GB2607931A

GB2607931A - Indentation plastometry

Info

Publication number: GB2607931A
Application number: GB2108593.1A
Authority: GB
Inventors: Dean James; William Clyne Trevor; Edward Burley Max; Edward Campbell James
Original assignee: Plastometrex Ltd
Current assignee: Plastometrex Ltd
Priority date: 2021-06-16
Filing date: 2021-06-16
Publication date: 2022-12-21
Also published as: GB202108593D0

Abstract

A method of preparing an indenter for indentation plastometry includes providing an indenter with initial state contact surface, providing a pre-hardening sample, applying a pre-hardening load to the indenter to press the indenter into the pre-hardening sample to plastically deform the indenter to alter the contact surface, measuring the shape and size of the altered contact surface. A method of performing indentation plastometry includes: providing an indenter with the altered contact surface and a test sample; applying a test load to press the indenter into the test sample, the test load being less than the pre-hardening load, measuring shape and size of the indentation in the test sample, determining plastic properties of the material of the test sample from the shape and size of the altered-state contact surface, the measured indentation shape and size, the test load, and the elastic properties of the indenter and the test sample.

Description

INDENTATION PLASTOMETRY

Field of the Invention

The present invention relates to indentation plastometry.

Background

Inelastic mechanical properties materials are conventionally obtained via uniaxial (tensile or compressive) tests. However, use of an indentation-based technique brings important advantages. These include a reduction in the size and shape requirements for the sample. Commonly, only a (small) flat plate is used. Also, the fact that properties are being measured in a relatively small region allows local variations in properties to be examined over the surface of a component.

More particularly, indentation plastometry generally involves penetrating an indenter into a sample of the material, removing the indenter, measuring the residual indent profile, and then performing iterative numerical modelling of the indentation process. The indenters are typically made of a material much harder than the sample, so as to prevent deformation of the indenter, and usually have a convex surface for engaging with the sample. The convex surface causes the size and shape of the (concave) indentation to correspond to the depth of penetration. A typical type of indenter used in indentation plastometry has a spherical surface and is made of a ceramic or cermet material.

"Hardness" by contrast is a commonly-used material property measured in a traditional hardness test, with hardness values of a material typically being obtained by measuring just the lateral dimensions of a residual indent created by the application of a given load for a short period of time. Hardness is related to the yield stress and work hardening characteristics of the material, but is not a well-defined property. For instance, a variety of hardness values are obtained using different indenter shapes and applied loads As such, characteristics of materials such as yield stress and work hardening behaviour cannot be determined from traditional hardness tests.

In order to determine such characteristics, indentation plastometry, as mentioned above, generally involves iterative numerical modelling of the indentation process, using a Finite Element Method (FEM) for example, with the plastic deformation of the material being captured in the form of a constitutive law containing adjustable parameters, and repeated comparisons being made between experimental and modelled outcomes. In this way, software packages can be devised that allow automated extraction of best fit plasticity parameter values via processing of experimental indentation data. A similar methodology can be used to obtain information about residual stresses in samples. A number of papers [1-11] have been published regarding various details of such methodologies, which have also recently been extended to the characterisation of the creep behaviour of materials [12]. The performance of indentation, profilometry and iterative numerical modelling procedure is termed PIP (Profilometry-based Inverse FEM Indentation Plastometry).

PIP commonly involves use of indenter balls made of a cermet (such as WC particles bonded together by a small volume fraction of a metal such as Co). These are widely used (particularly for bearing applications) and hence are readily (and cheaply) available in the form of perfect spheres (in the required range of radius, which is around 1 mm). They exhibit an excellent combination of hardness and toughness [13-15] commonly being reported as having a yield stress of around 5 GPa and a fracture toughness, Kit, of at least about 10 MPaNim. This relatively high toughness ensures that cracking of the ball during testing is very unlikely. This would be a potential danger with harder balls, which would almost certainly have to be ceramics of some type.

