GB2569442A

GB2569442A - Materials testing apparatus

Info

Publication number: GB2569442A
Application number: GB1817596.8A
Authority: GB
Inventors: Frank Smith James
Original assignee: Micro Materials Ltd
Current assignee: Micro Materials Ltd
Priority date: 2017-10-27
Filing date: 2018-10-29
Publication date: 2019-06-19
Anticipated expiration: 2038-10-29
Also published as: GB2569442B; GB201817596D0; GB2569442B8; GB201717737D0

Abstract

Test apparatus for testing the surface properties of materials comprises a chassis (20, 21, 22, 23), a specimen stage (2) for mounting a specimen (3), a test probe (4) including an indenter; and a loading assembly for applying a load to the test probe, the assembly has a loading column (5) upon which the test probe is carried which extends generally horizontally and is supported on two or more leaf spring assemblies (13, 31). Load may be applied to the loading column electromagnetically by application of a current to one or more coils (11). The stiffness of one or more of the leaf spring assemblies may be varied to vary the force that they apply to the loading column.

Description

Materials Testing Apparatus

This invention relates to materials testing apparatus. In particular, it relates to apparatus for small-scale, localised testing of several surface mechanical properties of a specimen of material. These properties typically include one or more of: hardness, elastic modulus, complex modulus, impact resistance, scratch resistance and coefficient of friction.

This invention provides test apparatus comprising:

a chassis;

a specimen stage upon which a specimen to be tested can be mounted;

a test probe including an indenter of hard material; and a loading assembly for applying a load to the test probe to cause it to come into contact with a specimen in the specimen stage, the loading assembly having a loading column upon which the test probe is carried.

The loading column may be supported on two or more leaf spring assemblies. This provides high stiffness in the negative vertical (downward) direction, limited only by the spring material, a requirement for accurate testing in a scanning-type friction test. This also minimises the possibility of the column tilting during such testing. Preferably, the loading column extends generally horizontally, and is supported on one or more leaf spring assemblies. Each leaf spring assembly may include a leaf spring of continuous uniform length. Alternatively, it may include a leaf spring having first and second end portions projecting from an intermediate rigid portion. In a further alternative arrangement, the leaf spring assembly may include two leaf springs interconnected by a rigid component. Typically, each leaf spring extends generally vertically, the loading column being carried at a lower end portion of each leaf spring. In such embodiments, when moved through the distances typically encountered in testing, movement of the loading column is practically linear. Each leaf spring may be carried by a support, the position of which can be adjusted with respect to the specimen stage.

One or more leaf spring assembly may include an arrangement whereby the spring stiffness applied by the leaf spring assembly to the loading column can be adjusted. The leaf spring assembly, when unloaded may be curved, in which case the arrangement whereby the spring stiffness of the leaf spring assembly can be adjusted may be operable to increase or decrease the curvature of the unloaded leaf spring assembly.

One or more magnetic devices may be carried on the chassis and on the loading column. The magnetic devices include coils and permanent magnets. For example, the loading column may carry a plurality of coils and the chassis may carry a plurality of permanent magnets. In this way, a current flowing in a coil can cause a force to be applied to the loading column along a long axis of the loading column, thereby causing a force to be applied to the test probe. Differently-modulated signals can be applied to the coils to enable further control to be applied

Multiple coils may be disposed in the field of a permanent magnet. Such an arrangement permits driving signals to be applied from several independent sources, e.g. a sinusoidal signal from a signal generator together with a DC signal from a ramp generator. The magnet pole pieces are usually in saturated mode, for example by appropriate selection of their diameters. The loading column with coils may pass through a permanent magnet The coils are typically located in the gap between the pole pieces. One or more coils may be located in the fringing field of the pole pieces as opposed to being in the central position at the point of maximum field strength.

One coil may be used to effect variable eddy current damping. Damping control is achieved through variation of resistance in series with the coil. Maximum damping requires zero added resistance (short circuit) and minimum damping requires an open circuit. Variable resistance is advantageous for example for the determination of polymer damping properties and ideally should be optimised for particular test conditions.

