WO2008151581A1 - A method of measuring mechanical properties of materials when at least one parameter characterising viscoelasticity of materials is estimated, and an apparatus for carrying out such a method - Google Patents

A method of measuring mechanical properties of materials when at least one parameter characterising viscoelasticity of materials is estimated, and an apparatus for carrying out such a method Download PDF

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Publication number
WO2008151581A1
WO2008151581A1 PCT/CZ2007/000094 CZ2007000094W WO2008151581A1 WO 2008151581 A1 WO2008151581 A1 WO 2008151581A1 CZ 2007000094 W CZ2007000094 W CZ 2007000094W WO 2008151581 A1 WO2008151581 A1 WO 2008151581A1
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Prior art keywords
sample
inertial member
probe
weight
inertial
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PCT/CZ2007/000094
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French (fr)
Inventor
Stanislav Doubal
Petr Klemera
Petr Rejchrt
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Univerzita Karlova V Praze
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    • 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
    • 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
    • G01N2203/0089—Biorheological properties
    • 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
    • G01N2203/0092—Visco-elasticity, solidification, curing, cross-linking degree, vulcanisation or strength properties of semi-solid materials
    • G01N2203/0094—Visco-elasticity
    • 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
    • G01N2203/026—Specifications of the specimen
    • G01N2203/0262—Shape of the specimen
    • G01N2203/0278—Thin specimens
    • 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
    • G01N2203/06—Indicating or recording means; Sensing means
    • G01N2203/0617—Electrical or magnetic indicating, recording or sensing means
    • G01N2203/0635—Electrical or magnetic indicating, recording or sensing means using magnetic properties

Definitions

  • a method of measuring mechanical properties of materials when at least one parameter characterising viscoelasticity of materials is estimated, and an apparatus for carrying out such a method
  • the present invention generally relates to a method of measuring viscoelasticity and an apparatus for measuring mechanical properties of viscoelastic bodies.
  • it relates to a dynamic viscoelastometer with an inertial member.
  • the viscoelastometer according to the invention is intended namely for measuring biological materials, such as blood-vessel walls, skin, bones, cartilages, ligaments etc.
  • a and b are coefficients, i and; are orders of derivatives, s is the input (usually force or mechanical stress), d is the output (usually absolute or relative deformation).
  • the rheological model contains a combination of Hooke bodies (Fig. IA), representing the elastic component of behaviour, and Newton bodies (Fig. IB) representing the viscous component of behaviour, and thus also the energy dissipation in the dynamic response (Meyers and Chawla (1999): Mechanical Behaviors of Materials, Mechanical behaviour of Materials, 570-580. Prentice Hall, Inc.).
  • Fig. IIA Hooke bodies
  • Fig. IB Newton bodies
  • the Voight model Fig. 2
  • F(t) - F s (t) 0 (1) wherein F(t) is the external force acting on the body, Fs(t) is the force acting in the body in reaction to the external force,
  • N] 1 N 2 ... are Newton coefficients
  • H / , H ⁇ ... are ⁇ ooke coefficients
  • Lj, L 2 ... are lengths of individual "parallel" combinations of ⁇ ooke and Newton bodies.
  • the overall length of the body is:
  • L 1O are static lengths and AL t are deformations of partial ,,parallel" combinations of ⁇ ooke and Newton bodies
  • the overall length is:
  • the overall deformation is:
  • US Patent No. 3,470,732 describes an apparatus intended for measuring a modulus of elasticity and so called ,,loss" modulus (which relates to the viscous properties of materials).
  • the apparatus measures the deformation response to the sinusoidal force course, i.e. frequency characteristics, and it does not use any inertial member. It is intended for samples of plastic.
  • US Patent No. 4,165,634 relates to an apparatus for measuring samples of nylon, PE, rubber, composite material fibres etc. Measuring is performed on samples of materials. Based on the measurement of frequency characteristic, it enables to obtain a complex modulus of elasticity and so called mechanical loss.
  • the document JP 62 250 336 describes an apparatus intended for measuring samples of surface layers of composite materials.
  • the sample is clasped between two clamps; in the middle of the sample an inertial body of a special construction is placed.
  • the system of the sample and the inertial body is vibrated and dumped oscillations are measured. From the frequency and dumped oscillations, viscoelastic properties of the sample are calculated. Twist torsion is measured. Measuring is based on the generally known principle of calculation of viscoelasticity from dumped torsion oscillations. Measuring is possible only in torsion, it is impossible to measure the surfaces.
  • the document JP 63196838 relates to an apparatus determined for measuring skin viscoelasticity.
  • the apparatus does not use an inertial body and enables to measure only responses to torsion.
  • the document DE 4 040 786 describes an apparatus intended for measuring viscoelastic properties of surfaces.
  • the measuring principle consists in measuring the functionality between forces and deformations in static states (static measurements) and from measuring deformation reactions on a harmonious course of the force at different static tensions.
  • the apparatus does not contain an inertial member and measures frequency characteristics.
  • US Patent No. 6,609,428 relates to an apparatus for determining the so called real and imaginary modulus (Young modulus and shear modulus) and the Poisson ratio.
  • a ,,weight inertial member
  • the Czech Patent No. 292 284 described an apparatus for measuring viscoelasticity of tissues of live organisms. The measurement principle consists in measuring the time functionality of the deformation of a body surface as a response to the action of a rectangular impulse of the straining force. Based on this response, parameters of the rheological model are determined.
  • the apparatus does not contain an inertial member, the sensing is not contactless and the apparatus is conceived only for measurement of objects under compressive stress.
  • the first main subject-matter of the invention is the method of measuring mechanical properties of materials, when at least one parameter characterising viscoelasticity of materials is estimated, consisting in attaching an inertial member to the measured sample of the viscoelastic material or to a probe put in contact with the sample of the viscoelastic material, the weight of the inertial member being substantially higher than the weight of the sample or the probe; the inertial member or the probe being at least partially formed of ferromagnetic material, and by means of the contactless sensor the impulse and/or transient characteristics are measured when straining the sample in tension, compression, bending or torsion.
  • the second main subject-matter of the present invention is the apparatus for carrying out the above mentioned method, i.e. the inertial viscoelastometer.
  • the inertial viscoelastometer according to the present invention is provided with an inertial member.
  • the inertial member is chosen so that its weight be higher than the weight of other mobile parts of the measured system (i.e. the probe and the moving part of the sample).
  • the error of measurement which originates due to the influence of inertial forces is thus eliminated, as the weight of the inertial member is known with a high accuracy and also the place of acting of its inertial force is well defined.
  • Another advantage of this construction is the considerable improvement of sensitivity of measuring. This setting enables to use ferromagnetic material for manufacturing of the inertial member (or a part thereof). This technical solution is connected with the possibility of simple, contactless scanning of deformations.
