Fatigue and Progressive Damage of Thin Woven CFRP Plates Weakened by Circular Holes

Abstract

Background In order to design thin-walled components it is necessary to consider the presence of holes and their effects. For high performance composite structures, this is still an issue, since usually only coupons are used in experimental observations and the influence of free edges and the hole affects the fatigue behavior mutually. Objective This work aims to find, through experimental trials, an empirical model that can be used to describe and predict the damage propagation, originating from a circular hole. Methods A fatigue test series is performed and the damage initiation and propagation is monitored with three-dimensional digital image correlation, with which the occurring damage can be measured. Validation of the experimentally induced damage size measured with digital image correlation is performed intermediate with an in-situ measurement with active thermography and phased array ultrasonic. The novelty of this approach is that wide specimens are used, where the influence of the free edges on the hole does not occur. Results The progression of the detected damage over the test reflects the applied loads, where higher loads cause larger damage. For all defined load levels a similar damage propagation is identified, allowing to establish an empirical model and fit it to the test data. Conclusion The proposed empirical model provides a novel approach to describe and predict damage propagation originating from a circular hole in thin-walled composite plates. In addition, it is shown that the damage propagation ceases for the selected plate configuration and thus does not lead to a complete failure.

Damage Evaluation in Plain-Woven CFRP Plate with Circular Hole under Cyclic Tensile Loading Using Electrical Impedance Tomography

“…The repeated quasi-static loading is always the same for comparability of the measurements. The load levels of the fatigue loading blocks vary and are chosen to be comparable to previous results of the authors [34]. The associated strains are between 3000 µm/m and 12000 µm/m, thus, well represent a realistic operational loading of CFRP components, leading to realistic damage.…”

“…cracks, also cause a reduction in the correlation confidence. Thus, this quantity is frequently considered as an indicator for damage [34,39,40].…”

“…Due to the evaluations of 3D-DIC images taken at the same load level (S max,i ), changes to the correlation confidence are attributed to damage, since it is assumed that any other influences on the correlation confidence are negligible with the selected evaluation. In order to identify the shape and size of the damage, the spatial correlation coefficient of the subsets are evaluated by a threshold, a method that was demonstrated in [34].…”

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

“…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.…”