A first conductive plate may be carried on the loading column to move with it and a second conductive plate fixed to the chassis. The first conductive plate has a surface normal to the motion of the loading column. The second conductive plate is closely spaced from and parallel to the first conductive plate. Movement of the loading column during testing (e.g., caused by the magnetic devices) changes the distance between the plates and therefore the effective capacitance of the plates. Preferably the first plate is remote from the test probe; e.g., at opposite ends of the loading column. This separation allows the use of a test probe heater which produces minimal thermal disturbance of the displacement sensor.

An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic view of apparatus embodying the invention; and

Figure 2 is a is a diagrammatic view of a second apparatus embodying the invention;

Figure3 shows, in more detail, a horizontal stiffness adjusting arrangement being part of the embodiment of Figure 2, the view being directed as Figure 2;

Figures 4 shows, in more detail, a horizontal stiffness adjusting arrangement being part of the embodiment of Figure 2, the view being directed perpendicular to that of Figure 2; and

Figures 5, 6 and 7 are simplified views of three alternative leaf spring assemblies suitable for use in embodiments of the invention.

With reference to the drawings, testing apparatus embodying the invention comprises a stage assembly constructed upon a chassis 20, 21, 22, 23.

The apparatus includes a specimen stage assembly. The specimen stage assembly includes an XYZ micrometer stage 1 that carries a specimen mounting stage 2. A test specimen 3 can be rigidly mounted upon the specimen mounting stage. The XYZ micrometer stage 1 allows the specimen mounting stage 2 (and therefore a specimen 3 mounted thereon) to be fixed in position for tests that require a static specimen or to be continuously moved for other types of tests, such as scratch testing. The specimen mounting stage 2 may be heated or cooled and may or may not contain piezo elements to permit specimen oscillation.

A test probe 4 is carried on a loading column 5. The test probe 4 is typically a pyramidal or spherical indenter of diamond or other hard material. The loading column 5 in this embodiment is made of high-density alumina but could also be fused silica to achieve low thermal expansion and low thermal conductivity. Other materials may be used for specific analytical tasks, e.g., metals for certain applications or different ceramics.

The loading column 5 is suspended from two leaf spring assemblies 13, as shown in Figure 5. Each leaf spring assembly 13 includes two resilient leaf springs 30 that are clamped at opposite ends of a rigid member 31. The upper portions of the leaf springs 30 project from the rigid member 31 and connect to a support bar 36. The lower end portions of the lower leaf springs 30 are connected by a connecting block to the loading column 5. The support bar 36 is carried on chassis member 21 by means of alignment adjustment screws 38 and 40. These components are arranged, when the apparatus is in use, such that the loading column 5 is horizontal, and each leaf spring assembly 13 extends vertically upwardly to the leaf spring support assembly 6. Supporting the loading column 5 on springs provides essentially frictionless motion. Thus no (or virtually no) component of the applied force is used to overcome unpredictable frictional forces.

The position of the support bar 36 is adjustable with respect to the chassis member 21. The leaf spring support assembly 6 includes a main screw 38 for moving the support bar 36 in one direction (e.g. upwards) and four secondary screws 40 for moving the support bar 36 in the opposite direction. The set of five screws 38, 40 are adjusted to rigidly lock the support bar 3 6 and hence the loading column 5 in its correct position.

Two or more coils 11 are attached to the loading column 5. The coils 11 are located within a magnet 12 that is secured to the chassis. The magnet 12 has pole pieces that have a through hole along an axis within which the coils 11 are located. The preferred magnetic material is a neodymium-iron alloy and the pole pieces are soft iron or mild steel. The coils 11 are placed in the gap between the central and outer pole pieces.

The coils 11 include one coil for general electromagnetic force application, one coil for electromagnetically imposing an oscillatoiy motion on the loading column, and one coil for eddy current damping. The damping may be adjusted by varying a resistor placed in series with the coil, maximum damping occurring when the coil is shorted (i.e., when the resistor is bypassed or close to 0Ω).

A mechanical stop 9 is provided. This interacts with a stop plate 42 carried on the loading column 5 to limit the extent of movement of the loading column in a direction towards the specimen 3. The stop 9 is carried on a micrometer stage 7 so that its position, and consequently the limiting position of the loading column 5 and the test probe 4, can be accurately adjusted.

