Free-edge effects at circular holes in composite laminated structures

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Stress concentration problems in composite laminated structures such as the so-called free-edge effect consist of significant and potentially singular interlaminar stresses in the vicinity of free laminate edges which often leads to according failure modes such as edge delaminations. While the rather classical situation of a straight free laminate edge has been under scientific investigation for nearly 50 years, related stress concentration problems such as the free-edge effect at circular holes in composite laminates have been treated at much lesser depth. This project is devoted to the closed-form analytical and semi-analytical investigation of the free-edge effect in the vicinity of circular holes in plane composite laminated structures. Besides the rather straightforward determination of delamination forces and moments, several approximate layerwise analysis approaches shall be developed which can be subdivided into methods that work with adequate shape functions for the stresses and those that use layerwise approximations of the displacement field. A further analysis method that shall be employed is the so-called Boundary-Finite-Element-Method (BFEM) that uses a 2D discretization of the hole surface while in the radial direction semi-analytical formulations for all state variable can be derived from a variational principle. Consequently, the BFEM incorporates the distinct advantages of the Finite Element Method and the Boundary Element Method and thus is an attractive analysis approach for sophisticated stress concentration problems such as the free-edge effect. Lastly, the free-edge effect problem shall be transformed into an optimization problem by employing the means of the methods mathematical programming in order to clarify which optimization potential can be exploited when free-edge stresses at circular laminate holes are the focus of the investigations.