Effects of defects on interlaminar tensile fatigue behavior of carbon/epoxy composites

This work presents a methodology for accurate assessment of the effects of porosity defects on the interlaminar tensile (ILT) fatigue behavior of carbon/epoxy tape composites that can be used in aircraft fatigue-critical structural designs. A primary failure mechanism for such structures is delamination. The ILT fatigue material properties may be helpful for the development of the analysis methods able to predict initiation of delamination at sites that give rise to interlaminar stresses and where a fracture based approach may be more applicable [1–4]. However, the engineering community lacks reliable ILT fatigue data available in the public domain. A major technical challenge is extreme sensitivity of ILT properties to manufacturing defects including porosity that could lead to unacceptable scatter in the test results. In this work, X-ray Computed Tomography (CT) measurements of porosity defects present in curved-beam test specimens are integrated into finite element stress analysis to capture the effects of defects on the ILT fatigue behavior. Once the effects of manufacturing defects are captured through transfer of the CT measurements into a three-dimensional finite element model, the scatter in the curved-beam fatigue test data is reduced; and material ILT fatigue properties can be determined. In this work, CT and finite element modeling are used to determine a reliable ILT S–N curve for a popular IM7/8552 carbon/epoxy tape composite system at 0.1 load ratio.