The position of the loading column 5, and therefore the test probe 4, is monitored by detecting variations in a capacitance. The capacitance is formed from two plates: a moving plate 18 that is carried on an end of the loading column 5 remote from the test probe 4, and a static plate 8. During operation of the apparatus, the static plate 8 is unmoving with respect to the chassis, and the distance between the plates 8,18 varies with movement of the loading column, hence varying the capacitance. Although the static plate 8 is immobile when the apparatus is in use, it is carried on an adjustable support 14. This enables the initial position of the static plate 8 with respect to the moving plate 18 to be adjusted and it also allows adjustment to ensure that the plates 8,18 are parallel to one another.

An ancillary spring assembly 10 provides for initial mechanical positioning of the loading column. The spring assembly 10 includes a serpentine spring 44 extending between a block carried on the chassis and the loading column 5. By adjusting the lateral position of the block, an axial force can be applied to the loading column 5 through the serpentine spring to cause the loading column 5 to move axially for accurate initial positioning in which the leaf spring assemblies 13 adopt a neutral position (typically with the leaf springs straight and vertical) when no loading is applied to the column by the coils 11.

The leaf spring assemblies 13 have fixed stiffness but the overall horizontal stiffness of the loading column may be adjusted by means of horizontal stiffness addition or subtraction.

The embodiment of Figure 2 includes all the components ofthe embodiment of Figure 1 and further includes an arrangement 50 for adjustment of horizontal spring stiffness. This arrangement allows a wide variation in horizontal stiffness, including to very low values, to give low oscillation frequencies. It is useful in combination with variable damping for analysis of certain polymeric materials.

The arrangement of Figures 3 and 4 includes two horizontally-extending leaf springs 52, 54 inner ends of which are secured to a mounting block 56 fixed to the loading column 5. The leaf springs 52, 54 are curved with a large radius about a vertical axis.

The outer end of each leaf spring 52, 54 is secured to a respective spring block 58, 60. Each spring block 58, 60 has a through bore that is internally threaded, the threads of the two blocks being oppositely handed. The spring blocks 58, 60 are carried on an adjusting rod 64 that is externally threaded along its length, half of the length being of a left-hand thread and the other half being of a right-hand thread, the threads being compatible with those of the spring blocks 58, 58. Centrally, an adjustment wheel 66 is fixed to the adjusting rod 64.

The adjusting rod 64 is carried for rotation about is axis in two bearing blocks 70, 72, that are each rigidly supported on the chassis 22 by a respective support strut 74, 76. Axial movement of the adjusting rod 64 with respect to the bearing blocks 70, 72 is substantially prevented by washers 78, 80 that are carried on the adjusting rod 64 and which abut the bearing blocks 70, 72.

The adjusting rod 64 can be rotated by manually turning the adjusting wheel 66. Interaction between the threads of the adjusting rod 64 and those of the bearing blocks 70, 72 causes the bearing blocks to move towards or away from one another, increasing or reducing the curvature and therefore the spring rate of the leaf spring 52. This allows the horizontal stiffness of the spring force applied by the leaf spring 53 to the loading column 5 to be adjusted. Each bearing block 70, 72 moves by exactly the same distance (but in the opposite direction) such that the transverse position of the mounting block 56 remains unchanged.

Alternative leaf spring assemblies are shown in Figures 6 and 7. In the arrangement of Figure 6, in place of each leaf spring 13, two narrower leaf springs 13a and 13b are provided. As compared with the arrangement of Figure 5, this allows for a lower spring stiffness to be applied to the loading column 5 without a commensurate reduction in torsional stiffness. As a further alternative as shown in Figure 7, a single leaf spring 13c may extend between the support bar 36 and the loading column 5. Various embodiments may include one or more of the spring arrangements shown in Figures 5 to 7.

The principle of operation of the apparatus will now be discussed.

A range of force profiles are applied to loading column 5 by driving a variable current through one or more of the coils 11. These interact with the magnetic field of the magnet 12 to apply a force to the column. This allows the test probe 4 to interact with a surface of the specimen 3.

The apparatus also includes several electronic units for supplying and controlling the coil currents and for monitoring capacitance changes due to test probe movement

The apparatus can be used in several modes to achieve different types of test

Dynamic contact testing. In one type of test, an oscillatory force is applied to the probe 4 whilst it is in contact with a polymer, and the phase and amplitude of the probe are monitored using the capacitor 8, 18. This allows viscoelastic properties of the polymer to be determined.

