No 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.

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.