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Measurement of Intrinsic Residual Stresses in 3D Woven Composites Using Measurement of the Displacement Fields from Hole Drilling by Electronic Speckle Pattern Interferometry and Digital Image Correlation

Intrinsic residual stresses in woven composites result from the coefficient of thermal expansion mismatch between the fibers and the matrix. Extrinsic residual stresses result from large scale thermal gradients during curing and cooling. Intrinsic residual stresses in 3D woven composites are sometimes severe enough to cause micro-cracking in the matrix. They are also expected to impact the fatigue resistance and the impact resistance. To the best of our knowledge, there have been no spatially resolved measurements of the intrinsic residual stress field as a function of position in the repeating weave pattern. We used digital image correlation (DIC) and electronic speckle pattern interferometry (ESPI) to measure the surface displacement field resulting from drilling a 1 mm diameter hole at four selected locations in two different 3D woven composite architectures that represent low and high through-the-thickness constraint. The two methods are used because the displacements sometimes on the lower end of the resolution for the DIC method and the displacement gradients are sometimes too steep to resolve the fringes for the ESPI method. Finite element models constructed with realistic fiber geometry using Dynamic Fabric Mechanic Analyzer software were utilized to estimate the residual stress field from cooling from the curing temperature. Holes were manually inserted by deactivating the elements in the hole region and the resultant displacement fields were compared to the measurements. In general, the measured displacement fields were lower in magnitude than the model predictions. In some cases, the sign of the predicted displacement field is opposite to the observed field which could be attributed to differences between the actual hole location and the hole in the model.