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.