Spallation is a specific kind of fracture in solids which is developed by tensile stress pulses produced by reflection of compression waves at the interfaces adjacent to low impedance media. Such waves can be caused by the impact of a projectile against a target, the detonation of high explosives in direct contact with the target, or by sudden deposition of an intense pulse of energy on the target surface. In laboratory experiments, a commonly used method to produce tensile pulses is shown in Fig. 1. A thin plate is launched with pre-programmed velocity by a gas gun and strikes a flat stationary plate specimen of the same material. Usually the specimen is about twice the thickness of the impactor. The reflected stress waves in the flyer and the target are also shown schematically in Fig. 1. The incident compressive wave is developed in the target by the flyer impact. The incident wave is purely elastic when the stress amplitude is below the Hugoniot Elastic Limit. If the impact velocity is high enough, the HEL is exceeded causing the elastic and slower plastic compressive waves to propagate towards the free surface of the target. At the same time identical waves propagate in the flyer. The elastic-plastic incident wave is reflected at the specimen free surface as the tensile one, but in the middle of the specimen the material is under tension. If the amplitude and duration of the tensile reflected wave is high enough, the spall occurs in the middle of the target.
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