13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- (b) (a) Figure 3. (b) The geometry and loading of SCK and CT specimens; (a) Slow crack growth test setup. Various specimen geometries have been used in the tests. One is the standard compact tension (CT) specimen. Another is a specially design stiff constant-K (SCK) specimen. SCK specimen poses the highest stiffness among comparable specimen geometries and provides a constant K within the crack size range 0.15 0.4 aW . The diameters and locations of holes are designed to maintain the constancy of SIF while crack is propagating. The side grooves are introduced to reduce the plane stress effect within the surface layers and guide the crack growth along the plane of symmetry. The CT specimen has the same shape and dimension as that in SCK, but does not have holes, as shown in Figure 3 (b). Both types CT and SCK specimens were machined from a commercial large diameter HDPE pipe with 3 wall thickness. The SIF is continuously increasing with crack growth in CT specimen. We selected HDPE material for this study since it displays the simplest, but not trivial PZ geometry observable in fracture experiments: a narrow wage shape domain of cold drawn fibers and membranes with sharp boundaries separating PZ and original bulk material. Figure 4 shows the records of the load-point displacement vs. time monitored in the creep tests at various loads (SIFs) and temperatures. Curve a in Figure 4 (initial SIF: 0 10 K MPa mm and o 80 C T ) clearly shows a transition from continuous to stepwise growth. The same but subtler transition is also observed in Curve b (initial SIF 0 12 K MPa mm and the same temperature compared to Curve a). At the same temperature, if the SIF is increased to 0 18 K MPa mm (initial SIF), a pure stepwise growth is observed as displayed by Curve c. The length of individual step in CT specimen is continuously increasing due to the increasing SIF in contrast with that in SCK specimen (reported in [6]). However, there are no visible steps in load-point displacement record (Curve d), even for a significantly higher load ( 0 48 K MPa mm initial SIF), if the temperature is reduced to room temperature ( o 23 C T ),. The continuous, discontinuous as well as transitional mechanisms of SCG are revealed even more explicitly by CL side view and fracture surfaces. For example, a homogeneous and continuous CL growth occurs at an elevated temperature with a much lower initial SIF ( 0 10 K MPa mm ) (see Figure 5 (a)). The upper picture in Figure 5 (a) presents a general view of CL developed during 290 hours of creep with initial SIF 0 10 K MPa mm at 80°C. There are no steps as well, rather monotonically decreasing width of the triangular CL and crack opening toward a very weakly visible PZ tip. The bottom picture shows a homogeneous surface without any striations. At the very end, we can see a smooth strip that corresponds to the last process zone broken in liquid nitrogen.
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