The stress intensity factor (SIF), K, is an important parameter to predict the stress state (“stress intensity”) near the tip of a crack caused by a remote load or residual stresses. It can determine the probability of crack propagation and failure of the material. To study the use of high-strength material, IM7/8552 in the crack prone region is the main focus of this present study. A semi-elliptical surface flaw in a typical Boeing-747 rib section having circular cut out and experiencing an in-plane shear loading of 10.21 MPa was considered for analysis. A parametric study on crack initiation is done by having different size of cracks at different locations across the layers. The values of SIF for all the three modes were calculated using the contour integral method. In the present study, we have considered IM7-8552/carbon-epoxy composite due to its high performance and intermediate modulus property. As there are no theoretical solutions for mixed mode loading problems, finite element packages like HYPERMESH and ABAQUS were used to obtain the SIF along the crack edge. The corresponding stress intensity factor values were compared to the fracture toughness of the material to determine the probability of crack initiation. It was observed that the mode of failure changes along with shape of the crack. The analysis results showed a high probability of failure. A comparative study on T300-5208/carbon-epoxy and IM7-8552/carbon-epoxy was performed. IM7-8552/carbon-epoxy composite showed higher resistance to failure. By modifying the fiber orientations, stress concentrations were minimized to a tangible limit.
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Dr. Sivakumar V., Dani, K. P., and Sriram, S., “Prediction of Stress Intensity Factor on Precracked Composite Wing Rib Made up of Carbon-Epoxy IM7-8552”, Journal of Failure Analysis and Prevention, pp. 1-12, 2016.