Quantification of the effects of reinforcement distribution and morphology on fatigue in al-sic p composites

Boselli, Julien (1999) Quantification of the effects of reinforcement distribution and morphology on fatigue in al-sic p composites. University of Southampton, Doctoral Thesis.

Abstract

Various reports in the literature have highlighted the effects of particle distribution and morphology on the fatigue and fracture behaviour of discontinuously reinforced metal matrix composites (DMMCs) although few attempts have been made at quantifying and/or modelling such effects. In the present study, a fundamental investigation was undertaken on the effects of reinforcement distribution and morphology on fatigue behaviour in 2124-based Al-SiC_P composites. Finite body tessellation methods have been specifically developed for characterising the spatial distribution of high volume fractions of second phase particles with wide reinforcement size and/or shape distributions, as observed in DMMCs. Quantitative comparisons with established Dirichlet methods indicate that finite body tessellation methods are more physically representative as well as more sensitive to local distribution characteristics in such materials.

Smooth specimen fatigue behaviour has been investigated in two powder-processed 2124-based A1-18.7 vol% SiC composites with nominal particle sizes of 6.5 and 3 μm. Tesselation analysis of the larger particle composite has demonstrated that crack-tip interactions with the reinforcement occurred preferentially in small particle/low reinforcement content regions. Further analysis of the data indicated a controlling influence of particle size on crack path behaviour. In addition, regression analysis particularly identified a combined influence on growth rates in the 'discontinuous' regime of reinforcement clustering (in terms of the spread of neighbouring particle separations) and reinforcement alignment in the regions where crack growth occurred. Reinforcement alignment was found to act as a large-area dependent effect, consistent with mesoscopic residual stress variations introduced by quench and stretch operations. Clustering was seen to act over the scale of a few particle diameters, consistent with localised load transfer effects.

A preliminary micromechanical understanding of the effects of clustering on crack growth behaviour in A1-SiC_P composites has been achieved via finite element modelling. Consistent differences were identified between models with regular vs. clustered particle arrangements in terms of crack path morphologies and local crack-tip stress intensity fluctuations.

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