GB2584856A

GB2584856A - Indentation creep plastometry

Info

Publication number: GB2584856A
Application number: GB1908656.0A
Authority: GB
Inventors: William Clyne Trevor; Dean James; Edward Campbell James; Edward Burley Max
Original assignee: Plastometrex Ltd
Current assignee: Plastometrex Ltd
Priority date: 2019-06-17
Filing date: 2019-06-17
Publication date: 2020-12-23
Anticipated expiration: 2039-06-17
Also published as: GB2584856B; GB201908656D0

Abstract

Indentation creep plastometry is performed on a sample 4 by applying a load with an indenter 5. The sample 4 has a recess 6 shaped to match the shape of the contact surface of the indenter 5, which may be spherical. The load is applied over a time period and a creep characteristic of the sample 4 is determined based on the shape and size of the contact surface, the applied load and the progressive penetration of the indenter 5 into the sample 4. The progressive penetration of the indenter 5 may be numerically modelled to determine the time-dependent plastic deformation of the sample 4. There may be a furnace 7 for heating the indenter apparatus to a homologous temperature of the sample greater than 0.5. The indenter 5 may have a second contact surface and may be placed between two samples (4a, 4b, fig 3) and a load applied to press the indenter 5 into both samples to determine a creep characteristic.

Description

Indentation Creep Plastometry

Field of Invention

The present invention relates to methods, apparatuses, computer programs, computer readable media, and computers configured for the performance of indentation creep plastometry.

Background

In materials science, "creep" is a term used to define the tendency of a solid material to plastically deform under a deviatoric stress which is below the yield stress of the material. Creep behaviour is normally observed at high homologous temperatures, i.e. the temperature of a material normalised by its melting temperature. Commonly, creep is exhibited by materials at homologous temperatures of over 0.5, and typically requires an applied deviatoric stress to be maintained for a significant length of time to be observed. As a result, creep is often described as "time-dependent plastic deformation", distinguishing such deformation from typical yielding which occurs at stresses above the yield stress of the material (time-independent plastic deformation).

A conventional approach for characterising the creep behaviour of a material is to subject a sample of the material to a uniaxial test. Uniaxial tests are used to characterise a variety of mechanical properties of materials, and typically involve applying a tensile or compressive load along a single axis of a sample, and monitoring the deformation of the sample which occurs as a result of the load. For creep characterisation uniaxial tests, a series of samples are each subjected to a different fixed load, each load generating a different deviatoric stress level in the respective sample. These loads are applied for extended periods of time, and length changes of the sample are monitored and analysed to determine the creep characteristics of the material at different deviatoric stress levels.

In recent years, a number of scientific papers [9-16] have been published relating to an alternative methodology for characterising the creep behaviour of materials. This methodology is known as indentation creep plastometry, and is based on the more general practice of indentation plastometry.

Indentation plastometry concerns the characterisation of inelastic mechanical properties of materials, by penetrating an indenter into a sample of the material, and analysing the depth of penetration in the sample. 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, and thus provides another means of determining the extent of penetration. A typical type of indenter used in indentation plastometry has a spherical surface and is made of a ceramic material.

A common material property measured by indentation plastometry is "hardness". In a traditional hardness test, hardness values of a material are obtained by measuring 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, iterative numerical modelling of the indentation process, using a Finite Element Method (FEM) for example, can be carried out, with the plastic deformation of the material being captured in the form of a constitutive law containing adjustable parameters, and repeated comparisons can be 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 number of papers [1-6] have been published regarding various details of this methodology. A similar methodology [7, 8] can be used to obtain information about residual stresses in samples.

Indentation creep plastometry applies the discussed principles of indentation plastometry to characterise the creep behaviour of a material. A creep plastometry test typically involves applying a constant load to a sample via an indenter. The load is held at a particular level for an extended period of time, during which progressive penetration of the indenter into the sample is monitored. As with other indentation plastometry techniques, FEM modelling of the process can be carried out to capture the creep characteristics of the sample in a constitutive law.

