Fatigue Crack Paths 2003

(a)

(b)

(C)

Figure 9. Crack path from the initial crack with length of 400 Pm: (a)

Reversed torsion with nt/Nf,t=0.4 followed by push-pull, 3.97×105

push-pull cycles, npp/Nf,pp=1.16; (b) Combined push-pull/torsion

with npp/t/Nf,pp/t=0.4

followed by push-pull, 1.72×105 push-pull

cycles, npp/Nf,pp=0.5; (c) Push-pull with npp/Nf,pp=0.4 followed by

torsion, 6.24×104 torsion cycles, nt/Nf,t=0.2.

Crack propagation curves and fracture mechanics evaluation

Although the reality of fatigue damage of a specimen should be related to the size of

crack [15], the term “fatigue damage” will be used in this paper as the value defined

conventionally by Miner’s rule. The fatigue damage calculated by Miner’s rule will be

discussed from the viewpoint of crack propagation. Figure 10 shows crack propagation

curves. Crack propagation under the first loading is shown by a dotted line and crack

propagation under the second loading is plotted by open marks connected with solid line.

Crack length is denoted by the surface length projected onto the axial direction.

The crack propagation curves for pure push-pull or reversed torsion is also shown in

Figs. 10(a) to (c). Immediately after switching from the first loading to the second

loading, a reduction in crack growth rate compared to the single loading occurred, i.e.,

compared to push-pull in Figs.10(a) and (b) and reversed torsion in Fig. 10(c).

Comparing Figs. 10(a) and (b), the reduction in the crack growth rate is larger for the

sequence of T–to–PP than for PP/T–to–PP. Thus, D obtained in the sequence of T–to–

PP was larger than D of PP/T–to–PP.

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