for measuring the frame compliance of a durometer (and hence for calibrating it).
In particular, the ISO 14577-2 standard specifies five different methods, which essentially share the need for making indentations on reference samples, calibrated by Young's modulus (indicated as E and also referred to as elastic modulus), and which exploit constitutive equations of the instrumented hardness test to obtain the frame compliance value. However, such methods (which concern the micro and nano range) couple the frame compliance calibration method with the methodological characterization system by using the same equations, without decoupling the force path from the displacement path.
Other known methods are based on sets of indentations on materials calibrated by mechanical properties (see review by Ullner et al. 2010 Measurement 43:216-222). The considerations previously made herein with reference to the ISO 14577-2 standard also apply to such methods, with the addition that the mechanical properties for calibrating the reference materials (penetration modulus, Martens hardness, instrumented hardness) require that the same be evaluated by instrumented penetration test. Although such test can be carried out by using Primary Sample Machines, the problem of characterizing the frame compliance of the latter still remains unsolved. A device called "Gali rock" (Officine Galileo) is also known in the art, wherein, during calibration, the indenter is supported by a support that can be made to translate vertically to explore the displacement stroke while the test load is being applied to the indenter. However, not even this device permits evaluating the frame compliance curve during a force application cycle, nor the deformability contribution given by the durometer's indenter.
Patent EP2075566B1 discloses a solution wherein a test machine is developed which permits an inherent characterization of frame compliance, and which is based on the measurement, by means of displacement sensors that are also used during the test, of the displacement of mechanical components of the test platform in a conditions of first contact between the indenter and the test surface and in a fullload condition, wherein the relative displacement and the applied force make it possible to estimate the frame compliance. However, this solution has some drawbacks as well, since the method of patent EP2075566B1 lacks generality; in fact, being strictly associated with the indentation platform for which it has been proposed, such method measures displacements of mechanical components of the test platform, without measuring the displacement at the very point where the load is applied during the characterization phase, and uses sensors incorporated in the test platform that do not provide a calibration of the frame compliance, but only an estimate of such frame compliance.
As concerns the method by Brand et al. (2015, International Journal of Materials Research), it is based on the measurement of the displacement of a reference spring made of silicone in order to compare the displacement read by the instrument's scale with the displacement actually undergone by the sample. The displacement due to the frame compliance of the test machine is then evaluated by difference. This method suffers from some drawbacks as well, in that it is only suitable for tests in the nano range and does not permit evaluating the indenter's contribution to the frame compliance. Moreover, the production of silicone springs is extremely costly. It is therefore apparent that, in the current state of the art, the error due to the durometer's frame compliance is not detected during the usual calibration operations, wherein the displacement scale is checked under almost zero-load conditions, with a procedure that is wholly separate from the procedure for checking the scale of the force applied by the durometer.
In this context, it is the main object of the present invention to provide a system, a method and a device for measuring the frame compliance of a durometer, in particular for hardness scales based on the measurement of the depth of penetration, wherein said system, method and device are suitable for overcoming the drawbacks suffered by the solutions currently known in the art.
In particular, it is one object of the present invention to provide a system, a method and a device for measuring the frame compliance of a durometer which have been conceived and realized to permit decoupling the force transmission path from the displacement measurement path, so that the displacement of the indenter tip can be measured without any errors due to elastic deformation and settling of the mating areas between the parts transmitting the force by contact, which are always present in the durometers' mechanisms.
It is another object of the present invention to provide a system, a method and a device for measuring the frame compliance of a durometer which can be used for all hardness scales based on the measurement of the depth of penetration.
It is a further object of the present invention to provide a system, a method and a device for measuring the frame compliance of a durometer, wherein said device is designed with an easy-to-make geometry that can even be obtained by means of traditional chip-removal precision machining, and wherein said measurement device is also applicable to any existing durometer to accurately characterize its frame compliance.
It is a further object of the present invention to provide a system, a method and a device for measuring the frame compliance of a durometer which implement adequate metrological characteristics and which also permit the characterization of Primary Sample Machines, providing actual referrability to the metrological chain. It is yet another object of the present invention to provide a system, a method and a device which can also characterize, at least in terms of resolution, the frame compliance of micro-range and nano-range durometers, not only of macro-range ones. Further objects, features and advantages of the present invention will become apparent in light of the following detailed description and of the annexed drawings, which are provided herein merely by way of non-limiting explanatory example, wherein:
- Figs. 1A and IB show, respectively, a perspective view and a plan view of a system and a device for measuring the frame compliance of a durometer according to the present invention;
- Fig. 2A shows a sectional side view along the half-planes A-A of the system and device of Fig. IB, while Fig. 2B shows a sectional side view along the half-planes B-B of the device of Fig. IB.
Describing now the annexed drawings, reference numeral 100 designates as a whole a system for measuring the frame compliance (referred to as "Cf") of a durometer according to the present invention.
It should be noted that, in the accompanying drawings, the durometer (the frame compliance Cf of which is to be measured by means of the system 100, the device 1 and the method according to the present invention) is not depicted in its entirety; in fact, the drawings only show an indenter 2, an optional zero reference ratchet 3 and a support surface (also called "anvil") 4 belonging to said durometer. As is known in the art, the anvil 4 is adapted to house at least one sample of a material to be tested.
