Mechanical Behavior of Advanced Aerospace Materials

This report summarizes investigations involving the characterization of second tier mechanical properties of current and advanced aerospace materials and the development of damage tolerant design approaches and durability analyses for the applications of these materials. Specifically, studies of gamma-titanium aluminides have dealt with the identification and origins of defects that are associated with processing procedures and with the role of defects and grain orientation on fatigue crack initiation. Growth of both through-the-thickness and surface bridged cracks in unidirectional metal matrix composites was studied over ranges of stress ratios and temperatures. The shear lag model that was developed to predict through-the-thickness crack growth was modified to successfully predict the effect of stress ratio and temperature on the surface crack growth. Damage evolution, associated with fiber breakage, and deformation models were developed for sustained loading of unidirectional metal matrix composites and extended to fatigue conditions. Studies of ceramic composite systems were conducted to understand the mechanics and mechanisms of damage initiation and crack growth in smooth sided and notched specimens under monotonic and fatigue loads. The influences of high cycle fatigue, fretting fatigue. and fatigue-load interactions on damage tolerance and mixed-mode crack growth of current monolithic structural alloys have been investigated for an enhanced understanding of improved life prediction methodologies. In support of the preceding investigations. the following experimental tools and methodologies were investigated andor developed - a surface wave amplitudes for correlation with the formation and the opening and closing of ceramic matrix cracks during loadunload cycles. b broadband sensors for detection of surface waves and rod waves to locate fiber breaks in metal matrix composites.