There is, however, a danger that a cermet indenter could undergo at least some plastic deformation during indentation. In general, it is observed [16] that such deformation tends to become significant when the ratio of the yield stress of the indenter to that of the sample is no greater than about 2, although there is naturally also a dependence on the load being applied (and hence on the penetration depth). Since there is interest in testing hard metals, particularly steels, with yield stresses of up to about 2 -3 GPa, this is a problem. Moreover, the problem cannot readily be avoided by limiting the applied load (and hence the penetration depth) during the test. In order to obtain a stress-strain curve that is reliable up to useful levels of plastic strain, the indentation test must involve the generation of strains in an appropriate range, which typically requires the penetration ratio (depth, 5, over indenter radius, R) to be of the order of 20% The present invention has been devised in light of the above considerations. 20 Summary of the Invention The present invention is at least partly based on a realisation that, rather than ensuring that the indenter retains its original shape during testing, it is possible to deliberately subject it to an initial treatment that causes plastic deformation of the indenter, which is then suitably characterised for use in subsequent testing. The deformation can be such that there will then be no further change to the indenter during the subsequent testing, even if the testing is on samples that have a very high hardness (up to a prescribed level).

Accordingly, in a first aspect, the present invention provides a method of preparing an indenter for the performance of indentation plastometry, the method including steps of: providing an indenter having an initial-state contact surface; providing a pre-hardening sample; applying a pre-hardening load to the indenter to press the indenter at its initial-state contact surface into the pre-hardening sample, the pre-hardening load being such and the hardness of the pre-hardening sample being such that the indenter is plastically deformed by the application of the pre-hardening load to alter the contact surface; and measuring the shape and size of the altered-state contact surface.

Advantageously, the plastically deformation of the indenter can produce work hardening that raises the flow stress of the indenter to such an extent that it does not plastically deform during the subsequent testing. The measured shape and size of the altered-state contact surface are characteristics of the indenter which allow it to be used in that testing.

The shape and size of the altered-state contact surface are typically measured by profilometry, i.e. by measuring one or more profiles of the altered-state contact surface The shape of the initial-state contact surface is preferably axisyrinnetric, and more preferably lies on a surface of a sphere. Conveniently, for example, the indenter before application of the pre-hardening load can be a sphere. The shape of the altered-state contact surface is also preferably axisymmetric, and more preferably also lies on a surface of a sphere. However, the sphere which may define the altered-state contact surface is typically different from the sphere which may define the initial-state contact surface. Generally the sphere which may define the altered-state contact surface has a larger radius.

The ratio of the yield stress of the material of the indenter prior to being plastically deformed to the yield stress of the material of the pre-hardening sample may be 4 or less, and preferably is 2 or less. This helps to ensure a suitable amount of plastic deformation of the indenter. Conveniently, the material of the pre-hardening sample, may be the same as the material for which the indenter is to be used in performance of subsequent indentation plastometry, or it may be a material of the same or higher hardness.

The ratio of the depth of penetration formed in performance of subsequent indentation plastometry using the indenter to the radius of that indentation may be at least 0.1, and preferably is at least 0.15 or 0.2.

Accordingly, the ratio of the depth of penetration formed in the pre-hardening sample by the indenter to the radius of that indentation may be at least 0.2, and preferably is at least 0.3. This helps to ensure a suitable amount of plastic deformation of the indenter such that it will not deform further in the subsequent indentation plastometry.

The material of the indenter may be a cermet, such as a WC-Co cermet.

In a second aspect, the present invention provides a method of performing indentation plastometry, the method including steps of: providing an indenter that has been plastically deformed by application of a known pre-hardening load which pressed the indenter at a contact surface thereof into a pre-hardening sample, thereby altering the contact surface, the altered-state contact surface having a predetermined shape and size; providing a test sample; providing elastic properties of the indenter and the test sample; applying a test load to press the indenter at its altered-state contact surface into the test sample, the test load being less than the pre-hardening load; measuring the shape and size of the indentation formed in the test sample by the pressing of the indenter therein; and determining plastic properties of the material of the test sample on the basis of the predetermined shape and size of the altered-state contact surface, the measured shape and size of the indentation, the test load, and the elastic properties of the indenter and the test sample.

The step of providing an indenter can be implemented by performing the method of the first aspect.

The shape and size of the indentation are typically measured by profilometry, i.e. by measuring one or more profiles of the indentation.

Thus, the altered-state contact surface may be axisymmetric, and preferably lies on a surface of a sphere. The ratio of the yield stress of the material of the indenter prior to being plastically deformed to the yield stress of the material of the test sample may be 4 or less, and preferably is 2 or less. The ratio of the depth of the indentation formed in the test sample by the indenter to the radius of that indentation may be at least 0.1, preferably is at least 0.15, and more preferably is at least 0.2. The material of the test sample may be the same as the material of the pre-hardening sample. The material of the indenter may be a cermet, such as a WC-Co cermet.

