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.
Highlights
Plain-woven carbon fiber reinforced plastics (CFRP) are widely used due to their good handling usability and tolerance against impact damages
As already emphasized by Mittelstedt,[3] the calculations were initially based on the classical laminate plate theory (CLPT), but in this theory, stress components in thickness direction are not taken into account
The reference submodels, which obtained their boundary conditions from the straight reference specimen, are used to determine the relevant stress components caused by the free-edge effect
Summary
Plain-woven carbon fiber reinforced plastics (CFRP) are widely used due to their good handling usability and tolerance against impact damages. The two variants of the submodel, which are defined for the reference specimen, the center of the half-wave imperfection and for the transition radius, are used to describe the free-edge effect. In order to evaluate the strain components relevant for the free-edge effect, paths are defined for the individual submodels and associated variants As the 3D-DIC system determine strains within the X-Y plane during experimental testing, the numerical results of the model of the specimen with a half-wave imperfection are transformed in the same global directions. The reference submodels, which obtained their boundary conditions from the straight reference specimen, are used to determine the relevant stress components caused by the free-edge effect. The final crack state was different for each specimen
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