13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- analysis of the geometry in 3D and the right boundary conditions and material properties to improve the solutions in the future. For plates there exist several reference stress solutions [11-15] which use Eqns. (4),(6) to derive C*. However in the VAMAS procedure [1] one reference stress solution is chosen. It has been shown that for small partially penetrating defects in plates subjected to combined tension and bending loading, these reference stresses can significantly over-estimate creep crack growth rates. Therefore a recommended reference stress, which is based on a global collapse mechanism is expressed as follows: 2 3 1 2 1 9 3 3 2 2 1 2 2 m m b m b ref Plate (12) where γ = (a·c) / (W·l) and α = a / W. In these equations, a is crack depth, c is half crack length at the surface, W is the thickness of the plate and l is the half-width of the plate, respectively. a) b) Figure 3: Effect of static and cyclic loading on crack growth rate, for 316LN at 650°C for a) C(T) specimens [3] showing the range of data scatter and b) comparison of plate crack growth data at different frequencies with the same C(T) databand as in Fig. 5(a) Fig. 4 gives an example of comparing the effects of frequency and the plate geometry for a 316LN type stainless steel tested at 650 oC [3]. Fig. 5a highlights that for low frequencies the crack growth data for this steels lies within the scatter of the static load data, suggesting that the cracking is time dependent and due to creep at low frequencies. Fig. 5b compared the same databand with data from plate tests. In this case there is a clear difference between negative R ratios and the rest of the cyclic test data of the plate lying at the upper and lower bounds of the C(T) databand respectively. This suggests that caution would be needed in using standard laboratory tests to predict component behaviour where negative R-ratios are present. 2.8. Geometry definitions for pipe components In the same way as the plates Eqns. (4),(6) are used to derive C* for pipe geometries. Equations For a range of pipes also exist to derive K and reference stress. It has been shown that solutions for K
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