  • the subject-matter of the invention is therefore the apparatus for carrying out the method according to the invention, the apparatus containing an inertial member tailored to be attached to the sample of the viscoelastic material or to the probe put into contact with the sample of the viscoelastic material, the weight of the member is substantially higher than the weight of the sample or probe, while the inertial member or probe are at least partially formed of ferromagnetic material, and further comprising a contactless sensor for sensing changes of the electromagnetic field of the contactless sensor, and a source of driving force for inducing dumped oscillating movement of the inertial member or probe.
  • the subject-matter of the invention is the inertial viscoelastometer, depicted schematically on Fig. 4, which contains the fixation means 1 for fixation of the sample 2 of the measured material, the inertial member 3 . , the contactless sensor 4 of the position, the electronics 6 processing the signal from the sensor and transmitting the information of the inertial member to a digital form, and further the computer 7 provided with software for identification of the model of the mechanical behaviour of the measured material and calculation of its parameters.
  • the weight of the inertial member 3 is chosen so that the motion equation of the system consisting of the measured sample 2 and the inertial member 3 has a periodical solution, and concurrently, that the weight of the inertial member 3_ is substantially higher than the weight of the sample 2.
  • the inertial member 3_ is at least partially formed of ferromagnetic material.
  • the contactless sensor 4 generates a magnetic field in the vicinity of the inertial member 2 and detects the changes in the electromagnetic field induced by the motion of the inertial member 3.
  • the information from the sensor 4 is processed by means of the computer.
  • Si is the vibration direction for tension and pressure
  • S2 is the vibration direction for bending.
  • the computer software solves the inversion problem.
  • the energy for inducing motion of the system sample - inertial member is generated into the system by means of the source 5 of driving energy. The following is measured: responses to the force or energy impulse (impulse characteristics), responses to the force or energy jump (transitional characteristics), reactions on the rectangular force or energy impulse (creep curves).
  • This basic embodiment of the invention may be modified for different types of measuring, such as measuring in straining of the sample in bending, tension, compression or torsion, and also for measurement of surfaces and membranes, as will be demonstrated in examples of advantageous embodiments of the apparatus according to the invention.
  • Viscoelastic bodies constitute an important group of biological materials from the mechanical point of view. Blood vessels, ligaments, cartilages, skin and bones belong to this group. The generally used quantification of stiffness (elastic modules) of these materials or other static characteristics (stress diagrams, differential modules, or strength limits) is not sufficient for description of mechanical behaviour of viscoelastic bodies.
  • stiffness elastic modules
  • static characteristics stress diagrams, differential modules, or strength limits
  • the solution according to the present invention has two substantial features: (1) connecting the sample or probe with the inertial member; and (2) detection using contactless sensor:
  • the inertial member e.g. 3 on Fig. 4 or 33 . on Fig. 15
  • the weight of the inertial member is chosen so that the following conditions are concurrently met:
  • the weight of the inertial member is higher than the limit for the periodical character of the response.
  • the periodicity limit stems from the solution of the motion equation (see the below mentioned equation (6)).
  • the motion equation has a periodical solution under the N 2 condition M) at least for one of the ,,parallel" combinations of Hooke and Newton
  • the weight of the inertial member is substantially higher than the weight of the moving parts of the measured body.
  • the weight of the inertial member is chosen at least 5 times higher than the weight of the sample.
  • the weight of the inertial member is chosen at least 5 times higher than the weight of a cylinder of the measured material, having the area of the size of the opening in the fixation apparatus, and the length of 0,5 mm.
  • Identification of parameters of the measured object is carried out with a sufficient accuracy and sensitivity based on impulse or transitional characteristics, which is simpler and consequently cheaper than the usual measuring of frequency characteristics.
  • M is the weight of the inertial member
  • Li, L 2 ... are lengths of individual ,,parallel" combinations of Hooke and Newton bodies (see the model on Fig. 6).
  • the overall length of the body is:
  • the overall deformation is:
  • the weight M is known with a high accuracy. It is given by the weight of the inertial member, which is very precisely determined by weighing.
  • the above equation system enables to express the functionality of ⁇ L(t) on the driving force F(t) and on the parameters of the system. Subsequently, it is possible based on measurement of the particular courses of ⁇ L(t), i.e. based on dynamic characteristics, to determine parameters of the model.
  • the detection of length changes (when measuring in tension) and measuring the position (when measuring in bending), optionally also measuring the acting forces is usually carried out by sensors which are in contact with the measured object. This implementation leads to error occurrence, due to the influencing of the measured object by sensors.
  • detection is carried out without contact, based on the principle of the electromagnetic induction.
  • Induction and inductive sensors in their regular implementation do not work without contact, they require that their mobile part be mechanically connected with the measured system.
  • this necessity is avoided by the fact that the inertial member is at least partially formed of ferromagnetic material and substitutes the mobile part of classical sensors.
  • the contactless sensor 4 in the embodiment according to the invention generates a magnetic field in the vicinity of the inertial member 3_ and scans the changes of the electromagnetic field induced by the motion of the inertial member 3.
  • Fig. 1 represents basic rheological bodies.
  • Fig. IA represents the Hooke body and the equation describing its behaviour
  • Fig. IB represents the Newton body and the equation describing its behaviour.
  • Fig. 2 represents the Voigt model consisting of the parallel combination of Hooke and Newton bodies.
  • Fig. 3 represents the rheological model containing ,,serial-parallel" combination of Hooke and Newton bodies.
  • Fig. 4 shows a general scheme of dynamic inertial viscoelastometer — i.e. the apparatus according to the invention.
  • Fig. 5 represents the rheological model of the inertial member.
  • Fig. 6 represents the rheological model with the inertial member M.
  • Fig. 7 shows schematically the apparatus according to the invention, modified for measuring viscoelasticity of a sample in straining of the sample in bending.
  • A is a front view
  • B is a ground plan.
  • Fig. 8 is a scheme of the contactless sensor with the probe 8 of the sensor.
  • Fig. 9 is a scheme of the source of the driving force and/or the lifting electromagnet.
  • Fig. 10 represents the schematic process of data processing.
  • Fig. 11 represents the rheological model containing 2 components.
  • Fig. 12 is a graph of the course of deformation when measuring according to Example 1. The authentic record of measuring.
  • Fig. 13 is a graph of dumped oscillations of component 1 of the model, with a periodical response. The data are re-calculated from the course of deformations according to Fig. 12.
  • Fig. 14 shows schematically the apparatus according to the invention modified for measuring viscoelasticity of a sample in straining of the sample in tension and pressure.
  • Fig. 15 shows schematically the apparatus according to the invention modified for measuring surfaces of materials.
  • Fig. 16 shows schematically the apparatus according to the invention modified for measuring membranes.
  • Fig. 17 shows schematically the fixation apparatus for membranes.
