THE SIGNIFICANCE OF CRACK DEPTH IN FRACTURE-TOUGHNESS SPECIMENS FOR HSLA STEEL WELDS

This work deals with the influence of crack depth on the plastic rotation factor (r(p)) and fracture toughness at crack initiation and maximum load. A series of tests was performed on high strength low alloyed (HSLA) steel welds, where the ratio of crack length to specimen width was about 0.05-0.5, keeping a/W constant, at room temperature and transition temperature. It is found that the r(p) is not a fixed value for both deep and shallow crack specimens during the loading process. At first the r(p) value increases from a threshold crack opening displacement (COD) value rapidly with increased crack opening, then the rising rate of r(p) decreases. Finally, r(p) tends toward a constant. At the transition temperature, much the same trend is expected, but there exists a large scatter of fracture toughness. Moreover, the threshold COD condition, in which r(p) is zero, is established. Three characteristic values of r(p) on the r(p)-COD curves are defined corresponding to the local and apparent crack initiation and maximum load toughness, respectively. The r(p) values at the initiation or maximum load are not sensitive to the crack depths and the mechanical properties of crack tip materials for the welded joints tested. In addition, it is indicated that there exist different rising rates with decreasing crack depths between the initiation toughness described by the crack tip opening displacement (delta(i)) and the J-integral (J(i)), and between the initiation (delta(i) or J(i)) and maximum load toughness (delta(m) or J(m)). Finally, the effect of mechanical heterogeneity on fracture toughness for specimens with shallow cracks has been considered.