Crack Paths 2009

Crack Propagation Rate

Fatigue cracks

The projection of the maximumcrack length on the plane perpendicular to the

longitudinal direction of the specimen was measured for both tension-compression and

torsion loadings. The half of the sum of the actual crack and the notch length was taken

as the crack length a. The J integral was determined from the relation between the load

and the displacement recorded during fatigue using simple estimation methods

described above.

For the displacement-controlled tests including pp, cp, and cc waves, the crack

propagation rate and the J integral range were nearly constant while the crack length

extended from 2.7 to 5.0 mm.Since the crack propagation rate and the fatigue J integral

were nearly constant for the displacement-controlled tests, the measured values were

averaged for each case. On the other hand, in load-controlled tests including pp, cp-th,

and cc-th waves, both the crack propagation rate and the J integral increase with the

crack length.

Figure 7 shows the relation between the crack propagation rate and the fatigue J

integral for pp waves under tension compression. For both cases of dispalcement- and

displacement-controlled tests, the crack propagation rate da/dN (m/cycle) is expressed

as the following power function of the fatigue J integral (N/m) : ( )1.34 12 7.96 10 f d a d N J∆ − = ×

(5)

Figure 7. Relation between crack porpagation rate and fatigue J integaral for pp

waves under dsialacement- and load-controlled tension-compression.

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