The durometer further comprises actuating means for applying a force F to said indenter 2 (said actuating means are not shown in the accompanying drawings), and measuring means associated with processing means for obtaining, for each force F applied to the indenter 2, a measurement of at least one first displacement value hi indicating the displacement of the tip of said indenter 2. It should be noted that neither the measuring means nor the processing means of the durometer are shown in the accompanying drawings, and that they may respectively comprise at least one measurement sensor and a control unit of the durometer. It should also be noted that, depending on the type of durometer under examination, either the zero reference ratchet 3 or the anvil 4 of the durometer is used as a reference to provide said first displacement value hi. The system 100 according to the present invention further comprises a device (designated as a whole by reference numeral 1 in the accompanying drawings) adapted to be coupled to the durometer, in particular for measuring its frame compliance Cf.
The device 1 comprises a plate 10 provided with a hole 11 positioned substantially in the centre of said plate 10.
In this context, when the durometer to be tested is equipped with a zero reference ratchet 3, it lies on a top surface 10A of the reference plate 10, causing it to undergo elastic deformation and settling of the contact conditions. In this regard, it can be observed that the contact force is applied before the indentation starts, and that it is nominally constant, so that its variations during the test according to the present invention are substantially negligible and are not taken into account by the device 1 and by the method according to the present invention. Moreover, by appropriately sizing the plate 10 (which, as aforementioned, constitutes the reference surface), the elastic deformation due to possible variations of the force applied by the zero reference ratchet 3 can be made certainly negligible. In addition to this, any variations in the contact conditions between a zero reference ratchet 3 and its reference surface will also occur during the normal use of the durometer; therefore, the errors that such components may produce are included in the frame compliance Cf of the durometer.
The device 1 according to the present invention further comprises an external body 20 having a substantially tubular shape, i.e. a shape with a hole or cavity developing longitudinally in said external body 20, wherein said plate 10 is fixed to a top end 20A of said external body 20. Basically, the assembly consisting of the plate 10 and the external body 20 is shaped substantially like a cup, in particular like an upsidedown cup (as particularly visible in Figures 1A, 2A and 2B) when the device 1 is associated with the durometer to measure its frame compliance Cf. In this regard, it should be noted that, in the embodiment shown in the accompanying drawings, the plate 10 and the external body 20 are mutually coupled by fastening means 12 of a type known in the art. However, such coupling may also be obtained otherwise, e.g. by screwing the plate 10 to the external body 20 or by means of a different type of connection (e.g. a bayonet coupling) between said components. Moreover, the plate 10 and the external body 20 may also be made as one piece, even though such an embodiment is unfavourable because it is too costly and difficult to implement.
According to a preferred embodiment, said external body 20 has a tubular and cylindrical shape, since such a cylindrical shape can be easily made (e.g. by turning) and has no geometrical edges that might act as stress concentration points. In this context, the plate 10 has a substantially circular shape in a plan view, i.e. a shape substantially corresponding to that of the top end 20A of the external body 20 to which said plate 10 is fixed. It is however clear that the shape of the external body 20 (and hence the shape of the plate 10) may also be different, e.g. prismatic and such as to have a hole or cavity along its longitudinal development.
The device 1 according to the present invention comprises an internal body 30 positioned within the external body 20, wherein said internal body 30 is coupled with the external body 20 by coupling means that allow the internal body 30 to make a linear and substantially coaxial movement relative to the external body 20, and wherein a top portion 31 of the internal body 30 is positioned at the hole 11 of the plate 10.
Essentially, in an operating condition of the device 1 according to the present invention (i.e. a condition in which the device 1 is used for measuring the frame compliance Cf of a durometer), the top portion 31 of the internal body 30 is positioned under the hole 11 of the plate 10. Preferably, the coupling between the external body 20 and the internal body 30 is such as to leave a gap between a bottom surface 10B of the plate 10 and the top portion 31 of the internal body 30.
In the embodiment shown in the accompanying drawings, the internal body 30 has a substantially cylindrical shape, in particular said internal body 30 having a longitudinal axis substantially coinciding with a longitudinal axis of the external body 20 (said longitudinal axes are not shown in the accompanying drawings); in this context, the centre of the hole 11 of the plate preferably lies on the longitudinal axis of the external body 20 and of the internal body 30.
The device 1 according to the present invention further comprises a replaceable test button 50 having a top wall 51, in particular consisting of a flat surface, adapted to receive the tip (or contact zone) of the indenter 2, said test button 50 being positioned in the hole 11 of the plate 10, so that said top wall 51 is flush with the top surface 10A of the plate 10, and so that a bottom wall 52 of said test button 50 lies on the top portion 31 of the internal body 30.
In this context, the external body 20 of the device 1 comprises a bottom end 20B adapted to lie on a top plane of the support surface or anvil 4 of the durometer, in particular the external body 20 being positioned on said support surface or anvil 4 in such a way that the indenter 2 of the durometer is positioned at the test button 50 and/ or at the hole 11 of the plate 10 (or on top of such elements).
In this regard, it should be noted that, since the evaluation of the frame compliance Cf of the durometer is made, according to the present invention, by using the very indenter 2 of the durometer (which may give an important contribution to the frame compliance Cf), it is necessary to adopt replaceable or interchangeable test buttons 50 because during the test the indenter 2 will form an impression on the surface whereto it will be applied; it is therefore apparent that such surface may not be a fixed or non-replaceable surface of the device 1 of the present invention, since errors would be generated in the evaluation of the frame compliance Cf.
In such a context, the interchangeability of the test buttons 50 implies that the force is transferred between two surfaces in mutual contact, i.e. between the bottom wall 52 of the test button 50 and the top portion 31 of the internal body 30; this may inevitably result in possible spurious displacements between said two surfaces in mutual contact, which may induce an error in the displacement reading.
