Residual stress refers to internal stresses that remain within a material either before any external load is applied or after such a load has been removed. In composite materials, these stresses are particularly significant, as they can arise to considerable magnitudes due to manufacturing processes such as curing, cooling, and ply placement.

In other words, residual stresses in composites are internal forces “remembered” by the structure—though invisible to the eye—typically formed during production, especially in autoclave-based processes.

• Demolding After Cooling
• Differences in Coefficients of Thermal Expansion
• Volumetric Changes Between Resin and Reinforcement

Over time, these invisible residual stresses can lead to the formation of cracks or geometric distortions in critical structural components.

^{Composite polimeric structure virtualization (Generated by AI)}

Key Sources of Residual Stress in Composites

• Demolding after Cooling: The mismatch between the thermal contraction of the composite part and the mold can generate internal stresses.
• Differences in Thermal Expansion Coefficients: The matrix (e.g., epoxy) and reinforcement (e.g., carbon fiber) have distinct coefficients of thermal expansion (CTE). Upon cooling after curing, this mismatch induces internal stresses.
• Volumetric Changes during Resin Cure: Thermoset resins such as epoxy shrink during polymerization, which can generate substantial internal stress as the matrix contracts.
• Ply Stacking Sequence and Orientation: Placing reinforcement layers in various directions—especially in asymmetric laminates—can lead to non-uniform stress accumulation under thermal or hygroscopic effects.
• Processing Errors and Manufacturing Defects: Inadequate temperature control, uneven resin distribution, or voids may result in localized stress concentrations.

Consequences of Residual Stress

• Increased risk of delamination (interlaminar separation).
• Dimensional instability, such as warping or distortion of parts.
• Reduced mechanical performance, particularly in terms of fatigue and impact resistance.
• Compromised structural stability, especially in aerospace applications, where even minor deformations can impair aerodynamic performance and part reliability.

Understanding and managing residual stresses is thus essential for ensuring the long-term durability and functionality of composite structures.