A discussion on deformation of solids by the impact of liquids, and its relation to rain damage in aircraft and missiles, to blade erosion in steam turbines, and to cavitation erosion - Stress waves, deformation and fracture caused by liquid impact

Abstract

When a liquid mass strikes a solid surface, compressible behaviour, giving rise to a sharp peak of pressure, may occur in the initial stages of the impact. The duration of the peak depends on the dimensions and impact velocity of the liquid mass, and also on the compressional wave velocity for the liquid. There are similarities between this type of loading and that produced by the detonation of small quantities of explosive, since both give intense pressure peaks of only a few microseconds’ duration. The fracture and deformation of glasses, hard polymers, single crystal and ceramic materials by liquid impact at velocities up to 1000 m/s is described and briefly compared with that produced by solid/solid impact and explosive loading. The detailed development of fracture has been followed by high speed photography. In brittle solids the main characteristics of damage on the front surface is a ring fracture surrounding a largely undamaged area. The ring fracture forms at the edge of the loaded area where high tensile forces develop during impact. Outside this main ring of fracture short circumferential cracks occur; these are shown to be initiated by the Rayleigh surface wave at points where flaws existed. More complex fracture patterns which appear on the front surface of plates are due to the reinforcement of the surface wave with components of stress reflected from the back surface. Thin plate specimens often exhibit ‘scabbing’ fracture at the rear surface; in brittle materials of low attenuation this form of damage can be of prime importance. Since the stress pulses producing fracture during liquid impact are short the fractures themselves remain short and discrete. By a combination of impact loading and etching it is possible to investigate the distribution and depth of flaws, their role in the fracture process, and the effect which etching has upon them.