An analysis of plastic instability in pure shear in high strength AISI 4340 steel

Abstract

The phenomenon of plastic instability in pure shear was studied at room temperature in heat treated high-strength AISI 4340 steels, employing the torsion test. The instability occurs after saturation of strain hardening by the dispersed carbide particles. The strain at onset of instability is sensitively dependent on rate of straining. The effective stress (total stress minus the stress resulting from dispersion hardening) may be a result of Snoek relaxation or the Cottrell drag, depending on strain rate; the magnitude of this stress is linearly proportional to the dissolved carbon content. Experimental observations indicate that instability interfaces advance spirally in the axial direction of the specimen. It is proposed that the interface is stable in a thermodynamic sense and that the driving force for migration depends only on ratio of the speed of the front to the macroscopic strain rate. When the interface becomes stationary, fracture is nucleated at an axial distortion in the interface. Fracture by the instability occurs by a translatory motion of one now rigid body with respect to a second rigid body along the characteristic (slip) surface. Pores may be found in the fracture surface, but these are incidental to the intense flow along the characteristic, and are not the cause of the instability.