Investigating the Effects of Cure Pressure on Tool-Part Interaction and Process-induced Deformations in Composites

Abstract

The interaction between a tool and part during composites manufacturing may significantly contribute to the formation of residual stresses and process-induced deformations (PIDs). High levels of uncertainty are often associated with tool-part interaction and resulting PIDs due to the complex underlying physics, and variabilities and uncertainties in processing. During the assembly of aerospace composite structures, PIDs may create significant geometry mismatches and lead to a loss of mechanical performance. Aerospace manufacturers commonly use costly and time-consuming techniques such as shimming to compensate for PIDs and meet process specification requirements. Although process simulation tools have evolved significantly in recent years to enable mitigation of PIDs via tool geometry compensation, in practice, they are not implemented often due to challenges associated with the characterization and calibration of numerical models. One such challenge is accurately measuring the degree of tool-part interaction and interfacial stresses during processing cycles. This paper presents a custom-built test fixture to directly measure tool-part interfacial stress development during processing of composites. The experimental setup is installed in a Dynamic Mechanical Analyzer (DMA) for in-situ measurement of tool-part interfacial stresses. The technique allows for the evaluation of the effects of process parameters, including pressure, temperature, lay-up, and tool surface condition. In this study, tool-part stress development was characterized as a function of the applied pressure for Toray T800S/3900-2 laminates cured on steel tools treated with Frekote release agent. The characterized stresses were then validated by measuring warpages of long and symmetric laminates cured on flat steel tools using different pressures similar to DMA tests. Results demonstrated that cure pressure significantly impacts the multi-physics interactions between fibers, resin, interply tougheners, and the tool surface throughout a cure cycle. The results of this paper can be used to expand the current understanding of tool-part interaction and potentially mitigate PIDs in composites processing.