4 c 2 (0.3~0.4) δ δ = eff . It should point that for elastic-plastic materials, the calculating validity of the crack tip open displacement is restricted by different work stresses σ, their calculation expressing forms would be different for work stress under s σ σ< or s σ σ> condition. Such, calculations of the crack growth rate in equation (3), in addition to directly calculate from the experiment achieved data, and can also use the following varied calculation methods to calculate it. 2.1 Calculations for crack growth rate under work stress s σ σ< condition According to the D-M’ as discussed by [9] mathematical model to calculate the crack tip open disparagement tδ, it can be adopted as following equation. ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × ⋅ = s s t E a σ σ π π σ δ 2 lnsec 8 2 (7) Under c t δ δ −> , c K K1 1−> condition, and the work stress 1 / << s σ σ ( s σ σ 0.5 ≤ ), for the D-M’ model is made by simplify treatment, it is s t E y K σ β δ ⋅ = 2 2 2 (8) Here 2y is correction coefficient related to crack size and shape. The coefficient β is 1=β under plane stress condition, and (1 )/2 2ν β= − under plane strain condition. νis Poisson's ratio. If 1=β and under cyclic loading, the crack tip open displacement can adopt three of kinds calculations methods as following forms. (1) The calculations methods used by the stress intensity factor ) ( I 2K K= For this method, the comprehensive material constant eff B2 and the crack tip open displacement tδΔ range in the crack propagation rate equation 2 2 / da dN are all to adopt the stress intensity factor to express. Here the crack tip open displacement range tδΔ can be [9],
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