4.3 Crack Opening Profiles
The effect of creep-fatigue loading condition on the crack opening profile is shown in Fig. 10. The crack-opening displacement in Fig. 10 is defined as the half of the vertical displacement of the crack surface (considering the symmetry). When the crack opening displacement becomes zero, it suggests the crack closure. Figures 10(a) and (c) show the crack-opening displacement under the maximum load (P =Pmax ) at Δa = 0 mm and Δa = 2 mm, respectively, whereas Figs. 10(b) and (d) show the crack-opening displacement under the minimum load (P = Pmin ) at Δa = 0 mm and Δa = 2 mm, respectively.
At initiation (Δa = 0 mm), crack blunting occurs at the maximum load in all cases, as shown in Fig. 10(a). The most blunting occurs for the specimen 1 (R = 0.833 and th = 10 hrs), and blunting is depressed with decreasing the load ratio and hold time. As shown in Fig. 10(b), crack closure is observed only in the specimen 5 (R = -0.92 and th = 10 min).
At crack growth (Δa = 2 mm), a large difference in the crack opening profile between tension-tension and tension-compression loading is observed. For the tension-compression loading cases (specimen 3 and 5), both crack blunting and closure occur, as shown in Figs. 10(c) and (d). On the other hand, in tension-tension loading case (specimen 1), the sharper crack profile is maintained during loading and unloading cycles.
As the specimen is unloaded and re-loaded elastically for the tension-tension loading, the crack opening profile changes only slightly during loading and unloading and thus the crack growth behavior is similar to that under pure creep loading. On the other hand, the crack growth behavior under the tension-compression creep-fatigue loading is quite different from that under the pure creep loading. The crack closure occurs under compressive loading, which initializes creep-redistributed crack-tip stress states, re-activates the transient deformation, and as a result, the crack blunting occurs.