Strain Development During Cure of Partially Pre‐Cured Thermosetting Composites Measured With Distributed Fiber Optic Sensors

Detection of manufacturing‐induced defects in thermosetting composites is crucial to guarantee part quality, safety, and improve manufacturing efficiency. Residual stresses from cure and cool‐down often lead to matrix microcracking and geometrical distortion, with potential detrimental effects on mechanical performance and non‐fulfillment of geometrical requirements. Partial pre‐cure of the material prior to layup allows part of the exothermic reaction to exhaust before stacking the composite; thus, it can potentially mitigate thermally induced defects. To investigate such possibility, strain measurement experiments during cure of partially pre‐cured resin and composite samples were carried out, together with final warpage measurements. Distributed fiber optic sensors were selected, which allow strain and temperature measurement with high spatial resolution. Increasing pre‐cure level up to 50% in thin CFRP laminates was found to reduce the amount of residual strain and warpage generated by up to 31% and 14%. Further measurements showed that a strong temperature overshoot and higher longitudinal residual strain were generated during cure of thicker laminates. Thermal overshoot and internal strain were reduced by 29% and 17%, respectively, by initially pre‐curing the plies to 28% conversion.