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Development of Indentation Methods and Numerical Models for Characterising Shape Memory Material Behaviour

Indentation hardness testing is commonly employed in the materials science and engineering communities. The technique allows rapid characterisation (and ranking) of material hardness, and various indentation hardness scales have been developed over many years. The most common is the Vickers hardness scale, in which a 4-sided pyramidal indenter is forced under some fixed load into a specimen surface of lower hardness. Simple measurements of the geometrical features of the indent can be used, along with the applied load, to calculate the hardness.

A more sophisticated hardness test is one in which the applied loads and accompanying displacements are continuously varied and/or monitored. These are instrumented indentation tests, and they are used extensively in academia and industry. In fact, there are many reasons why instrumented indentation testing has become so popular but the ability to continuously measure the relationship between the applied force and the indenter displacement is the most prominent amongst them. This is because the load-displacement-time characteristics are governed entirely by the shape of the indenter and (most importantly) the constitutive material behaviour. There is enormous scope therefore for using these data to infer (or measure directly) mechanical properties of practical interest. These include the yield stress and work-hardening characteristics of metallic materials as well as primary and secondary creep parameters. The proposed methodologies, some of which have already been developed involve systematic comparisons between experimental outputs (load-displacement-time measurements) and iteratively altered model predictions, until an acceptable level of agreement is reached between the two.

My current research work is focussed on extending these methodologies to include characterisation of superelastic and shape memory alloys, exploiting the high temperature capabilities of the vacuum-enclosed nanoindenter and customised shape memory material model subroutines for use in ABAQUS.

Developing methodologies for inferring these parameters in a robust and reliable way will significantly enhance the capability envelope of the instrumented indentation test, which is currently used in a rather routine way to measure just the hardness and Young’s modulus of materials. What’s more, by developing methodologies that depend on the generation of indentation test data, many of the best attributes of indentation testing can be exploited. For instance, data acquisition is fast and simple (unlike many conventional mechanical test arrangements), only small volumes of material are required (which is highly beneficial when limited supplies are available), and specimen machining difficulties are avoided. The technique is also very well suited to the study of thin films, for which mechanical characterisation techniques are commonly unavailable, and a project in this are is currently ongoing.

This project falls under the"Fine Scale Mechanical Interrogation" research theme, where published papers in this area can be found.

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