Abstract
Mitigation of cure-induced defects in thermoset composite parts has always been a challenging problem for manufacturers especially when it comes to high dimensional accuracy of components. Thus, it is crucial to understand the evolution of the thermo-chemical properties of these materials during the totality of the curing cycle. In this paper, a new methodology is presented to characterize the process-induced strains throughout the cure. The investigation is based on the development of an existing laboratory bench named as PvT-HADDOC. The tests were performed on an interlayer toughened aerospace carbon/epoxy prepreg. Unidirectional laminate samples (105x105 mm2) of almost 6 mm of thickness were manufactured by hand lay-up then debulked at room temperature under full vacuum. The PvT-HADDOC device allows a manufacturing process following the recommended cure cycle of epoxy composites under 7 bars pressure and a temperature up to 180°C. It enables the measurements of the process-induced strains, simultaneously, along two directions: through-thickness and in-plane. Results show a complex behavior of an assumed unidirectional composite. It exhibits a temperature and time dependent compaction behavior through the thickness only. The measured thermal expansion coefficients are proved to be higher in the thickness direction for the uncured as well as for the cured state of the material. Most of the chemical shrinkage occurs along the thickness direction. This unexpected complexity is mainly attributed to the presence of interleaf layers of resin in the laminate structure. Thus, the investigated M21/IMA material is considered fully orthotropic.
Highlights
The composite manufacturing industry faces enormous challenges to understand and control thermosetting composite behavior throughout the production processes
The coefficients of chemical shrinkage and thermal expansion are among the significant parameters that are used for numerical simulation of residual stress development and final shape of composite parts
The manufacturer‘s recommended thermal cure cycle for the M21/IMA system based on the epoxy resin and according to the thickness of laminate samples used in this investigation, consists of a heating ramp of 2°C/min up to 180°C and an isothermal curing dwell for 120 min at this temperature before the cooling stage of the cured material to the ambient temperature
Summary
The composite manufacturing industry faces enormous challenges to understand and control thermosetting composite behavior throughout the production processes. Cure shrinkage in the thermoset matrix is the major source of cure induced defects in composite parts. Due to its coupling with thermal expansion or contraction, the linear chemical shrinkage is difficult to characterize when it comes to orthotropic materials. The coefficients of chemical shrinkage and thermal expansion (noted CCS and CTE, respectively) are among the significant parameters that are used for numerical simulation of residual stress development and final shape of composite parts. Experimental investigations are usually conducted on partially or fully cured samples limited to single plies of prepregs to estimate these parameters and lack reliable data about the total cure behavior. The aim of this study is to obtain a further understanding of linear thermal expansion and chemical shrinkage developments during the whole cure cycle of composite materials. A multi-axial investigation of the thermo-chemical behavior of an epoxy/carbon laminate is conducted using a new experimental device
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