An analysis of the tensile elongation to failure of laminated metal composites in the presence of strain-rate hardening

Cécile Bernardi, Arda Cetin, Andreas Mortensen
Elsevier, 2012
Article

We extend the analysis of Hutchinson and Neale (Hutchinson JW, Neale KW. Acta Metall 1977;25:839) and Ghosh (Ghosh AK. Acta Metall 1977;25:1413) predicting the tensile elongation to failure of strain-rate-dependent plastic materials to two-phase composites deforming quasistatically according to the equistrain rule of mixtures. The analysis incorporates the influence of work hardening and strain-rate hardening in both composite constituent phases. It is shown that the problem can be formulated in a manner that condenses the seven underlying material parameters into four dimensionless numbers for composites of power-law hardening phases, the number of parameters falling to two for linear hardening. It then emerges that the stabilizing influence of both work hardening and strain-rate hardening is, within assumptions of the model, always predominantly exerted by the phase that carries the greater share of the composite stress. It is also shown that the prediction can be simplified so as to enable an approximate but convenient direct graphical deduction of the tensile elongation of ductile laminated metal composites (LMCs), knowing the work hardening and strain-rate hardening characteristics of the two phases making the composite. The utility of this graphical scheme is illustrated with two examples, namely LMCs containing one phase of (moderately ductile) aluminium alloy or of a (low-ductility) nanocrystalline metal. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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Cécile Bernardi, Arda Cetin, Andreas Mortensen
Elsevier, 2012
Article

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https://infoscience.epfl.ch/record/175388?ln=fr

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Designing laminated metal composites for tensile ductility

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This contribution draws practical implications of a recently published estimation of the tensile ductility in laminated composites made of two ductile materials, typically metals or alloys, which harden as both the strain and the strain-rate increase. To this end, the literature is surveyed to collect values for the strain hardening exponent, the strain-rate sensitivity and the strength constant for a wide range of engineering metals and alloys. Material combinations that might produce ductile laminated metal composites are then examined in light of the data and theory. A simple graph is proposed, which gives a direct reading of the predicted elongation to failure of composites containing equal volume fractions of any two materials among those surveyed. The resulting plots show material combinations in which a more ductile material can significantly increase, within a laminated metal composite (LMC), the tensile elongation of a less ductile material. In this role, 304 stainless steel and commercial purity iron emerge as sensible possibilities. (C) 2014 Elsevier Ltd. All rights reserved.

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We use the long-wavelength model of Hutchinson and Neale (1977) and Ghosh (1977) to estimate the uniform tensile elongation of two-phase composites deforming quasistatically according to the equistrain rule of mixtures, in which one phase is ductile while the other fractures progressively according to two-parameter Weibull statistics. We use shear-lag models in the literature to quantify load transfer from the ductile phase to the fractured brittle phase, and to estimate the influence of matrix strain and strain-rate hardening, of brittle phase fracture characteristics, and of phase volume and strength ratios, on the composite strain to failure as dictated by the onset of unstable necking. Calculations show that strain and strain-rate hardening of the ductile phase do relatively little to increase the ductility of the composite. Two parameters play a dominant role, namely the brittle-phase Weibull modulus and a dimensionless parameter describing load transfer across the two phases. The main practical implication of this analysis is that, to produce reasonably ductile two-phase composites, the best strategy is to aim for small layer thicknesses. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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Infiltrated particle reinforced composites combine a dense matrix with particles that are in mutual contact and can therefore transfer compressive stress directly from one particle to the next. As a result, these composites may combine characteristics of the plasticity of their matrix with those of granular matter plasticity. We measure here the influence of a 200 MPa superimposed fluid hydrostatic pressure on the flow stress of high volume fraction (56-62 vol pct) particulate Al2O3-Al composites produced by infiltration and show that the yield response of such composites is indeed pressure-sensitive. A simple analysis that transposes to metal matrix composites the theory of fluid-saturated granular media mechanics explains the phenomenon quantitatively. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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