ANALYSIS OF STRESS-STRAIN CURVE OF MATERI AL CONTAINING BUBBLES ON GRAIN-BOUNDARY BY 2-DIMENSIONAL ELASTIC-PLASTIC FINITE-ELEMENT METHOD

The helium embrittlement is analyzed by applying a two-dimensional ela stic plastic finite element method based on continuum mechanics. This program was written by the present author and can treat large scale pl astic deformation. In this program, triangular mesh elements are used assuming plane stress condition. The Mises yield criterion, the J(2) f low theory, and the power workhardening law are assumed. The morpholog y of grain boundary helium bubbles is approximated by cyclic boundary conditions. The effect of workhardenability and bubble morphology on t he conventional stress strain curves is surveyed. The deformation mode is divided into tyro categories. (1) Grain boundary deformation mode, in which localization of strains near the grain boundary occurs. (2) Grain interior deformation mode, in which deformation near the grain b oundary ceases and deformation proceeds in the grain interior. Only in the former case, the reduction of total elongation results in and thi s is interpreted as helium bubble embrittlement. The reduction of work hardening exponent causes severe helium embrittlement in the case of t he strain localization mode near the grain boundary. The loss of total elongation has been considered to be controlled by the bubble areal f raction on the grain boundary. This is partly true and it is revealed that the bubble density has an essential impact on helium embrittlemen t. The existence of bubbles on the grain boundary enhances stress mult iaxiality near the grain boundary. This results in an increase of the hydrostatic component of stress and a different behavior of deformatio n near the grain boundary. In view of the prevention of helium embritt lement, the high workhardenability is essential. The high bubble densi ty is also effective, but attention must be paid to bubble coalescence due to the temperature excursion during transient phenomena.