The Effect of Hydrostatic Pressure on Plastic Flow under Shearing Stress

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Abstract
This paper presents measurements of the forces required to drive a punch into plates of several grades of steel as a function of the distance of penetration, the entire punching operation being conducted in a fluid medium subjected to hydrostatic pressures up to 30,000 kg/cm2. Qualitatively, the effects are similar to those already found for the tensile properties of steel subjected to hydrostatic pressure, namely, ductility is greatly increased, and greatly increased distortion is tolerated without fracture. The effects are similar in general character for all the steels experimented on here, but are quantitatively accentuated for the softer steels. At a pressure of 20,000 kg/cm2 or more a punch may be driven completely through a plate of mild steel, with no loss of coherence at any stage of the process, and with strain hardening, when expressed in terms of true shearing stress, which may increase by a factor of as much as 3. If the punching operation is suspended at any intermediate stage before complete penetration and afterward completed at atmospheric pressure, very material strengthening will be found as compared to virgin material of the same geometrical configuration. There are certain qualitative differences in the details of the ductility exhibited during the processes of punching and pulling. At intermediate pressures and penetrations the true shearing stress in punching may, under proper circumstances, exhibit maxima, for which there is no analog in the pulling operation. This is to be understood in terms of a difference of geometry, there being greater opportunity for self-healing during the punching operation, and, furthermore, partial deterioration of the coherence of the metal not necessarily leading to complete catastrophe.

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