  • FIG. 7 The embodiment of the inertial viscoelastometer for measuring samples of bones in bending is schematically depicted on Fig. 7.
  • the sample 2J_ of the measured material has the following dimensions: 15 to 50 mm (length), 1 to 10 mm (lateral dimensions) and it is mechanically attached to the fixation apparatus 1.1.
  • the inertial member 3J of ferromagnetic material (steel) is attached.
  • the weight of the inertial member 3J is chosen from a set of inertial members.
  • the weight of the inertial member 3J must be at least 5 times higher than the weight of the sample 2J,.
  • Inertial members 3JL have the shape of a block.
  • the length (in the direction of vibrations) is 2 times longer then the dimension perpendicular to the vibrations (depth).
  • the sensor 4J is formed by a cylindrical coil (schematically depicted on Fig. 8) containing 6000 turns of enamelled wire, Cu, 0.14 mm, with a ferromagnetic core, length 55 mm, diameter 5 mm, which constitutes the probe 8 of the contactless sensor.
  • the probe 8 projects 10 mm above the upper end of the coil. It is attached in parallel to the edge of the inertial member 3J,. In a static state, the distance between the edge of the inertial member 3.1 and the near edge of the probe 8 ranges from 1 to 2 mm.
  • the system consisting of the measured sample 2 ⁇ . and the inertial member 3J ⁇ is mechanically vibrated by the source of the driving force 5J,.
  • the vibration amplitude must be lower or identical compared to the distance of the probe 8 of the sensor from the inertial member 3J,.
  • the introduction of the driving force is done by switching on and off the source of the driving force 5J ⁇ , which is the electromagnet schematically depicted on Fig. 9, formed for example of a coil, 5 500 turns of enamelled wire, Cu, 0.22 mm, with ferrous core, length 45 mm, diameter 10 mm.
  • the signal from the sensor 4_ ⁇ _ is processed by the electronics 6A_ and converted into digital form and further processed by the computer.
  • the block diagram of the data processing process is on Fig. 10.
  • block A represents the measured system
  • B is A/D converter
  • C represents loading and saving measured data
  • D is the model selection
  • E is identification of parameters.
  • the magnitude X represents the time flow of the driving force or energy.
  • the magnitude Y represents the time flow of the deformation response.
  • the output Z represents rheological parameters (N 1 , N 2 , H 1 , H 2 , see Fig. 11) of the model.
  • the course of responses is recorded and parameters of the sample materials are calculated for the rheological model which is depicted on Fig. 11.
  • the weight of the inertial member is chosen so that the first component of the model have
  • N 2 a periodic response to the impulse or jump of the driving force (M) — — ) and the second
  • the time response of the second component of the model is aperiodic.
  • the time response of the second component of the model to a jump is in this case:
  • the measurement was carried out on a cut of the spongy part of caput femoris, which forms the head of the hip joint from a human donor (healthy male, 70 years).
  • the material was obtained after a hip joint surgery. After the surgery, it was preserved for two weeks in a physiological solution at a temperature of - 15 °C. Before the experiment, a sample was cut from the joint head at a laboratory temperature (24 0 C).
  • the weight of the first component represents the ratio between the deformation of the first (quick) component related to the overall deformation in a steady state.
  • the obtained parameters show the important influence of viscose components in dynamic straining.
  • the influence of viscose components on the responses grows with the frequency of the driving force and manifests itself with the growth of dynamic stiffness of materials.
  • the growth of the dynamic stiffness (compared to the stiffness at static straining) will be substantial (over 10 %) in the first component in frequencies over 7 Hz. In case of the second component, already over 0.05 Hz.
  • the inertial member 32 is in this embodiment of a cylindrical shape.
  • the diameter of the cylinder is within the range from 5 to 15 mm, its length is from 10 to 30 mm.
  • the material is steel.
  • the weight is within the range from 1.54 to 50 g.
  • the inertial member 32. is lead by using of the guidance means 12 of the inertial member.
  • the guidance means 12 is formed for example of a glass pipe having the diameter larger by 0.1 mm than the diameter of the inertial member 32.
  • the guidance means 12 of the inertial member prevents the lateral deviations in vibration.
  • the guidance means JJ2 of the inertial member, the inertial member 3 ⁇ 2 and the sample 22 . are attached coaxially, the axis being vertical.
  • the apparatus represents a fundamental improvement of an older apparatus according to the Czech Patent No. 292284.
  • the inertial member 33 . is of a cylindrical shape in this embodiment.
  • the diameter of the cylinder ranges from 5 to 15 mm, the length from 10 to 30 mm.
  • the material is brass.
  • the weight is within the range from 1.7 to 50 g.
  • the inertial member 33 . is placed on the probe J3, between the deformation sensor 16 and the fixation apparatus 1_5.
  • the probe 13 . is formed of a cylindrical stick, for example from glass or plastics, having diameter for example 4 mm.
  • the lifting electromagnet 53 may be made for example as depicted on Fig. 9.
  • the measurement is carried out so that the probe J3 with the inertial member 33 . is lifted by the electromagnet 53 . 5 to 10 mm above the surface of the measured sample 23.
  • the sensor 16 (according to the patent No. 292284) measures the time course of the deformation.
  • the electronics 63_ and the computer 73 . provided by the software may be optionally the same as in the measurement according to Ia).
  • the measurement was carried out on the skin of the left palm of a female, 58 years, in the area in the middle and above facilis brevis by the method described above.
  • the palm was degreased and heated by an IR lamp to about 30°C. Subsequently, it was fixed pneumatically by the pressure of 60 mmHg.
  • the weight of the inertial member and probe was 37.8 g.
  • the opening of the fixation apparatus had a diameter of 20 mm.
  • FIG. 16 there is a scheme of modification of the inertial viscoelastometer modified for measuring membranes.
  • the apparatus differs from the variant 2a by a different method of fixation of the sample.
  • the rest of the setting is virtually identical.
  • Fig. 17 the fixation apparatus 15.1 (see Fig. 16) for measuring membranes is depicted in detail.
  • the sample 2 ⁇ of the measured membrane is clasped between two desks 17, provided with a concentric and evenly big opening 18 for passage of the probe.
  • the measured membrane 2_5_ is attached between the fixation desks 17 by means of the screws 19.
  • the size of the opening 18 is within the range from 5 to 50 mm (diameter).
  • the present invention will find its use in medicine and in the biological research, in the industry of prosthetic and auxiliary materials for medicine, optionally in textile, rubber, and plastic industries.
  • the research of viscoelastic properties of biological materials is crucial for understanding the dynamics of mechanical reactions of cardiovascular system, bones, ligaments and other structures.
  • the importance of these characteristic is diagnostic and is crucial also for a suitable selection of mechanical parameters of auxiliary and prosthetic materials.
