Research Papers

This paper explores anisotropy in stress–strain responses of steel pipelines, where traditional hoop-direction tensile testing, requiring flattening, may distort results. Profilometry-based indentation plastometry (PIP) was used to evaluate stress–strain curves and detect anisotropy directly on pipeline surfaces. Testing on eight pipes revealed no true anisotropy, indicating that axial-direction testing might be a more accurate, flattening-free alternative.

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...

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.

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.

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.

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.

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.

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.

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

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

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.

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.

This paper concerns, the effect of (unknown) residual stresses in the near-surface region of a sample on outcomes of an indentation plastometry technique for obtaining stress–strain relationships, using relatively large spherical indenters.

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.

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.

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.

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!

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.