Indentation creep plastometry has significant advantages over conventional uniaxial tests. For example, the size and shape requirements of a sample for indentation-based techniques are less restrictive compared to those of uniaxial tests. Additionally, as the indenters are relatively small compared to the samples, multiple tests can be performed on a single sample, and hence any local variations in properties across the sample may be investigated. Moreover, indentation creep plastometry provides the ability to analyse creep behaviour of a sample, over a range of deviatoric stress levels, during a single test. This is not the case for uniaxial tests, which require individual tests for different deviatoric stress levels. Furthermore, in conventional uniaxial testing, the tests are typically based on "engineering" stress levels, which are approximations of actual stresses that do not account for changes in stress as a result of cross-sectional area changes of the sample during the test. In order to account for this approximation, uniaxial tests are often required to vary the applied load according to a feedback loop. However, FEM modelling used in indentation creep plastometry can be easily based on "true" stress levels, i.e. the actual stresses present in the sample, and thus variable loading and feedback loops are not required.

In view of these advantages, there are significant prospects for indentation creep plastometry to become widely used for creep characterisation of materials. However, in current indentation creep plastometry techniques there exists a source of error which may limit the accuracy of their data. This source of error is caused by the generation of large deviatoric stresses in the sample, at the beginning of the indentation process, which exceed the yield stress of the material.

Specifically, when an indenter with a convex surface, such as a spherical indenter, comes into initial contact with a sample, the surface area of the sample with which the indenter is in contact is significantly smaller than the surface area of the convex surface of the indenter. Therefore, when a load is initially applied to the sample via the indenter, the local deviatoric stresses generated in the sample can approach levels significantly above the yield stress of the sample.

In such cases, time-independent plastic deformation of the sample occurs at the site of initial penetration, and thus the penetration of the indenter cannot solely be attributed to creep deformation of the sample. The exact ratio of time-dependent and time-independent deformation is difficult to determine, and therefore the occurrence of time-independent deformation lowers the accuracy of the determined creep characteristics. Furthermore, yielding can also change the microstructure of the sample so as to affect the creep behaviour of the sample, which contributes further errors to the subsequent analysis.

Therefore, there is a need for an indentation creep plastometry technique which avoids the generation of large deviatoric stresses in a sample which exceed the yield stress of the sample material, and which can thus determine a creep characteristic accurately and consistently.

Summary

According to a first aspect of the present invention, there is provided a method of performing indentation creep plastometry, the method including steps of: providing an indenter having a contact surface, and a sample of a material with a surface recess pre-formed therein to match the contact surface; offering the indenter to the sample such that the contact surface rests in the recess; applying a load over a period of time to press the indenter further into the sample at the recess; measuring progressive penetration of the indenter into the sample over that period; and determining a creep characteristic of the material on the basis of: the shape and size of the contact surface and the recess, the applied load, and the measured progressive penetration.

The method of the first aspect differs from known indentation creep plastometry techniques in that a surface recess which matches the contact surface of the indenter is formed in the sample before the indenter is offered to the sample. The shape and depth of the recess can be determined so as to provide an initial surface area of contact between the indenter and the sample that is sufficiently large to avoid the generation of deviatoric stresses above the yield stress of the material at the site of penetration when the load is applied. In doing so, it can be ensured that substantially all plastic deformation of the sample is attributed only to creep phenomena, rather than time-independent plastic deformation, thus enabling more accurate determination of the creep characteristic than is possible using conventional indentation plastometry techniques.

According to a second aspect of the present invention, there is provided an apparatus for performing indentation creep plastometry, the apparatus including: an indenter having a contact surface for pressing into a sample of a material with a surface recess pre-formed therein to match the contact surface of the indenter; a testing machine configured to: hold the indenter relative to the sample such that the contact surface rests in the pre-formed recess, apply a load over a period of time to press the indenter further into the sample at the recess, and measure progressive penetration of the indenter into the sample over that period; and a computer programmed to execute a numerical model which determines a creep characteristic of the material on the basis of: the shape and size of the contact surface and the recess, the applied load, and the measured progressive penetration.