  • the apparatus according to the invention will enable precise and at the same time cheap measurement of these properties. It will also enable obtaining of a more detailed description of viscoelastic properties of materials compared to the current possibilities. It will enable an analysis of risks of harm in dynamic straining with respect to diseases (osteoporosis, sclerotic changes etc.), nutrition and ageing.
  • the invention will enable technology enhancement, especially in textile and shoe manufacturing industries, its use is prospective also in rubber industry and in research and production of plastics.

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Abstract

The subject-matter of the invention is the method of measuring mechanical properties of materials, namely biological materials, when at least one parameter characterising the viscoelasticity of the material is detected; the method consists in attaching an inertial member to the measured sample of the viscoelastic material or to the probed put into contact with the sample of the viscoelastic material, the weight of the inertial member being substantially higher than the weight of the sample or probe, while the inertial member or probe are at least partially formed of ferromagnetic material; by means of a contactless sensor, impulse and/or transient characteristics in straining the sample in bending, tension, compression or torsion are measured. Another subject-matter of the invention is an apparatus for carrying out the method according to the invention, i.e. the inertial viscoelastometer. In an advantageous embodiment, the apparatus according to the invention is modified for determining viscoelasticity of materials in straining the sample in bending, tension or compression. Another advantageous embodiment enables to determine the viscoelasticity based on measuring the surface of the sample or to determine the viscoelasticity of a sample in the form of a membrane.

Description

A method of measuring mechanical properties of materials when at least one parameter characterising viscoelasticity of materials is estimated, and an apparatus for carrying out such a method
Field of the Invention
The present invention generally relates to a method of measuring viscoelasticity and an apparatus for measuring mechanical properties of viscoelastic bodies. In particular, it relates to a dynamic viscoelastometer with an inertial member. The viscoelastometer according to the invention is intended namely for measuring biological materials, such as blood-vessel walls, skin, bones, cartilages, ligaments etc.
Background of the Invention
For describing the mechanical behaviour of bodies, it is important to know the relation between forces acting on the body and the resulting deformation responses. These responses may be non-linear in the general sense. Adequate mathematical models in this situation have the character of non-linear differential equations. Identification of these equations and determining their parameters is difficult (Albert Tarantola, Inverse Problem Theory Society for Industrial and Applied Mathematics, 2005. ISBN 0-89871-572-5; Richard Aster, Brian Borchers, and Cliff Thurber, Parameter Estimation and Inverse Problems, Academic Press, 2004. ISBN 0-12-065604-3). Linear models are therefore used for practical purposes.
It is usually assumed that the behaviour of a body, the mechanical parameters of which are distributed in the whole space occupied by the body, may be substituted by a model with lumped parameters. Classically, behaviour of real bodies is described as behaviour of a solid elastic body, in case of liquids as behaviour of the Newton liquid. In case of solid bodies, this model is acceptable for small relative deformations in a static regime of straining. They fit also in situations when the changes in straining are substantially slower than the time constants of the motion equation of the body. In viscoelastic bodies, these assumptions are not met. Therefore, models which describe behaviour of bodies under dynamic stress are used for describing deformation behaviour of viscoelastic bodies. It is usually assumed that it is a behaviour that can be considered linear within a certain range of straining powers.
In linear mechanical systems, the relations between input and output may be fully described by the differential equation of the type n m
(=1 7=1
wherein a and b are coefficients, i and; are orders of derivatives, s is the input (usually force or mechanical stress), d is the output (usually absolute or relative deformation).
Practically, mathematical description of mechanical behaviour usually comes from a suitable rheological model (from this the left part of the above equation comes out). The calculation of parameters of the left side of the equation is derived from measuring the mechanical deformation response to the chosen course of the deformation force (which determines the shape and magnitude of parameters of the right side of the equation). In other words, it is derived from measuring the chosen characteristic.
The rheological model contains a combination of Hooke bodies (Fig. IA), representing the elastic component of behaviour, and Newton bodies (Fig. IB) representing the viscous component of behaviour, and thus also the energy dissipation in the dynamic response (Meyers and Chawla (1999): Mechanical Behaviors of Materials, Mechanical behaviour of Materials, 570-580. Prentice Hall, Inc.). For describing mechanical properties, differently complex rheological models are used. Usually, the Voight model (Fig. 2) is used.
The more complex model according to Fig. 3, which is a serial-parallel combination of Hooke and Newton bodies, is described by the following equation system:
F(t) - Fs(t) = 0 (1) wherein F(t) is the external force acting on the body, Fs(t) is the force acting in the body in reaction to the external force,
while the forces acting on individual ,,parallel" combinations of Hooke and Newton bodies are identical:
FMy=N1 ^+H1AL1 =N2 ^+H2AL2 =.... = Nn ^+HnALn (2) at at at
wherein N]1 N2 ... are Newton coefficients, H/, H^ ... are Ηooke coefficients, Lj, L2 ... are lengths of individual "parallel" combinations of Ηooke and Newton bodies.
The overall length of the body is:
Figure imgf000004_0001
wherein L1O are static lengths and ALt are deformations of partial ,,parallel" combinations of Ηooke and Newton bodies
The overall length is:
Lo =∑ 1 L* (4)
The overall deformation is:
AL = ^AL1 (5)
Concurrent methods predominantly use frequency characteristics or creep curves for identifying the model. Apparatuses are very complicated and usually also costly. Literature also describes methods based on measuring of further characteristics, for example transitional characteristics (e.g. Czech Patent No. 292 284).
US Patent No. 3,470,732 describes an apparatus intended for measuring a modulus of elasticity and so called ,,loss" modulus (which relates to the viscous properties of materials). The apparatus measures the deformation response to the sinusoidal force course, i.e. frequency characteristics, and it does not use any inertial member. It is intended for samples of plastic.
US Patent No. 4,165,634 relates to an apparatus for measuring samples of nylon, PE, rubber, composite material fibres etc. Measuring is performed on samples of materials. Based on the measurement of frequency characteristic, it enables to obtain a complex modulus of elasticity and so called mechanical loss.
The document JP 62 250 336 describes an apparatus intended for measuring samples of surface layers of composite materials. The sample is clasped between two clamps; in the middle of the sample an inertial body of a special construction is placed. The system of the sample and the inertial body is vibrated and dumped oscillations are measured. From the frequency and dumped oscillations, viscoelastic properties of the sample are calculated. Twist torsion is measured. Measuring is based on the generally known principle of calculation of viscoelasticity from dumped torsion oscillations. Measuring is possible only in torsion, it is impossible to measure the surfaces.
The document JP 63196838 relates to an apparatus determined for measuring skin viscoelasticity. The apparatus does not use an inertial body and enables to measure only responses to torsion.
The document DE 4 040 786 describes an apparatus intended for measuring viscoelastic properties of surfaces. The measuring principle consists in measuring the functionality between forces and deformations in static states (static measurements) and from measuring deformation reactions on a harmonious course of the force at different static tensions. The apparatus does not contain an inertial member and measures frequency characteristics.
