Issue34

Y. Sumi, Frattura ed Integrità Strutturale, 34 (2015) 43-59; DOI: 10.3221/IGF-ESIS.34.04 53 Figure 8 : A Mode-I crack intersecting a line-degradation at angle  . Figure 9 : A brittle crack propagating parallel to a welded joint; (a) crack propagation in the base metal; (b) crack propagation along butt-weld joint. Early experimental results of the ESSO tests, which were performed by Kihara et al . [28, 29] using welded mild steel plates, indicate that a brittle crack initiated along the welded joint turns off the welding line and propagates into the base metal as illustrated by the crack path (a) in Fig.9. A brittle crack propagates in a region a finite distance away from the welding line, where the distance increases with decreasing the ratio of applied and residual stresses. These morphological characteristics of brittle fracture of welded mild steel plates had been confirmed later by more sophisticated dynamic crack propagation and crack arrest tests using middle and large size specimens, SR147 [30] and SR153 [31]. In the case of welded mild steel plates, since cracks are expected to turn off the welding line due to the high tensile welding residual stress acting parallel to the welding line [32], the localized decrease of fracture toughness in the heat-affected zone does not play an important role in relation to the crack arrest capability of a welded total structure. Kihara and Ikeda [33] observed crack paths of welded high tensile steel plates, which exhibited different behavior from what was observed in welded mild steel plates. These morphological aspects of brittle fracture of welded plates, which were made of high tensile steel for ship structural use, were later investigated in detail by the Shipbuilding Research Association of Japan, SR147 [30], and SR153 [31], and SR169 [34]. It is observed in these experiments that although in some cases cracks turn off the welding line and propagate in the base metal, they sometimes propagate along the welding line as is illustrated by the path (b) in Fig. 9. Since the ratio of applied stress a  of high tensile steel plates and the maximum tensile residual stress is relatively high in comparison with that for the mild steel plates, and since the relatively large decrease of the fracture toughness may be expected near the bond zone, these effects could cause the brittle fracture along the butt-weld for high tensile steel plates.