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A study has been made of the deformation of solids at high rates of strain which are produced by the impact of a small cylinder or jet of liquid on the surface of the solid. A method is developed for projecting this jet against the solid at velocities up to 1200 m/s. The subsequent deformation of the solid under impact and the behaviour of the liquid is observed by high-speed photographic methods. The magnitude and duration of the impact load are also measured by using a piezo-electric transducer. The mode of deformation of the solid has been investigated for plastic, elastic and brittle materials. There is evidence that the liquid jet, on impact, behaves initially in a compressible manner. Part of the deformation is due to these compressible effects and part to the shearing action of the liquid flowing at very high speeds across the surface. If the head of the jet has an appropriate shape (e.g. wedge shaped) the velocity of flow across the surface may be much greater than the velocity of approach. It is found that there are five general types of deformation produced in the solid. There are (i) circumferential surface fractures, (ii) subsurface flow and fractures, (iii) large-scale plastic deformation, (iv) shear deformation around the periphery of the impact zone, and (v) fracture due to the reflexion and interference of stress waves. The predominating mode of deformation depends primarily on the mechanical properties of the solid and on the velocity of impact. The observations have a bearing on the practical problem of the erosion of aircraft flying at high speed through rain and on the erosion of turbine blades.

The object of the work has been to investigate experimentally the mechanisms of erosion in metals and alloys under drop impingement attack. For this purpose an apparatus of the wheel and jet type has been used to erode aluminium, copper, iron, cobalt and alloys of these metals. The various stages in the process from the first detectable microplastic deformation to the eventual pitting and removal of material from the surface have been investigated. In addition, experiments were carried out with the purpose of examining the effects of the normal impact pressure of a liquid on a surface in the absence of shear forces associated with liquid flow. This was achieved by propagating impact-generated compression waves through a liquid column in a filled and sealed cylinder onto a specimen surface inside the cylinder. With this arrangement the initial damage—small shallow depressions in the specimen surface— was identical with that produced under standard drop impact conditions in the wheel and jet apparatus. In either case the calculated values of the maximum impact pressure were lower than the average yield strength of each metal investigated. A complementary series of experiments was carried out in order to examine the erosive effects of liquid flow over the surface in the absence of high impact pressures. The technique used here involved a continuous high-speed water jet impinging against a solid surface at glancing incidence. This study showed that while flat well polished surfaces were apparently unaffected by the flow, lightly roughened surfaces or surfaces which contained the shallow impact depressions were severely eroded in regions adjacent to discontinuities. These various experiments suggest that the initial yielding which gives rise to the depression is associated with non-uniformity in the strength, structure and shape of the solid surface rather than with local variations in the impact pressure over the surface. The subsequent acceleration in the erosion rate is linked with the increased roughening of the surface and with an increase in the shear damage. When the surface becomes very rough and pitted, the impinging drop is deflected into less damaging streams by surface projections. This effect would account for the eventual decrease observed in the erosion rate. Further studies of the structure of the eroded surfaces have shown that the fractures have a number of features which are characteristic of metal fatigue failure. The connexion between erosion and fatigue is illustrated by similarities between the endurance curves for erosion and for the same metal in a standard fatigue test. As in the case of fatigue failure, strain energy to fracture appears to be one of the most important mechanical properties determining the erosive behaviour of a ductile metal.

A study has been made of the deformation at high strain rates of solids under the impact of liquids. A method is described for projecting a short liquid jet against a solid surface at speeds up to 1200 m/s. The flow of the liquid and the deformation of the solid during impact have been examined by high speed photographic methods. An attempt has been made to measure the magnitude and duration of the load by means of a piezoelectric pressure transducer. There is evidence that the liquid behaves initially on impact in a compressible manner. Part of the deformation of the solid is due to this compressible behaviour and part to the erosive shearing action of the liquid flowing at very high speeds out across the surface. The mode of deformation in brittle and in plastically deforming materials has been investigated. The deformation patterns produced are shown to be characteristic of liquid impact. The predominating mechanism of deformation depends on the mechanical properties of the solid and on the velocity of impact.