A Multi-Dwell Temperature Profile Design for the Cure of Thick CFRP Composite Laminates

This paper develops a multi-objective optimization method for the cure of thick composite laminates. The purpose is to minimize the cure time and maximum temperature overshoot in the cure process by designing the cure temperature profile. This method combines the finite element based thermo-chemical coupled cure simulation with the non-dominated sorting genetic algorithm-II (NSGA-II). In order to investigate the influence of the number of dwells on the optimization result, four-dwell and two-dwell temperature profiles are selected for the design variables. The optimization method obtains successfully the Pareto optimal front of the multi-objective problem in thick and ultra-thick laminates. The result shows that the cure time and maximum temperature overshoot are both reduced significantly. The optimization result further illustrates that the four-dwell cure profile is more e ective than the two-dwell, especially for the ultra-thick laminates. Through the optimization of the four-dwell profile, the cure time is reduced by 51.0% (thick case) and 30.3% (ultra-thick case) and the maximum temperature overshoot is reduced by 66.9% (thick case) and 73.1% (ultra-thick case) compared with the recommended cure profile. In addition, Self-organizing map (SOM) is employed to visualize the relationships between the design variables with respect to the optimization result.

Assessing residual stress redistribution during annealing in thick thermoplastic composites using optical fiber sensors

In thick thermoplastic composite laminates, nonuniform temperature and cooling rate distribution arises in the through-thickness direction during cost-effective high-rate manufacturing processes. Annealing is often carried out after molding to homogenize degree of crystallinity (DOC) and to reduce residual stress. Even though the change in the residual stress/strain distribution occurring inside thick laminates by this heat treatment is practically important, the changing process and the detailed mechanism are not sufficiently clarified. This present study addresses development and redistribution behavior of residual stress through both molding and annealing using multiple optical fiber sensors deployed in the thickness direction. This article begins by explaining about process monitoring of thick laminates to discuss process-induced strain distribution depending on cooling conditions during molding. Next, strain monitoring is performed during annealing, and the strain change caused by cold crystallization is clarified. Finally, the residual stress distribution is evaluated by a transverse three-point bending test, and the validity of the redistribution mechanism deduced from the strain measurement is confirmed.

The influence of curing cycle and through thickness variability of properties in thick laminates

Mechanical properties of glass fibre reinforced polymers are dependent on the manufacturing curing cycles. During the laminate manufacturing process, each thickness position experiences a different local curing cycle. Therefore, it can be expected that mechanical properties vary through the thickness, particularly for thick laminates. To study the through-thickness variation of static and fatigue mechanical properties, thick laminates were divided into sub-laminates and these sub-laminates were separately tested. The present work reports temperature profiles through the thickness recorded during the manufacturing of thick laminates, as well as experimental data from static and fatigue tests (S–N curves) of sub-laminates obtained at different thickness positions. The variation of the mechanical properties through the thickness is discussed and related to the local curing temperatures experienced by each sub-laminate.