Issue34

Y. Sumi, Frattura ed Integrità Strutturale, 34 (2015) 43-59; DOI: 10.3221/IGF-ESIS.34.04 55 disturbances in the system with a small initial kink angle ߙ at the original crack tip, which can be interpreted as the initial imperfection of the system. The crack path stability is then examined by taking into account the second and third terms of Eq. (1). The crack path stability is determined by the condition (40). Fig. 11 shows the crack path destabilizing factors, * / α S    and * / α S    , for the present case, where the ratios of applied stress and maximum residual stress are chosen as 0.25 and 0.5. In the case where the applied stress level is relatively low, these parameters have high values, so that crack paths of low stress brittle fracture initiated from a small initial crack may be curved to the base metal. As the crack further extends, it begins to intersect the line degradation zone. At this point the question arises as to whether the crack intersects the zone and penetrates into the base metal, or it kinks to the degradation zone. In order to answer this question, we calculate the angle *  formed by the crack and the degradation line, which will lead to condition of kinked crack extension along the degradation line by the inequality (55). Effect of Residual Stress and Toughness along Butt-Weld Case-studies are shown for the initial half crack length a = 100, 200, 300, 400, 500 mm. Since the distance, S L between the initial crack line and the degradation line is of the order of the bead-width, it is selected as 50 mm. The imperfection parameter is selected as 2.5    . Based on the theory presented in the previous subsection, we can determine the critical curves which distinguish the crack propagation in the base metal and in the degraded zone, respectively. Such curves are illustrated in Fig. 12, in which if the ratio of the applied stress and the residual stress, / a r   , and the ratio of the toughness of degraded zone and the base metal, * / C C G G fall into the lower right-hand side of the respective curves, a brittle crack may propagate along the degradation zone. It can be quantitatively understood that if the tensile residual stress acting parallel to the welded joint is relatively low, and if the decrease of fracture toughness in the degraded zone is relatively large, cracks may propagate along the degradation zone. This result qualitatively explains the difference of brittle fracture behavior observed in welded mild steel plates and high tensile steel plates. As observed in Fig. 12, brittle fracture initiating from a longer straight crack has a tendency to propagate along the degradation zone. This means that once a brittle crack has begun to propagate along a welded joint, it is hard to turn it into the base metal. Figure 12 : Critical relations between the material degradation and the applied stress; cracks penetrate into the base metal for the conditions corresponding to the upper left regions with respect to the curves [7, 16]. F RACTURE CONTROL OF BUTT - WELD OF LARGE CONTAINER SHIPS ith the rapid increase of the size of container ships, fracture control of extremely thick plates employed in the deck structure of large container ships has been studied in a project organized by Japan Ship Technology Research Association (JSTRA) [17], which includes the crack arrest design of a brittle crack having unexpectedly started and propagating in the deck structure. Possible scenarios to ensure the crack arrest performance in the hatch-side coaming and upper deck plates have been proposed by ClassNK [35] focusing attention on the practical W