The determining of the plastic properties of the material of the test sample may be performed by numerically modelling the penetration of the indenter into the sample, for example by FEM modelling.

The test load may be up to 75% of the pre-hardening load. For many cases, by avoiding test loads which are higher than 75% of the pre-hardening load, it is possible to prevent further plastic deformation of the indenter during the indentation plastometry.

Additionally to, or separately from, preparing an indenter for the performance of indentation plastometry on a test sample as discussed above, the present invention is at least partly based on a realisation that deliberately plastically deforming an indenter provides a convenient method for determining plastic properties of the material of the indenter.

Accordingly, in a third aspect, the present invention provides a method of determining plastic properties of the material of an indenter, the method including steps of: providing an indenter having an initial-state contact surface of known shape and size; preparing the indenter by performing the method of the first aspect; providing elastic and plastic properties of the material of the pre-hardening sample, and elastic properties of the material of the indenter; and determining plastic properties of the material of the indenter on the basis of: the change in the shape and size of the contact surface between its initial and altered-states, the pre-hardening load, the elastic and plastic properties of the material of pre-hardening sample, and the elastic properties of the material of the indenter.

The method may include an in initial step of measuring the shape and size of the initial-state contact surface of the indenter. Typically, these are measured by profilometry, i.e. by measuring one or more profiles of the initial-state contact surface.

The determining of the plastic properties of the material of the indenter may be performed by numerically modelling the penetration of the indenter into the pre-hardening sample, for example by FEM modelling.

In a fourth aspect, the present invention provides an apparatus for performing indentation plastometry, the apparatus including: an indenter that has been plastically deformed by application of a known pre-hardening load which pressed the indenter at an initial-state contact surface thereof into a pre-hardening sample, thereby altering the contact surface, the altered-state contact surface having a predetermined shape and size; a testing machine configured to: apply a test load to press the indenter at its contact surface into a test sample, the test load being less than the pre-hardening load; a profilometer configured to: measure the shape and size of the indentation formed in the test sample by the pressing of the indenter therein (i.e. by measuring one or more profiles of the indentation); 15 and a computer programmed to execute a numerical model which determines plastic properties of the material of the test sample on the basis of: the predetermined shape and size of the altered-state contact surface, the measured shape and size of the indentation, the test load, and elastic properties of the indenter and the test sample.

Accordingly, the apparatus is suitable for performing the method of the second aspect.

The shape and size of the altered-state contact surface are typically measured by profilometry, i.e. by measuring one or more profiles of the altered-state contact surface.

Thus the shape of the initial-state contact surface may be axisymmetric, and preferably lies on a surface of a sphere. The altered-state contact surface may be axisymmetric, and preferably lies on a surface of a sphere. The sphere which may define the altered-state contact surface is typically different from the sphere which may define the initial-state contact surface, generally the sphere which may define the altered-state contact surface having a larger radius. The ratio of the yield stress of the material of the indenter prior to being plastically deformed to the yield stress of the material of the test sample may be 4 or less, and preferably is 201 less. The ratio of the depth of the indentation formed in the test sample by the indenter to the radius of that indentation may be at least 0.1, preferably is at least 0.15, and more preferably is at least 0.2. The material of the indenter may a cermet, such as a WC-Co cermet. The numerical model may be an FEM model.

In a fifth aspect, the present invention provides a computer program comprising code which, when the code is executed on a computer, causes the computer to execute a numerical model (such an FEM model) which determines plastic properties of the material of an indenter on the basis of an indentation plastometry test in which: an indenter has a pre-hardening load applied thereto to press the indenter at an initial-state contact surface thereof into a pre-hardening sample, the pre-hardening load being such and the hardness of the pre-hardening sample being such that the indenter is plastically deformed by the application of the pre-hardening load to alter the contact surface, the shape and size of the initial-state contact surface being measured, and the shape and size of the altered-state contact surface being measured; wherein the numerical model determines the plastic properties of the material of the indenter on the basis of: the change in the shape and size of the contact surface between its initial and altered-states, the pre-hardening load, elastic and plastic properties of the material of pre-hardening sample, and elastic properties of the material of the indenter.

Accordingly, the computer program is suitable for performing the method of the third aspect.

