Predicting Onset of Damage in Special Class of Laminates under Tension

i Abstract In design with composite materials, the capability of tailoring the properties of the structure with respect to the specific load requirements in each direction, allowed by their anisotropic nature, is coupled with the development of interlaminar stresses transferred through the thickness. Specifically, at the free edge of the laminate, due to the change in fiber orientation and the different response of the constituent materials, stresses that act in the thickness direction are developed. Starting at the free edge and moving towards the center of the laminate up to a certain threshold, the stress state of the plies is further complicated by the interlaminar stresses. The widely employed Classical Laminated Plate Theory does not account for the aforementioned effects, since each ply is considered to be in plane stress state. The primary goal of the present thesis project was to develop an analytical stress model that includes expressions for all six stresses. By construction, the stresses acting through the thickness have an exponential decay behavior until they become zero, once the afore mentioned threshold is reached. Parallel to that, the in-plane stress expressions gradually recover the CLPT predicted values. Furthermore, the model was intended to capture a specific sequence of damage occurrences in laminates of the family [0 2 /θ 2 /-θ 2 ] s loaded under tension, as observed by O'Brien. Namely, during loading matrix cracks initiating at the free edge of the off-axis plies and propagating along the respective fiber orientation are anticipated to occur first. While these cracks increase in leng th and number, at some point a +θ crack will intersect with a – θ one, thus forming an envelope with the free edge. As loading continues, a local delamination is expected within this envelope. In order to predict that, the stress expressions would have to be implemented in a failure theory that is capable of accounting for the ongoing phenomena and of distinguishing not only between matrix and fiber failure, but also between different matrix failure modes. These requirements led to the selection of Puck Failure Theory. The process proved to be more complex than estimated and numerous issues occurred during the formulation of the model. At first, results from literature could not be duplicated. After an extensive period of thoroughly checking and editing the approach, the expressions on a symbolic level and the code in which they were implemented, it was realized that …