The apparatus of the second aspect is thus configured to perform the method of the first aspect.

The apparatus may further include a rotating lapping tool having a lapping head which is the same shape as the contact surface of the indenter for forming the surface recess in the sample. Such a lapping tool allows the recess to be accurately formed to match the contact surface of the indenter. It can also help to avoid producing undesirable microstructural changes, such as work hardening, in the sample at the recess.

The apparatus may further include a profilometer for measuring the shape of the contact surface of the indenter and/or the shape of the recess. This can help to ensure that the recess accurately matches the indenter. The measured shape can also be used by the numerical model for the determination of the creep characteristic.

The apparatus may further include a furnace for heating the indenter and testing machine to an elevated temperature and maintaining the elevated temperature during the load application and the measurement of progressive penetration. In this way, a testing temperature which is greater than a homologous temperature of 0.5 for the material can be established.

According to a third aspect of the present invention, there is provided a computer program comprising code which, when the code is executed on a computer, causes the computer to execute a numerical model which determines a creep characteristic of a material on the basis of an indentation creep plastometry test in which: an indenter having a contact surface is offered to a sample of a material with a surface recess pre-formed therein to match the contact surface of the indenter such that the contact surface rests in the recess; a load is applied over a period of time to press the indenter further into the sample at the recess; and progressive penetration of the indenter into the sample over that period is measured; wherein the numerical model determines the creep characteristic of the material on the basis of: the shape and size of the contact surface and the recess, the applied load, and the measured progressive penetration.

Thus the computer program can be used in the performance of the method of the first aspect, and/or can be the computer program of the second aspect.

According to a fourth aspect of the present invention, there is provided a computer readable medium storing the computer program according to the third aspect.

According to a fifth aspect of the present invention, there is provided a computer programmed to execute the computer program according to the third aspect.

Optional features of the present disclosure will now be set out.

The contact surface of the indenter may be axisymmetric. This facilitates the forming of the matching recess, e.g. using rotating tools. For example, an axisymmetric recess may be formed by rotationally lapping the sample using a lapping tool. Additionally, having an axisymmetric contact surface can substantially simplify numerical modelling of the progressive penetration.

Conveniently, the contact surface of the indenter may lie on a surface of a sphere. Such a contact surface is relatively easy to measure, define and model. Also indenters having such a surface can be readily sourced or produced. In addition, matching recess are relatively easy to form, e.g. using a lapping tool have a lapping surface which also lies on a surface of such a sphere.

The load may be selected such that over the time period the maximum deviatoric stress generated in the sample at the indenter remains below the yield stress of the material. This helps to ensure that time-independent plastic deformation does not occur in the sample during performance of the method of the indentation creep plastometry.

The determining may be performed by numerically modelling the progressive penetration of the indenter into the sample. This numerical modelling may be based on a finite element model. Numerically modelling the penetration of the indenter into the sample allows quick and accurate determination of the creep behaviour of the sample. For example, the creep characteristic may take the form of a constitutive law. In this case, numerical modelling may iteratively simulate the penetration to determine parameter values of the constitutive law.

The method may further include a preliminary step of measuring the shape of the contact surface of the indenter and/or the shape of the recess. As mentioned above, this can help to ensure that recess accurately matches the indenter. Additionally, the measured shape can be used by a numerical model for the determination of the creep characteristic.

The method may further include a preliminary step of numerically modelling progressive penetration of an indenter into the material to determine suitable sizes of the contact surface and the recess and a suitable load, the sizes of the contact surface and the recess of the provided indenter and sample and the applied load used in the subsequent testing being as thus-determined. This step can help to ensure that the determined recess-indenter-load combination avoids time-independent plastic deformation occurring in the sample during the subsequent plastometry testing.

The method may further include a preliminary step of forming the surface recess in the sample of the material. This forming step may be performed by lapping the surface of the sample thereat using a rotating lapping tool having a lapping head which is the same shape as the contact surface of the indenter. This helps to ensure that the recess formed in the forming step is a close match to the contact surface.

