Composites made of glass fibers and unsaturated polyester resins are widely applied for various products. The failure processes of such composites are insufficiently understood. Fracture of the resin often initiates bulk composite fracture. Residual stresses occur during the curing of the resin due to the resin volumetric shrinkage. These unfavorable stresses may enhance resin fracture. Moreover, resin shnnkage may decrease the surface quality of the product. The addition of Low Profile Additives &PA) to the resin may decrease or even remove these problems. The shrinkage behavior of unsaturated polyester resin with varying amounts of LPA is investigated in this work. The emphasis is on the development of curing shnnkage occurring after the gel point of the resin and on accompanying shnnkage stresses. These stresses can be reduced and even reversed by the addition of LPA.

Notches accelerate the initiation and the growth of the matrix-dominated damage in composites. Different forms of matrix-dominated damage are observed in a notched cross-ply laminate, where transverse cracks (TC), notch-induced splits (NIS), and notch-induced delaminations (NID) occur simultaneously. These matrix-dominated damage systems, particularly NIS and NID, interact with each other and are important mechanisms for the failure process of a notched cross-ply laminate. In this article, a progressive failure approach is proposed and it is used to model the NIS growth in cross-ply laminates under the influence of other forms of matrix failure, transverse cracks (TC), and notch-induced delamination (NID). The numerical results indicate that a strong interaction effect does exist, which is obviously indicated by the NIS growth rates under different combined damage cases. As one of the most important results, the present modeling approach leads to a better understanding of the failure mechanisms of notched composite laminates. The numerical estimation of the NIS growth shows a good agreement with the experimental observations.

A new ultimate strength model is developed to predict the final failure, with the influence of matrix failure, of notched cross-ply laminates. The present model is based on the estimation of the local stresses and the redistribution of the local stresses in the critical damage zones around the notches. The new ultimate strength model can be stated as: when the local stresses in the critical-load-carrying elements reach their failure strength, the critical-load-carrying element will fail and the ultimate failure will occur. In order to estimate the local stress concentration in the critical damage zones, a method is developed and applied to decompose the local stress concentration into several parts, e.g., the geometrical contribution, the stacking-ratio contribution, and the damage contribution. Combined with a single numerical simulation for a notched laminate, the damage-dependent stress concentration of the laminate can be established for different notches and it is then utilized to predict the ultimate strength of notched laminates with different notches. The applications of this method show that the model prediction agrees with the experimental observations satisfactorily.

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