Research Papers

See our latest peer-reviewed publications.

Profilometry-Based Inverse Finite Element Method Indentation Plastometry

This 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 a 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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