4.4 Variation of Equivalent Stress Ahead of the Crack Tip
The effect of the creep-fatigue loading condition on the equivalent stress distribution at the maximum load (P =Pmax ) is shown in Fig. 11. At initiation (Δa = 0 mm), equivalent stresses near crack tip in all cases are almost same as shown in Fig. 11(a), but with increasing the distance from the crack tip, the equivalent stress for the tension-tension loading case (specimen 1) decreases more rapidly than that for the tension-compression loading case (specimen 3 and 5).
At growth (Δa = 2 mm), the equivalent stress for the tension-compression loading case is overall higher than that for the tension-tension loading case, as shown in Fig. 11(b). It is because of the crack closure under compressive loading, which initializes stress relaxation by creep; under the tension-tension loading, initially creep-relaxed stress distribution is maintained during crack growth. High equivalent stress at growth under tension-compression loading causes rapid creep deformation, which is previously reported as re-activation of transient creep deformation by Takahashi [28]. Meanwhile, both at initiation and growth (Δa = 2 mm), the hold time rarely affects the equivalent stress distribution.