The Analysis of Process-induced Deformations in Advanced Composite Aero-structures

Abstract

Advanced composite structures cured in the autoclave under high temperature and pressure cycles often generate residual stresses upon cooling. The stresses arise due to different factors such as the anisotropy nature of the composite material, and the tool-part interaction. The relaxation of residual stresses leads to process-induced deformation (PID) in the final manufactured structure. Therefore, PID is a concern during the structure's design phase as it makes the assembly stage of the manufacturing process difficult and challenging. The objective of this research is to analyze and predict the process-induced deformations of composite sandwich structures, mainly the warpage in flat panels and the spring-in of curved structures, as very few research have considered these types of structures. This is achieved through a systematic experimental and numerical studies involving complex representative aero-structures. Key processing characteristics of a representative aerospace grade matrix system are evaluated through experiments to understand the curing process of a typical single-part resin system. Warpage measurement was performed using a Romer Arm, feeler gauge and HandyScan 3D by Creaform Thermochemical and thermomechanical models of a flat composite panel and a U-shaped structure are employed to predict the warpage and the spring-in of the structure, respectively, using ABAQUS/COMPRO simulation tools. The finite element models were validated through experiments to be used in investigating the effect of the tooling material, the core material, and the effect of the core as a design feature in composite structures PIDs. Cure kinetics and rheology models were successfully defined for a single-part resin matrix as the models showed a good correlation with the experimental results. The research discussed the modeling of process-induced deformation in flat and U-shaped sandwich structure warpage and spring-in, and the comparison of numerical and experimental results. For the U-shaped sandwich structures, an error of less than 5% is reported between the predicted and measured spring-in. This indicates that simulation could predict the PID of complex aero-structures with high accuracy. Based on numerical studies, the research showed that an aluminum tool and an aluminum core could reduce the structure spring-in or warpage for the specific structure shape and lay-up used in this research. Furthermore, it showed that curved sandwich structures deform less than laminated structures, and the latter deform with a combined effect of spring-in and warpage. Finally, the simulation results showed that the structure deforms after tool removal due to the relaxation of the high residual stresses that were locked in the structure before it is demoulded from the tool.

Date of Award	Oct 2018
Original language	American English