The Influence of Internal Stresses on the Aging of Polymer Composite Materials: a Review

The formation of internal stresses in polymer composite materials (PCMs) caused by different elastic moduli and thermal expansion coefficients of polymer resin and reinforcing fibers, as well as by swelling due to the moisture uptake is discussed. The influence of thermal cycles on the internal stresses and strength of the materials was studied in dry and wet atmospheres. It shown that thermal cycles cause a periodic jumps in the stresses at lowfrequency mechanical loadings, during which the mechanical properties are degraded due to the formation of microscopic cracks in the polymer matrix. The relative changes in the strength, elastic modulus, glass-transition temperatures, moisture diffusion coefficient, and other PCM physical characteristics are proportional to the logarithm of the number of cycles and also depend on the form and size of specimens, amplitude, conditions, and length of thermal cycles. A prolonged action of external actions relaxes the internal stresses and reduces their influence on the aging of PCMs.