Effect of loading mode on the fracture toughness of a ferritic/martensitic stainless steel

The critical J integrals of mode I (JIC), mixed-mode I/III (JTC), and mode III (JIIIC) 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 JIIIC was determined by means of specially designed specimens. The results showed that F-82H steel had high fracture toughness. Both JIC and JIIIC were about 500 kJ/m2, and the mode I tearing modulus (dJI/da) was about 360 (kJ/m2)/mm. However, JTC and mixed-mode tearing modulus (dJT/da) values varied with the crack angles and were lower than their mode I and mode III counterparts. Both the minimum JTC and dJT/da values occurred at a crack angle between 40 and 50 deg, at which the load ratio of σiii/σi was 0.84 to 1.2. The Jmin was 240 kJ/m2, and ratios of JIC/Jmin and JIIIC/Jmin were 2.1 and 1.9, respectively. The morphology of the fracture surfaces was consistent with the change in JTC and dJT/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 Jmin