抄録
Fatigue cracks start to initiate as surface cracks at high stress/strain concentration field such as structural discontinuity parts in weld steel structures. These small surface cracks lay almost on a straight line and coalesce from repetition to repetition as the cracks advance. Finally, these cracks come to be a larger one surface crack. Single edge notched specimens with a projection in which breadth is 0.3mm at a notch root are prepared. A slit is introduced into the projection in advance. In these specimens, the projection is removed just after fatigue crack propagates at the end of projection. In this test, only one surface crack propagates at a notch root, and the configuration is obtained by means of beach marks. From this test, the changes of aspect ratio as equilibrium crack growths are investigated which may relate to a stress distribution beneath a notch root. On the other hand, we carried out fatigue tests using single edge notched specimen subjected to tension with notch radius of 0.5mm, 1mm and 2mm. During testing, ink with different colors, was permeated into crack surfaces and was dried while keeping some constant loading just below maximum load for the purpose of obtaining crack shapes and sizes at some intervals of cyclic loading. It is very difficult to analyze each crack growth because of the interference each other and crack opening/closing behavior. Therefore increase rate of K value at the deepest point of the largest crack among surface craks due to the interference effect is obtained from previous studies for surface elliptical surface cracks in a semi-infinite plate subjected to tension. Then a supposed representative surface crack of which K value at the deepest point is the same as the increased K value by the interference is obtained, which has the same depth of the largest surface crack. It becomes clear that the aspect ratio of the supposed representative surface crack emanated form a notch changes along a tangent line that touches the curve of aspect ratio for equilibrium surface crack growth from a semi-circular small crack with the depth of a grain size of micro structure. Consideration of the change of aspect ratio for the supposed representative surface cracks may play a role of the indirect consideration of the crack growth behavior for plural surface cracks emanated from a notch.