The method may further include heating the sample to an elevated temperature and maintaining the elevated temperature during the applying and measuring steps. The heating can establish a testing temperature which is greater than a homologous temperature of 0.5 for the material.

The indenter may have a second contact surface on an opposite side thereof. In this case: in the providing step, a second sample of the material with a surface recess preformed therein to match the second contact surface may also be provided; in the offering step, the indenter may be sandwiched between and offered to both samples such that the contact surfaces rest in the respective recesses; in the applying step, the load may be applied over the period of time to press the indenter further into both samples at the recesses; in the measuring step, the progressive penetrations of the indenter into both samples over that period may be measured; and in the determining step, the creep characteristic of the material may be determined on the basis of: the shape and size of the contact surfaces and the recesses, the applied load, and the measured progressive penetrations.

By having two samples sandwiching the indenter, the indenter can be held in place without the need for an indenter housing to hold the indenter. Preferably the first and second contact surfaces are identical, and the recesses are also identical. For example, the indenter may be a sphere, and the spherical indenter may rest in identical matching recesses in the two samples. This eliminates a potential source of error which may arise as a result of deformation of the indenter housing when the load is applied. Additionally, the combined depths of penetration into the two samples is approximately twice the depth of penetration into of one sample for a given load, thus providing a larger measurable quantity to improve accuracy.

Brief Description of the Figures

Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically a testing machine configured to perform indentation creep plastometry according to a first embodiment of the present invention; Figure 2 shows plots of penetration against time obtained in an indentation creep plastometry experiment, and plots of penetration against time obtained by converging curves of a constitutive law onto the experimental plots; Figure 3 shows schematically a variant testing machine configured to perform indentation creep plastometry according to a second embodiment of the present invention; Figure 4 shows plots of penetration against time obtained in a uniaxial creep experiment, and plots of penetration against time obtained by converging curves of a constitutive law onto the experimental plots; and Figure 5 shows a comparison of the uniaxial experimental plots of Figure 4 and plots obtained using the parameters of the constitutive law derived in Figure 2.

Detailed Description

Figure 1 shows schematically a testing machine configured to perform indentation creep plastometry according to a first embodiment of the present invention.

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 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 has a spherical shape, and is typically made of a ceramic material. This ensures that the indenter does not deform significantly during performance of indentation creep 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, and has a surface recess 6 pre-formed therein which is shaped to match a contact surface of the indenter 5, the contact surface being that part of the indenter 5 which contacts the sample 4 when the indenter 5 and the sample 4 are brought together.

The shape and depth of the pre-formed recess 6 are determined so as to avoid time-independent plastic deformation of the sample at the site of the recess 6 when a load is applied to the indenter 5 pressing the indenter into the sample 4. This can be achieved by applying a finite element model (FEM) to predict the stresses generated in the sample 4 for different recess-load combinations, and based on these predictions choosing an appropriate recess size for which the maximum deviatoric stresses generated are below the yield stress of the sample material. In particular, such FEM modelling assists selection of the recess/indenter radius, the applied load, and the depth of the recess 6.

Conveniently, the recess 6 may be formed in the sample 4 by a lapping process, described below, although other materials removal techniques may be used.

At a site on the surface of the sample 4 at which the recess 6 is to be formed, a preliminary recess can be drilled into the surface using a conventional drill bit with a spherical tip. The preliminary recess is slightly smaller than the recess 6 to be formed. To remove machining marks as a result of the drilling, and to finish the recess 6, an abrasive powder is placed in the preliminary recess, and a lapping tool is used to lap the preliminary recess. The lapping tool has a lapping head which has the same shape as the contact surface of the indenter 5, which ensures the recess 6 formed by the lapping has the same shape as the contact surface of the indenter 5.

