13th International Conference on Fracture June 16–21, 2013, Beijing, China 4 will keep constant but the load amplitude will reduce to 50% for several cycles. By comparing successive crack profiles obtained after loading for designated cycle numbers of each interval, the crack growth rate at different points on the crack front can be evaluated. The crack growth length in CT specimens and the crack depth as well as the surface crack arc length in rod specimens will be measured under light microscope. Fig. 4 shows typical beach marked crack surfaces for CT specimen and rod specimen. 4. Crack propagation behavior of CT specimen A simple and well known method for predicting fatigue crack propagation is a power law described by Paris and Erdogan [19], and it is also known as Paris law. Forman [20] improved the model by considering effects of KC and stress ratio. Hartman and Schijve [21] proposed the following equations which is the continuation of Forman’s work by including the threshold stress intensity factor ΔKth: R K K C K K dN da C m th − −Δ ⋅ Δ −Δ = (1 ) ) ( (4) where C is the intercept and m is the slop on the double logarithmic plot of da/dN versus ΔK. Kc is the fracture toughness for the material and ΔKth is the threshold stress intensity factor value for fatigue crack growth which is sensitive to R. For AISI304 material, the fracture toughness Kc is 133 MPa m and the threshold value ΔKth with R=0 8.053 MPa m. Fitting with the experiment data, the coefficient can be fixed as C=10!!.!"#$# and m=2.80192. The experiment data are shown in Fig. 5 with identified Forman model curves. In the figure the loading ratio effect is clearly illustrated and experimental data agree with the Forman model. Figure 5. Crack growth curve of CT specimens. 5. Crack propagation in cracked rod specimens The crack geometry of the rod specimen is described in Fig. 1. An equivalent elliptical arc curve is used to indicate the actual crack front after certain number loading cycles. 3D FE model is generated with symmetric boundary conditions, as shown in Fig. 6. The location of the points on the crack front can be described by the location x/h. with x=0 for the middle of the specimen and x=h for the specimen surface. The crack growth of the deepest point A and the surface point C are represented as the depth length Δa and the chord length Δc,
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