Papers by Keyword: Hygrothermal Environment

Abstract: The effects of moisture content on the bending strength of T300/914 composite laminates that immersed in water for 7 days and 14 days was discussed in this paper. The three-point bending tests were conducted on the composite laminates. Experimental results reveal that the moisture content in the laminates increased with immersion time and that moisture absorption accelerated damage propagation in the composite laminates. The bending strength of the unaged, aged specimens were characterized and analyzed. Compared to the unaged specimens, the bending strength of the composite laminates immersed for 7 and 14 days decreased by 6.62% and 16.98%, respectively. The results revealed that the bending strength of the aged specimens decreased with the increasing immersion time.

Abstract: This study investigates the effect of moisture content on the residual bending strength after bending fatigue of T300/914 composite laminates immersed in water for 7 days and 14 days. Displacement-controlled three-point bending fatigue tests were conducted on specimens. After 40,000 cycles the fatigue test was stopped and the residual properties were measured on the tested specimens. Reduction in material strength was found to depend on the level of moisture content. Experimental results reveal that the moisture content in the laminates increased with immersion time. Compared to the unaged specimens, the residual bending strength after bending fatigue decreased by 6.67% and 37.04%, respectively. The residual bending strength and strength retention decreased with increased immersion time.

Abstract: This study investigates the effect of moisture content on the bending fatigue properties of T300/914 composite laminates immersed in water for 7 days and 14 days. Displacement-controlled three-point bending fatigue tests were conducted on specimens. After 40,000 cycles the fatigue test was stopped and the properties were measured on the tested specimens. Reduction in material strength was found to depend on the level of moisture content. Experimental results reveal that the moisture content in the laminates increased with immersion time and that moisture absorption accelerated damage propagation in the composite. Hygrothermal ageing lowered the threshold level for the onset of fatigue. The experimental results were further validated by the supportive micrographs that illustrate different moisture content and their morphology before and after moisture absorption.

Abstract: The dynamic stability behavior of laminated composite shells subjected to hygrothermal loadings are studied in the present investigation. A simple laminated model is developed for the vibration and stability analysis of laminated composite shells subjected to hygrothermal conditions. A computer program based on FEM in MATLAB environment is developed to perform all necessary computations. An eight-node isoparametric element is employed in the present The analysis with five degrees of freedom per node. Element elastic stiffness matrices, mass matrices, geometric stiffness matrix due to mechanical and hygrothermal loads and load vectors are derived using the principle of minimum potential energy. Quantitative results are presented to show the effects of curvature, ply-orientation, degrees of orthotropy and static load factors of laminate on dynamic stability of composite shells for different temperatures and moisture concentrations.

Abstract: The effects of voids on the bending strength of T300/914 laminates that exposed to room temperature, hygrothermal and drying environment was discussed in this paper. The experimental results revealed that the saturated moisture content and the rate of water uptake increased with porosity increasing from 0.71% to 1.50%, which proved that voids facilitate moisture absorption. The bending strength of the unaged, aged and dried specimens were characterized and analyzed. The results revealed that the bending strength of the aged specimens decreased with the increasing void contents and immersion time. Compared to the unaged specimens, the bending strength of the aged specimens decreased 13.33% and 18.78% with porosity of 0.71% and 1.50%, respectively. The bending strength of the dried specimens was higher than that of the aged specimens and lower than that of the unaged specimens in the case of similar porosity.

Abstract: The effect of voids on moisture absorption of T300/914 laminates was discussed in this paper. The experimental results revealed that the moisture content increases with the immersion time at first and then reach a pseudo equilibrium state for the compressive and bending specimens. The saturated moisture content and the rate of water uptake increased with porosity increasing from 0.71% to 1.50%, which proved voids accelerate moisture absorption. The geometry of the specimens influences the moisture behavior. The saturated moisture content increased with the decreasing width of the specimens. The most significant increase in dimensional change is in the thickness and length direction, followed by the width direction.

Abstract: Stitched laminates is a low-cost structure panels with carbon fiber face sheets, and through-the-thickness Kevlar stitching. Through-the thickness stitching is proposed to increase the interlaminar strength and damage tolerance of composite laminates. Tensile and shear experiment of stitched laminates at room temperature and in hygrothermal environment was carried out according to corresponding national standards. Experiment results indicate that the tensile and shear modulus and strength were much reduced by the stitching, especially in hygrothermal environment. Micrographs of fracture appearance showed that the exist of resin-rich area is the source of crack both in normal room temperature environment and hygrothermal environment. It is concluded that hygrothermal environment and initial crack in resin-rich area were prime reason for performance lost of stitched laminates.

Abstract: In this paper, a thermodynamic approach is presented to model coupled fluid transport, heat transfer, long-term deformation and damage in polymeric materials. The well-known Gibbs free energy is expressed as a functional of stress, temperature and fluid concentration with damage being introduced as an internal state variable. Constitutive equations for nonlinear viscoelastic materials in hygrothermal environments are derived in memory functional forms. The kinetics of damage evolution induced by stress, temperature and fluid is described by a damage function with thermodynamic driving force. Governing equations for mass and heat transfer are obtained from transport laws relating fluid and heat fluxes to gradients of chemical potential difference and temperature. A superposition principle of time, temperature, fluid concentration, stress, and aging is proposed so that long-term property functions may be derived from momentary master curves by horizontal and vertical shifting. The approach provides a theoretical framework for evaluating longterm behavior of polymeric materials in hygrothermal environments from short-term experiments.