Research on Notch Plasticity May Contribute to the Understanding of Stress Corrosion Cracking

RESEARCH JUST COMPLETED AT THE OHIO STATE UNIVERSITY ON FACTORS LIMITING PLASTIC STRAIN UNDER A NOTCH (OR CRACK) MAY PROVE IMPORTANT IN ELUCIDATING CERTAIN ASPECTS OF PLASTIC ZONE BEHAVIOR IN STRESS CORROSION CRACKING. IT WAS FOUND THAT THE PLASTIC ZONE AT THE TIP OF THE CRACK EXPANDS ON LOADING UNTIL A CRITICAL STRAIN AT THE NOTCH ROOT IS ATTAINED. AT THIS STRAIN, THE MATERIAL UNDERGOES A RHEOLOGICAL TRANSFORMATION INTO FLUID-LIKE FLOW (VISCO- PLASTIC), IN WHICH IT IS SUSCEPTIBLE TO LOCALIZATION OF FLOW ALONG "CHARACTERISTICS" (SLIP LINES IN FIELDS), WHICH ARE DIRECTIONS OF CRITICAL MAXIMUM SHEAR STRESS AND ALSO DIRECTION OF PURE SHEAR. PLASTIC INSTABILITIES MAY BE MOUNTED ALONG THESE DIRECTIONS. WEAK INSTABILITIES LEAD TO STABLE SLOW CRACK PROPAGATION; STRONG INSTABILITIES LEAD TO UNSTABLE FAST PROPAGATION (BRITTLE FRACTURE). AGGRESSIVE CHEMICAL ENVIRONMENTS MAY AFFECT ONE OR BOTH OF THESE NEW MATERIAL PARAMETERS WHICH HAVE BEEN DISCLOSED, NAMELY THE CRITICAL STRAIN AT WHICH FLOW IS CONCENTRATED ALONG CHARACTERISTICS AND THE NATURE OF THE INSTABILITIES (STRONG OR WEAK) THAT CAN BE GENERATED ALONG CHARACTERISTICS. PLASTIC ZONE BEHAVIOR, THUS, INVOLVES METALLURGICAL STRUCTURE, RHEOLOGY, AND FORMAL PLASTICITY THEORY. /AUTHOR/