In a sixth aspect, the present invention provides a computer readable medium storing the computer program according to the fifth aspect.

In a seventh aspect, the present invention provides a computer programmed to execute the computer program according to the fifth aspect.

The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

Summary of the Figures

Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows schematically a testing machine configured to perform indentation plastometry; Figure 2 is a nominal stress-strain plot from tensile testing of a Maraging 350 steel, up to the onset of necking; Figure 3 shows experimental data for the effective radius of the contact surface of a cermet ball near its indentation axis, after being pushed into a Maraging 350 steel pre-hardening sample, with progressively increasing loads; Figure 4 is a comparison between an experimental profile of a ball contact surface after application of a 11.4 kN load and the best fit circle (radius of 1.06 mm); Figure 5 is a comparison of an experimentally determined ball contact surface after application of a 11.4 kN load, and a corresponding FEM modelled ball contact surface using Voce parameter sets; Figure 6A shows a modelled plastic strain field in the Maraging 350 steel pre-hardening sample, and Figure 6B shows the shows the corresponding modelled plastic strain field in the cermet ball after application of the 11.4 kN load; and Figure 7 shows FEM-predicted ball contact surface profiles before and after pressing a pre-plastically deformed ball into a test sample.

Detailed Description of the Invention

Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

Figure 1 shows schematically a testing machine configured to perform indentation plastometry. The apparatus includes a sample base 1 and an indenter housing 2. The testing machine is configured to move the indenter housing 2 towards and away from the sample base 1, such that the relative displacement between the sample base 1 and the indenter housing 2 along the loading axis changes. This relative displacement is measurable by a displacement measurement system 3 connected to the indenter housing 2 and the sample base 1. The displacement measurement system may comprise a linear variable displacement transducer (LVDT) or another device for measuring displacement.

Sandwiched between the sample base 1 and the indenter housing 2, are a test sample 4 and an indenter 5. The sample 4 is mounted onto the sample base 1 so as to be fixed relative thereto, and the indenter 5 is held by the indenter housing 2.

The indenter 5 is typically made of a ceramic or cermet material. This ensures that the indenter does not deform significantly during performance of indentation plastometry. In general, the indenter 5 is made of a material that is significantly harder than the sample material under the conditions of the plastometry testing. The sample 4 is significantly larger than the indenter 5. The contact surface is that part of the indenter 5 which contacts the sample 4 when the indenter 5 and the sample 4 are brought together.

When performing the indentation plastometry, the testing machine offers the indenter 5 to the sample 4 such that a contact surface of the indenter 5 rests on the sample 4. The testing machine then applies a load to the indenter 5, to press the indenter 5 into the sample 4, forming an indentation in the sample matching the shape of the contact surface.

As the applied load causes the indenter 5 to progressively penetrate into the sample 4, the displacement measurement system 3 measures and records the relative positions of the indenter housing 2 and the sample base 1, which corresponds to the depth of penetration into the sample 4 by the indenter S. The contact surface has a known shape and size. The shape and size of the corresponding indentation are directly measured, typically by profilometry, after release of the load.

After the indenting and profilometry are completed, FEM meshes superimposed onto the sample 4 and the indenter 5 are used to numerically model the penetration of the indenter 5 into the sample 4 by a computer using the known shapes and sizes of the contact surface and the indentation, and a constitutive law, which typically includes the elastic properties of the indenter and the sample.

In conventional indentation plastometry, measures are taken to ensure that the indenter does not plastically deform during indentation, for example by ensuring that the ratio of the yield stress of the indenter to that of the sample greater than about 2. However, as discussed above, the present invention is based on a different concept which involves deliberately plastically deforming the indenter prior to performing indentation plastometry.

In order to confirm what is expected to happen during such deliberate "over-loading" indentation, it is desirable to have agreement between measured and modelled indenter profiles after such a procedure. A (hard) sample can be used that has been well-characterized in terms of its yielding and work hardening characteristics (stress-strain curve). However, obtaining appropriate yielding and work hardening characteristics of the indenter presents more of a challenge. In particular, work hardening of the indenter can be significant, since the avoidance of later plasticity is at least partly dependent on the prior plastic deformation which causes the effective yield stress for subsequent loading to rise. Moreover, since the volume fraction of metal in cermets tends to be low (-10%), and the plastic deformation effectively takes place exclusively within it, there is an expectation that its work hardening rate will be (beneficially) high.