After the lapping process, or periodically throughout the lapping process, a profilometer (not shown) is used to capture the shape of the recess 6, allowing a computer to determine the quality of the surface finish and the depth of the recess 6. Once the surface finish is determined to be of a suitable quality and the lapping process is ended, an FEM mesh can be generated by a computer to model the sample 4 including the recess 6, and to generate a corresponding FEM mesh to model the indenter 5 resting in the recess.

Frequently, creep is only exhibited at homologous temperatures of over 0.5, and thus many samples need to be heated for the determination of their creep characteristics. Thus the sample base 1, sample 4, indenter 5, and the indenter housing 2 can be located inside a furnace 7 which is configured to control the temperature of the sample 4. This allows the creep response of the sample 4 to be characterised at different test temperatures. . When performing the indentation creep plastometry, the testing machine offers the indenter 5 to the sample 4 such that the contact surface of the indenter 5 rests in the recess 6. The testing machine then applies a load to the indenter 5 for a period of time, to press the indenter 5 further into the sample 4. Even at the very first stages of the load application, the sample 4 is in intimate contact with the indenter 5 over the total area of the recess 6. Thus, the maximum deviatoric stresses induced in the sample 4 as a result of the applied load always remain lower than the yield stress of the sample 4.

As creep of the material induced by 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 5. Separately, and typically after the creep plastometry is completed, the FEM meshes superimposed onto the sample and the indenter are used to numerically model the penetration of the indenter 5 into the sample 4 by a computer using a constitutive law. For example, the constitutive law can be the Miller-Norton law: C6ntm+1 -Q) Ecreep m+1 eXP(RT.)I in which C is a constant (units of MPa-" s-(m-'1)), t is the time (s), n is a stress exponent and m is a dimensionless constant. The law captures both primary and secondary regimes of creep and the transition therebetween. This is important [9] for analysis of indentation creep in which a steady state, i.e. purely secondary creep, is never established.

The equation can be differentiated with respect to time to give: Ecreep = Ccflm exp(-

RT

Time t can thus be expressed in terms of both a strain rate and a strain: Ecreep Can exp RT(Q) Co-" exp CR t

-

(1+ m) Ecreep 1 ( 1 + m) Eliminating t and rearranging allows the strain rate to be expressed as a function of the strain: Tm) Ecreep-are. =[Can exp[-Q){(1+ m) E I RT creep The numerical model assumes that the cumulative creep strain defines the "state" of a volume element of the sample 4, with the instantaneous creep strain rate determined by the current stress, in the volume element concerned, and the prior strain. The creep strain rate thus has no explicit dependence on time. Details of this algorithm are supplied in reference [13].

The modelled penetration data and the measured penetration data are subsequently compared, and the parameters (C, n and m) of the constitutive law are iteratively adapted to converge the modelled data with the measured data as closely as possible. This can be achieved with systematic migration in parameter space until convergence is obtained, using a "goodness-of-fit' parameter to characterise the level of convergence between the measured and modelled penetration data. The Nelder-Mead algorithm can be used for this process, as described in reference [5]. For this modelling, a friction coefficient at the indenter/sample interface is specified as 0.3. However, the outcome of the modelling is relatively insensitive to this value, and a different coefficient may be used. Other constitutive laws known to the skilled person may be used in the modelling.

Figure 2 illustrates plots of measured and modelled penetration data after iterative convergence, the Examples section providing more details of the materials and testing method. The converged modelled constitutive model provides an accurate characterisation of the creep behaviour of the sample 4, based on the measured penetration data.

A variant testing machine for performing indentation creep plastometry according to a second embodiment of the present invention is illustrated schematically in Figure 3. Features of Figure 3 that are substantially the same as those shown in Figure 1 are referenced with the same numbers.

The variant testing machine shown in Figure 3 differs from that of Figure 1 in that two (typically identical) samples 4a, 4b, with respective recesses pre-formed therein, sandwich a free-standing indenter 5 therebetween, with the indenter held in the recesses. This removes a need for the indenter housing of Figure 1, thereby avoiding a potential source of error that can arise if the indenter housing deforms as a result of the applied load.

