| The nature of damage evolution in a discontinuously reinforced metal matrix composite subjected to fatigue loading is investigated in this study. The objective is to understand the stages in the damage process and the dominant mechanisms controlling failure. The macroscopic manifestation of damage, as indicated by changes in the mechanical response with continued exhaustion of life, is studied by monitoring the peak strains, hysteresis energy and fatigue modulus during cycling. The microscopic evidence of damage, on the other hand, is characterized at the surface by optical and electron microscopy, and monitored nondestructively for the bulk using acoustic emission. The alumina-silicate, short fiber reinforced aluminum alloy (A356) composites used were fabricated by the high pressure infiltration casting method (HiPIC). Nondestructive techniques were used successfully, both during and after processing, to detect preform crushing and porosity, respectively, to ensure that only sound specimens were tested. After characterizing the composite for microstructural features that could affect performance, modified dog-bone specimens were tested in tension-tension fatigue (R = 0.1).; The development of damage in the composite is driven by, and sensitive to, the applied strains because of the limited elongation to failure. Microstructural damage evolves by three distinctly different mechanisms--by cracking at hollow shot particles, by microcracking of fibers throughout the bulk, and by void nucleation at stress concentrations, such as fiber ends. The ductile, proeutectic phase acts as an effective barrier against the growth of cracks and linking of microcracks. Just prior to catastrophic failure, the cracks initiated at the shot particles link with each other through the intervening microcracked material, causing rupture. Acoustic emission results corroborate the observed microstructural evidence. The fatigue modulus reveals the accumulation of strain (dynamic or cyclic creep) due to the tensile mean stresses. However, the microstructural damage evolution is not reflected strongly in the variation of Young's modulus and hysteresis energy in a cycle. The above sequence of events, broadly consisting of initiation of cracks at hollow shot, growth of distributed damage (by microcracking of fibers), followed by crack linking just prior to failure, defines the failure process. |