However, obtaining these yielding and work hardening characteristics in a conventional way, such as by uniaxial testing, would be highly challenging. Tensile testing (if a suitable sample shape could be created) would result in early fracture. Compression testing might be possible, but friction would lead to barrelling, such that both the generation and measurement of relatively large, uniform plastic strains would be difficult. Thus the interpretation of test data would be complicated. Furthermore, the manufacturing procedure would have to be different for the production of samples suitable for uniaxial loading, since machining of indenter balls into such shapes would be very problematic. The suppliers of such balls tend, at best, to quote only hardness numbers. These are not uniquely or accurately related to the stress-strain curve. For a material exhibiting no work hardening, the ratio of Vickers hardness to yield stress is often taken to be between 2.5 and 3. An Hv value quoted by suppliers for a typical (WC-Co) cermet is usually -1600 kgf mm-2, corresponding to a yield stress of the order of 5-6 GPa. In fact, this probably represents some kind of "flow stress" over a (poorly-defined) plastic strain range, although it should not really be regarded as more than a semi-quantitative estimate.

A better approach is therefore to treat the yield stress and work hardening characteristics (i.e. the stress-strain curve) of the indenter as parameters that can be adjusted to obtain the best fit between measured and modelled indenter profiles (and residual indentation profiles) after loading to different levels. Once this stress-strain curve has been established, further modelling (using the new starting shape of the indenter, with its altered-state contact surface) can be used to explore what will happen during subsequent indentation tests. A particular objective of such testing can be confirming the envelope of material hardness and applied load within which further plastic deformation of the indenter can be avoided.

This iterative FEM approach to evaluating the plasticity characteristics of the indenter material, using the measured shape of the indenter after high load indentation as the target outcome, is itself of value.

Experimental Set-Up Using a testing machine like that shown schematically in Figure 1, a series of loads were applied to a (2 mm diameter) cermet indenter ball to push the ball into a maraging steel pre-hardening sample. The profiles of both the ball and the residual indentation in the pre-hardening sample were then measured using a stylus profilometer. Details of such a procedure are described in [7].

The balls, which were supplied by RPGBalls, are described as "Tungsten Carbide (WC) K20 Co Binder (Alloy) Balls". Relevant properties quoted by the supplier for this product are a Young's modulus of 650 GPa, a Poisson ratio of 0.21, a Vickers Hardness, I-Iv, of 1550-1780 kgf mm-2 and a range of "ultimate compressive strength" of 4.6-5.8 GPa, although it is not clear exactly how the latter has been obtained or how it relates to the actual yield stress and work hardening characteristics.

The maraging steel was supplied by Dynamic Metals, with the designation of "Maraging 350 Stainless Steel". It was subjected to the recommended age hardening treatment, which was 6 hours at 480°C. Its stress-strain characteristics were obtained by conventional tensile testing. An example outcome is shown in Figure 2, which is a nominal stress-strain plot from tensile testing of the Maraging 350 steel, up to the onset of necking. The level of reproducibility was high. The measured yield stress and UTS values seen in Figure 2 (-2.3 GPa and 2.4 GPa) are close to the values quoted by the supplier (-2.24 GPa and 2.31 GPa). Also shown is a comparison between experimental and modelled plots of the nominal stress -nominal strain curve, with the latter having been obtained using the Voce parameter set (defining the true stress -true strain curve) shown in Table 1.

Table 1: Voce plasticity parameter sets used for the pre-hardening sample and the ball Voce Parameter Maraging 350 Steel WC-Co Cermet Yield stress, ay (MPa) 2,415 3,875 Saturation stress, as (MPa) 2,675 8,875 Characteristic strain, Co (%) 10 20 Ball Deformation Several loads were applied to each indenter ball penetrating into a pre-hardening sample. After each load, the profile of the contact surface of the ball (in the region near to the axis of loading) was measured, using the stylus profilometer. The tendency was observed, once plastic deformation starts, for the contact surface to remain lying on the surface of a sphere, but with an increasing radius. Accordingly, each profile was fitted to a circle. Figure 3 shows experimental data for the effective radius of the contact surface with progressively increasing loads. It can be seen that, once plastic deformation starts, at an applied load of about 4 kN, this radius progressively increases, reaching about 1.06 mm for a load of 11.4 kN. This is significantly above the peak load of about 7.5 kN expected to be applied to test pre-hardening samples of Maraging 350 steel during normal indentation plastometry testing of the steel.

