A Hybrid Damage Modeling and Experimental Study of Composite Tee-joints under Pull-off Loading

It is a great challenge to perform an accurate and efficient fatigue life prediction of a bonded composite structure with the presence of geometry and material heterogeneity induced stress concentration. The present fatigue damage characterization of composite structures is still dominated by the use of a phenomenological stress-life (S-N) approach due to the availability of extensive S-N data and lower cost in generation of S-N data from fatigue tests at different applied stress ratios. Because of the inaccurate life prediction using the S-N approach for the structure with stress concentrators, a more rational fracture mechanics approach based on a Paris type crack growth law can be applied to compute the crack growth driving force provided that an initial flaw has to be introduced. In order to simulate both the crack initiation and propagation, a dual spring model is implemented at each nodal point where the static failure is simulated using springs of a cohesive type material model while fatigue crack propagation is calculated using springs of an elastic penalty stiffness coupled with a virtual crack closure technique (VCCT). In order to validate the dual spring model for the fatigue damage prediction, two types of Tee-joints are fabricated and tested by the National Institute for Aviation Research (NIAR) with and without a Teflon insertion. A calibration analysis is performed to determine the fatigue crack growth parameters using Tee-joints with a Teflon insert followed by the blind fatigue prediction of the specimens without a Teflon insert.