Therefore, the bottom wall 52 of the test button 50 and the top portion 31 of the internal body 30 are so designed as to create a conical coupling (e.g. ASME) or to create an isostatic abutment in order to reduce or eliminate said possible spurious displacements between their surfaces in mutual contact.
Preferably, the test button 50 is made of high-hardness material, e.g. tungsten carbide, the compliance of which will prove negligible when measuring the frame compliance Cf of a durometer in accordance with the present invention.
The device 1 according to the present invention further comprises at least one measurement sensor associated with a control unit (not shown in the accompanying drawings) to provide, based on the measurement of the first displacement value hi taken by the durometer, a measurement of a second displacement value he of the internal body 30 relative to the external body 20, produced by the force F applied to the indenter 2 (and hence to measure the displacement of the indenter 2 under actual load conditions, in particular for the purpose of performing a full calibration of a durometer that comprises said indenter 2).
Note that the designations of the displacement hi of the tip of the indenter 2 as the "first" displacement value and of the displacement he of the internal body 30 relative to the external body 20 as the "second" displacement value should not be considered as time-related indications, in that such designations are solely useful to better identify such different displacement values. For example, since said first displacement value hi represents the measurement of the displacement of the tip of the indenter 2, it may also be defined as "durometer reading hi". As to the second displacement value he, since it represents the measurement of the displacement of the internal body 30 relative to the external body 20 of the device 1, it may also be defined as "device reading he".
In this context, the frame compliance Cf of the durometer is determined by evaluating the ratio between:
- the difference between the first displacement value hi (or "durometer reading hi") of the indenter 2, detected by the measuring means and the processing means of the durometer, and the second displacement value he (or "device reading he") of the internal body 30 relative to the external body 20, detected by the measurement sensor associated with the control unit of the device 1, and
- the test force F.
In accordance with the present invention, the frame compliance Cf of the durometer can thus be expressed by the following formula:
Cf= (ht - he)/F.
Or its effect can be eliminated by correcting the indication of the durometer reading hi by removing the systematic error es given by the following formula: es = hi - he.
It should be noted that the processing means of the durometer and the control unit of the device 1 have been described above as separate components; in such a case, they can be designed to be able to communicate with each other, whether via wired or wireless connection. However, even though the device 1 should be considered as a calibration tool with no substantial connection with the instrument to be calibrated (i.e. a durometer), it is also possible to conceive an embodiment of the present invention wherein said processing means and said control unit are implemented as a single control unit (which may also be defined as "main control unit"), in particular associated with the actuating means and the measuring means of the durometer and also with at least one measurement sensor of the device 1 to control the application of the control forces F, acquire the durometer reading hi from the durometer, acquire the device reading he from the device 1, and make the calculations of the frame compliance characteristic Cf.
In this context, it is necessary to point out that the frame compliance Cf of the durometer (or the correction of the durometer reading hi by removing the systematic error es) can be determined:
- by the processing means of the durometer and/or by the control unit of the device 1 (whether alternatively or jointly, e.g. when the processing means and the control unit are implemented as a single control unit or designed to communicate with each other), or
- by an additional control unit, i.e. a control unit distinct from the processing means of the durometer and the control unit of the device 1, or
- manually (e.g. by an operator) on the basis of the separate readings of the force F, the durometer reading hi and the device reading he.
In accordance with a preferred embodiment, said coupling means comprise a plurality of laminae 40 extending from said internal body 30 and mating with an inner wall 21 of the external body 20.
In this preferred embodiment, the measurement sensor of the device 1 comprises at least one electric resistance strain gauge associated with at least one lamina 40 for measuring the displacement of said at least one lamina 40 (and hence for measuring the displacement of the internal body 30 relative to the external body 20); preferably, in particular in order to attain the necessary accuracy, the measurement sensor of the device 1 comprises at least one pair of strain gauges, wherein each strain gauge is associated with a respective lamina 40. Basically, the device 1 according to the present invention comprises at least one complete strain gauge bridge, wherein each strain gauge is suitably associated with the different laminae 40.
In this regard, it should be noted that, considering the equations that describe the displacement (arrow f) and the deformation (E) of at least one of said elastic laminae 40: one can see that the relationship between displacement (arrow f) and deformation (E) is a linear one:
Consequently, by measuring the deformation of the elastic lamina 40 by means of at least one electric resistance strain gauge, along with the instrumentation necessary for handling the output thereof, it is possible to obtain a measurement of the displacement f of said elastic lamina 40 (and hence a measurement of the displacement of the internal body 30 with respect to the external body 20).
In the previously described preferred embodiment, shown in the accompanying drawings, each lamina 40 is a sort of elastic hinge which may perform the task of guiding the linear and substantially coaxial movement of the internal body 30 relative to the external body 20, and also the task of measuring the displacement of the internal body 30 relative to the external body 20. Moreover, such an embodiment also turns out to be particularly suitable for measuring the frame compliance Cf of a durometer with limited testing space, since it permits the construction of a device 1 which is short in height.
It is nevertheless clear that said coupling means may also be implemented otherwise, and that they may comprise, for example, a recirculating-ball sleeve, a ball-bearing guide or other similar systems.
In this context, the measurement sensor of the device 1 may be implemented in several ways as well, e.g. including at least one linear displacement transducer (also known as LVDT, "Linear Variable Displacement Transducer"), or optical-scale systems, or piezoelectric transducers, etc.) for measuring the displacement of the internal body 30 relative to the external body 20. Although such transducers are a few tens of millimetres long or tall and, therefore, increase the space occupation of the device 1 according to the present invention, they can ensure accurate measurements and can therefore be considered to be suitable for implementation in the device 1 according to the present invention for measuring the frame compliance Cf of a durometer.
