Processing-Induced Residual Stresses in Composites

Composite materials inherently develop residual stresses during processing. This happens because the two (or more) phases that constitute the composite behave differently when subjected to nonmechanical loading. For example, consider a reinforcing phase that has low thermal expansion characteristics embedded in a matrix phase with high thermal expansion characteristics. If the material is initially stress free and the temperature is decreased, then the matrix will try to shrink more than the reinforcement. This places the reinforcement in a state of compression (i.e. a compressive residual stress). If the phases are well bonded, then models can be developed to predict the residual stress field that is induced during processing. 

The purpose of the second dwell is to allow crosslinking of the matrix to take place. It is during the second dwell when the strength and related mechanical properties of the composite are developed. To characterize the exothermic crosslinking reaction of a thermosetting polymer matrix, a thermal cure monitor technique such as Differential Scanning Calorimetry 

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Chapman. T.J., Gillespie, J.W. and Pipes, R.B. (1990). Prediction of process-induced residual stresses in thermoplastic composites, J. Composite Mater. 24, 616-643. 

This process uses a beam of high-energy electrons from a device called an accelerator to initiate resin cure in place of the usual autoclave that applies heat and pressure. EB curing is suitable for composite parts as thick as 5 cm. Parts are cured by spraying these electrons onto the composite part. The cure is controlled through the application rate of the electrons and the total number of electrons or dose applied to the part. It is conducted at room temperature so that normal thermal stresses associated with heat curing are minimized across the part however, local residual stresses are induced due to the localized curing that occurs only within the exposure area. 

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