A new method to determine the compression after impact (CAI) strength of composite laminates was presented. In this method, an impact damage zone was modeled as an equivalent hole. The most outstanding characteristic of the method was that the simplification of the impact damage was based on the compressive failure mechanisms of impacted laminates. Such a simplification has not been discussed in the literature before. A technique was established for determining the shape and size of the equivalent hole. To the authors' knowledge, it is the first practical method for simplifying the impact damage as a hole, and also the present work was the first attempt to predict the CAI strength by modeling the impact damage as a hole. The stress distribution around damage was calculated using the complex potential method and the classical lamination theory. A lay-up independent failure criterion was used to predict CAI strength. The predictions of the present approach were compared to test results and found to be in very good agreement over a wide variety of materials. The study indicated that the width of damage zone was a key factor governing CAI strength. The influence of damage area, impact energy, impactor shape and dimension, etc. on CAI strength can be essentially characterized by their effects on the damage width. The study provided a very simple and effective approach for CAI strength prediction compared with previous methods.KEY WORDS: impact damage, compression after impact strength, damage width, elliptic hole, failure criterion. the behavior of composite aircraft structures subjected to impact by foreign objects

An approach based on a displacement compatibility model is presented for both riveted and bonded stiffened composite panels containing impact damage. This represents the first application of the displacement compatibility model to the failure analysis of impacted stiffened composite panels, and some newresults are obtained for the impact damage tolerance properties of stiffened composite panels. In the present analysis, the previous displacement compatibility model for a stringer/orthotropic skin panel is improved for a stringer/unbalanced skin panel. The impact damage is simplified as an elliptic hole based on the compressive failure mechanisms of impacted composite laminates and stiffened panels. Predictions for failure loads and damage arrest capability agree well with experimental results for several composite panel configurations. The important results obtained in this study are: (1) damage arrest capability is dominated by the strength of the skin/stringer attachment; (2) distinct two-stage failure shows that the riveted panels have significant damage arrest capability due to the high shear strength of the rivets; and (3) since the very low shear strength of the skin/stringer attachment, the co-cured panels have no damage arrest capability.