Continuing the description of the preferred embodiment, i.e. the one wherein the coupling means comprise a plurality of laminae 40, Fig. IB (wherein the plate 10 is represented as being substantially transparent, in order to make visible the components of the device 1 located within the external body 20) and Figs. 2A and 2B show that said plurality of laminae 40 preferably comprise two superimposed tiers of laminae 40 to ensure an axial translation of the internal body 30 relative to the external body 20, wherein each tier comprises three laminae 40 that are substantially equally spaced (i.e. mutually spaced by 120°), and wherein the laminae 40 of the first tier are offset by approximately 60° from the laminae 40 of the second tier. This embodiment makes it possible to restrain the spurious movements of the internal cylinder 30, thus absorbing any spurious Force-Moment components; in addition, with such an architecture the device 1 according to the present invention can be manufactured by simple, low-cost machining on a machine tool.
The features of the system 100 and device 1 according to the present invention clearly emerge from the above description, and their functional scheme can be summarized as follows.
The plate 10, in particular the top surface 10A of said plate 10, provides a reference surface whereon the zero reference ratchet 3 (if present) of a durometer rests. Directly connected to such reference surface is the fixed part of the device 1 (i.e. the external body 20), which is laid on the anvil 4 of a durometer (which acts as a reference in the absence of the zero reference ratchet 3), while the movable part of the device 1 (i.e. the internal body 30, the test button 50 lying on the internal body 30, and the laminae 40 that join the internal body 30 to the external body 20) is directly connected to the tip of the indenter 2 of the durometer. With such a scheme, therefore, the displacement of the movable part of the device 1 (that is, under settled penetration conditions and with a preload corresponding to the maximum value of the scale to be tested, the actual displacement of the tip of the indenter 2) is measured by means of at least one measurement sensor associated with a control unit of the device 1. For example, said at least one measurement sensor may comprise at least one electric resistance strain gauge associated with at least one lamina 40 (preferably, the measurement sensor comprises at least one pair of strain gauges, wherein each strain gauge is associated with a respective lamina 40, or the measurement sensor comprises at least one complete strain gauge bridge, wherein each strain gauge is suitably associated with the different laminae 40) for measuring the displacement of said at least one lamina 40, and hence for measuring also the displacement of the internal body 30 relative to the external body 20 (i.e. obtaining the device reading he); as an alternative, said at least one measurement sensor may comprise at least one linear displacement transducer for directly measuring the displacement of the internal body 30 relative to the external body 20 (i.e. the device reading he).
It is therefore apparent that the device 1 according to the present invention permits decoupling the force transmission path from the displacement measurement path, so that the displacement of the tip of the indenter 2 can be measured while excluding any errors due to elastic deformation and settling of the mating areas between the parts transmitting the force by contact, which are always present in the durometers' mechanisms.
The device 1 according to the present invention can be manufactured easily, in particular by traditional chip-removal precision machining, and can be used for calibrating any existing durometer to accurately characterize its systematic errors due to frame compliance.
The device 1 according to the present invention is suitable for the implementation of adequate metrological characteristics, and also permits the characterization of Primary Sample Machines, providing actual referrability to the metrological chain. In addition, the device 1 can also characterize, at least in terms of resolution, the frame compliance of micro-range and nano-range durometers, not only of macrorange durometers.
The device 1 according to the present invention further comprises a bottom 60 secured to the external body 20 under the internal body 30 and adapted to act as a stroke limiter for said internal body 30 (in particular, to prevent overloading the laminae 40 in the preferred embodiment shown in the accompanying drawings, i.e. to limit the displacement of the internal body 30 to the maximum value which is considered to be safe for the laminae 40).
Preferably, the bottom 60 comprises a central portion 61 and a peripheral portion 62, in particular said central portion 61 being taller than the peripheral portion 62 and having substantially the same width (or diameter) as a bottom portion 32 of the internal cylinder 30 (as clearly shown in Figures 2 A and 2B).
It should be noted that the coupling between the external body 20 and the internal body 30 is effected in such a way as to leave a gap between a bottom portion 32 of the internal body 30 and the central portion 61 of the bottom 60, which keeps the displacement of the internal body 30, and hence the bending of the laminae 40, within safety limits.
Furthermore, the peripheral portion 62 of said bottom 60 and the inner wall 21 of the external body 20 are provided with respective threads allowing them to be coupled to each other. In this regard, it should be noted that the bottom 60 is coupled to the external body 20 in such a way that it is not at the same level as a bottom end 20B of the external body 20. Essentially, the bottom 60 is coupled to the external body 20 in such a way as to be positioned within said external body 20 at a given distance from the bottom end 20B of the external body 20 (as shown in Figures 2A and 2B).
Therefore, the device 1 according to the present invention comprises two reference surfaces which can be used for measuring the second displacement value he (or "device reading he"), wherein a first reference surface consists of the top surface 10A of the plate 10, for calibrating durometers equipped with a zero reference ratchet 3, and wherein a second reference surface consists of the bottom end 20B of the external body 20, for calibrating durometers having, as a zeroing surface, the top plane of the anvil 4. Moreover, said top plane of the anvil 4 is normally used in known durometers (e.g. durometers that are not equipped with a zero reference ratchet) as a reference surface, in particular for measuring the first displacement value hi (or "durometer reading hi").
