13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- on both sides of the crack. The numbers above the lines indicate the magnitude of amplification and reduction of SIF. The external solid line indicates schematically the boundary of the process zone and therefore CL. Figure 1. Dimensionless Mode-I SIF Green’s Function due to a unit normal displacement dipole with orientation ( = 0°) with respect to the x-axis. By distributing the individual micro-cracks and passing to a continuum damage, we introduce damage density as a bivector ( ) that represents the total micro-cracks area per unit volume (with dimensions [ ] = mm2/mm3). There is a domain of PZ, where the defects constituting PZ strongly interact with crack by shielding and/or amplifying the crack tip fields. The crack in turn causes the new defects formation and growth, i.e., 0 in that domain. Such domain is referred to as active zone (AZ) of CL. It leads to material degradation in the immediate vicinity of crack front and crack movement into PZ. As the crack advances through AZ, the latter “moves” away from the crack by forming new defects and leaves behind a wake zone (WZ) constituted by the defects that remain after unloading of AZ. Thus, CL propagation occurs as AZ “movement” in material frame of reference and crack advancement into AZ. The total SIF is the sum of SIF KI 0 due to remote load and the increment of SIF due to distributed micro-cracks with opening b dependent on the main crack – micro-cracks array interaction [3–5]: 0 ( ) ( )^ ( ) tot SIF I I I i i i V K K G b dV . (1) The calculations suggest that the shielding effect (a negative SIF increment) is stronger than the amplification and the total SIF in some cases is vanishing [4]. In such cases the “beak” type crack tip opening displacement profile is observed [1]. 2. Experimental and Results To observe the details of CL evolution a set of creep tests was conducted at various loads and temperatures. The creep testing machine equipped with high temperature chamber with four creep stations and two observation windows, pneumatic system of graduate load application and continuous displacement monitoring device is used for tests (see Figure 3 (a)).
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