13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 1.5 4 3 2 W a 1 W a 5.6 W a 14.72 W a 13.32 W a 0.886 4.64 W a f − ⋅ − ⋅ + ⋅ − ⋅ + = (2) where F is load, B is specimen thickness, BN is net thickness of side grooved specimens, a is crack length and W is test piece width. - Components with stationary creep (not only at the crack tip) are assessed using the integral C*. In case of the experimental determination of the parameter C* approximation formulae can be used for a number of fracture mechanics specimens [5], which are basically functions of the load line displacement rate due to creep [3]: = ⋅σ ⋅η ⋅ net C C* v (3) with load line displacement rate ⋅ Cv , net section stress net σ and the factor η depending on specimen geometry (η = [2+0.522•(1-a/W)] for C(T)-specimens). For the evaluation of C(T)25-specimens the following equation was used: β⋅η − ⋅ ⋅ ⋅ = ⋅ ⋅ W/ a 1 h h B c F C* v 3 1 N (4) where h1 and h3 are geometrical functions, c=W-a, and β: 1,071 for plain stress condition. One of the validity criteria, which decide on the suitability of the relevant parameter, is the transition time t1 [3]: C* (n 1) E' K t 2 I 1 ⋅ + ⋅ = (5) If t1 is smaller than the crack initiation time t1<<ti for the initiation criterion (∆ai=0.5 mm) the parameter C* is more appropriate for the description of crack behaviour under creep conditions. In order to describe the creep crack growth rate either the parameters C* or stress intensity factor KI can be used, dependent on the situation in the component. Typically, the creep crack growth rate depending on the respective fracture mechanics parameter is presented as follows: m1 I 1C (K ) dt da ⋅ = (6) m2 2C (C*) dt da ⋅ = (7) where C1, C2, m1 and m2 are material constants. Fatigue crack behaviour can be described by threshold values for the beginning of cyclic crack growth and by the Paris-law [6] for the crack propagation: m3 3C ( K) dN da = ⋅ ∆ (8) with C3 and m3 as material specific constants.
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