EFFECT OF LOADING MODE ON THE FRACTURE-TOUGHNESS OF A FERRITIC/MARTENSITIC STAINLESS-STEEL

The critical J integrals of mode I (J(IC)), mixed-mode I/III (J(TC)), and mode III (J(III)C) were examined for a ferritic stainless steel (F-82H) at ambient temperature. A determination of J(TC) 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 in J(TC) and dJ(T)/da values. While the upper shelf-fracture toughness of F-82H depended on loading mode, the J(min) 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).