In thick thermoplastic composite laminates, nonuniform temperature distribution arises in the through-thickness direction during high-rate manufacturing processes. This causes the so-called thermal skin-core effect. The surface region solidified in advance constrains shrinking of the inside region, so nonuniform residual stress/strain distribution arises in the through-thickness direction. This study quantitatively clarified this mechanism and identified the amount of residual stress/strain by utilizing fiber optic–based internal strain measurement and process simulation. First, in-plane transverse strain of thin carbon fiber/polyphenylenesulfide laminates was measured using fiber Bragg grating sensors to determine two key parameters for stress/strain simulation; thermal/crystalline shrinkage strain and composite stiffness. Abaqus-based simulation using these properties was then performed to calculate stress/strain distribution in thick laminates. The simulated strain agreed well with the measured value and it was confirmed that the residual stress developed in a relatively low temperature range. In addition, transverse three-point bending tests were conducted to validate the amount of residual stress calculated by the simulation. The bending strength increased by the thermal skin-core effect and the amount of strength increase coincided with the simulation, confirming the validity of the simulation. Extension of the proposed approach to the evaluation of the morphological skin-core effect is also discussed.

This study investigates the influence of cooling rate on the residual strain of the carbon fibre/polyphenylenesulfide unidirectional laminates. Three different cooling rates (−300℃/min, −100℃/min and −10℃/min) were applied to simulate a wide range of cooling conditions. The crystallisation behaviour which depends on the cooling rate was evaluated using differential scanning calorimetry. A process monitoring test was then conducted using embedded fibre Bragg grating sensors. In-plane transverse strain of carbon fibre/polyphenylenesulfide unidirectional laminates was measured and the results were presented based on the crystallisation behaviour determined by differential scanning calorimetry. Furthermore, residual strain change after subsequent annealing was examined. This study successfully demonstrates the effectiveness of in situ process monitoring using fibre Bragg grating sensors for evaluating material behaviour of thermoplastic composites during rapid heating and cooling under realistic processing conditions.

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This study investigates the influence of macroscale skin-core residual stress and cooling rate on the impact response of aerospace grade carbon fibre/polyphenylenesulphide (CF/PPS). Numerical simulations are developed which analyse the thermal shrinkage and residual stress development of unidirectional (UD) lay-up configurations. Macroscale skin-core residual stresses are then incorporated into low-velocity impact simulations based on an orthotropic elastic material model. Interlaminar delamination is modelled using a bilinear cohesive traction–separation law, and intralaminar failure is modelled using the Chang–Chang strength-based failure criterion. The simulation results are compared with the results of drop tower impact tests showing qualitative agreement in terms of maximum impact force and delamination. The results of this work highlight the importance of cooling rate on the interlaminar delamination and intralaminar failure of CF/PPS.