Furthermore, the profile remains very close to that of a sphere On this region, over this range). Figure 4 is a comparison between the experimental profile after application of the 11.4 kN load and the best fit circle (radius of 1.06 mm), and demonstrates that the contact surface indeed still lies on the surface of a sphere. This spherical form is helpful, since it means that the inverse FEM modelling, which is part of the PIP procedure, can be carried out in the normal way, with the indenter taken to be a sphere (that remains elastic during the test) albeit with adjustment made for the change in the effective radius of the ball.

FEM Modelling of Ball Deformation In order to be confident that the plastic behaviour of the indenter ball is being well captured, and that it can be ensured that the ball does not undergo further plastic deformation during subsequent normal testing, various scenarios were studied using FEM modelling. For this purpose, true stress-strain curves are needed for both ball and pre-hardening sample. Elastic properties for the cermet and the steel, and plastic characteristics for the steel are readily available. However, as mentioned above, plastic characteristics are not generally available for cermet indenters, except in very approximate form. Thus it was obtained in best-fit form by comparing modelled and measured ball deformation outcomes. Figure 5 shows such a comparison between experiment and FEM modelling for the contact surface of the ball after application of the 11.4 kN load, the modelling using the Voce parameter sets shown in Table 1 for the cermet and the Maraging 350 steel.

The level of agreement is very good, with the sensitivity to the ball properties being quite strong. This approach is thus a useful way of obtaining the plasticity properties of an indenter, such as a cermet ball, which are otherwise inaccessible. The value for the yield stress of the cermet determined in this way is about 3.9 GPa, which is lower than commonly-quoted figures of 5-6 GPa. However, the initial work hardening rate represented by the saturation stress and characteristic strain values in Table 1, which is about 20 GPa (i.e. 0.2 GPa %-1), is high. This probably accounts for the apparent discrepancy in yield stress, since a value obtained via a hardness measurement really represents a kind of average "flow stress" over a significant range of plastic strain. Figure 6A shows the modelled plastic strain field in the pre-hardening sample and Figure 6B shows the shows the corresponding modelled plastic strain field in the ball after application of the 11.4 kN load. It can be seen that the strain levels in the ball are fairly high -of the order of 4-5% in a relatively large volume. The strain hardening effect is thus expected to be significant, particularly in view of the high work hardening rate that the cermet exhibits. It may also be noted that, since plastic deformation of a cermet effectively takes place only in the metallic constituent (typically -5-10% Co), the strain there will be much higher than the macroscopic strain -probably by a factor of at least about 10. A high work hardening rate is therefore to be expected.

PIP Testing Using a Pre-Deformed Ball Further testing confirms that in the deformed (and strain-hardened) state produced by this initial treatment, any further plastic ball deformation during PIP testing can be avoided. For example, a 2 mm diameter cermet indenter ball of the type described above, and subjected to a pre-hardening load of 11.4 kN by pressing into the Maraging 350 steel pre-hardening sample to plastically deform the ball such that its contact surface adopts an altered state of defined shape and size can then be used to perform subsequent PIP testing, e.g. on similar steels. By limiting the applied load in such testing (e.g. capping it at 7.5 kN), further plastic deformation of the ball can be avoided. Figure 7 shows, for example, FEM-predicted ball contact surface profiles before and after a subsequent operation of indenting into a test sample formed of a material with a YS of 2 GPa and similar work hardening characteristics to that of the Maraging steel. There is no detectable change in the profile of the contact surface or in the distribution of plastic strain in the ball.

Thus the use of such pre-deformed indenters allows plastic properties of the material of test samples to be obtained by performing indentation plastometry even in cases where the ratio of the yield stress of the material of the indenter, prior to its pre-deformation to the yield stress of the material of the pre-hardening sample is relatively low, e.g. 4 or less, or even 2 or less.

The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and "include", and variations such as "comprises", "comprising", and "including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about" in relation to a numerical value is optional and means for example +/-10%.

References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below.

The entirety of each of these references is incorporated herein.

1. Heinrich, C., A.M. Waas and A.S. Wneman, Determination of material properties using nanoindentation and multiple indenter tips. Int. J. Solids and Structures, 2009. 46: p. 364-376.

