Pneumatic behavior of woven glass/epoxy composite laminates

Abstract

It is a well-known phenomenon that temperature and moisture concentration may cause dimensional changes in a composite lamina. This hygrothermal behavior can be characterized in terms of two principal coefficients of thermal expansion (CTEs), α 1 and α 2, and two principal coefficients of moisture expansion (CMEs), β 1 and β 2. Recently, a novel discovery ascertained that fluctuations in ambient air pressure may also cause dimensional change of a composite lamina. This behavior of composite material is very similar to the hygric behavior of the same material. It is possible that air permeation induces the material expansion just as moisture concentration induces moisture expansion. This behavior is refered to as pneumatic behavior. As for hygrothermal behavior, it can be characterized in terms of two principal coefficients of pneumatic expansion (CPEs), γ 1 and γ 2. Experiments verify that the pneumatic strains in woven glass/epoxy lamina are not negligible in comparison with hygrothermal strains in the same material. When the pneumatic strains are not uniformly distributed within a composite laminate, the strain mismatch through the thickness will result in unexpected stress. Thus, cyclic fluctuations of ambient air pressure may induce fatigue loading within the composite laminate and endanger the composite structure. Design of composite structures exposed to an environment of drastic cyclic air-pressure fluctuation, such as gas pipes, pressure vessels, and aircraft and aerospace structures, should consider these pneumatic effects.