Numerical investigation of the loading frequency for fatigue test of C/SiC materials

The S-N curve of material and dynamic stress responses are two necessary elements for acoustic fatigue life assessment of hot structures. However, the S-N curves of C/SiC material for hot structures are of great frequency-dependence. In order to obtain the proper S-N curve of C/SiC material for the fatigue life assessment of typical hot structures subjected to the random acoustic loading, the level-crossing problem of a random signal was analyzed. The expected level-up crossing was deduced. The zero-up crossing was demonstrated to characterize the expected frequency of a random process. And the zero-up crossing can be calculated using the power spectral density function of the process. The finite element model of a stiffened panel was constructed and the power spectral density of stress responses of the panel at three representative locations were calculated under the excitation of acoustic loading. Numerical analysis was conducted to examine the accuracy of zero-up crossing to represent the average cycles per second of a Gaussian-distributed random signal. Numerical results show a good agreement. The maximum of the three zero-up crossings is 532 Hz. Thus, the fatigue test to obtain S-N data need to be conducted at the loading frequency of 532 Hz as far as possible. © 2016, Nanjing Univ. of Aeronautics an Astronautics. All right reserved.