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.