Damage initiation, propagation and final failure of GFRP cross- and angle-ply laminates tested in the very high cycle fatigue rangeWednesday (28.09.2016) 10:00 - 10:15 Part of:
In contrast to fatigue in low and high cycle ranges, the very high cycle fatigue range has barely been investigated yet . However, as there are several applications facing cycle numbers beyond 1E+08 (e.g. wind turbine rotor blades), a strong need for VHCF data arises. For this reason, a high frequency bending test rig (50 Hz) with integrated damage monitoring systems has been developed.
So far, cross-ply [90/0]s and angle-ply [+45/-45]s GFRP flat specimens have been tested. At higher loads, cracking and delamination are the major damage mechanisms. Cracks mainly initiate at free edges and subsequently grow across the width and in thickness direction. Crack growth rate in thickness direction is crucial for delamination initiation. Damage initiation and progression are delayed at lower loads. Whereas cracks and delamination come to rest in the case of the cross-ply laminate, angle-ply specimens undergo the typical three-staged fatigue life including initiation, progression and final failure. Stiffness degradation correlates well with damage progression and is verified by numerical modelling.
Concerning the existence of a fatigue limit, which has not been established yet , two important tendencies have been observed. Firstly, beneath a certain stress level, no progressive cracking is found up to 1E+08 cycles in the cross-ply tests. Secondly, despite cracking, delamination does not occur in both laminates under comparably low loadings. Consequently, the angle-ply laminate does not reach final failure up to 1.6E+08 cycles. At higher loads, delamination growth-based life prediction indicates that the SN-curve will flatten out. However, as damage growth is rather slow, the identification of a true fatigue limit is difficult.
On the whole the fatigue under VHCF-loadings has been comprehensively characterized for the first time. As the experiments indicate a fatigue limit, further investigations are of great importance, especially with respect to prospective advances in design of fatigue-loaded structures.
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 R. P. L. Nijssen: Fatigue Life Prediction and Strength Degradation of Wind Turbine Rotor Blade Composite, Technical Report, SAND2006-7810P, Sandia National Laboratories, Albuquerque, NM., 2006