The laminae 40, which have been described above, and which are included in the preferred embodiment of the device 1 according to the present invention, are sized to undergo their maximum safety deformation when a given force F is applied. In a preferred embodiment, the selected applied force F corresponds to the lowest full- scale value in the category of the durometers to be calibrated (the sizing of the laminae 40 must, of course, be different for the macro, micro and nano ranges). It is however known that durometers, although they have an almost constant displacement full-scale value, can operate with different force scales; as a consequence, in order to allow the device 1 according to the present invention to be used with loads higher than those of the lowest scale, it must be possible to modify the stiffness of the device 1. To this end, the device 1 comprises an elastic element 70, which can be changed from time to time in accordance with the maximum load to be applied, interposed between the internal cylinder 30 and the bottom 60. In this regard, by selecting suitable elastic elements 70 having different stiffness, to be placed between the internal cylinder 30 and the bottom 60, it is possible to adapt the device 1 to the different force scales that may be found in durometers.
According to a preferred embodiment, said elastic element 70 comprises a spring, in particular a coil spring, preferably housed in a suitable centring seat 71 formed in the bottom 60 and/ or in the bottom portion 32 of the internal body 30.
It is therefore apparent that the device 1 according to the present invention has been conceived for use with any hardness scale based on the measurement of the depth of penetration.
The following will describe a method for measuring the frame compliance Cf of a durometer by means of the above-described device 1.
The method according to the present invention comprises the following steps: a) activating, in particular via processing means of a durometer, actuating means of said durometer in order to apply a force F (which may also be defined as "test force" F) to an indenter 2, suitable for obtaining at least one impression on the test button 50 of the device 1; b) measuring, in particular via measuring means associated with said processing means of the durometer, at least one first displacement value hi (which displacement may also be defined as "durometer reading hi") indicating the displacement of a tip of said indenter 2 produced by said force F applied to the indenter 2; c) providing a measurement, in particular via said measurement sensor associated with the control unit of the device 1, of at least one second displacement value he (which displacement may also be defined as "device reading he") of the internal body 30 relative to the external body 20 of said device 1, in particular said second displacement value he being produced by said force F applied to the indenter 2; d) determining the frame compliance Cf of the durometer by evaluating the ratio between:
- the difference between said first displacement value hi of the indenter 2 (detected by the measuring means and the processing means of the durometer) and said second displacement value he of the internal body 30 relative to the external body 20 (detected by the measurement sensor and the control unit of the device 1) and
- the force F applied to the indenter 2.
In substance, during step d) the frame compliance Cf and the systematic error es of the tested durometer are determined by means of the following formulae:
Cf= (ht - he)/F es = hi - he
It must be pointed out that said steps b) and c) can be carried out substantially at the same time; it should be reminded, in fact, that the designations of the displacement hi of the tip of the indenter 2 as the "first" displacement value hi and of the displacement of the internal body 30 relative to the external body 20 as the "second" displacement value he should not be considered as time-related indications, since such designations are only useful to better identify such different displacement values.
Furthermore, steps b) and c) can be carried out considering, as a reference surface (or zeroing surface) for measuring the first displacement value hi (or durometer reading hi), either the zero reference ratchet 3 or the top plane of the anvil 4; as concerns the second displacement value he (or device reading he), one may consider, as a reference surface (or zeroing surface), either the zero reference ratchet 3 and/ or the top surface 10A of the plate 10, for calibrating durometers equipped with a zero reference ratchet 3, or the bottom end 20B of the external body 20, for calibrating durometers lacking a zero reference ratchet 3 and using, as a zeroing surface, the top plane of the anvil 4.
In one embodiment, said step a) is carried out through a continuous activation of said actuating means in order to obtain a continuous loading cycle on the indenter 2 and take an isochronous recording of the first displacement value hi (or durometer reading hi) and of the second displacement value he (or device reading he) of the internal body 30 relative to the external body 20.
Alternatively, said step a) is carried out through a discontinuous activation of said actuating means in order to obtain different values of the load on the indenter 2 and record, for each value of the force F, displacement values hi, he of the indenter 2, so that the systematic error es and the frame compliance Cf can be calculated off-line. In a preferred embodiment, said step a) is preceded by a step a-1) of executing a series of consecutive activations of the actuating means of the durometer, for the purpose of obtaining series of impressions on the test button 50, from a minimum load, such as to ensure contact between the indenter 2 and the test button 50, to a maximum load equal to the force F or, preferably, higher than such force by approximately 10%, until appreciably constant displacement values hi, he are obtained when returning to the minimum load.
In substance, step a-1) is preferably carried out by means of the following load cycle: - activating, in particular via the processing means of the tested durometer, the actuating means of said durometer in order to apply a maximum force FM of the examined scale to the indenter 2, in order to obtain an impression on the test button 50, - returning to a force Fo close to 0 (zero) without separating the indenter 2 from the impression,
- taking note of the corresponding displacement indication heo given by the system 100,
- repeating this load cycle until the difference in heo compared with the previous cycle is negligible. In such condition, the test button 50 and the impression will have settled and will no longer produce any displacement when subjected to the load cycle.
Therefore, said step a-1) is useful to stabilize the contact between the bottom wall 52 of the test button 50 and the top portion 31 of the internal body 30 (and also between the top wall 51 of the test button 50 and the tip of the indenter 2).
Furthermore, said step a-1) is preferably preceded by the following steps:
- a step a-2) of positioning an elastic element 70 between a bottom 60 of the device 1 and a bottom portion 32 of the internal body 30, in particular said elastic element 70 having a stiffness suitable for the force F to be tested;
- a step a-3) of inserting a new test button 50 into the hole 11 of the plate 10; in particular, such insertion must be done in such a way that a top wall 51 (consisting of a flat surface) of the test button 50 is flush with a top surface 10A of the plate 10, and that a bottom wall 52 of said test button 50 rests on the top portion 31 of the internal body 30.