US Patent No. 6,609,428 relates to an apparatus for determining the so called real and imaginary modulus (Young modulus and shear modulus) and the Poisson ratio. At one end of the sample, a ,,weight" (inertial member) is attached. When measuring, acceleration is scanned in a contact way. Frequency characteristics are thus measured. Moreover, parameters of rheological models are not determined from these measurements. The Czech Patent No. 292 284 described an apparatus for measuring viscoelasticity of tissues of live organisms. The measurement principle consists in measuring the time functionality of the deformation of a body surface as a response to the action of a rectangular impulse of the straining force. Based on this response, parameters of the rheological model are determined. However, the apparatus does not contain an inertial member, the sensing is not contactless and the apparatus is conceived only for measurement of objects under compressive stress.
The main disadvantages of methods and apparatuses used so far may be summarised as follows:
1) The influence of inertia of the measured material and inertia of the mobile part of the sensor constitute an important problem. Inertial forces principally influence the dynamics of the deformation response. The solutions so far either neglected or eliminated this influence by calculations. The correction of errors by calculations is complicated by the fact that the weight of mobile parts of the measured system (i.e. probe and the moving part of the measured sample) is difficult to determine. Hence the accuracy of the measurements is principally limited.
2) The sensitivity of methods used is limited by low values of deformations measured.
3) The influence of sensors, if they are in contact with the measured material, also deteriorates the accuracy of methods used.
The insufficiencies and problems of methods and apparatuses used so far for determining rheological characteristics of materials, in particular viscoelasticity, namely of biological samples, are solved simply and at the same time effectively by the method and apparatus according to the present invention.
Summary of the Invention
The above mentioned insufficiencies known from the state of the art are eliminated by the new method of measuring viscoelasticity and the new apparatus according to the present invention - a dynamic viscoelastometer with an inertial member (hereinafter referred to as the ,,inertial viscoelastometer").
The first main subject-matter of the invention is the method of measuring mechanical properties of materials, when at least one parameter characterising viscoelasticity of materials is estimated, consisting in attaching an inertial member to the measured sample of the viscoelastic material or to a probe put in contact with the sample of the viscoelastic material, the weight of the inertial member being substantially higher than the weight of the sample or the probe; the inertial member or the probe being at least partially formed of ferromagnetic material, and by means of the contactless sensor the impulse and/or transient characteristics are measured when straining the sample in tension, compression, bending or torsion.
The second main subject-matter of the present invention is the apparatus for carrying out the above mentioned method, i.e. the inertial viscoelastometer. Generally speaking, the inertial viscoelastometer according to the present invention is provided with an inertial member. The inertial member is chosen so that its weight be higher than the weight of other mobile parts of the measured system (i.e. the probe and the moving part of the sample). By attaching the inertial member, the error of measurement which originates due to the influence of inertial forces is thus eliminated, as the weight of the inertial member is known with a high accuracy and also the place of acting of its inertial force is well defined. Another advantage of this construction is the considerable improvement of sensitivity of measuring. This setting enables to use ferromagnetic material for manufacturing of the inertial member (or a part thereof). This technical solution is connected with the possibility of simple, contactless scanning of deformations.
The subject-matter of the invention is therefore the apparatus for carrying out the method according to the invention, the apparatus containing an inertial member tailored to be attached to the sample of the viscoelastic material or to the probe put into contact with the sample of the viscoelastic material, the weight of the member is substantially higher than the weight of the sample or probe, while the inertial member or probe are at least partially formed of ferromagnetic material, and further comprising a contactless sensor for sensing changes of the electromagnetic field of the contactless sensor, and a source of driving force for inducing dumped oscillating movement of the inertial member or probe.
In particular, the subject-matter of the invention is the inertial viscoelastometer, depicted schematically on Fig. 4, which contains the fixation means 1 for fixation of the sample 2 of the measured material, the inertial member 3., the contactless sensor 4 of the position, the electronics 6 processing the signal from the sensor and transmitting the information of the inertial member to a digital form, and further the computer 7 provided with software for identification of the model of the mechanical behaviour of the measured material and calculation of its parameters. The weight of the inertial member 3 is chosen so that the motion equation of the system consisting of the measured sample 2 and the inertial member 3 has a periodical solution, and concurrently, that the weight of the inertial member 3_ is substantially higher than the weight of the sample 2. The inertial member 3_ is at least partially formed of ferromagnetic material. The contactless sensor 4 generates a magnetic field in the vicinity of the inertial member 2 and detects the changes in the electromagnetic field induced by the motion of the inertial member 3. The information from the sensor 4 is processed by means of the computer. Si is the vibration direction for tension and pressure, S2 is the vibration direction for bending. The computer software solves the inversion problem. The energy for inducing motion of the system sample - inertial member is generated into the system by means of the source 5 of driving energy. The following is measured: responses to the force or energy impulse (impulse characteristics), responses to the force or energy jump (transitional characteristics), reactions on the rectangular force or energy impulse (creep curves). This basic embodiment of the invention may be modified for different types of measuring, such as measuring in straining of the sample in bending, tension, compression or torsion, and also for measurement of surfaces and membranes, as will be demonstrated in examples of advantageous embodiments of the apparatus according to the invention.
Viscoelastic bodies constitute an important group of biological materials from the mechanical point of view. Blood vessels, ligaments, cartilages, skin and bones belong to this group. The generally used quantification of stiffness (elastic modules) of these materials or other static characteristics (stress diagrams, differential modules, or strength limits) is not sufficient for description of mechanical behaviour of viscoelastic bodies. The fundamental limitation of these classic methods consists in the fact that they render information on the mechanical behaviour only in static state (under static straining). However, in real situations, materials are strained dynamically, which applies namely to biological structures.
When determining an adequate model for the particular material and for calculation of its parameters, dynamic reactions of the sample are measured. Usually, frequency characteristics are used. Identification of the model and calculations of parameters comes out from the mathematical description of the above mentioned rheological models.
The fundamental disadvantage of the above mentioned approach is the neglecting of the impact of the inertial component of the material, and in most cases also of the impact of inertia of the mobile part of the measuring apparatus (clamps, sensor etc.) This approach may lead to crucial errors. Even in case the neglecting of inertia is rightful and the system behaves according to the simple model on Fig. 3, this approach is less advantageous than the invention described in the present application. The dynamic responses measured have in this case a non-periodical character (the roots of the motion equation are real numbers). This fact limits the sensitivity and accuracy of measuring.
