PSI - Issue 7

Kentaro Wada et al. / Procedia Structural Integrity 7 (2017) 391–398 K. Wada et al./ Structural Integrity Procedia 00 (2017) 000–000

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2.3. Evaluation of crack closure behavior Crack closure behavior was investigated using the unloading elastic compliance method. Two strain gages were attached onto the specimen surface, one in the region of the crack and the other in the smooth area. By subtracting the strain values obtained from the two gages, the resulting difference in compliance between the cracked segment and the intact zone was thereby measured. In order to amplify the change in compliance as caused by the crack closure, the subtracted strain value was adjusted to zero, within the range wherein the crack had opened fully. The knee point was then determined from the load-normalized strain curve, and the stress at the knee point was defined as the crack closing stress, σ cl . The effective stress range, Δ σ eff , and the effective stress intensity factor range, Δ K eff , were calculated using the following equations:

(3)

eff max cl σ σ σ ∆ = −

0.65 σ ∆ = ⋅ ∆ K

π

area

.

(4)

eff

eff

3. Results and discussion 3.1. Results of fatigue tests

Fig. 2 presents the endurance limit diagram, i.e. , the relationship between the fatigue limit and the mean stress. The circles represent the results obtained from unbroken specimens, whereas the ‘x’ marks denote the outcomes derived from specimens which had been broken. The presence or absence of non-propagating cracks in the un-failed specimens is distinguished by either a filled or an empty circle, respectively. The solid line in the figure illustrates the modified Goodman prediction, as given by the following equation:

a σ σ σ σ w B

m + = . 1

(5)

Crack initiation limits for all R conditions were found to be in good agreement with the modified Goodman prediction. In contrast, the crack growth limit at R = − 10 was 900 MPa, which was more than twice as high as the modified Goodman prediction; σ a = 334 MPa.

Fig. 2: Endurance limit diagram for sharp-notched SAE52100 steel.