Co-authors​ :

 Francesco TADDEI (SWITZERLAND), Giacomo STRUZZIERO , Véronique MICHAUD (SWITZERLAND) 

As the composite industry is considerably expanding its boundaries, thick section components are also being increasingly employed in key industrial sectors. However, the manufacturing of thick composites poses several challenges in terms of process time and arising of process-induced defects [1]. In the curing stage, the strong temperature spikes and thermal gradients are source of residual stresses, which might cause shape distortion, matrix microcracking, hardly predictable changes in performance, and ultimately may lead to part rejection. All these contributions affect the manufacturing efficiency and reliability. The additive curing concept consists in the additive deposition of impregnated tows while being partially cured through a moving heat source and after they have been possibly pre-cured (Fig. 1). This approach shows the potential to reduce cure-induced defects and process time, while preserving the mechanical performance of the final component [2, 3].
The present work exploits numerical simulations to study the influence of additive curing on thermal overshoot, residual stress and process time in the manufacturing of a 100x100x30mm carbon/epoxy laminate with cross layup. The role of the pre-cure level, deposition speed, and heat source dimensions and intensity will be investigated, and the outcomes will be compared to the standard batch curing.
The relevant material properties of the hot-melt resin system to feed the Finite Element model have been characterised as a function of the degree of cure and temperature by using Differential Scanning Calorimetry (cure kinetics, glass transition, specific heat capacity), Dynamic-Mechanical Thermal Analysis (elastic modulus), and Distributed Fibre Optic Sensors (DFOS, thermal expansion); the remaining properties of the epoxy resin and carbon fibres have been taken from literature. Strain and temperature monitoring during cure experiments have been performed embedding DFOS in thin and thick laminates [4], and compared with simulations in order to validate the thermo-mechanical problem. A sequential thermal-static analysis in Abaqus is built using the Additive Manufacturing plugin [5] and user-defined subroutines to simulate the tow deposition and the action of the moving heat source as event series, while computing the cure evolution; fixed temperature is applied on the mould side and equal to the deposition temperature (80°C), while natural convection at room temperature is applied anywhere else (Fig. 2). A preliminary analysis proves that a partial pre-cure level up to 0.3 degree of cure mitigates the thermal spike by 90%, the longitudinal residual stress by 8% and the stress gradient through the thickness by 45%.
This study explores the possibilities of additive curing, which is envisaged to combine an efficient curing strategy with the benefits from additive manufacturing, opening new design opportunities through a reliable and flexible manufacturing of composites.