Study of the Onset of Delamination at Holes in Composite Laminates

Abstract

For laminated composites, delamination can be a controlling failure mode and a major damage mode. The problem of delamination growth has been widely studied, especially for idealized situations such as the edge delamination of specially designed plate specimens. However, comparatively little systematic study has been made of the ini tiation of delamination around notches such as cutouts and holes in laminates of practical interest. Such work is essential to efforts to establish criteria for the design and optimiza tion of laminates to withstand the important practical requirements of notched strength, especially under long term static or cyclic loading. Composite laminates are prone to delamination at free edges, straight edges, or at holes, due to the mismatch of deformations at interfaces where two adjacent plies have different fiber orientations and/or different material properties. The linear analysis of the mismatch at the edge results in a mathe matical singularity. That phenomenon occurs in a boundary layer and has to be treated mathematically and physically as such. It is of great importance to recognize and be able to predict delamination locations at edges prone to such events. The goal of the research presented here was to create a model capable of providing such a prediction and to verify that model. In an effort to generalize the model, the more complicated case of a free edge at holes in a composite laminate was considered rather than the case of a straight free edge. Once the development of the analysis of the free-edge effect at a hole in a composite lami nate was completed, a sequel of two major efforts was employed: 1) performance of an ex tensive experimental program to provide data for the creation of the prediction model, and 2) establishment of the model based on the analysis, and comparison of predictions with the experimental results. The final formulation of the stress field near the hole edge con sists of the determination of the mathematical order of the singularity and a series of near- field parameters. Both terms play an important role in the analysis, and were required for the prediction model. The testing program employed two different material systems and two types of stacking sequences under quasi-static loading. Delamination locations as well as threshold loads for those locations are reported and discussed in terms of common philosophies regarding the subject. Some unexpected contrasts in the behavior of the two material systems are observed.