Implications of free-edge effect at thin plain-woven carbon fiber reinforced plastic laminates with out-of-plane waviness under cyclic loading

Abstract

The onset of damage caused by the free-edge effect in plain-woven carbon fiber reinforced plastic (CFRP) specimens with an out-of-plane waviness under tension-tension fatigue loading is investigated. Numerical calculations show that interlaminar and intralaminar stresses close to the out-of-plane waviness are higher than the equivalent stresses at the surrounding edge regions. Using submodels, the influence of the chosen out-of-plane waviness can be better assessed. The free-edge effect of the considered specimens, which originates from stress gradients between plies of different orientation, is altered by the change in the stress field caused by the out-of-plane waviness. Large interlaminar stresses between plies of the same orientation are obtained, which contrasts with existing literature. In experimental fatigue testing it is found that cracks at the free edge appeared at the predicted locations, and after reaching crack saturation, in regions close to the out-of-plane waviness, interlaminar and intralaminar stresses lead to additional cracks along the whole free edges. The experimental tests are supported by a three dimensional image correlation system (3D-DIC), a thermal-imager and a digital photographic camera, which allows detailed examination of selected areas. Visual observation during fatigue testing and post-mortem inspection show good agreement between experimental data and numerical calculations in relation to the location of the damage initiation. As a result, out-of-plane waviness at free edges must be considered as an additional significant fatigue damage initiation location in laminate analysis.

Progressive Damage in thin 2D Woven CFRP Laminates due to Stress Concentrations at Free Edges and Notches

“…Performed experimental testes and numerical computations revealed relevant stress components that were responsible for damage through superposition of the waviness and the free edge. However, for detailed information see Heinzlmeier et al [36].…”

Progressive Damage in thin 2D Woven CFRP Laminates due to Stress Concentrations at Free Edges and Notches

“…Performed experimental testes and numerical computations revealed relevant stress components that were responsible for damage through superposition of the waviness and the free edge. However, for detailed information see Heinzlmeier et al [36].…”

“…Hence, a larger specimen width is necessary to avoid any interaction between the stress concentrations of the circular hole and the free edge. This important aspect was addressed by the author's institute by contributing research at 2D plain-woven CFRP plates with holes that allowed the description of damage that originates and propagates from a circular hole without any influence of the free edge [67]. In the experimentally identified damage propagation emanating from the hole, dam- age ceased to grow under the selected loads.…”

“…Since larger specimens have been used, no redistribution of damage progression was observed, in contrast to the investigations on smaller specimens [66,37]. Within the experiments performed at the author's institute, damage was measured optically and an empirical model was established, which could be used to describe the subcritical damage as a function of the load level [67]. During these investigations, damage to the hole as illustrated in Figure 2 appeared.…”