No abstract

No abstract

The effects of grain size on the spall response were investigated for high purity copper materials by plate-impact experiments including real-time measurements of the free surface velocity profiles as well as post-impact fractography studies on the soft-recovered samples. High purity copper plates were cold rolled and heat treated to produce recrystallized samples with average grain sizes of 78, 273 and 400 μm, respectively. The spall strength estimated from the free surface velocity profile is nearly constant with no significant effect on the grain size. However, differences are observed in the acceleration rate of velocity rebound beyond the minima. This may be attributed to the effect of grain size on the growth rate of damage. Metallographic analyses of the fracture surface show that the characteristic feature of the fracture surface clearly depends on the grain size. In the 78-and 273-μm samples, the fracture surfaces are decorated with large, high-density ductile dimples suggesting that the preferential failure mode is ductile intergranular fracture. In the 400-μm samples, the fracture surfaces have a rock candy appearance with small, high density brittle dimples as well as large ductile dimples suggesting that the fracture mode is a mix of both brittle intergranular fracture and ductile transgranular fracture.

A series of uniaxial compression experiments on unpoled porous lead zirconate titanate Pb(Zr0.95Ti0.05)O3 (PZT95/5) ceramics were performed to investigate the influence of porosity on the mechanical properties, domain switching and phase transformation via non-contact digital image correlation optical technique to measure the full-field strain. Four unpoled PZT95/5 ceramics were fabricated with a range of porosity levels via systematic additions of pore formers. The nonlinear compressive mechanical behaviors of unpoled PZT95/5 ceramics were measured and evaluated as a function of porosity. The nonlinear deformation mechanism of unpoled PZT95/5 ceramics was attributed to domain switching and ferroelectric (FE) to antiferroelectric (AFE) phase transformation processes, which was verified by analyzing the anomalous behavior between axial strain and radial strain. Normalized critical stresses for the onset of domain switching as well as the onset and exhaustion of phase transformation resulted in a very similar linear decrease with increasing porosity. We suggest that a critical macroscopic volumetric strain is needed for the onset of phase transformation that is nearly independent of porosity.