PSI - Issue 4

David Simunek et al. / Procedia Structural Integrity 4 (2017) 27–34 D. Simunek/ Structural Integrity Procedia 00 (2017) 000 – 000

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Figure 6 illustrates the crack propagation rate of the constant amplitude tests (EA4T 1 - EA4T 4) compared to the overload tests (EA4T 5, EA4T 6 and EA4T 9). The evaluation of the overload tests shows that after overload and retardation the crack growth rate converges again to the level of the constant amplitude tests.

Fig. 6 Comparison of constant amplitude (specimens EA4T 1 to 4) and overload (specimens EA4T 5 to 9) tests

3. Fracture surface analysis

After testing, all specimens are cooled down in liquid nitrogen atmosphere and burst fractured. Subsequent microscopy of the fracture surfaces is conducted to analyse the crack growth behaviour in depth and the shape evolution of the half-elliptical crack. To the latter purpose, the a/c ratio, beginning from the starter notch to final fracture, is studied. Beach marks provide information about the behaviour of the a/c value in the course of the test. Figure 7 represents the evaluation of the a/c ratio at the starter notch and the final fracture surface. Based on selected points along the semi-elliptical crack front a half-ellipse is fit.

Fig. 7 Evaluation of a/c-ratio at fractured surface

The summary of the a/c ratio measurements is shown in Figure 8. At all fracture surface investigations an increase of the a/c ratio right after crack initiation is observed, which means faster crack growth in depth. Due to the fact, that CA - tests (EA4T 1 to EA4T 4) exhibit no beach marks after crack initiation, no conclusion of the a/c ratio behavior can be drawn. To this purpose EA4T 3 was already stopped at approximately a/D ~ 0.17 (see Figure 8). The evaluation shows, that the a/c ratio of the final fracture surface was in the range of the beach marks of the OL - tests. The overloads at the OL - tests were applied at a/D ~ 0.065. Subsequently, a decrease of the a/c ratio is noticed with increasing crack depth. Although some beach marks were not clearly detectable, a characteristic evolution of the crack geometry can be observed.