13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 3.2 Prediction Results of Different Initiate Crack Shape Taking the Al 7075-T6 alloy for example, the crack front change of the elliptic surface crack with different shape parameters is simulated. The initiate geometry parameters are: thickness d equals 10 mm, width W equals 20 mm, and the shape parameters α0 are 2.5, 1, 0.8, 0.6, 0.4 and 0.2 respectively. The prediction results of shape change of the crack front during the extending process are shown in Fig. 5. Meanwhile, the relationship curves between the shape parameter α and the depth ratio β=a/t are also shown in Fig. 6. Seen from Fig. 6, when initiate shape parameter α0<1, the maximum damage is found in the controller point A, which defines the corresponding crack growth life of the whole surface crack. Otherwise, the maximum damage turns to the deepest point B. Combined with Fig. 4, it shows that the same crack growing trend is found with different initiate crack shape parameter, the crack front shape all become oval after getting round, which is consistent with results in some recent reports[20-21] (Wu zhixue, 2007; Brennan et al., 2008). What’s more, every two parameters of the three initiate factors: shape parameter α0, crack depth a0 and crack width c0 control the change of the surface crack front. 0.0 0.3 0.6 0.9 1.2 0.0 0.3 0.5 0.8 1.0 a/t a/c α0=0.4(prediction) α0=0.6(prediction) α0=0.8(prediction) α0=1(prediction) α0=0.2(prediction) α0=0.2(test) α0=0.4(test) α0=0.6(test) α0=0.8(test) α0=1(test) Fig. 4. Comparison of the shape change of the elliptic crack front 4 Conclusions From the point of low cycle fatigue strain damage, this paper proposed a new fatigue crack growth rate prediction model. It has been well testified with Al 7075-T6 alloy. Meanwhile, combined with the famous Newman-Raju formula used to solve the stress intensity along the surface crack front, a numerical approach is obtained to predict the fatigue crack growth behavior of elliptic surface crack. The prediction results of different initiate crack shape are all consistent with the reference test, and they all present a conversion process of rounding first to oval again. And every two parameters of the three initiate factors: shape parameter α0, crack depth a0 and crack width c0 control the change of the surface crack front.
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