The Plastic Flow and Fracture of a ‘Brittle’ Material (Grey Cast Iron) with Particular Reference to the Effect of Fluid Pressure

Abstract

This paper describes tests carried out on a ‘brittle’ material in the form of grey cast iron to investigate the criterion of plastic flow and ultimate failure, and in particular torsion tests carried out with superimposed fluid pressures of up to 35 ton/in2. It is suggested that the results show that the mode of failure is dependent on the volumetric stress acting. For volumetric compressive stresses greater than approximately zero, failure occurs at a critical value of the shear stress, a value which is some function of the volumetric stress. At small or negative values of the volumetric compressive stress failure occurs at a critical value of the maximum tensile stress, a value which may be slightly dependent on the volumetric stress. The shear stress-strain curve is apparently also related to the volumetric stress, and the shear stress-strain curve deduced from a compression test on the shear-stress hypothesis coincides with that found from a torsion test carried out under a pressure approximately equal to the volumetric stress in the compression test. In torsion the strain to failure is raised by pressure; the rate of change of strain with pressure being considerably increased when the fluid pressure is sufficient to suppress completely any tensile component of stress. The strain to failure in compression is greater than in the comparable torsion tests, but this may be explained by the weakening effect of the fluid or the rubber protective film when they are forced into incipient cracks.