Effect of Loading Mode on the Fracture Toughness of a Ferritic/Martensitic Stainless Steel

摘要

The critical / integrals of mode I (J_(IC)), mixed-mode I/III(J_(TC)), and mode III (J_(IIIC)) were examined for a ferritic stainless steel (F-82H) at ambient temperature. A determination of JTC was made using modified compact-tension specimens. Different ratios of tension/shear stress were achieved by varying the principal axis of the crack plane between 0 and 55 deg from the load line. The value for J_(IIIC) was determined by means of specially designed specimens. The results showed that F-82H steel had high fracture toughness. Both J_(IC) and J_(IIIC) were about 500 kJ/m~2, and the mode I tearing modulus (dJ_I/da) was about 360 (kJ/m~2)/mm. However, J-TC and mixed-mode tearing modulus (dJ_T/da) values varied with the crack angles and were lower than their mode I and mode III counterparts. Both the minimum J_(TC) and dJ_T/da values occurred at a crack angle between 40 and 50 deg, at which the load ratio of sigma_(iii)/sigma_i was 0.84 to 1.2. The J_(min) was 240 kJ/m~2, and ratios of J_(IC)/J_(min) and J_(IIIC)/J_(min) were 2.1 and 1.9, respectively. The morphology of the fracture surfaces was consistent with the change J_(TC) and dJ_T/da values. While the upper shelf-fracture toughness of F-82H depended on loading mode, the Jmin value remained high. Other important considerations include the effect of mixed-mode loading on the ductile-brittle-transition temperature and effects of hydrogen and irradiation on J_(min).