13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 4. Load-point displacement vs. time recorded in creep test conducted at a) 0 10 K MPa mm , o 80 C T ; b) 0 12 K MPa mm , o 80 C T ; c) 0 18 K MPa mm , o 80 C T ; d) 0 48 K MPa mm , o 23 C T . (a) (b) Figure 5. CT specimens tested at 80°C with initial 0 10 K MPa mm : (a) tested for 290 hours, Upper: side view; Lower: fracture surface. (b) tested for 680 hours, Upper: side view; Lower: fracture surface. The striking observation is that, given the same testing conditions ( 0 10 K MPa mm , 80°C), if the test is conducted longer (680 hours), a transition from continuous to stepwise growth occurs. The upper picture in Figure 5 (b) depicts a monotonically decreasing opening of crack toward the tip, and pairs of symmetrical “flankers” emanating from two sides of the boundary in the region close to the crack tip. The positions of flankers correspond to white shining striations observed in the lower image, and are signs of discontinuous growth. This evolution from continuous to stepwise growth is manifested in the lower image in such a way that fracture surface first shows homogeneity then striations gradually appear with a wider and wider spacing due to increasing SIFs. When the initial SIF is increased to certain level, the discontinuous fracture propagation takes place from the beginning. It is illustrated in Figure 6 that shows CL in a CT specimen tested at 80°C with initial SIF 0 18 K MPa mm (Note: the sample was step loaded from 12 K MPa mm to 18 K MPa mm during a short period, and the latter is considered as the initial SIF.). A wedge-type pulsating domain indicates a discontinuously CL grown with six well defined steps in upper image
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