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Residual stresses are detrimental to composite structures as they induce processing defects like debonding, delamination, and matrix cracking which significantly decrease their load-bearing capability. In this research, a new in-situ approach using digital image correlation is utilized to analyze the effect of the cure cycle modification on residual stress evolution during processing. It was found that the modified cure cycle comprising abrupt cooling after gelation reduces the residual stresses. Five different layup configurations are investigated to examine the effect of fiber direction. A maximum average residual stress reduction of 31.8% is observed for the balanced unsymmetric [30/-30/60/-60] laminate. The residual stress reduction results in an increase in failure strength between 4 and 12% in the different layups and can lead up to a 22% increase in first-ply failure strength. 

Thin-ply composite laminates are of interest for several applications in aerospace and other high-performance industries due to their ability to delay transverse microcracking and delamination in static, fatigue, and impact loadings. It is essential to understand the evolution of thermal residual stresses during cure to optimize the manufacturing process of thin-ply composites for deep-space applications. In this research, processing induced residual stresses in thin-ply laminates are evaluated by devising a novel in-situ experimental approach. Thin-ply prepreg laminates are cured in a specially designed autoclave with viewports with plies laid upon a flat tool and a curved tool. The curved tool configuration used in this research is designed to simulate cryogenic fuel tank surfaces. The evolution of residual stresses in terms of out-of-plane displacement is characterized using Digital Image Correlation (DIC) during the autoclave cure cycle.