Separating sources of manufacturing distortion in laminated composites

Manufacturing distortion in composites has been a problem for many years and continues to impede the development of composite materials. Previous efforts have shown these manufacturing distortions to be related to differences in the in-plane and through-thickness properties. Often tooling is modified in an attempt to compensate for these fabrication induced distortions; however, the various sources of the problem have not been clearly identified, measured, nor accepted. Earlier efforts have suggested that both the cooldown from process temperature and shrinkage during cure play a role, yet no single work has measured the relative contributions of these effects. Further, researchers have suggested numerous other factors as being responsible for all, or some portion, of the measured distortion. In this work, distortion in autoclave-cured laminated composites has been experimentally measured, at temperatures ranging from 24 degrees C to 178 degrees C, to determine the reversible and irreversible contributions to manufacturing warpage and to attempt to separate the distortion into components related to different mechanisms. Three mechanisms, anisotropy, material property gradients, and stress gradients are described, and are further separated into thermoelastic and non-thermoelastic components. The measured reversible response closely matches computed distortion, based solely on the in-plane and through-thickness coefficients of thermal expansion. Further, it seems that the irreversible contributions can be separated into material dependent effects, such as cure shrinkage, and process related effects, such as part/tool interactions. Calculations, based on anisotropy, indicate that both temperature change and cure shrinkage contribute significantly to manufacturing distortion; but, their contribution does not account for the total distortion. The irreversible contribution is much greater than predicted, indicating the action of a mechanism which only involves a non-thermoelastic contribution, such as a part/tool interaction. Trends in the experimentally observed distortion indicate the presence of other non-thermoelastic process related contributions and show that these other contributions seem to diminish as the thickness, corner radius and included angle increase.