Abstract
Many failures of fatigue cracks from a boxing fillet weld toe have been experienced in ship structures. Fatigue surface cracks emanate from plural points on fillet weld toe line and coalesce each other until becoming a larger surface crack during crack propagation. The life during plural surface cracks usually occupies the major portion of total fatigue life. Therefore it is necessary to establish the estimation method of fatigue crack propagation life before completion of crack coalescence. For the purpose of grasping the clue, we carried out fatigue crack propagation tests with single edge notch and investigated the change of aspect ratio of surface cracks emanated from notch root. We could not direct the search for a new law about aspect ratio of each surface crack because the relation between crack depth and aspect ratio for each surface crack has a very large scatter. Considering and expecting that crack may propagete continuously in main direction under constant amplitude loading, K value at the deepest point of the deepest crack among surface cracks was studied with considering interference effect from near cracks. Then a supposed surface crack which exist alone at the notch root and of which K value is the same as above K value is considered. As a result, it becomes clear that the aspect ratio of the supposed crack changes linear with respect to a crack depth during plural crack growth. However it is expected that the aspect ratio may be affected by residual stress because residual stress controls crack opening/closing behavior. In this paper, we carried out fatigue crack propagating tests with gusset specimen built up welding and studied the change of aspect ratio for the supposed crack at boxing fillet weld toe where large tensile residual stress exist. As a result, there is a important law about the change of ratio for the supposed crack with respect to K value which may represent indirectly crack propagation behavior for multiple surface cracks emanated from notch root with welding tensile residual stress field.
