13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- FELIX 12 with mixed load amplitudes tend to equal lifetimes. Otherwise, the reconstructed load time history FELIX 11 with increasing load amplitudes tends to result in lower lifetimes than FELIX 10 and FELIX 12. The analyses of the fracture surfaces show circular arrest marks formed around non-metallic inclusions within the fish-eye (Fig. 6b-d) for variable amplitude loading in contrast to the fracture surfaces of specimens subjected to constant amplitude loading (Fig. 6a). This could be an indicator for load interaction effects on the crack growth within the fish-eye, as it is well known in the regime for long crack growth. The arrest marks of FELIX 11 (Fig. 6c) are obviously more pronounced than in the case of FELIX 10 (Fig. 6b) and FELIX 12 (Fig. 6d). However, the sizes of the area, where arrest marks occur, as well as the spacings of the arrest marks are different for the investigated VAL. Figure 6. Fracture surfaces with typical fish-eye fracture around non-metallic inclusions: a) constant amplitude loading and FELIX-tests with arrest marks within fish-eye for b) FELIX 10, c) FELIX 11 and d) FELIX 12 By counting and measuring the arrest marks an average crack growth rate da/dN for the crack propagation within the fish-eye can be calculated, as plotted in Fig. 7. By using the √area-approach [8], the stress intensity factor ΔK was determined. Therefore, the stress amplitude of 386 MPa with the highest amount of cycles within the load sequence has been used. The load sequences of FELIX 10 and FELIX 12 lead to similar crack growth rates. In comparison, the crack growth rate of FELIX 11 is shifted to higher crack growth rates. Nevertheless, all three investigated load sequences lead to crack growth rates beneath the threshold value ΔKth for long crack growth. c) b) a) d) 1000 µm 500 µm 500 µm 500 µm 200 µm
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