The interplay of microstructure, geometry, and stress state on the mechanical performance of MMC joints (Squeeze casting)

Model MMC joints were fabricated to provide an avenue for assessing the failure mechanisms which limit the performance of composite joints. The joints consisted of aluminum matrix composite subelements separated by thin metal interlayers, and were manufactured by pressure infiltration of an Al-4.5% Mg alloy into preforms of polycrystalline alumina fibers containing planar discontinuities. This technique minimized processing defects and ensured interface integrity. The high degree of constraint and concurrent buildup of large hydrostatic stresses in the metal interlayers allowed simple butt joints to support applied loads in excess of two times the metal yield strength. The composite-interlayer interface fails at these stress levels due to the nucleation, growth, and coalescence of voids at the fiber ends.; The performance of joints with interlayers inclined at various scarf angles was investigated and revealed that the applied load at failure increased substantially when the interlayers were inclined at angles greater than 45°. The strain to failure also increases significantly under these conditions, although strength remains limited by the load carrying capacity of the composite-interlayer interface. The change in performance is due to the dependence of the interlayer stress state on scarf angle, an effect modeled with both analytical and finite element techniques. Microstructural changes associated with varying scarf angles are also discussed.; The addition of particulate reinforcement to the interlayers provides an alternative manner in which to change the interlayer stress state. Model butt joints with reinforced interlayers exhibited strength increases of up to 50% over unreinforced joints. More importantly, joint strength was not limited by failure along the composite-interlayer interface when particles were present. Failure instead occurred through the reinforced interlayer, often at substantially higher loads. The change in failure mode is associated with changes in the interlayer stress state on two length scales, as has been demonstrated with finite element calculations of the entire joint and individual fiber ends.; A macroscopic description of the interface stress state was found to correlate with fracture in all the model joint specimens tested. The criterion is based on work by Rice and Tracey [69RiT], and accounts for the growth and coalescence of voids under different combinations of hydrostatic stress and plastic strain. In addition, stresses normal to individual fiber ends were found to be consistent between joints with varying scarf angles when this criterion was satisfied.