13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 1/4 1 min min B B 2 1 2 B K K K K B ⎛ ⎞ ⎡ ⎤ = + − ⋅ ⎜ ⎟ ⎢ ⎥ ⎣ ⎦ ⎝ ⎠ , (2) The parameters B1 and B2 correspond to respective specimen thickness (length of crack front). On the lower shelf of fracture toughness (KIC << 50 MPa√m) the equations may be inaccurate. The model is based upon the assumption that brittle fracture is primarily initiation controlled, even though it contains the conditional crack propagation criterion. On the lower shelf, the initiation criterion is no longer dominant, but the fracture is completely propagation controlled. In this case there is no statistical size effect (Eq. 2) and also the toughness distribution differs (not very much) from Eq. (1). In the transition region, where the use of small specimens becomes valuable, Eqs. (1) and (2) are valid. For structural steels, a “Master Curve” describing the temperature dependence of fracture toughness is assumed in the form of Eq. (3). [ ] ( ) 0 0 K 31 77 exp 0.019 T T = + ⋅ ⋅ − , (3) T0 is the transition temperature (°C) where at which the mean fracture toughness, corresponding to a 25 mm thick specimen, is 100 MPa√m and K0 is 108 MPa√m. The original data used to define Eq. (3) are shown in Fig. 1. It should be pointed out that the temperature dependence is purely empirical, even though it has been found to provide a rather good description of a large number of structural steel. The assumption is that the dislocation mobility in the ferrite matrix controls the temperature dependence. So far, attempts to provide a theoretical derivation of the temperature dependence have not been successful. One reason for this may be that most theoretical models only deal with cleavage fracture initiation. However, it is not only the probability of initiation that is affected by temperature. The theoretical temperature dependence is considered next. -100 -50 0 50 100 0 100 200 300 400 K0 [MPa√m] T-T0 [ oC] 73 W T0 = -63 oC 73 Wi T0 = +36 oC 72 W T0 = -54 oC 72 Wi T0 = +34 oC A533B Cl.1 T0 = -109 oC HSST 02 KIC T0 = -22 oC HSST 02 KJC T0 = -21 oC HSST 02i KJC T0 = +51 oC 10MnNi2Mo T0 = -70 oC 10MnNi2Mo T0 = -74 oC PTSE-2 [TS] T0 = +14 oC PTSE-2 [TL] T0 = +20 oC K0 = [31 + 77⋅exp{0.019⋅(T-T 0)}] (MPa√m, oC) Individual Ko estimates are based on more than 3 tests Figure 1. Original data used to define the temperature dependence of the MC. Each point denotes a K0 estimate based on more than three tests [1]. 1.2. Basis for the temperature dependence The different possible mechanisms of cleavage fracture are qualitatively rather well known. Primarily the initiation is a critical stress controlled process, where stresses and strains acting on the
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