UC San Diego

A systematic investigation of the evolution of damage and the residual stresses in Ti6Al4V-Al3Ti metal-intermetallic laminate (MIL) composites were carried out. Crack morphology and density of as-processed MIL composites were quantified, and quasi-static and dynamic compression tests were conducted on pure Al3Ti, as well as on MIL composites, with different volume fractions of Ti6Al4V (14%, 20% and 35%) under different loading directions (perpendicular and parallel directions to laminate plane), to different strains (̃1%, ̃2%, ̃3%), and at different strain rates (0.0001/s and 800-2000/s). Crack densities and distributions were measured, and the differences in crack propagation and damage evolution in MIL composites under quasi-static (0.0001/s) and dynamic (800-2000/s) deformation were observed anddiscussed. The fracture stresses under different testing conditions do not exhibit significant strain-rate sensitivity, which is indicative of the dominance of microcracking processes in determining strength. The crack density after dynamic deformation is higher than that after quasi-static deformation. This is attributed to the decreased time for crack interaction in high-strain rate deformation. The effect of crack density, quantified by a damage parameter, on elastic modulus were quantified and the results were compared with experimental results. The principal damage evolution mechanisms were identified. The elastic properties and anisotropy of the laminates were calculated and successfully compared with Resonant Ultrasonic Spectroscopy (RUS) measurements. The residual stress in MIL composites was evaluated from the differences in the thermal expansion coefficients of Ti6Al4V and Al3Ti. The residual stress evolution during cooling process was modeled by incorporating two stress release mechanisms: creep and crack propagation, and implemented through analytical modeling and finite element simulation. The obtained results indicate good agreements with the X-ray measurements. The fracture toughness of MIL composites was modeled as a combination of the crack initiation toughness and the stress intensity, and the calculation predicts the maximum fracture toughness occurs at 57% volume fraction of Ti6Al4V