13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- initially semi-circular crack gradually evolves into a mushroom-shaped crack. The crack mouth is kept close and the crack, initially, is only given an opportunity of extending in a small region near the crack depth. As the cracked area becomes large, the portion of the crack front in a state of closure gradually shrinks. This is because of the increasing positive resultant stress. The maximum crack dimension can be seen to occur nearly at the t/4 position from the front surface, which is in accordance with the loading configuration that has the maximum tensile stress at the t/4 position. This phenomenon should be considered to be reasonable although no direct experimental result can be compared. The predicted crack fronts are obviously far from the semi-elliptical shape, and thus the widely used semi-elliptical crack shape assumption is not adequate for such load case. The crack shape evolution in Fig. 5 (Case V) also demonstrates that surface cracks can be effectively delayed by creating a compressive stress field near the plate surface, such as using the shot peening method. The crack shape change illustrated in Fig. 5 for Case VI also shows that the semi-elliptical shape is not proper to be approximated to the crack fronts in this case. The lagging of crack growth along the surface can be observed, which is due to the zero stress value at the surface. The crack develops mainly in the depth direction at the early stage but the crack growth in this direction is slowed down and even stopped finally by the descending stress level as the crack extends further, see Fig. 3. The maximum crack growth is subsequently transferred to the crack front position, where the highest stress happens, i.e. at the t/4 position from the front surface. CASE I CASE II CASE III CASE IV CASE V
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