It must be pointed out that said steps a-2) and a-3) can be carried out substantially at the same time or, alternatively, in succession.
Of course, all of the above steps a) to d) (and also steps a-1) to a-3), if envisaged) are preceded by a step 0) of positioning the device 1 onto a support surface or anvil 4 of the durometer, such positioning being carried out in such a way that a bottom end 20B of the external body 20 of the device 1 rests on a top plane of the support surface or anvil 4, and that the indenter 2 of the durometer is positioned at the test button 50 and/ or the hole 11 of the plate 10 (or on top of such elements); such positioning makes it possible, in particular after the activation of the actuating means of the durometer, performed during step a), to obtain at least one impression on said test button 50 as a consequence of the force F applied to the indenter 2. As concerns step c), it may be carried out by constructing the coupling means in such a way that they comprise a plurality of laminae 40 and by measuring the displacement of at least one lamina 40 by means of at least one electric resistance strain gauge associated with said at least one lamina 40. Preferably, especially for the purpose of attaining the necessary accuracy, said step c) is carried out by using a measurement sensor of the device 1 comprising at least one pair of strain gauges, wherein each strain gauge is associated with a respective lamina 40. Essentially, said measurement sensor comprises at least one complete strain gauge bridge, wherein each strain gauge is suitably associated with the different laminae 40.
Alternatively, said step c) may be carried out by measuring the displacement of the internal body 30 relative to the external body 20 by means of at least one linear displacement transducer (also known as LVDT, "Linear Variable Displacement Transducer"), or optical-scale systems, or piezoelectric transducers, etc.).
The features of the system 100, of the device 1 and of the associated method for measuring the frame compliance Cf of a durometer according to the present invention, as well as the advantages thereof, are apparent in light of the above description.
Indeed, the special provisions of the system 100, device 1 and method for measuring the frame compliance Cf of a durometer according to the present invention provide a reference measurement, taken by means of a system wherein the force transmission path is substantially decoupled from the displacement measurement path, so that such paths are not affected by the error due to the frame compliance Cf, and it is thus possible to correct the durometer reading hi, (i.e. the measurement taken by a durometer), which is always affected by errors due to elastic deformation and settling of the mating areas between the parts transmitting the force by contact. In this context, the device 1 according to the present invention can be manufactured easily, in particular by traditional chip-removal precision machining, and can be used for calibrating any existing durometer to accurately characterize its systematic errors caused by its frame compliance Cf.
The device 1, system 100 and method according to the present invention are suitable for the implementation of adequate metrological characteristics, and also permit the characterization of Primary Sample Machines, providing actual referrability to the metrological chain. In addition, the device 1 and the system 100 can also characterize, at least in terms of resolution, the frame compliance of durometers in the micro and nano ranges, not only in the macro range.
The particular provisions of the bottom 60 and the elastic element 70 make it possible to use the device 1 according to the present invention with different loads, in that they allow the device 1 to be adapted to different forces that may be applied by the durometers, while at the same time preventing the laminae 40 from being overloaded by limiting the displacement of the internal body 30 to a maximum value considered to be safe for the laminae 40.
The system 100, device 1 and method described herein by way of example may be subject to many possible variations without however departing from the novelty spirit of the inventive idea; it is also clear that in the practical implementation of the invention the illustrated details may have different shapes or be replaced with other technically equivalent elements.
It can therefore be easily understood that the present invention is not limited to the above-described system 100, device 1 and method, but may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the inventive idea, as clearly specified in the following claims.

Claims

1. System (100) for measuring the frame compliance (Cf) of a durometer, said system (100) comprising a durometer equipped with an indenter (2), actuating means for applying a force (F) to said indenter (2), and measuring means associated with processing means for obtaining, for each force (F) applied to the indenter (2), a measurement of at least one first displacement value (hi) indicating the displacement of a tip of said indenter (2), said system (100) being characterized in that it comprises a device (1) comprising:
- a plate (10) provided with a hole (11), in particular said hole (11) being positioned substantially in the centre of said plate (10);
- an external body (20) having a substantially tubular shape, wherein said plate (10) is fixed to a top end (20A) of said external body (20);
- an internal body (30) positioned within the external body (20), wherein said internal body (30) is coupled with the external body (20) by coupling means that allow the internal body (30) to make a linear and substantially coaxial movement relative to the external body (20), and wherein a top portion (31) of the internal body (30) is positioned at the hole (11) of the plate (10);
- a replaceable test button (50) having a top wall (51), in particular consisting of a flat surface, adapted to receive a tip of the indenter (2) of the durometer, said test button (50) being positioned in the hole (11) of the plate (10), so that said top wall
(51) is flush with a top surface (10A) of the plate (10), and so that a bottom wall
(52) of said test button (50) lies on the top portion (31) of the internal body (30);
- at least one measurement sensor associated with a control unit to provide a measurement of at least one second displacement value (he) of the internal body (30) with respect to the external body (20), in particular said second displacement value (he) being produced by said force (F) applied to the indenter (2), wherein the frame compliance (Cf) of the durometer is determined by evaluating the ratio between:
- the difference between said first displacement value (hi) of the indenter (2), detected by the measuring means and the processing means of the durometer, and said second displacement value (he) of the internal body (30) relative to the external body (20), detected by the measurement sensor and the control unit of the device (1), and
- the force (F) applied to the indenter (2).
2. System (100) according to claim 1, characterized in that the bottom wall (52) of the test button (50) and the top portion (31) of the internal body (30) are so designed as to realize a conical coupling.
3. System (100) according to claim 1, characterized in that the bottom wall (52) of the test button (50) and the top portion (31) of the internal body (30) are so designed as to realize an isostatic support.
