ELSI conference: invited lecture - Liquid impact: theory, experiment, applications

Liquid impact comes into a wide range of technological problems. The theory has developed historically from two areas: those interested in "water entry" and the action of waves on ships and structures; and those studying liquid drops or jets impinging on surfaces. Erosion problems caused by liquid drops impacting surfaces occur with steam-turbines and with aircraft or missiles travelling at high speed through rain. Cavitation damage is also closely related to liquid impact erosion. More beneficial uses of liquid drops and jets are for cleaning surfaces or cutting materials. The reason a liquid/solid impact can cause so much damage is that in the initial stages of impact the contact periphery expands supersonically (i.e., faster than stress waves in either liquid or solid). Then is, therefore, a period (the duration depending on the impact velocity and geometry in the contact region) when compressible loading results and pressures are not released by flow. This paper describes the background theory, the various areas of application, techniques for producing controlled impacts in the laboratory for the velocity range from a few meters per second to several thousands of meters per second, the use of high-speed photography for studying such impacts, and the damage processes in the solid. An area of current interest is producing IR transmitting ''window" materials for aircraft which have sufficient strength to resist erosion damage by rain, sand, ice and bird-strike. IR materials such as zinc sulphide and germanium are relatively weak. However, it is now possible to produce free-standing "windows" and domes made from chemically vapour-deposited diamond (CVDD). Recent advances in this area are presented. (C) 1999 Elsevier Science S.A. All rights reserved.