13th International Conference on Fracture June 16–21, 2013, Beijing, China 6 Fig. 7(b) shows variations of F along the crack front during crack shape evolution in the rod specimen, as shown in Fig. (6). For the initial notch, a straight crack front with the crack aspect ratio α=0 reveals the maximum SIF in the specimen middle and the SIF decreases with the distance x/h. It implies that the crack propagation starts at Point A and the crack front tends to become curved. With the crack developed α increases, the distribution of SIF reaches gradually constant in the whole crack front. In this region the crack front attains a uniform growth rate. Due to varying geometry of the circular rod, the stationary crack growth cannot maintain long. The SIF at the crack front near rod surface increases and becomes maximal in the SIF distribution. It follows that the crack propagation near the rod surface is quicker than elsewhere. Variations of the geometry factor F agree with experimental observation, as shown in Fig 6. The variations of the geometry factor F can be further found in fatigue crack growth. In Fig. 9 ΔK is plotted as a function of crack growth for Points A and C with different loading ratios. Due to transient behavior the crack growth near the initial straight notch is not included in the figure, so that the SIF at C is mainly larger than that at A in the whole crack growth process. The curvature of the crack front decreases with crack growth, that is, the crack rate in the middle of the specimen is lower than that near the specimen surface. The difference of the crack rate can be further observed in Fig. 10, in which the crack length at A and C is plotted as functions of loading cycles. The predictions are based on the Forman model identified from the CT specimens. The figure shows that the prediction of the Forman model agrees with the experiments well for all investigated loading ratios. Note that the crack length at A is represented by the crack depth a and the surface point B is given by the arc length s respectively. Fatigue crack growth in the 3D surface cracked specimen can be described by the conventional long crack model. Figure 9. Development of ΔK in the crack front Point Figure 10. Crack growth in a surface cracked and Point C in the cracked rod specimen in fatigue tests. The crack rate curves for surface crack at Points A and C are plotted together with the experimental data for CT specimen in Fig. 11. The Forman model baselines obtained from CT specimens, Eq. (6), are illustrated in the figure. In the range with lower ΔK, the crack growth at the internal crack front is faster than that at the surface point. For high loading range, the crack growth at the surface becomes higher. This phenomenon is also reported by Shin [15]. Generally, the surface crack data agree with the Forman model.
RkJQdWJsaXNoZXIy MjM0NDE=