Cohesive zone modelling of crack growth in particle reinforced metal matrix composites

Metal matrix composites have been produced by gas pressure infiltration of ceramic particle beds with pure aluminium. Two different types of alumina particles have been employed: 35 μm angular and 15 μm polygonal particles. The resulting composites feature a homogeneous microstructure that is free of processing defects. Crack growth in these materials was characterized via single specimen J-integral testing according to ASTM E-1737. Both types of material exhibit stable crack growth and marked R-curve behaviour. Stereophotogrammetric measurements were used in order to infer the amount of energy dissipated in the process zone, by particle cracking or and matrix voiding. In the process zone and for both types of damage, the main dissipative mechanism is via the stretching of metal ligaments. Crack growth is modelled in CT specimens with a cohesive model, with parameters fitted from experimental load vs. CMOD records. Computed J-Δa curves exhibit somewhat stronger crack growth as compared to experimental ones. We attribute this offset mainly to the limited sample dimensions, and to the difficulty of representing a 3-D sample with a 2-D plane strain analysis. On the other hand, the agreement is sufficiently good to confirm the metallic character of the examined half-ceramic metal matrix composites, in so far as their toughness is dominated by remote plasticity. The amplification factor in our analysis is about half as large as computed from the stereophotogrammetric measurements, but probably more reliable than the latter, which was obtained in a very simplistic analysis. Both analyses confirm that a large peak stress in the process zone is attained, which explains why energy amplification by remote plasticity becomes operational and significantly toughens these composites.