Unloading waves and unloading failures in structures under impact loading

In studies of dynamic inelastic response of structures under high strain rates, a great deal of attention has been paid to inertia and strain rate effects. Most of the published works have concentrated upon the dynamic response during loading. However dynamic failure of structures is not unusual during unloading. Some examples are presented, including how “front spalling” can be caused by the interaction of the incident unloading/reverse plastic waves and the following incident elastic unloading waves; how the surface damage on the target in the form of annular short circumferential surface cracks is induced by the unloading Rayleigh surface waves, and how a series of stress unloading waves predominantly propagate in viscoelastic media due to relaxation effects. It is emphasized that more attention should be paid to studies on unloading waves, unloading constitutive relations and failure criteria.