Residual stresses in composites

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

The discussion on the residual stresses in composites in Section ll.C.8.d had already provided an example of how composite theory can be used to correlate and/or to predict the thermoelastic properties and the related performance characteristics of a multiphase polymeric system. A much broader discussion of the use of composite theory will now be provided. 

Residual stresses in composite materials Edited by M. Shokrieh... 

A side effect of the use of a second phase with markedly different linear coefficients of thermal expansion (CTEs) is the residual stresses that are generated during cooling from the processing temperature. Residual stresses in composite materials, and how they impact mechanical properties, have become an increasingly important subject in materials research in recent years." As researchers continue to push materials closer to their property limits, it has become mote important to understand and control failure. Residual stresses are important because, vriien combined with applied stresses, they can lead to premature structural failure. Thus, the ability to limit residual stresses could improve the thermo-structural capabilities of a material. These stresses in multi-phase materials such as ZrB2-SiC, arise from the difference in the CTE of the phases (for ZrB2 CTE = 6.7 x lO" K for SiC CTE = 4.7x 10- K- ). 

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Harper BD, Weitsman Y (1985) On the effects of environmental conditioning on residual stresses in composite laminates. International Journal of Solids and Structures 21(8) 907-926 Harper B, Rao J, Kermer V, Popelar C (1997) Hygrothermal effects upon stress relaxation in a polyimide film. Journal of Electronic Materials 26(7) 798-804 Jackie J, Frisch HL (1986) Properties of a generalized diffusion equation with a memory. J Chem Phys 85(3) 1621-1627... 

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