Stress and Strain Rate Effects on Incipient Spall in Tantalum

Spall fracture is a high-rate tensile damage phenomenon associated with impulsive and shock-load events. Typically, the material undergoes a sequence of compression followed by release into high rate (on the order of 10⁴ s^-1 and up) tension, causing voids to nucleate and grow, which can then coalesce into a crack and the material fails. We present a series of experiments on high purity, well characterized tantalum samples subjected to shock-loading via gas-gun plate impact. Through careful selection of the flyer-plate velocity and material we have independent control over the peak compressive stress and the tensile strain rate in the sample. At all times, the spall damage remains incipient, i.e. in the early stages of void formation and the material does not fully fracture. Velocimetry was used on the rear of the sample to record the wave-profiles and determine spall strength. Soft recovery and sectioning of the samples allowed the internal damage to be observed, quantifying the damage amount, distribution, and relationship to microstructural features with both optical and electron based microscopy.