Damage property of incompletely spalled aluminum under shock wave loading

Abstract

The nucleation, growth, and coalescence of microscopic voids are induced inside ductile metal when it is subjected to dynamic tension, and this eventually results in a catastrophic fracture of the specimen. In the present work, this failure property is studied by using ultrapure aluminum (99.999%) as a representative candidate for the ductile metal and, further, the micro-void growth behavior (or the damage evolution) before the catastrophic fracture has been investigated. Measurements of the free surface velocity profile and statistical analysis of micro-voids were used to determine the spall characteristic and damage evolution behavior of dynamic tensile fracture in ultrapure aluminum. Through comparison of the differences between the incompletely and completely spalled signals, the spall characteristics of ultrapure aluminum from incomplete to complete spall were analyzed. Moreover, an obvious critical characteristic is found on the basis of the association between the maximum damage caused within samples and the product of peak stress and tensile duration. The damage variable slowly increased in a linear manner in the initial stages but changed to nonlinear growth and rapidly approached a fracture state as the damage variable extended beyond the critical value, which is approximately 0.09. A physical explanation for this transition is discussed and implicates micro-void linkage behavior during the dynamic failure.

Critical Damage Evolution model for spall failure of ductile metals

“…Recently, damage property of incompletely spalled aluminum under shock wave loading was explored by our research group (Qi et al, 2012). Experimental results of statistical analysis of micro-voids clearly indicate a two-stage damage-evolution pattern with a transition at a critical damage.…”

“…The evolution above the threshold is governed by another behavior (i.e. ''shoulder effects'' for example, Chen et al, 2006;Qi et al, 2012) and thus, it does not allow estimating the spall strength accurately. Second, as the impact velocity increases, the loading rate increases as shown in Fig.…”

“…Based on the assumption that the stress rapidly decreases to porositydependent values given by Cochran and Banner (1977), a simple phenomenological model is constructed here to describe the critical transition behavior from void growth to void coalescence and the rapid stress relaxation due to void coalescence. Inspired by the experimental observations of damage evolution in aluminum (Qi et al, 2012) and the percolation theory, the Percolation-Relaxation (P-R) model can be written as…”

Spallation caused by the diffusion and agglomeration of vacancies in ductile metals

“…It is like the ferromagnetic transition associated with the nucleation, the growth of clusters and the percolation of clusters, which are usually encountered in condensed matter physics. The critical behaviors of the spallation have been discovered by the experimental results [14,15],…”

High-power laser shock-induced dynamic fracture of aluminum and microscopic observation of samples

“…When the surface polishing scratches is very small, polish with diamond polishing liquid; Finally, reveal metallographic structure by the erosion corrosion liquid, and further metallographic analysis can be done. Figure 7 is the metallographic section pictures of laser loading and gas gun driven flyer impact samples [15] respectively. It is obvious that the voids are smaller and more irregular in samples of high-power laser loading than the gas gun loading, demonstrate that the growth time of voids of laser loading is much shorter which is closely related to strain rate, the strain rate of high-power laser loading is higher than that of gas gun loading.…”