Creep plastometry of the second embodiment performed using the variant testing machine is similar to that of the first embodiment. However, in the second embodiment, an applied load is applied directly to sample 4a to push the indenter 5 into both sample 4a and sample 4b. The relative displacement of sample 4a and the sample holder 1, which holds sample 4b, is measured by the displacement measurement system 3. In general, because the indenter 6 penetrates progressively into both samples, the measured displacement is twice that measured in the first embodiment, thereby reducing measurement error. The increased displacement is readily taken into account by the subsequent numerical modelling.

Experimental Testing Two types of creep test were undertaken. The first test was a compressive uniaxial test, and the second test was an indentation creep plastometry test according to the present invention. The tests were performed at 20°C (293K) on samples of pure tin (Sn), obtained from commercial suppliers in the form of extruded cylindrical rods having a diameter of approximately 10 mm, and a grain size of the order of 100 pm. At room temperature, the homologous temperature of Sn is approximately 0.59. This value is large enough to ensure that considerable creep takes place at such temperatures, even at relatively low stress levels.

The uniaxial test was conducted using dead-weights to produce the applied loads, with applied stress levels of 9, 10.5, and 12 MPa. These values were chosen to be less than the yield stress of the material of about 15 MPa, as determined by uniaxial stress-strain testing (carried out using an InstronTM 3367 screw-driven machine). Displacement measurements for the uniaxial test were carried out using a Linear Variable Displacement Transducer.

The indentation creep plastometry testing according to the first embodiment described above was carried out using the testing machine of the type shown in Figure 1. The indenter was a WC-Co cermet sphere having a diameter of 4mm. The recess in the sample had a depth of 1.1 mm, and was produced by using a spherical end drill having a diameter of 2mm to form a preliminary recess, and then using an identical indenter to that of the test attached to the end of a drill bit to lap the preliminary recess to complete the recess. SiC polishing powder having a particle diameter of the order of 1 pm was inserted into the preliminary recess for the lapping process. FEM simulation was used to ensure that, with this configuration, and with the levels of applied load used, the maximum deviatoric stress created under the indenter was below the yield stress. As the tests were conducted at 20°C, the furnace of the testing machine was not required.

Results Figure 4 shows creep strain vs time plots of each of the uniaxial tests, corresponding Miller-Norton curves which have been converged using a best-fit algorithm to the uniaxial test plots, and the parameter values of the Miller-Norton curves. The agreement between the uniaxial test plots and the Miller-Norton curves is good, thus confirming that the creep behaviour can be captured well using the Miller-Norton constitutive law. The uniaxial test plots were determined using a constant value of engineering stress, rather than the true stress. Thus, the agreement would likely be improved if the true stresses in the samples were accounted for.

Figure 2 shows corresponding plots for penetration as a function of time during the indentation creep plastometry tests, with two different applied loads of 140 N and 160 N respectively. In this case the predicted plots were obtained via iterative FEM simulation of the process (requiring about 150 iterations). It can be seen that the level of agreement is again very good, with the value of the goodness-of-fit parameter, which is zero for perfect agreement, being below 10-4. Such values indicate that a high level of confidence can be placed in the reliability of the FEM simulation [5]. While the Miller-Norton parameter values of Figure 2 are similar to those shown in Figure 4, which were obtained by direct comparison with the uniaxial experimental data, they are not quite the same.

Figure 5 is a comparison between the uniaxial test plots obtained experimentally and those corresponding to the Miller-Norton equations using the parameter values derived from the indentation creep plastometry tests in Figure 2. It can be seen that the uniaxial behaviour is well-captured by the parameters derived from the indentation creep plastometry experiments. In fact, if allowance were made for the experimental data corresponding to constant values of the nominal stress, rather than the true stress, the agreement would be likely to improve further.

