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Extensive practical applications of pretwisted shells such as in turbomachinery blades can be found in mechanical, aerospace, nuclear, and marine industries. These structural elements with low aspect ratio can be idealized as pretwisted conical shells. While in service the composite materials are highly sensitive to various environmental factors like temperature variations, moisture diffusion, exposure to chemicals, radiations, thermal oxidation, etc. The combined effect of two such prominent factors namely temperature and moisture on the vibration characteristics of composite laminates has been considered in the present work. Thermal stresses are developed inside the composites during processing when it is heated to high temperatures and then cooled to room temperature. In addition, the composite laminates are exposed to severe temperature conditions in power plants and nuclear applications. Different composites absorb moisture to a varying degree in humid working conditions like steam turbine blades, submarine propellers and aircraft structures or windmill blades operating in high moisture zones or high rainfall areas. This absorbed moisture results in swelling of the matrix. Both temperature variations and absorbed moisture leads to resultant strains being developed due to mismatch in the thermal and moisture coefficients of the matrix and the fiber comprising the composite laminate. The residual strains may lead to the development of micro-cracks inside the composites resulting in strength degradation. Delamination or interlaminar debonding is one of the most commonly occurred damage mode in laminated composite structures and the failure of a delaminated composite may become much more drastic because of the induced residual internal stresses at high temperature and moisture concentrations. In general, all vibrations of blades are closely related to their natural frequencies. Hence, knowledge of the natural frequencies of turbomachinery blades is very essential for the designers in order to ensure the safety of operation. The worth mentioning work on pretwisted composite plates was carried out by Qatu and Leissa (1991) to Abstract This paper presents a finite element method to investigate the hygrothermal effects on the free vibration characteristics of delaminated carbon-epoxy composite pretwisted rotating conical shells. The generalized dynamic equilibrium equation is derived from Lagrange’s equation of motion neglecting Coriolis effect for moderate rotational speed. In the present formulation, an eight-noded isoparametric plate bending element is used based on the Mindlin’s theory. The multi-point constraint algorithm has been incorporated to ensure the compatibility of deformation and equilibrium of resultant forces and moments at the delamination crack front. Numerical results have been generated to study the combined effects of temperature and moisture concentration on the natural frequencies of delaminated composite conical shells in respect of twist angle, stacking sequence, rotational speed and location of delamination. The mode shapes for a typical laminate configuration in hygrothermal environment is also presented. Received 7 May 2014

Hygrothermal effects on the free vibration characteristics of delaminated composite pretwisted rotating conical shells