Laser-induced spall in metals: Experiment and simulation

Abstract

Spall at ultra high strain rate (107 s−1) was investigated using short pulsed laser-induced shock waves in copper and aluminum foils. The intensities of the 3.9-ns Nd:Glass laser were in the range of 1010–1012 W/cm2, and the foil thickness was in the 100–600 μm range. The laser-generated shock wave pressure was in the range of a few hundred kilobars (kb). The shock wave traversed the foils in a few tens of nanoseconds. The controlled stepwise increase in laser energies allowed the stages of damage evolution from incipient to complete perforation of the target foils to be found. The energy threshold for spall and the spall width at that energy was measured as a function of the foil thickness for both materials. At threshold energy conditions, spall width of 25–65 μm for Al and 15–45 μm for Cu were obtained for foil thicknesses of 100–600 μm. Computer simulations of the laser-induced spall were performed, including the laser absorption, shock wave travel through the foil, and the spall phenomena. The simulations were based on the one-dimensional medusa code, which was expanded to include the spall phenomena, using simple spall criteria. An estimate of the strain rate was derived from the simulations and it was shown that the strain rates in the present experiments are about an order of magnitude larger than those obtained in spall experiments using other methods. The experimental results of energy threshold for spall and spall width at this energy were compared with the numerical simulations. The experimental results are in good agreement with the simulation results, indicating that spall strength for both materials, Al and Cu, are in the −50 to −60 kb regime.

Study of dislocation walls evolution during spall in pure aluminum

“…The spall strength in the experiments can be calculated from the acoustic approximation P spall =ρ 0 C 0 ∆u/2, where ∆u=u max -u min is the difference between the maximum and the first minimum of the free surface velocity -the pull-back velocity [5]. The free surface velocity was measured with a VISAR technique.…”

Ultra-high-speed studies of shock phenomena in a miniaturized system: A preliminary evaluation

“…This information is particularly vital to any attempt to correlate the response of shock-loaded thin films with the known behavior of well-characterized bulk materials. To address this need, we developed a sample characterization strategy that involves the following procedures: [ 13 film deposition on a low-mass substrate (i.e., the 0.5-mm-thick sapphire discs) to permit accurate measurement of the thin film mass using a microbalance with 0.1 pg sensitivity, [2] measurement of the flatness and uniformity of the thin film samples using an optical, non-contact profilometer, and [3] determination of grain sizes and void distributions from SEM images of sectioned samples. The microbalance and non-contact profilometry measure-…”

“…15; [2] Refinement of thin-film energetic material deposition processes to generate film densities and porosities that match properties of currently used bulk explosives as closely as possible-in particular, fine-grain formulations of explosives such as PETN or HNS that can undergo transition to detonation with extremely short time scales; [3] Detailed tests of the response of PETN and HNS thin films to short-pulse shock loading (to promote and assist development of models of the initiation and transition-todetonation behavior of these materials under these conditions)--high-speed velocity interferometry could be used to probe the equation of state of the unreacted explosive at previously inaccessible shock conditions and to measure time-to-reaction in the nanosecond regime; additional fundamental physical and chemical information might be obtained using the Raman probe; [4] Evaluation of the response of energetic material thin films exhibiting significantly different morphologies than those occurring in bulk materials; [5] Evaluation of strategies to extend the maximum available velocity of laser-generated flyers (presently -4 km-s-') to the hypervelocity regime (>7 km-s-'); factors contributing to flyer "failure" at elevated drive conditions include rapid diffusion of heat from the hot, driving plasma (leading to flyer melt and vaporization) and rapidly developing Ray1 eigh-Tayl or i nstabili ti es-possi bl e approaches to achieve acceptable mechanical integrity and higher velocities include the use of different composite flyer target materials and flyer generation with a broadband laser driver (to minimize instabilities driven by variations in the laser intensity at the fibedflyer interface); as illustrated in Fig. 16, an extended velocity range would enable impact studies on high-density materials (such as lead) at shock pressures well in excess of 100 GPa; [6] Detailed measurements of the dynamic material properties of a high-density metal foil 7 -1 (e.g., Ta or Pb) at high-strain-rate (>lo s ) conditions using ORVIS; results from a full test series would provide an important test for existing equation-of-state and dynamic material models in the short-pulsey high-strain-rate regime.…”

“…This system combines [ 13 well-characterized techniques for nearly planar shock compression using laser-driven flyer plates, [2] thin-film or foil sample materials, and [3] highspeed, real-time optical diagnostics including optically recording velocity interferometry and single-pulse Raman spectroscopy. Experiments are performed with a variety of miniature test platforms that are designed for precisely timed optical access to the target material.…”

In situ measurement of the particle size distribution of the fragmentation product of laser-shock-melted aluminum using in-line picosecond holography

“…For the laser-shock experiments discussed here, reasonable estimates would indicate a strain rate on the order of 10 7 −10 8 s −1 and an average particle diameter on the order of a micrometer. [19][20][21] Therefore, the minimum T predicted should ranges from a few tens of microseconds to a few tens of milliseconds. Generally the velocity of the fragmentation product front is on the order of 10 3 m/s; [13][14][15][20][21][22] thus, at the imaging moment of image capture for the given time delay range, the front has correspondingly advanced a distance ranging from a few tens of centimeters to a few meters.…”