Integrated Structural Monitoring of Composite Materials via Distributed Optical Sensor

Measuring the strain history in pre-impregnated thermoset composites during the curing process provides valuable data for manufacturing specification development, quality control, diagnostics of dimensional stability, and validation of cure models. Unlike traditional Fiber-Bragg Grating-based methods, the Distributed Optical Sensors (DOS) provide information along the entire optical fiber distance (optical path length) of the sensor embedded in the laminate for strain measurement. This study’s unique contribution to the field is the coupling of the optical sensor monitoring of composite cure strain with models of the cure kinetics, viscosity, and glass transition temperature of the thermoset matrix. Coupling the strain measurements to the material models facilitates coherent comparisons between strain sensor output and thermoset material behavior during the cure process rather than making suppositions of material behavior based on the strain measurement alone. In this research, two laminate types were manufactured with an embedded optical sensor from IM7/5320-1 prepreg tape; a [0]20 Unidirectional (UD) laminate and a 50/40/10 (%0°/%±45°/%90°) Structural Laminate (SL) with the optical sensor routed along three different ply interfaces including [0/0], [45/90], and the [0/45]. The internal residual strain developed during each stage of the thermal cure cycle is examined, including after cooling. Results show that the local strain has variable magnitude, which depends on the ply configuration. The laminate micro-structure was also investigated by optical microscopy for selected cross-sections to provide the resin pocket geometry created by different optical sensor placements in the laminates. Lastly, a Micromechanics-based approach was used to calculate the chemical shrinkage and the residual strain in the UD laminate during curing and compared with the measured results from the DOS to further validate the strain measurements. The Classical Laminate Plate Theory (CLPT) was used to calculate the residual strain in the SL after curing and compared with the measured results from the optical sensor. A continuous DOS was embedded inside the SL and attached to the surface of a coupon, along with traditional Strain Gauges (SG) and the Digital Image Correlation (DIC). The results from the SG and the DIC were compared to the results from the DOS. Tensile and bending tests were performed on a notched composite laminate coupon with embedded DOS. Tensile test results show that the SG and the DIC reading match closely with the results from the DOS at the far region of the notch. Near the notch, the DOS was able to monitor a strain concentration during tensile and bending tests. Finite element analysis was performed to obtain the theoretical distribution of axial strain along the inspection lines corresponding to the locations of the physical DOS segments to compare the theoretical and experimental strains. As the results indicate, the DOS readings agree well with the theoretical results. The strain measurements from the DOS overcome some of the limitations from the SG and the DIC. Therefore, it was demonstrated that a DOS system can deliver reliable and robust continuous monitoring of mechanical strains in a composite laminate