13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- For the investigation of realistic variable amplitude loadings, the standardized helicopter load spectrum FELIX [20] has been used. Therefore, the stress amplitudes of the FELIX spectrum have been transformed to a constant R-ratio of -1 using the equations accounting for the mean stress effect. After dividing the spectrum into different classes, they have been reconstructed to different load sequences (Fig. 3). The load sequence FELIX 10 was reconstructed with decreasing load amplitudes starting with the maximum amplitude and ending with the lowest amplitude. The load sequence FELIX 11 starts with the lowest amplitude and is step-wise increased up to the maximum load amplitude. The load amplitudes of the last reconstructed sequence FELIX 12 is randomly mixed. Each sequence is repeated until failure occurs or the limit of 109 cycles is reached. Figure 3. Reconstructed load time history FELIX with a) decreasing load amplitudes, b) increasing load amplitudes and c) mixed load amplitudes 3. Results The influence of the size of non-metallic inclusions on the number of cycles to failure is shown in Figure 4, where areainc is the inclusion size projected on the plane perpendicular to the maximum principal stress. The data of constant amplitude (CA) loading tests show no significant influence of the inclusion size on the lifetime. In contrast, the size of inclusions tends to decrease with increasing lifetime for all variable amplitude (VA) loading tests. b) FELIX 10 FELIX 11 FELIX 12
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