The solution according to the present invention has two substantial features: (1) connecting the sample or probe with the inertial member; and (2) detection using contactless sensor:
1) Connection with the inertial member
On the mobile part of the measuring apparatus, which consists of the measured sample (e.g. 2 on Fig. 4) of the material or the probe (e.g. H on Fig. 15), the inertial member (e.g. 3 on Fig. 4 or 33. on Fig. 15) is mechanically attached. The weight of the inertial member is chosen so that the following conditions are concurrently met:
a) The weight of the inertial member is higher than the limit for the periodical character of the response. The periodicity limit stems from the solution of the motion equation (see the below mentioned equation (6)). The motion equation has a periodical solution under the N2 condition M) at least for one of the ,,parallel" combinations of Hooke and Newton
AH bodies (see Fig. 6).
b) The weight of the inertial member is substantially higher than the weight of the moving parts of the measured body. In case of measuring samples of materials, the weight of the inertial member is chosen at least 5 times higher than the weight of the sample. In case of measuring surfaces, the weight of the inertial member is chosen at least 5 times higher than the weight of a cylinder of the measured material, having the area of the size of the opening in the fixation apparatus, and the length of 0,5 mm.
When the above conditions are met, the sensitivity of determining parameters is increased, and the accuracy of measurement is improved.
Identification of parameters of the measured object is carried out with a sufficient accuracy and sensitivity based on impulse or transitional characteristics, which is simpler and consequently cheaper than the usual measuring of frequency characteristics.
After attaching the inertial member (see Fig. 5), the simple model according to Fig. 3 is changed to a more complex model according to Fig. 6.
The model according to Fig. 6 is described by the following equation system, which differs from the equation system ((1) — (4)) in that the influence of inertial forces is included:
For equilibrium of forces, it applies:
F (O=M^-+*, f- + lW =M N2 2AL2 =... =M^+N,, ^+Hn Mn (6)
wherein M is the weight of the inertial member, Ni1 N2 ...are Newton coefficients, Hi1 H2 ...are Hooke coefficients, Li, L2 ... are lengths of individual ,,parallel" combinations of Hooke and Newton bodies (see the model on Fig. 6).
The overall length of the body is:
Z(O =∑ (Ao +Ai1) (7) wherein Lto are static lengths and ΔZ,- are deformations of partial ,,parallel" combinations of Hooke and Newton bodies
The overall static length is
Lo =∑Lw (8) i
The overall deformation is:
AL=∑AL, (9) i
It must be emphasised that the weight M is known with a high accuracy. It is given by the weight of the inertial member, which is very precisely determined by weighing.
The above equation system enables to express the functionality of ΔL(t) on the driving force F(t) and on the parameters of the system. Subsequently, it is possible based on measurement of the particular courses of ΔL(t), i.e. based on dynamic characteristics, to determine parameters of the model.
2) Detection by contactless sensor
The detection of length changes (when measuring in tension) and measuring the position (when measuring in bending), optionally also measuring the acting forces is usually carried out by sensors which are in contact with the measured object. This implementation leads to error occurrence, due to the influencing of the measured object by sensors.
In the apparatus according to the present invention, detection is carried out without contact, based on the principle of the electromagnetic induction. Induction and inductive sensors in their regular implementation do not work without contact, they require that their mobile part be mechanically connected with the measured system. In case of the apparatus according to the invention, this necessity is avoided by the fact that the inertial member is at least partially formed of ferromagnetic material and substitutes the mobile part of classical sensors. The contactless sensor 4 (see Fig. 4) in the embodiment according to the invention generates a magnetic field in the vicinity of the inertial member 3_ and scans the changes of the electromagnetic field induced by the motion of the inertial member 3. The advantages are the elimination of the influence of the sensor 4 on the measured sample 2 and a high sensitivity of detection.
The advantageous embodiments of methods of measurement and apparatus according to the present invention are for better understanding described in detail in the following examples 1 and 2 and the advantageous embodiments of the apparatus according to the invention are shown on the attached figures 7 to 9 and 14 to 17.
Description of figures on the drawings
Fig. 1 represents basic rheological bodies. Fig. IA represents the Hooke body and the equation describing its behaviour, Fig. IB represents the Newton body and the equation describing its behaviour.
Fig. 2 represents the Voigt model consisting of the parallel combination of Hooke and Newton bodies.
Fig. 3 represents the rheological model containing ,,serial-parallel" combination of Hooke and Newton bodies.
Fig. 4 shows a general scheme of dynamic inertial viscoelastometer — i.e. the apparatus according to the invention.
Fig. 5 represents the rheological model of the inertial member.
Fig. 6 represents the rheological model with the inertial member M. Fig. 7 shows schematically the apparatus according to the invention, modified for measuring viscoelasticity of a sample in straining of the sample in bending. A is a front view, B is a ground plan.
Fig. 8 is a scheme of the contactless sensor with the probe 8 of the sensor.
Fig. 9 is a scheme of the source of the driving force and/or the lifting electromagnet.
Fig. 10 represents the schematic process of data processing.
Fig. 11 represents the rheological model containing 2 components.
Fig. 12 is a graph of the course of deformation when measuring according to Example 1. The authentic record of measuring.
Fig. 13 is a graph of dumped oscillations of component 1 of the model, with a periodical response. The data are re-calculated from the course of deformations according to Fig. 12.
Fig. 14 shows schematically the apparatus according to the invention modified for measuring viscoelasticity of a sample in straining of the sample in tension and pressure.
Fig. 15 shows schematically the apparatus according to the invention modified for measuring surfaces of materials.
Fig. 16 shows schematically the apparatus according to the invention modified for measuring membranes.
Fig. 17 shows schematically the fixation apparatus for membranes. Examples
Example 1
Determining parameters of viscoelasticity of materials during bending stress or tensile stress
Ia) Measuring based on bending stress
The embodiment of the inertial viscoelastometer for measuring samples of bones in bending is schematically depicted on Fig. 7.
The sample 2J_ of the measured material has the following dimensions: 15 to 50 mm (length), 1 to 10 mm (lateral dimensions) and it is mechanically attached to the fixation apparatus 1.1.
At the lower end of the measured sample 2Λ_, the inertial member 3J, of ferromagnetic material (steel) is attached. The weight of the inertial member 3J, is chosen from a set of inertial members. The weight of the inertial member 3J, must be at least 5 times higher than the weight of the sample 2J,. Inertial members 3JL have the shape of a block. The length (in the direction of vibrations) is 2 times longer then the dimension perpendicular to the vibrations (depth). There are inertial members ranging from 2 to 150 g of weight in the set.
The sensor 4J, is formed by a cylindrical coil (schematically depicted on Fig. 8) containing 6000 turns of enamelled wire, Cu, 0.14 mm, with a ferromagnetic core, length 55 mm, diameter 5 mm, which constitutes the probe 8 of the contactless sensor. The probe 8 projects 10 mm above the upper end of the coil. It is attached in parallel to the edge of the inertial member 3J,. In a static state, the distance between the edge of the inertial member 3.1 and the near edge of the probe 8 ranges from 1 to 2 mm. The system consisting of the measured sample 2Λ. and the inertial member 3J^ is mechanically vibrated by the source of the driving force 5J,. The vibration amplitude must be lower or identical compared to the distance of the probe 8 of the sensor from the inertial member 3J,. The introduction of the driving force is done by switching on and off the source of the driving force 5J^, which is the electromagnet schematically depicted on Fig. 9, formed for example of a coil, 5 500 turns of enamelled wire, Cu, 0.22 mm, with ferrous core, length 45 mm, diameter 10 mm.
