Distributed optical sensing for monitoring strain evolution during mechanical testing of composite laminates

Abstract

A Distributed fiber Optical Sensor (DOS) was embedded inside and attached to the surface of a multi-directional continuous fiber composite laminate coupon, along with the traditional foil of Strain Gages (SG) and Digital Image Correlation (DIC), to compare the DOS measure strain response to traditional strain acquisition methods. Tensile and bending tests were performed on an Open Hole composite laminate coupon to study different modes of loading and the performance of the DOS in the presence of a strain gradient. Tensile test results show good agreement between the SG, the DIC, and the DOS strain measurements in the region far from the strain concentration. Near the hole, the DOS repeatably measured the strain concentration during tensile and bending tests. The DOS was able to measure the residual strain in the composite laminate coupon through the hole-drilling method. Finite Element Analysis was performed to obtain the theoretical distribution of axial strain along the locations corresponding to the physical placement of the DOS in the coupon. As the results indicate, the DOS readings agree well with the theoretical results for the strain distribution in the test coupons. The results obtained during these evaluations provide confidence that the DOS strain measurement method and equipment are a robust strain acquisition option with the capability to overcome some of the limitations of the traditional foil SG and DIC methods.