4. System (100) according to one or more of the preceding claims, characterized in that the test button (50) is made of high-hardness material.
5. System (100) according to one or more of the preceding claims, characterized in that said coupling means comprise a plurality of laminae (40) extending from said internal body (30) and mating with an inner wall (21) of the external body (20).
6. System (100) according to claim 5, characterized in that said at least one measurement sensor comprises at least one electric resistance strain gauge associated with at least one lamina (40) for measuring the displacement of said at least one lamina (40), wherein said displacement of the lamina (40) corresponds to said second displacement value (he).
7. System (100) according to one or more of claims 1 to 4, characterized in that said at least one measurement sensor comprises at least one linear displacement transducer for measuring said second displacement value (he) of the internal body (30) relative to the external body (20).
8. System (100) according to one or more of the preceding claims, characterized in that said plurality of laminae (40) comprise two superimposed tiers of laminae (40) to ensure an axial translation of the internal body (30) relative to the external body (20).
9. System (100) according to claim 8, characterized in that each tier of laminae (40) comprises three laminae (40) that are substantially equally spaced, in particular the laminae (40) of the first tier being offset by approximately 60° from the laminae (40) of the second tier.
10. System (100) according to one or more of the preceding claims, characterized in that the device (1) comprises a bottom (60) secured to the external body (20) under the internal body (30) and adapted to act as a stroke limiter for said internal body (30).
11. System (100) according to claim 10, characterized in that the device (1) comprises an elastic element (70) interposed between the internal cylinder (30) and the bottom (60).
12. System (100) according to claim 11, characterized in that said elastic element
(70) comprises a spring, in particular a coil spring housed in a suitable centring seat
(71) formed in the bottom (60) and/ or in the bottom portion (32) of the internal body (30).
13. System (100) according to one or more of the preceding claims, characterized in that the device (1) comprises two reference surfaces which can be used for measuring the second displacement value (he), wherein a first reference surface is the top surface (10A) of the plate (10), for calibrating durometers equipped with a zero reference ratchet (3), and wherein a second reference surface is the bottom end (20B) of the external body (20), for calibrating durometers using, as a zeroing surface, the top plane of an anvil (4).
14. Method for measuring the frame compliance (Cf) of a durometer by means of a device (1) comprising:
- a plate (10) provided with a hole (11), in particular said hole (11) being positioned substantially in the centre of said plate (10);
- an external body (20) having a substantially tubular shape, wherein said plate (10) is fixed to a top end (20A) of said external body (20);
- an internal body (30) positioned within the external body (20), wherein said internal body (30) is coupled with the external body (20) by coupling means that allow the internal body (30) to make a linear and substantially coaxial movement relative to the external body (20), and wherein a top portion (31) of the internal body (30) is positioned at the hole (11) of the plate (10);
- a replaceable test button (50) having a top wall (51), in particular consisting of a flat surface, adapted to receive a tip of the indenter (2) of the durometer, said test button (50) being positioned in the hole (11) of the plate (10), so that said top wall
(51) is flush with a top surface (10A) of the plate (10), and so that a bottom wall
(52) of said test button (50) lies on the top portion (31) of the internal body (30);
- at least one measurement sensor associated with a control unit for measuring a second displacement value (he) of the internal body (30) with respect to the external body (20), said method being characterized in that it comprises the following steps: a) activating, in particular via processing means of a durometer, actuating means of said durometer in order to apply a force (F) to an indenter (2) suitable for obtaining at least one impression on said test button (50); b) measuring, in particular via measuring means associated with said processing means of the durometer, at least one first displacement value (hi) indicating the displacement of a tip of said indenter (2), in particular said displacement being produced by said force (F) applied to the indenter (2); c) providing a measurement, in particular via said measurement sensor associated with the control unit of the device (1), of at least one second displacement value (he) of the internal body (30) relative to the external body (20) of said device (1), in particular said second displacement value (he) being produced by said force (F) applied to the indenter (2); d) determining the frame compliance (Cf) of the durometer by evaluating the ratio between:
- the difference between said first displacement value (hi) of the indenter (2), detected by the measuring means and the processing means of the durometer, and said second displacement value (he) of the internal body (30) relative to the external body (20), detected by the measurement sensor and the control unit of the device (1), and
- the force (F) applied to the indenter (2).
15. Method according to claim 14, characterized in that steps b) and c) are carried out considering, as a reference surface for measuring the first displacement value (hi), a zero reference ratchet (3) of the durometer and/ or a top surface (10 A) of the plate (10), and considering, as a reference surface for measuring the second displacement value (he), the top surface (10 A) of the plate (10) and/ or the bottom end (20B) of the external body (20).
16. Method according to one or more of claims 14 and 15, characterized in that said step a) is carried out through a continuous activation of said actuating means in order to obtain a continuous loading cycle on the indenter (2) and take an isochronous recording of the first displacement value (hi) and of the second displacement value (he).
17. Method according to one or more of claims 14 and 15, characterized in that said step a) is carried out through a discontinuous activation of said actuating means in order to obtain different values of the load on the indenter (2) and record, for each value of the force (F), the displacement values (hi, he) of the indenter (2), in particular so that the frame compliance (Cf) can be calculated off-line.
18. Method according to one or more of claims 14 to 17, characterized in that said step a) is preceded by a step a-1) of executing a series of consecutive activations of the actuating means of the durometer, for the purpose of obtaining series of impressions on the test button (50), from a minimum load, such as to ensure contact between the indenter (2) and the test button (50), to a maximum load equal to the force (F) or, preferably, higher than such force by approximately 10%, until appreciably constant displacement values (hi, he) are obtained when returning to the minimum load.
