13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- CASE VI Figure 5. Fatigue shape development starting from an initially semi-circular surface crack under different crack face loads. 4.2. Aspect Ratio Change The crack shape change is usually described quantitatively by the aspect ratio (a/c) variation with the crack depth ratio (a/t), but it should be aware that it is incomplete. The aspect ratio changes are shown in Fig. 6 for all six load cases. Clearly, the aspect ratio relation is strongly dependent on the load, as mentioned previously. The load with decreasing stress along the plate thickness makes the crack advance more difficult in this direction and eventually leads to a smaller a/c value, as seen for Cases II and IV; whilst the increasing stress along the plate thickness causes more rapid crack growth in this direction than along the surface. This can be seen in Case III. The a/c value of the Case V crack increases linearly with the a/t ratio, quickly exceeding the maximum value set in the figure. This is because no crack extension occurs along the plate surface due to the large compressive stress acting in the surface layer of the plate. However, for Case VI, the a/c value re-drops rapidly after its initial rise similar to the Case V crack, which is caused by the stress distribution with a decreasing value when the crack depth is in the region of t/4 and 3t/4. Obviously, the aspect ratio cannot give a proper description for the cracks that are not of semi-elliptical shape, like the Cases V and VI cracks. CASE I II III IV V & VI VI Figure 6. Aspect ratio change for surface crack under different crack face loading Lin and Smith [8] compared the numerical results of aspect ratio predicted by the simulation technique with the experimental results for the pure tension case, i.e. Case I, and found that both the numerical and experimental results are in good agreement. This indicated that the simulation technique is effective. The numerical aspect ratio result of Case VI can be found to agree well with the experimental results of [9, 10], who carried out the fatigue test of welded joint containing a
RkJQdWJsaXNoZXIy MjM0NDE=