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3. Meng, L., P. Breitkopf, B. Raghavan, G. Mauvoisin, 0. Bartier and X. Hernot, Identification of material properties using indentation test and shape manifold learning approach. Computer Methods in Applied Mechanics and Engineering, 2015. 297: p. 239-257.

4. Patel, D.K. and S.R. Kalidindi, Correlation of spherical nanoindentation stress-strain curves to simple compression stress-strain curves for elastic-plastic isotropic materials using finite element models. Acta Materialia, 2016. 112: p.295-302.

5. Dean, J. and T.W. Clyne, Extraction of Plasticity Parameters from a Single Test using a Spherical Indenter and FEM Modelling. Mechanics of Materials, 2017. 105: p. 112-122.

6. Campbell, J.E., R.P. Thompson, J. Dean and T.W. Clyne, Experimental and Computational Issues for Automated Extraction of Plasticity Parameters from Spherical Indentation. Mechanics of Materials, 2018. 124: p.118-131.

7. Campbell, J.E., R.P. Thompson, J. Dean and T.W. Clyne, Comparison between stress-strain plots obtained from indentation plastometry, based on residual indent profiles, and from uniaxial testing. Acta Materialia, 2019. 168: p. 87-99.

8. Burley, M., J.E. Campbell, R. Reiff-Musgrove, J. Dean and T.W. Clyne, Measurement of Residual Stresses using Profilometry-based Inverse FEM Indentation Plastometry (PIP). Adv. Eng. Mats., 2021: article 2001478.

9. Tang, Y.T., J.E. Campbell, M. Burley, J. Dean, R.C. Reed and T.W. Clyne, Use of Profilometrybased Indentation Plastometry to obtain Stress-Strain Curves from Small Superalloy Components made by Additive Manufacturing. Materialia, 2021. 15: article 101017.

10. Campbell, J.E., H. Zhang, M. Burley, M. Gee, A.T. Fry, J. Dean and T.W. Clyne, A Critical Appraisal of the Instrumented Indentation Technique (//T) and Profilometry-based Inverse FEM Indentation Plastometry (PIP) for Obtaining Stress-Strain Curves. Adv. Eng. Mats., 2021: article 2001496.

11. Clyne, T.W. and J.E. Campbell, Testing of the Plastic Deformation of Metals. 2021, Cambridge, U.K.: Cambridge University Press.

12. GB A 2584856.

13. Lueth, R.C., Fracture Mechanics of Ceramics, R.C. Bradt, Editor. 1974, Plenum Press. p. 791-806.

14. Ravichandran, KS., Fracture Toughness of Two Phase WC-Co Cermets. Acta Metall. Mater., 1994. 42: p. 143-150.

15. Mad, D., Cermets and Hardmetals, in Encyclopaedia of Materials, votx -Composite Materials, A. Mortensen, Editor. 2001, Elsevier: Oxford.