The signal from the sensor 4_Λ_ is processed by the electronics 6A_ and converted into digital form and further processed by the computer. The block diagram of the data processing process is on Fig. 10. Here, block A represents the measured system, B is A/D converter, C represents loading and saving measured data, D is the model selection, E is identification of parameters. The magnitude X represents the time flow of the driving force or energy. The magnitude Y represents the time flow of the deformation response. The output Z represents rheological parameters (N1, N2, H1, H2, see Fig. 11) of the model. By means of the computer, the course of responses is recorded and parameters of the sample materials are calculated for the rheological model which is depicted on Fig. 11.
An example of identification of the model and relations for calculation of parameters
The rheological model mentioned on Fig. 11 is described by the following equation system:
d2L
Fs (t)=M-7^+N1 ^+H1L1 =jtf —- ++##,2 - ^^+H2L2 (10) dt2 λ dt λ l dt1 dt
wherein L is the deflection of the end point of the sample.
Figure imgf000015_0001
wherein Lift) and ∑2(t) are deflection falling on individual components of the model (Fig. 11).
Between members Tj and T 2 this ratio applies: MAX (12)
H1 L '2MAX
wherein LIMAX QΩA L2MAX^z deformations in individual components of the model in steady state in the beginning of measurement.
The weight of the inertial member is chosen so that the first component of the model have
N 2 a periodic response to the impulse or jump of the driving force (M) — — ) and the second
4H1 component have the reaction to impulse and jump of the driving force aperiodic
(M(^M.
4H2
It applies for the response to a jump force of the periodical component:
Figure imgf000016_0001
It applies for dumping the oscillation amplitude:
IZi = N1 / 2M (14)
wherein Mis the weight of the inertial member.
It applies for the frequency of dumped oscillations:
Figure imgf000016_0002
The time response of the second component of the model is aperiodic.
The time response of the second component of the model to a jump is in this case:
L2 (t)=^(l-e -k>1) (16)
I2 = H2 ZN2 (17) Process of measuring and calculating
1) A measurement of the time response of the sample to the bending deformation jump is carried out. The resulting response has a course determined by the sum of the partial deformation Lj(t) of the first component (see equation (13)) and the partial deformation ∑2(t) of the second component (see equation (16)). Using the equations (13) and (16), the
magnitudes k], Jζ2, (o, and the ratio XMAX are determined from the course of response.
2) Further, the parameter Nj is determined based on the equation (14).
3) The parameter Hj is determined from the equation (15).
4) The parameter H? is determined from the equations (12), (13) a (16).
5) The parameter N 2 is determined from the equation (17).
Application of the method according to the invention on a biological sample
The measurement was carried out on a cut of the spongy part of caput femoris, which forms the head of the hip joint from a human donor (healthy male, 70 years). The material was obtained after a hip joint surgery. After the surgery, it was preserved for two weeks in a physiological solution at a temperature of - 15 °C. Before the experiment, a sample was cut from the joint head at a laboratory temperature (240C).
Subsequently, the measurement in bending was carried out by the above mentioned method.
Sample: dimension 25 x 3.5 x 3.5 mm, weight 0.61 g
Inertial member: weight M= 141.6 g, dimensions 20 x 20 x 45 mm
On Fig. 12, the deformation measurement as a function of time is recorded. On Fig. 13, the values - dumped oscillations of component 1 of the model - obtained from values shown on Fig. 12 are calculated.
The results of measuring are summarised in Table 1.
Tab. 1. Parameters of the sample of the spongy part of caput femoris
Figure imgf000018_0001
The weight of the first component represents the ratio between the deformation of the first (quick) component related to the overall deformation in a steady state.
The obtained parameters show the important influence of viscose components in dynamic straining. The influence of viscose components on the responses grows with the frequency of the driving force and manifests itself with the growth of dynamic stiffness of materials. The growth of the dynamic stiffness (compared to the stiffness at static straining) will be substantial (over 10 %) in the first component in frequencies over 7 Hz. In case of the second component, already over 0.05 Hz.
As the time constants (N/H) of the first component are 2.3 ms (see Tab.l) and of the second component 0.29 S5 it is obvious that in case of an rectangular impulse of the straining force, the deformation in the first component will reach 90 % of the deformation in static state after 5 ms and in the second component after 0.65 s. This practically implies that responses of deformations on impulses of forces shorter than about 0.5 s are reduced significantly by the influence of the viscose component N2, compared to the static straining. In other words, the viscose component N2 limits the straining of the sample in straining by rectangular impulses shorter than the order of seconds. The implications for analysis of the risk of fractures are obvious.
Ib) Measuring based on tensile stress and compression stress
Determining viscoelastic parameters in tensile stress may be carried out by means of the modified apparatus according to Fig. 14. The inertial member 32 is in this embodiment of a cylindrical shape. The diameter of the cylinder is within the range from 5 to 15 mm, its length is from 10 to 30 mm. The ratio of the length to the diameter from 2 : 1 to 2,5 : 1. The material is steel. The weight is within the range from 1.54 to 50 g. The inertial member 32. is lead by using of the guidance means 12 of the inertial member. The guidance means 12 is formed for example of a glass pipe having the diameter larger by 0.1 mm than the diameter of the inertial member 32. The guidance means 12 of the inertial member prevents the lateral deviations in vibration. The guidance means JJ2 of the inertial member, the inertial member 3^2 and the sample 22. are attached coaxially, the axis being vertical.
Example 2
Determining parameters of viscoelasticity of materials in measuring surfaces of materials and membranes
2a) Measuring viscoelastic properties of surfaces of materials
On Fig. 15, there is a scheme of a modification of the inertial viscoelastometer for measuring surfaces of materials. The apparatus represents a fundamental improvement of an older apparatus according to the Czech Patent No. 292284. The inertial member 33. is of a cylindrical shape in this embodiment. The diameter of the cylinder ranges from 5 to 15 mm, the length from 10 to 30 mm. The ratio of the length to the diameter from 2 : 1 to 2,5 : 1. The material is brass. The weight is within the range from 1.7 to 50 g. The inertial member 33. is placed on the probe J3, between the deformation sensor 16 and the fixation apparatus 1_5. The probe 13. is formed of a cylindrical stick, for example from glass or plastics, having diameter for example 4 mm. At the lower end, it is closed by a spherical cap or flat. At the upper end, it is closed by a circular foot 14 having the diameter of 8 mm, the foot is of steel. The lifting electromagnet 53. may be made for example as depicted on Fig. 9. The measurement is carried out so that the probe J3 with the inertial member 33. is lifted by the electromagnet 53. 5 to 10 mm above the surface of the measured sample 23. By switching off the electromagnet 53. the probe J3 with the inertial member 33. is dropped in a free fall on the surface of the sample body 23.. The sensor 16 (according to the patent No. 292284) measures the time course of the deformation. The electronics 63_ and the computer 73. provided by the software may be optionally the same as in the measurement according to Ia).
