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

摘要

Intrinsic residual stresses in woven composites result from the coefficient ofthermal expansion mismatch between the fibers and the matrix. Extrinsic residualstresses result from large scale thermal gradients during curing and cooling. Intrinsicresidual stresses in 3D woven composites are sometimes severe enough to causemicro-cracking in the matrix. They are also expected to impact the fatigue resistanceand the impact resistance. To the best of our knowledge, there have been no spatiallyresolved measurements of the intrinsic residual stress field as a function of position inthe repeating weave pattern.We used digital image correlation (DIC) and electronic speckle patterninterferometry (ESPI) to measure the surface displacement field resulting from drillinga 1 mm diameter hole at four selected locations in two different 3D woven compositearchitectures that represent low and high through-the-thickness constraint. The twomethods are used because the displacements sometimes on the lower end of theresolution for the DIC method and the displacement gradients are sometimes too steepto resolve the fringes for the ESPI method.Finite element models constructed with realistic fiber geometry using DynamicFabric Mechanic Analyzer software were utilized to estimate the residual stress fieldfrom cooling from the curing temperature. Holes were manually inserted bydeactivating the elements in the hole region and the resultant displacement fields werecompared to the measurements. In general, the measured displacement fields werelower in magnitude than the model predictions. In some cases, the sign of thepredicted displacement field is opposite to the observed field which could be attributedto differences between the actual hole location and the hole in the model.