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Claims

Claims: 1. A method of preparing an indenter for the performance of indentation plastometry, the method including steps of: providing an indenter having an initial-state contact surface; providing a pre-hardening sample; applying a pre-hardening load to the indenter to press the indenter at its initial-state contact surface into the pre-hardening sample, the pre-hardening load being such and the hardness of the pre-hardening sample being such that the indenter is plastically deformed by the application of the pre-hardening load to alter the contact surface; and measuring the shape and size of the altered-state contact surface.
2. The method according to claim 1, wherein the shape of the initial-state contact surface is axisymmetric, and preferably lies on a surface of a sphere.
3. The method according to claim 2, wherein the shape of the altered-state contact surface is also axisymmetric, and preferably also lies on a surface of a sphere.
4. The method according to any one of the previous claims, wherein the ratio of the yield stress of the material of the indenter prior to being plastically deformed to the yield stress of the material of the pre-hardening sample is 4 or less.
5. The method according to any one of the previous claims, wherein the ratio of the depth of the indentation formed in the pre-hardening sample by the indenter to the radius of that indentation is at least 0.2, and preferably is at least 0.3.
6. A method of performing indentation plastometry, the method including steps of: providing an indenter that has been plastically deformed by application of a known pre-hardening load which pressed the indenter at a contact surface thereof into a pre-hardening sample, thereby altering the contact surface, the altered-state contact surface having a predetermined shape and size; providing a test sample; providing elastic properties of the indenter and the test sample; applying a test load to press the indenter at its altered-state contact surface into the test sample, the test load being less than the pre-hardening load; measuring the shape and size of the indentation formed in the test sample by the pressing of the indenter therein; and determining plastic properties of the material of the test sample on the basis of the predetermined shape and size of the altered-state contact surface, the measured shape and size of the indentation, the test load, and the elastic properties of the indenter and the test sample.
7. The method according to claim 6, wherein the altered-state contact surface is axisymmetric, and preferably lies on a surface of a sphere.
8. The method according to claim 6 or 7, wherein the ratio of the yield stress of the material of the indenter prior to being plastically deformed to the yield stress of the material of the test sample is 4 or less.
9. The method according to any one of claims 6 to 8, wherein the ratio of the depth of the indentation formed in the test sample by the indenter to the radius of that indentation is at least 0.1, preferably is at least 0.15, and more preferably is at least 0.2.
10. The method according to any one of claims 6 to 9, wherein the material of the test sample is the same as the material of the pre-hardening sample.
11. The method according to any one of claims 6 to 10, wherein the determining of the plastic properties of the material of the test sample is performed by numerically modelling the penetration of the indenter into the sample.
12. The method according to any one of claims 6 to 11, wherein the test load is up to 75% of the pre-hardening load.
13. The method according to any one of the previous claims, wherein the material of the indenter is a cermet, such as a WC-Co cermet.
14. A method of determining plastic properties of the material of an indenter, the method including steps of: providing an indenter having an initial-state contact surface of known shape and size; preparing the indenter by performing the method of any one of claims 1 to 5; providing elastic and plastic properties of the material of the pre-hardening sample, and elastic properties of the material of the indenter; and determining plastic properties of the material of the indenter on the basis of: the change in the shape and size of the contact surface between its initial and altered-states, the pre-hardening load, the elastic and plastic properties of the material of pre-hardening sample, and the elastic properties of the material of the indenter.
15. The method according to claim 14, wherein the determining of the plastic properties of the material of the indenter is performed by numerically modelling the penetration of the indenter into the pre-hardening sample.
16. An apparatus for performing indentation plastometry, the apparatus including: an indenter that has been plastically deformed by application of a known pre-hardening load which pressed the indenter at an initial-state contact surface thereof into a pre-hardening sample, thereby altering the contact surface, the altered-state contact surface having a predetermined shape and size; a testing machine configured to: apply a test load to press the indenter at its contact surface into a test sample, the test load being less than the pre-hardening load; a profilometer configured to: measure the shape and size of the indentation formed in the test sample by the pressing of the indenter therein; and a computer programmed to execute a numerical model which determines plastic properties of the material of the test sample on the basis of: the predetermined shape and size of the altered-state contact surface, the measured shape and size of the indentation, the test load, and elastic properties of the indenter and the test sample.
17. An apparatus for determining plastic properties of the material of an indenter, the apparatus including: a testing machine configured to: apply a pre-hardening load to press an indenter at an initial-state contact surface thereof into a pre-hardening sample, the pre-hardening load being such and the hardness of the pre-hardening sample being such that the indenter is plastically deformed by the application of the pre-hardening load to alter the contact surface; a profilometer configured to: measure the shape and size of the initial-state contact surface, and measure the shape and size of the altered-state contact surface; and a computer programmed to execute a numerical model which determines plastic properties of the material of the indenter on the basis of: the change in the shape and size of the contact surface between its initial and altered-states, the pre-hardening load, elastic and plastic properties of the material of pre-hardening sample, and elastic properties of the material of the indenter.
18. A computer program comprising code which, when the code is executed on a computer, causes the computer to execute a numerical model which determines plastic properties of the material of an indenter on the basis of an indentation plastometry test in which: an indenter has a pre-hardening load applied thereto to press the indenter at an initial-state contact surface thereof into a pre-hardening sample, the pre-hardening load being such and the hardness of the pre-hardening sample being such that the indenter is plastically deformed by the application of the pre-hardening load to alter the contact surface, the shape and size of the initial-state contact surface being measured, and the shape and size of the altered-state contact surface being measured; wherein the numerical model determines the plastic properties of the material of the indenter on the basis of: the change in the shape and size of the contact surface between its initial and altered-states, the pre-hardening load, elastic and plastic properties of the material of pre-hardening sample, and elastic properties of the material of the indenter.
19. A computer readable medium storing the computer program according to claim 18.
20. A computer programmed to execute the computer program according to claim 18.