Research Papers

See our latest peer-reviewed publications.

Use of hardness, PIP and tensile testing to obtain stress-strain relationships for metals

Both hardness testing and Profilometry-based Indentation Plastometry (PIP) can be used to obtain features of (tensile) stress-strain curves. The two tests are superficially similar, involving penetration (under a known load) of an indenter into the flat surface of a sample, followed by measurement of dimensional characteristics of the residual indent. The associated data handling procedures, however, are very different in the two types of test. Hardness numbers, which are commonly based on measurement of the lateral extent or depth of the indent, essentially give a semi-quantitative indication of the resistance to plastic deformation: going beyond this to infer features of the (nominal) stress-strain curve – notably the yield stress (YS) and Ultimate Tensile Stress (UTS) – can only be done via empirical correlations (often restricted to certain types of alloy). PIP testing, on the other hand, involves measurement of the complete indent profile, followed by (automated) iterative FEM modelling of the indentation, allowing the complete (true) stress-strain curve to be obtained. This paper covers application of both approaches to 12 different alloys, with inferred stress-strain characteristics being compared with those from tensile testing. Insights are provided relating to the very different levels of detail and reliability offered by the two procedures.

Indentation Plastometry for Study of Anisotropy and Inhomogeneity in Maraging Steel produced by Laser Powder Bed Fusion

An important capability of PIP testing is that it allows detailed study of anisotropy (detectable via a lack of radial symmetry in the indent profile) and inhomogeneity (on a scale of a few mm or above). The technique was used in this study to clarify that a particular (additively manufactured) material was isotropic everywhere, but that its stress-strain curve changed significantly with increased distance from the original growth plate. PIP has unrivalled potential for study of AM products.

PIP Testing for Characterization of Case Hardened Steels

This work concerns use of the PIP procedure to obtain stress-strain relationships for a series of incremental near-surface layers in a steel that had been hardened by diffusional penetration of carbon (to a depth of about 1 mm). This was done via serial sectioning, with several layers about 200 µm in thickness being tested. Residual stresses were also measured and taken into account. No other testing technique allows such characterization to be carried out.

Use of Profilometry-based Indentation Plastometry (PIP) to Study the Effects of Pipe Wall Flattening on Tensile Stress-Strain Curves of Steels

This paper is focussed on how PIP testing can satisfy the important technological requirement for non-destructive testing of pipes in situ ("in the ditch"), so as to obtain yield stress and UTS values that would be obtained by the "industry standard" (destructive) procedure of flattening a section and tensile testing it in the hoop direction. A number of different pipes are examined in the study. The reliability and practicality of the PIP procedure is clearly demonstrated.

Effect of Relatively Low Levels of Porosity on the Plasticity of Metals and Implications for Profilometry-Based Indentation Plastometry

The effect of dispersed porosity within a metal on its plastic deformation is examined, and the suitability of applying the PIP methodology is also assessed.

Tensile-compressive Asymmetry in Extruded AZ31B Rod and its Effect on Profilometry-based Indentation Plastometry

This paper concerns tensile-compressive asymmetry (TCA) in a Mg-based alloy (an unusual characteristic in some metallic systems) whereby the yield stress in tension differs from that in compression. We investigate whether the PIP testing methodology can be used to reliably infer stress-strain characteristics in material of this type.

Indentation Plastometry of Welds

This investigation concerns the application of the profilometry-based indentationplastometry (PIP) methodology to obtain stress-strain relationships for material in the vicinity of fusion welds.

Indentation Plastometry of Very Hard Metals

This investigation concerns the application of profilometry-based indentation plastometry (PIP) to metals with very high hardness, i.e., those with yield stresses of 1.5–3 GPa.

Profilometry-Based Inverse Finite Element Method Indentation Plastometry

This is a review paper covering Profilometry-based Indentation Plastometry (PIP), a novel method for rapidly inferring stress/strain curves from indentation test data.

Profilometry-based Indentation Plastometry to Obtain Stress-Strain Curves from Anisotropic Superalloy Components made by Additive Manufacturing

In this paper with Professor Roger Reed and Dr. Tony Tang at Oxford University, we explore the power of PIP testing to characterize the plasticity of additively manufactured alloys.

A Critical Appraisal of the Instrumented Indentation Technique and Profilometry‐Based Inverse Finite Element Method Indentation Plastometry for Obtaining Stress–Strain Curves

This is a collaborative study with the The National Physical Laboratory, the UK's national measurement facility. In this work we compare the accuracy of the PIP method to that of Instrumented Indentation Testing (IIT) at inferring stress/strain behaviour from indentation test data.

The Effect of Residual Stresses on Stress–Strain Curves Obtained via Profilometry‐Based Inverse Finite Element Method Indentation Plastometry

In this work the Plastometrex team explore the impact of residual stress on stress/strain curves derived from the PIP method.

Evaluation of the Fracture Energy of Magnesium via Ballistic Impact Experiments

This paper covers an interesting extension to our conventional (quasi-static) Indentation Plastometry methodology. It concerns the experimental study and modelling of the ballistic impact (indentation) of spherical projectiles into thick metallic samples, and resultant crack propagation. It outlines an approach that can be used to determine high strain rate plasticity parameters.

