Aashish Rohatgi

Resistance-curve and fracture behavior of Ti–Al3Ti metallic–intermetallic laminate (MIL) composites

The R-curve and fracture toughness behavior of single-edge notch beams of Ti–Al3Ti metallic–intermetallic laminate (MIL) composites has been investigated. Composites with 14, 20, and 35% volume fraction Ti, with a corresponding intermetallic layer thickness of ~540, ~440, and ~300 microns, respectively, were tested in crack arrester and crack divider orientations. In the arrester orientation, the R-curve could not be determined for the two highest Ti volume fraction compositions as the main crack could not be grown through the test samples. In the divider orientation, R-curves were determined for all three Ti volume fractions tested. The laminate composites were found to exhibit more than an order of magnitude improvement in fracture toughness over monolithic Al3Ti. Crack bridging and crack deflection by the Ti layers were primarily responsible for the large-scale bridging conditions leading to the R-curve behavior and enhanced fracture toughness. Estimates of steady-state toughness under small-scale bridging conditions were in close agreement with experimental results.

A METALLOGRAPHIC AND QUANTITATIVE ANALYSIS OF THE INFLUENCE OF STACKING FAULT ENERGY ON SHOCK- HARDENING IN Cu AND Cu-Al ALLOYS

Abstract This paper deals with the mechanical behavior of Cu and solid–solution Cu–Al alloys that were shock-deformed to 10 and 35 GPa. All the shock-deformed materials showed shock-strengthening that was greater at higher shock pressure and decreased with decreasing stacking fault energy (SFE) at both shock pressures. In the literature, shock-strengthening has been qualitatively ascribed to a greater dislocation density and the formation of deformation twins without addressing the question as to why shock-strengthening is lower in low SFE materials. This question is addressed in the present work by quantifying the twin contribution to the total post-shock strength. The twin contribution was found to increase with decreasing SFE suggesting that the contribution of dislocations concurrently decreases. The stored energy of as-shock-deformed materials was measured and found to decrease with decreasing SFE implying a lower net stored dislocation density in the lower SFE alloys. It is suggested that a lower net stored dislocation density in low SFE alloys results in the observed lower shock strengthening.

The variation of dislocation density as a function of the stacking fault energy in shock-deformed FCC materials

Abstract The variation of dislocation density with stacking fault energy (SFE) was measured in shock-deformed Cu and Cu–Al alloys. A differential scanning calorimeter (DSC) was used to measure the stored energy, from which the dislocation density was estimated. The energy released during recrystallization in the DSC experiments was attributed primarily to the annihilation of dislocations with the energy contribution from recovery, deformation twins and point-defects estimated to be relatively small. The dislocation density in the 10 GPa shock-deformed materials first increased and then decreased with increasing Al content (decreasing SFE) while the dislocation density in the 35 GPa shock-deformed materials initially decreased and then remained constant with increasing Al content. This variation in dislocation density in the shock-deformed materials is attributed to the nature of shock-deformation, the influence of stacking fault energy on the dislocation storage mechanisms, and the propensity for deformation twinning.

Analysis of the dynamic responses for a pre-cracked three-point bend specimen

In this paper, shear deformation and rotary inertia was introduced into the calculation of the dynamic stress intensity factor by means of solving the stiffness of a pre-cracked three-point bend specimen. A simple formula of dynamic stress intensity factor for a pre-cracked three-point bend specimen is derived using the vibration analysis method. Dynamic three-point bending tests were performed on a uniquely modified Hopkinson pressure bar, allowing the dynamic responses of the pre-cracked specimen, such as: the natural frequency, the period of apparent specimen oscillations, the dynamic loads, and the dynamic stress intensity factor to be analyzed experimentally and theoretically.