Laser-induced spallation of aluminum and Al alloys at strain rates above 2×106s−1

Abstract

Material microstructure is a significant determinant of the tensile stress at which materials fail. Using a high-energy laser to drive shocks in thin slabs, we have explored the role material microstructure plays on the spall strength of high-purity and alloyed aluminum at strain rates of (2–7.5)×106s−1. Slabs of pure recrystallized Al and recrystallized or cold worked Al+3wt% Mg were shock driven using the Z-Beamlet Laser at Sandia National Laboratories. Velocity interferometer measurements determined the spall strength of the materials, and postshot target analysis explored the microscopic fracture morphology. We observed the greatest spall strength for large-grained, recrystallized high-purity aluminum, with the dominant failure mode being ductile and transgranular. We observe for the first time at these strain rates fracture features for a fine-grained Al+3wt% Mg that were a combination of brittle intergranular and ductile transgranular fracture types. Postshot analysis of target cross sections and hydrocode simulations indicate that this mixed-mode failure results from spall dynamics occurring on spatial scales on the order of the grain size. Differences in spall strength between these Al samples were experimentally significant and correlate with the damage morphologies observed.

Spall fracture: methodological aspects, mechanisms and governing factors

“…18 was observed by Dalton et al (2008) at laser-induced spallation in a coarse-grain pure aluminum. Even larger difference in state of transgranular and intergranular parts of the fracture surface was observed by Rességu-ier et al (2007) at spallation in laser shock-loaded tin.…”

Computational modeling and simulation of spall fracture in polycrystalline solids by an atomistic-based interfacial zone model

“…There are two conflicting conclusions about the grain size effect on spall strength in metals. Some researchers presented that spall strength decreases with increasing grain sizes[52], while other studies have observed that spall strength increases as grain size increasing[45, 53]. In this work, the atomic-based interfacial zone model is used to study the grain size effects on spall strength for three different grain sizes.…”

“…Thus, the free surface velocity history at the opposite side of contact surface is widely utilized to observe the spall behavior as an approach for indirect measurement. Within the acoustic approximation, the spall strength is defined as[44, 45]: $${\sigma }_{sp}=\frac{1}{2}\rho c{V}_{sp}$$ where ρ is the material density, c is the sound velocity in the material and V sp = V o – V m . A compressive disturbance called a “spall signal” appears on the free surface velocity history as the tensile stress relaxes[4, 12].…”

“…The transition from intergranular to transgranular mode was observed by many researchers [47-49], which is determined by a variety of factors including loading conditions, microstructure of a material, interfacial properties etc. The fracture morphology, such as intergranular mode or transgranular mode, plays an important role on spall strength [45, 50]. …”

A grain-scale study of spall in brittle materials

“…Second, most experiments involving tension or expansion (e.g., rings) will be strongly affected by defects on the surface of the sample, but spall experiments generally only sample flaws on the interior. Finally, while spall experiments involving plate impact have length scales on the order of millimeters, laser spall has been used to study much smaller samples on the order of 100 µm (c.f., Dalton et al 2008) or even a few microns for coatings (Gupta and Yuan 1993). In fact, in the study of Dalton et al (2008), a line-VISAR was fielded on samples where the spall plane was approximately 200 µm from the free surface, though they did not attempt to obtain spatial variations from the measurement.…”