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Title: Extraction of mechanical properties over a range of strain rates from indentation data
Author: Burley, Max
ISNI:       0000 0004 9348 391X
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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A procedure is described for indentation creep plastometry. It is based on iterative numerical simulation of the indentation process, with repeated comparison between an experimental outcome and the corresponding model prediction, systematically varying the values of parameters in a constitutive law until optimal agreement is achieved. The experimental outcome is the penetration depth as a function of time, under a constant applied load. An important feature of the procedure is the prior creation of a spherical recess in the sample, having a pre-selected depth and a curvature radius equal to that of the indenter. This allows control over the stress levels created during the indentation creep testing and can be used to ensure that no (time-independent) plastic deformation is stimulated during the test. Confirmation of the viability of the procedure is provided via comparisons between the creep characteristics of pure nickel samples at 750 ̊C, obtained in this way and via conventional uniaxial tensile testing. A similar procedure has been applied for the study of rate-dependent plasticity in bulk metallic materials. Ballistic impact (indentation) of hard spherical projectiles was used to study a rigidly held target, with operative strain rates of the order 105 s-1. Input for the FEM model includes data characterizing the (temperature-dependent) quasi-static plasticity, obtained by conventional uniaxial testing. The experimental outcomes are displacement-time plots for the projectile and the residual indent profile. Since the strain rate sensitivity is characterised by a single parameter value (C in the Johnson–Cook formulation), convergence on its optimum value is straightforward, although a parameter characterizing interfacial friction is also required. Using experimental data from (both work-hardened and annealed) copper samples, this procedure has been carried out and best-fit values of C (∼0.016 and ∼0.030) have been obtained. This procedure has been extended for the study of fracture characteristics (under high imposed strain rates). The strain rate sensitivity of magnesium was evaluated (C~0.026) as above. The main emphasis, however, is on study of its fracture characteristics, with tomographic imaging being used to obtain crack patterns for different projectile velocities. An approach based on fracture mechanics, and on use of FEM modelling to estimate the strain energy release rate required for crack propagation (i.e. the fracture energy of the material) is proposed and applied to these experimental results, leading to a value of the order of 2 kJ m-2. While such a procedure is unlikely to produce accurate values, partly because the crack propagation takes place under local conditions that change rapidly and are not well-defined, this figure is plausible for the case concerned. While there are several sources of complexity, it may be possible to develop this methodology, both as a technique for fracture toughness measurement (requiring only small samples of simple shape) and as an improved approach to prediction of ballistic impact outcomes.
Supervisor: Clyne, T. W. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Indentation ; Fracture ; Creep ; Plasticity ; High strain rates ; Finite element analysis ; Ballistic indentation