19. Method according to claim 18, characterized in that said step a-1) is preceded by the following steps:
- a step a-2) of positioning an elastic element (70) between a bottom (60) of the device (1) and a bottom portion (32) of the internal body (30), in particular said elastic element (70) having a stiffness suitable for the force (F) to be tested;
- a step a-3) of inserting a new test button (50) into the hole (11) of the plate (10).
20. Method according to one or more of claims 14 to 19, characterized in that said step c) is carried out by constructing said coupling means in such a way that they comprise a plurality of laminae (40) and by measuring the displacement of at least one lamina (40) by means of at least one electric resistance strain gauge associated with said at least one lamina (40).
21. Method according to one or more of claims 14 to 19, characterized in that said step c) is carried out by measuring the displacement of the internal body (30) relative to the external body (20) by means of at least one linear displacement transducer.
22. Device (1) adapted to be coupled with a durometer, in particular in order to measure the frame compliance (Cf) thereof, said device (1) being characterized in that it comprises:
- a plate (10) provided with a hole (11), in particular said hole (11) being positioned substantially in the centre of said plate (10);
- an external body (20) having a substantially tubular shape, wherein said plate (10) is fixed to a top end (20A) of said external body (20);
- an internal body (30) positioned within the external body (20), wherein said internal body (30) is coupled with an inner wall (21) of the external body (20) by coupling means that allow the internal body (30) to make a linear and substantially coaxial movement relative to the external body (20), and wherein a top portion (31) of the internal body (30) is positioned at the hole (11) of the plate (10);
- a replaceable test button (50) having a top wall (51), in particular consisting of a flat surface, adapted to receive a tip of the indenter (2) of the durometer, said test button (50) being positioned in the hole (11) of the plate (10), so that said top wall
(51) is flush with a top surface (10A) of the plate (10), and so that a bottom wall
(52) of said test button (50) lies on the top portion (31) of the internal body (30);
- at least one measurement sensor associated with a control unit to provide a measurement of a displacement value (he) of the internal body (30) with respect to the external body (20), in particular said displacement value (he) being produced by a force (F) applied to an indenter (2) of the durometer.
23. Device (1) according to claim 22, characterized in that the bottom wall (52) of the test button (50) and the top portion (31) of the internal body (30) are so designed as to realize a conical coupling.
24. Device (1) according to claim 22, characterized in that the bottom wall (52) of the test button (50) and the top portion (31) of the internal body (30) are so designed as to realize an isostatic support.
25. Device (1) according to one or more of the preceding claims 22 to 24, characterized in that the test button (50) is made of high-hardness material.
26. Device (1) according to one or more of the preceding claims 22 to 25, characterized in that said coupling means comprise a plurality of laminae (40) extending from said internal body (30) and mating with an inner wall (21) of the external body (30), wherein said at least one measurement sensor comprises at least one electric resistance strain gauge associated with at least one lamina (40) for measuring the displacement of said at least one lamina (40), wherein said displacement of the lamina (40) corresponds to said displacement value (he).
27. Device (1) according to one or more of claims 22 to 25, characterized in that said at least one measurement sensor comprises at least one linear displacement transducer for measuring said displacement value (he) of the internal body (30) relative to the external body (20).
28. Device (1) according to one or more of the preceding claims 22 to 27, characterized in that said plurality of laminae (40) comprise two superimposed tiers of laminae (40) to ensure an axial translation of the internal body (30) with respect to the external body (20).
29. Device (1) according to claim 28, characterized in that each tier of laminae (40) comprises three laminae (40) that are substantially equally spaced, in particular the laminae (40) of the first tier being offset by approximately 60° from the laminae (40) of the second tier.
30. Device (1) according to one or more of the preceding claims 22 to 29, characterized in that the device (1) comprises a bottom (60) secured to the external body (20) under the internal body (30) and adapted to act as a stroke limiter for said internal body (30).
31. Device (1) according to claim 30, characterized in that the device (1) comprises an elastic element (70) interposed between the internal cylinder (30) and the bottom (60).
32. Device (1) according to claim 31, characterized in that said elastic element
(70) comprises a spring, in particular a coil spring housed in a suitable centring seat
(71) formed in the bottom (60) and/ or in the bottom portion (32) of the internal body (30).
33. Device (1) according to one or more of the preceding claims 22 to 32, characterized in that said device (1) comprises two reference surfaces which can be used for measuring the displacement value (he), wherein a first reference surface is the top surface (10A) of the plate (10), for calibrating durometers equipped with a zero reference ratchet (3), and wherein a second reference surface is the bottom end (20B) of the external body (20), for calibrating durometers using, as a zeroing surface, the top plane of an anvil (4).
EP23828474.9A 2022-11-29 2023-11-28 System, method and device for measuring the frame compliance of a durometer Pending EP4627317A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
IT102022000024549A IT202200024549A1 (en) 2022-11-29 2022-11-29 SYSTEM, METHOD AND DEVICE FOR MEASURING THE COMPLIANCE OF A HARDOMETER
PCT/IB2023/061974 WO2024116066A1 (en) 2022-11-29 2023-11-28 System, method and device for measuring the frame compliance of a durometer

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EP4627317A1 true EP4627317A1 (en) 2025-10-08

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EP (1) EP4627317A1 (en)
IT (1) IT202200024549A1 (en)
WO (1) WO2024116066A1 (en)

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JP5017081B2 (en) 2007-12-26 2012-09-05 株式会社ミツトヨ Indentation tester and indentation test method

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