Hardness and elastic modulus testing. In another type of test, indentation of the probe 4 into the surface of the specimen 3 is monitored as the force is increased and subsequently decreased in order to determine the hardness and elastic modulus of the surface.

Multi-mode impact testing. The column can be held at any position by means of a constant force (by application of a constant current) applied to one of the coils. An alternating force can be applied to another coil such that a known periodic displacement occurs. The specimen can then be brought into contact to a known distance, ensuring that the test probe can make repeated impacts whilst its displacement into the surface is limited (e.g. only to a thin film of material whilst not penetrating its substrate).

Impact testing from a probe position fixed electromagnetically as opposed to mechanically. The accelerating force is known and the rebound behaviour of the probe is studied. The conventional approach of mechanically holding the loading column at a fixed point prior to initiating an impact cycle can be problematical at ultra-low loads due to unpredictable surface forces attracting the column at the mechanical contact point, thus necessitating an excess force for column detachment.

Repetitive impact testing at ultra-low loads on thin films by employing variable damping to precisely control very low contact speeds.

Applying a small alternating signal to one coil throughout an impact trajectory and monitoring the amplitude and phase of this signal. The precise time at which the test probe makes contact with the surface can be determined. Conventionally, displacement vs. time data is differentiated to identify the contact point, but this approach has hitherto been problematical 5 for soft materials.

Claims

1. Test apparatus comprising:

a. a chassis; a specimen stage upon which a specimen to be tested can be mounted;

b. a test probe including an indenter of hard material; and

c. a loading assembly for applying a load to the test probe to cause it to come into contact with a specimen in the specimen stage, the loading assembly having a loading column upon which the test probe is carried, wherein

d. the loading column extends generally horizontally in use; and

e. the loading column is supported on two or more leaf spring assemblies.

2. Test apparatus according to claim 1 in which each leaf spring assembly is a continuous uniform length.

3. Test apparatus according to claim 1 or claim 2 in which each leaf spring assembly includes a leaf spring having first and second end portions projecting from an intermediate rigid portion.

4. Test apparatus according to any preceding claim in which each leaf spring assembly includes two leaf springs interconnected by a rigid component

5. Test apparatus according to any preceding claim in which each leaf spring assembly extends generally vertically, the loading column being carried at a lower end portion of each leaf spring.

6. Test apparatus according to any preceding claim in which each leaf spring is carried by a support, the position of which can be adjusted with respect to the specimen stage.

7. Test apparatus according to any preceding claim in which one or more leaf spring assembly includes an arrangement whereby the spring stiffness applied by the leaf spring assembly to the loading column can be adjusted.

8. Test apparatus according to claim 7 in which the leaf spring assembly when unloaded is curved, and the arrangement whereby the spring stiffness of the leaf spring assembly can be adjusted is operable to increase or decrease the curvature of the unloaded leaf spring assembly.

9. Test apparatus according to any preceding claim having one or more magnetic devices may be carried on the chassis and on the loading column.

10. Test apparatus according to claim 9 in which the magnetic devices include coils and/or permanent magnets.

11. Test apparatus according to claim 10 in which the loading column carries a plurality of coils and the chassis may carry a plurality of permanent magnets a current flowing in a coil causes a force to be applied to the loading column along a long axis of the loading column, thereby causing a force to be applied to the test probe.

12. Test apparatus according to any one of claims 9 to 11 having multiple coils disposed in the field of a permanent magnet.

13. Test apparatus according to claim 12 in which one or more coil is located in the gap between the pole pieces.

14. Test apparatus according to claim 12 or claim 13 in which one or more coil is located in the fringing field of the pole pieces.

15. Test apparatus according to any one of claims 9 to 14 in which one coil is operable used to effect variable eddy current damping by placing resistance in series with the coil.

16. Test apparatus according to any preceding claim having a first conductive plate

5 carried on the loading column to move with it and a second conductive plate fixed to the chassis.

17. Test apparatus according to claim 16 in which the first conductive plate has a surface normal to the motion of the loading column.

18. Test apparatus according to claim 16 or claim 17 in which the first conductive plate is

10 at the opposite end of the loading column from the test probe.

19. Test apparatus according to any one of claims 16 to 18 in which the second conductive plate is closely spaced from and parallel to the first conductive plate.