Predicting Scaling Effect on the Notched Strength of Prepreg and Fiber Tow-Placed Laminated Composites

The effect of scaling on the notched strength and response of composite plates was investigated. Unidirectional prepreg and tow-placed laminated composites, both of which were made of the same fibers and resin system, were considered. In order to predict the scaling effect, the local damage accumulative inside the notched composites was considered. A finite element analysis based on an accumulative damage model previously proposed was developed for analyzing the response and tensile failure of composite laminates containing a slit or a circular hole of various sizes. The model consists of a constitutive equation which relates the damaged material to an accumulated damage state; and damage accumulation criteria which predict the state of damage as a function of the applied stresses. The free-edge induced damage was not considered in the model.

The effect of scaling on the notched strength of laminated composites was evaluated by comparing the results of the calculations from the analysis with the test data generated at Stanford University and the Boeing Company. The predictions agreed with the test data very well for the notch sizes ranging from as small as 0.25 inch up to 2.5 inches.

It was found that the larger the notch size, the more accumulated damage was predicted. Hence, the scaling effect on the notched composite strength is strongly related to the extent of damage accumulated inside the composites. The model also shows that the fiber interaction zone affects significantly the residual strength of notched composites. The higher fiber interaction zone tends to reduce fiber tensile strength, but to increase tensile notched strength. With the same material system, although the fiber tensile strength of unidirectional tow-placed composites was lower than prepreg composites, tow-placed composites exhibited higher notched strength than did the prepreg composites.