A Methodology for Obtaining Primary and Secondary Creep Characteristics from Indentation Experiments using a Recess

This paper describes a procedure for performing Indentation Creep Plastometry, which is analogous to the methodologies developed by Plastometrex for conventional Indentation Plastometry. More specifically, the paper covers the creation of spherical recess in the sample prior to indentation testing, which allows control over the stress levels created during the indentation creep test and can be used to ensure that no (time-independent) plastic deformation is stimulated during the test.

Extraction of Superelasticity Parameter Values from Instrumented Indentation via Iterative FEM Modelling

This paper concerns the use of (load-displacement) data obtained during spherical indentation of a superelastic NiTi alloy, so as to obtain a stress-strain curve. The results indicate that the sensitivity of the load-displacement curve to alterations in the parameters controlling the superelastic behaviour are not sufficiently strong to enable accurate extraction of superelastic parameters.

Comparison between Stress-Strain Plots Obtained from Indentation Plastometry, based on Residual Indent Profiles, and from Uniaxial Testing

This paper is focussed on comparisons between stress-strain plots from conventional uniaxial testing and those obtained from indentation experiments, via FEM modelling of the process in which plasticity is represented using a constitutive law. The paper shows that there is excellent consistency with the outcomes of uniaxial tests, concluding that indentation plastometry has the potential to become a mainstream testing methodology!

Experimental and Computational Issues for Automated Extraction of Plasticity Parameters from Spherical Indentation

This paper outlines the main issues involved in optimisation of experimental conditions and model formulation for extracting stress-strain curves from indentation test data. It covers issues such as representative volumes, load-displacement curves versus residual profiles shapes, depth of penetration, and issues related to FE meshes, constitutive relations and convergence algorithms.

Mechanical Properties of Sprayed Overlayers on Superalloy Substrates, Obtained via Indentation Testing

This paper concerns the use of our PIP methodology on cold-sprayed over-layers on superalloy substrates, in as sprayed and annealed conditions. The information gathered from these tests could not have been obtained using conventional mechanical testing methods.

Extraction of Plasticity Parameters from a Single Test using a Spherical Indenter and FEM Modelling

A methodology is presented for obtaining plasticity characteristics of bulk metallic materials from single run indentation data. It involves repeated FEM modelling, with the predicted outcomes being systematically compared with experiment. The correct properties are found by searching for the combination giving the maximum value for a "goodness-of-fit" parameter that measures the level of agreement between measured and predicted outcomes.

Limit Case Analysis of the "Stable Indenter Velocity" Method for Obtaining Creep Stress Exponents from Constant Load Indentation Creep Tests

This paper concerned a commonly used procedure for evaluating the steady-state creep stress exponent from indentation data. Concerns had previously been expressed about the reliability of the procedure, and this paper demonstrates (unequivocally) that the method is fundamentally flawed and should not be used.

A Methodology for Obtaining Plasticity Characteristics of Metallic Coatings via Instrumented Indentation

This work covers a methodology for inferring yield stress and work-hardening characteristics of metallic coatings from indentation data. It is concluded that the methodology is basically reliable, with relatively good sensitivity and resolution, although this does depend on several factors that are highlighted in the paper.

A Critical Assessment of the "Stable Indenter Velocity" Method for Obtaining the Creep Stress Exponent from Indentation Data

This paper utilises the constant indenter velocity method for measuring steady-state creep stress exponents from indentation data. The results are used to explain why the method doesn't work, by exposing the grossly inaccurate assumptions that are built into the method - often leading to physically implausible values for the creep stress exponent.

A Procedure for Extracting Primary and Secondary Creep Parameters from Nanoindentation Data

A methodology is presented for the extraction of creep parameters from nanoindentation data. The procedure involves consideration of both primary and secondary creep regimes, and it is shown to be robust through comparisons with uniaxial creep data.

Use of Nanoindentation to Measure Residual Stresses in Surface Layers

In this paper, equal biaxial residual stresses were generated in thin copper foils via differential thermal contraction. The foils were then indented in order to assess the viability of such techniques for quantifying residual stresses.

Use of Quasi-Static Nanoindentation Data to Obtain Stress-Strain Characteristics for Metallic Materials

This paper concerns optimisation of procedures and algorithms for extraction of stress-strain relationships from quasi-static nanoindentation experiments, using finite element modelling. Several issues are highlighted, including the usefulness of incorporating residual indent shapes into the comparisons, as well as load-displacement-time data.

Nanoindentation of Binary and Ternary Ni-Ti-based Shape Memory Alloy Thin Films

This sputtered firms of binary (Ni-Ti) and ternary (Ni-Ti-Hf and Ni-Ti-Cu) shape memory alloys were subjected to nanoindentation testing over a range of temperature using a small diameter spherical indenter. The results indicated that ternary alloys with up to about 20 at.%Hf or 10 at.% Cu can exhibit superelastic behaviour over suitable temperature ranges.

Measurement and Modelling of the Nanoindentation Response of Shape Memory Alloys

In this paper a Ni-Ti shape memory alloy was subjected to nanoindentation over a range of temperature, such that the starting material was either predominantly martensitic or largely composed of the parent phase. The load-displacement data were interpreted to give information about whether the imposed strain was partly accommodated by the martensitic phase transformation - i.e. whether superelastic deformation was taking place.

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