Abstract
Microcrack formation and delamination growth are the main damage mechanisms in the fatigue of composites. They lead to significant stiffness loss, introduce stress concentrations and can be the origin of subsequent damage events like buckling or fibre breakage, especially in case of shear and compression stresses during load reversal. Fatigue experiments of carbon fibre reinforced laminates were conducted at several stress ratios and analysed in terms of crack and delamination growth. These investigations were accompanied by microscopic imaging, digital image correlation and finite element modelling to take into account the effects of residual stresses and crack closure. It was found that residual stresses significantly change the local stress ratio in off-axis layers and lead to residual crack opening of inter fibre cracks. These cracks remain open and close under high compression loadings only. Furthermore, crack formation under pulsating compression loading turned out to be driven by residual stresses leading to perpendicular cracks as observed under pure tension loading. The experimental findings further confirm the severe detrimental effect of tension-compression loading on crack formation and delamination growth compared to pulsating tension-tension or compression-compression loads.
Highlights
High performance composite structures usually undergo complex loadings throughout their service lifetime
Whereas failure of composite materials under static loading conditions is fairly well understood and many damage models exist [1,2,3,4,5,6,7,8], the damaging process becomes more complex in case of fatigue loading
The study presents experimental results for the microcracking process in CFRP-laminates in static and cyclic loading. It further provides data for the fatigue growth of inter-layer delaminations emanating from the tips of inter fibre microcracks and the remaining crack opening displacement due to manufacturing induced residual stresses in a cross-ply laminate
Summary
High performance composite structures usually undergo complex loadings throughout their service lifetime. Whereas failure of composite materials under static loading conditions is fairly well understood and many damage models exist [1,2,3,4,5,6,7,8], the damaging process becomes more complex in case of fatigue loading. It is common sense, that the failure mechanisms inter fibre failure (further on termed as microcracks), delamination and fibre failure remain the same in fatigue, but their order of occurence and magnitude may vary between static and fatigue loading [9,10]. In fatigue loading, many different factors influence the damaging process, e.g., the type of fatigue load (mean load, load ratio, testing frequency, load sequence, etc.), environmental conditions (temperature, humidity, etc.), material configuration and manufacturing processes [16,17].
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