Fatigue delamination propagation at high frequency of a CFRP UD material
Résumé
During their service life aeronautical structures are submitted to vibrations due to aerodynamic turbulent flows around structure and engines. These fluid-structure interactions induce not only sonic acoustic fatigue spectra with low load amplitudes at high frequency range but also buffeting or flutter type fatigue with high loads at lower frequency range. The current static design of composite aircraft covers fatigue loading cases. But with the need to more and more lighten the structure, and with today improvements in material behaviour knowledge, static reserve factors will likely be reduced in the near future. Sonic or flutter type fatigue might then become a major issue. So it seems necessary to evaluate the possibility of delamination propagation in a composite under vibratory loading. In this context, methods and fatigue tests have been developed to characterize delamination propagation under vibratory loading for the two main crack propagation modes: opening (I) and shear (II). These tests are performed at resonance of specimen with an electrodynamic shaker system taking advantage of the high frequency and large amplification factor at resonance. These dynamical tests allow an accurate measurement of the delamination size through the resonance frequency shift due to crack propagation. Loading was performed at sufficiently low levels to limit any detrimental self-heat generation during test. Results of these dynamical tests were compared to more classical tests performed at 10Hz with a hydraulic fatigue testing device, in order to investigate the effects induced by high frequency loading. For mode I, a decrease in the resistance to propagation of delamination is observed with the increase in frequency for the composite under study. This frequency effect seems to be a strain rate effect and was taken into account by using a dynamical critical energy restitution rate for the G-N curve plotting. For mode II data reduction is performed thanks to a numerical model determining ERR level available during the dynamical cyclic loading. Fatigue crack propagation results are presented taking into consideration the loading ratio difference between test configurations. All configuration results are found to follow a unique propagation rate curve. Frequency effect is negligible in mode II for the carbon/epoxy material under study and the range of frequency investigated. For these test configurations the CFRP material used for this study (T700-M21) seems to be little sensitive to high frequency effects. Delamination propagation behavior of carbon fiber epoxy resin composites of such type might be determined with high frequency tests. If so dynamical fatigue at resonance should largely accelerate fatigue tests.