Moisture effect on the response of orthotropic stiffened panel structures

This paper examines the effect of absorbed moisture on the dynamic response of stiffened panel structure from orthotropic material and a general external loading condition. An analysis of orthogonally anisotropic stiffened plates subject to loads induced by the moisture and random external pressure is presented. The procedure is based on the linear theory of plates and beams utilizing modified transfer matrices to accommodate the effect of the moisture. Ingestion of moisture was taken to vary linearly with the swelling which results in effective force resultants. Numerical results indicative of the moisture effect are offered. EROSPACE structures, aircraft, and other transporta- tion vehicles are composed of flat or curved panels. The panels usually have length-wise stiffeners. Vibrations induced by various sources such as thrusters, mechanical and electrical equipment, exhaust noise, turbulent boundary layer flow, oscillating shocks, jet noise, etc., could have a direct effect on the quality and limitation of the precision requirements and comfort levels inside the aerospace structures. Thus, there is a need for accurate and precise solutions.13 Approximate and numerical solutions for the vibration response of the surface protection systems of the aircraft utilize the finite-element methods. These solutions require large computer storage capabilities, and when the vibrations are coupled to the acoustic field, the results are limited to very low frequencies.3 Analytical solutions for the dynamic response to aircraft panels have also been developed. Analyt- ical models are usually limited to simplified geometries and do not include geometric details, but the solution is given in compact form, and the computed time required is limited. An analytical approach for an aluminum and composite shell is presented in Refs. 4 and 5. These models did not account for the discrete stiffening effect. Analytical solutions for dy- namic response of discretely stiffened panels from homoge- neous isotropic610 and composite11 materials have been developed. However, in all the previous approaches, the hy- grothermal effect is not present. Investigations have shown the deleterious effects of hygrothermal environments of struc- tural performance of nonisotropic materials.1216 The deleteri- ous effects consist of degradation of the mechanical properties and hygroscopic expansions.13 The hygrothermal effects are two. One is associated with the temperature, and the second with the moisture ingestion. At any temperature there is a shift in the stiffness and strength curves. These curves are lowered by the absorbed moisture.12 Because of the swelling, significant residual stresses are induced by moisture absorp- tion.15 Ingestion of moisture has an effect similar to the thermal effect.1215