It can therefore be seen that the method of the present invention is a reliable one. Under the applied loads, such reliability would not be possible if the recess were not present as time-independent plastic deformation would be stimulated early in the indentation process. This would introduce uncertainty into the displacement data and would also be likely to affect the microstructure and hence the creep response.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

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Claims

CLAIMS1. A method of performing indentation creep plastometry, the method including steps of providing an indenter having a contact surface, and a sample of a material with a surface recess pre-formed therein to match the contact surface; offering the indenter to the sample such that the contact surface rests in the recess; applying a load over a period of time to press the indenter further into the sample at the recess; measuring progressive penetration of the indenter into the sample over that period; and determining a creep characteristic of the material on the basis of: the shape and size of the contact surface and the recess, the applied load, and the measured progressive penetration.
2. The method according to claim 1, wherein the contact surface of the indenter is axisymmetric.
3. The method according to claim 2, wherein the contact surface of the indenter lies on a surface of a sphere.
4. The method according to any one of the previous claims, wherein the load is selected such that over the time period the maximum deviatoric stress generated in the sample at the indenter remains below the yield stress of the material.
5. The method according to any one of the previous claims, wherein the determining is performed by numerically modelling the progressive penetration of the indenter into the sample.
6. The method according to claim 5, wherein the numerical modelling is based on a finite element model.
7. The method according to any one of the previous claims, further including a preliminary step of measuring the shape of the contact surface of the indenter and/or the shape of the recess.
8. The method according to any one of the previous claims, further including a preliminary step of numerically modelling progressive penetration of an indenter into the material to determine suitable sizes of the contact surface and the recess and a suitable load, the sizes of the contact surface and the recess of the provided indenter and sample and the applied load being as thus-determined.
9. The method according to any one of the previous claims, further including a preliminary step of forming the surface recess in the sample of the material.
10. The method according to claim 9, wherein the forming is performed by lapping the surface of the sample thereat using a rotating lapping tool having a lapping head which is the same shape as the contact surface of the indenter.
11. The method according to any one of the previous claims, wherein: the indenter has a second contact surface on an opposite side thereof; in the providing step, a second sample of the material with a surface recess preformed therein to match the second contact surface is also provided; in the offering step, the indenter is sandwiched between and offered to both samples such that the contact surfaces rest in the respective recesses; in the applying step, the load is applied over the period of time to press the indenter further into both samples at the recesses; in the measuring step, the progressive penetrations of the indenter into both samples over that period is measured; and in the determining step, the creep characteristic of the material is determined on the basis of: the shape and size of the contact surfaces and the recesses, the applied load, and the measured progressive penetrations.
12. An apparatus for performing indentation creep plastometry, the apparatus including: an indenter having a contact surface for pressing into a sample of a material with a surface recess pre-formed therein to match the contact surface of the indenter; a testing machine configured to: hold the indenter relative to the sample such that the contact surface rests in the pre-formed recess, apply a load over a period of time to press the indenter further into the sample at the recess, and measure progressive penetration of the indenter into the sample over that period; and a computer programmed to execute a numerical model which determines a creep characteristic of the material on the basis of: the shape and size of the contact surface and the recess, the applied load, and the measured progressive penetration.
13. The apparatus according to claim 12, further including a rotating lapping tool having a lapping head which is the same shape as the contact surface of the indenter for forming the surface recess in the sample.
14. The apparatus according to claims 12 or 13, further including a profilometer for measuring the shape of the contact surface of the indenter and/or the shape of the recess.
15. A computer program comprising code which, when the code is executed on a computer, causes the computer to execute a numerical model which determines a creep characteristic of a material on the basis of an indentation creep plastometry test in which:; an indenter having a contact surface is offered to a sample of a material with a surface recess pre-formed therein to match the contact surface of the indenter such that the contact surface rests in the recess; a load is applied over a period of time to press the indenter further into the sample at the recess; and progressive penetration of the indenter into the sample over that period is measured; wherein the numerical model determines the creep characteristic of the material on the basis of: the shape and size of the contact surface and the recess, the applied load, and the measured progressive penetration.
16. A computer readable medium storing the computer program according to claim 15.
17. A computer programmed to execute the computer program according to claim 15.