Pneumatic behavior of woven glass/epoxy composite laminates

It is a well-known phenomenon that temperature and moisture concentration m ay cause dimensional changes in a composite lamina. This hygrothermal behav ior can be characterized in terms of two principal coefficients of thermal expansion (CTEs), alpha (1), and alpha (2), and two principal coefficients of moisture expansion (CMEs), beta (1) and beta (2). Recently, a novel disc overy ascertained that fluctuations in ambient air pressure may also cause dimensional change of a composite lamina. This behavior of composite materi al is very similar to the hygric behavior of the same material. It is possi ble that air permeation induces the material expansion just as moisture con centration induces moisture expansion. This behavior is referred to as pneu matic behavior. As for hygrothermal behavior, it can be characterized in te rms of two principal coefficients of pneumatic expansion (CPEs), gamma, and gamma (2). Experiments verify that the pneumatic strains in woven glass/ep oxy lamina are not negligible in comparison with hygrothermal strains in th e same material. When the pneumatic strains are not uniformly distributed w ithin a composite laminate, the strain mismatch through the thickness will result in unexpected stress. Thus, cyclic fluctuations of ambient air press ure may induce fatigue loading within the composite laminate and endanger t he composite structure. Design of composite structures exposed to an enviro nment of drastic cyclic air-pressure fluctuation, such as gas pipes, pressu re vessels, and aircraft and aerospace structures, should consider these pn eumatic effects. (C) 2001 Elsevier Science Ltd. All rights reserved.