Process of calculation:
AL1
1. The parameters of course are determined: kj, fø, ω, IMAX
AL '.2MAX
2. The following process is similar as the measurement according to Ia). The theoretical relations and calculations are analogous as well.
Application of the method according to the invention on a biological sample
The measurement was carried out on the skin of the left palm of a female, 58 years, in the area in the middle and above facilis brevis by the method described above. The palm was degreased and heated by an IR lamp to about 30°C. Subsequently, it was fixed pneumatically by the pressure of 60 mmHg. The weight of the inertial member and probe was 37.8 g. The opening of the fixation apparatus had a diameter of 20 mm.
Results of measurement are mentioned in the following Table 2.
Tab. 2. Parameters of the rheological model of behaviour of the surface of human body
Figure imgf000020_0001
The results show the behaviour of the surface of human body is in accordance with the model according to Fig. 3. They also show the importance of viscose components of the behaviour and their importance for cosmetics and dermatology. The intuitively used term ,,skin elasticity" may be more precisely replaced by speed constants (H//N; a T2/N2, see Tab. 2).
2b) Determining viscoelastic properties by measuring surfaces of membranes
On Fig. 16, there is a scheme of modification of the inertial viscoelastometer modified for measuring membranes. The apparatus differs from the variant 2a by a different method of fixation of the sample. The rest of the setting is virtually identical.
On Fig. 17, the fixation apparatus 15.1 (see Fig. 16) for measuring membranes is depicted in detail. The sample 2Λ of the measured membrane is clasped between two desks 17, provided with a concentric and evenly big opening 18 for passage of the probe. The measured membrane 2_5_ is attached between the fixation desks 17 by means of the screws 19. The size of the opening 18 is within the range from 5 to 50 mm (diameter).
The theories, relations and processes are analogous to the previous example.
A person skilled in the art will understand that the embodiments described in examples do not limit the scope of the invention only to those embodiments. Based on the invention idea, description of the invention and examples of preferred embodiments, a person skilled in the art would easily find other advantageous modifications of the embodiments, or other advantageous embodiments. A person skilled in the art can, for example, modify the apparatus according to the invention for measuring in torsion. All such modifications or advantageous embodiments fall within the scope of the present invention. Industrial applicability
The present invention will find its use in medicine and in the biological research, in the industry of prosthetic and auxiliary materials for medicine, optionally in textile, rubber, and plastic industries.
The research of viscoelastic properties of biological materials is crucial for understanding the dynamics of mechanical reactions of cardiovascular system, bones, ligaments and other structures. The importance of these characteristic is diagnostic and is crucial also for a suitable selection of mechanical parameters of auxiliary and prosthetic materials. The apparatus according to the invention will enable precise and at the same time cheap measurement of these properties. It will also enable obtaining of a more detailed description of viscoelastic properties of materials compared to the current possibilities. It will enable an analysis of risks of harm in dynamic straining with respect to diseases (osteoporosis, sclerotic changes etc.), nutrition and ageing.
Until now, little attention has been given to mechanical conformation of prosthetic and auxiliary materials in health service. The proposed device will enable precise and cheap solution of these problems.
The invention will enable technology enhancement, especially in textile and shoe manufacturing industries, its use is prospective also in rubber industry and in research and production of plastics.

Claims

PATENT CLAIMS
1. A method of measuring mechanical properties of materials when at least one parameter characterising the viscoelasticity of materials is estimated, characterised in that an inertial member is attached to the measured sample of the viscoelastic material or to a probe put in contact with the sample of the viscoelastic material, the weight of the inertial member being substantially higher than the weight of the sample or probe, while the inertial member or probe are at least partially formed of ferromagnetic material, and impuls and/or transient characteristics in straining of the sample in bending, tension, compression or torsion are measured by means of a contactless sensor.
2. The method according to Claim lcharacterised in that the weight of the inertial member is at least 5 times higher than the weight of the sample or probe, and at the same time the motion equation of the system consisting of the sample and the inertial member or the sample, the probe and inertial member has a periodic solution, while the system consisting of the sample and the inertial member or the sample, the probe and the inertial member is set into motion in the magnetic field of the contactless sensor by means of the source of driving force, and subsequently the changes of the electromagnetic field of the contactless sensor, induced by the dumped oscillating motion of the inertial member or probe, are detected.
3. The method according to Claim Ior2 characterised in that at least one parameter of the rheological model of viscoelastic material is determined from the measured values.
4. An apparatus to carry out the method according to claims 1 to 3 characterised in that it contains an inertial member modified for attaching to the sample of the viscoelastic material or the probe put into contact with the sample of the viscoelastic material, the weight of the inertial member being substantially higher than the weight of the sample or probe, while the inertial member or probe are at least partially formed of ferromagnetic material, and further comprises a contactless sensor for detecting the changes of the electromagnetic field of the contactless sensor, and a source of driving force for inducing the dumped oscillating motion of the inertial member or probe.
5. The apparatus according to Claim 4characterised in that the weight of the inertial member is at least 5 times higher than the weight of the sample or probe, and concurrently the motion equation of the system consisting of the sample and the inertial member or the probe and the inertial member has a periodic solution.
6. The apparatus according to Claims 4or 5 characterised in that the sensor for contactless detection of the electromagnetic field is formed of a coil with a ferromagnetic core.
7. The apparatus according to Claims 4to 6characterised in that the source of the driving force is an electromagnet.
8. The apparatus according to Claims 4to7characterised in that it is modified for measuring in straining the sample in bending, tension or compression so that an fixation means is tailored to fix the upper end of the sample and the inertial body is tailored to fix the lower end of the sample.
9. The apparatus according to Claims 4to7 characterised in that it is modified for measuring surfaces of materials so that the fixation means is tailored to fix the surface of the sample and the inertial body is tailored to be attached to the probe which is put into contact with the surface of the measured sample
10. The apparatus according to Claim 9 characterised in that it is modified for measuring membranes so that the fixation means is modified for fixation of membranes.
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WO2011048110A1 (en) * 2009-10-19 2011-04-28 Université Paris Diderot - Paris 7 Method for determining the response of a contractile or expansible biological material
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EP3112836A3 (en) * 2015-07-02 2017-01-11 Universita Degli Studi di Trieste Device and method for detecting the structural integrity of a sample object

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