Study of Interfacial Creep Strength of Polymer Matrix Composites

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Time-dependent behavior of carbon fiber-reinforced polymer matrix composites (CFRP) has been studied. The results obtained are summarized as follows :1. Time-Dependent Behavior of Unidirectional CFRP : The creep deformation is clearly observed for all off-axis angles, including the fiber direction. The magnitudes of the creep strains depend significantly on the creep stress and the off-axis angle. Unlike most conventional metallic materials, the creep rate of the composite rapidly decreases to disappear as the creep strain develops. The composite creep strain partly recovers with time after unloading the creep stress of the composite, but a certain amount of permanent strain eventually remains for every off-axis angle, regardless of the prior creep stress level. The creep strain gradually decreases with time after the stepped decrease in stress. Such a creep recovery after partial unloading reveals that the internal stress acting in the opposite direction of the applied stress exists a … More nd it has developed witlt prior creep.2. Time-Dependent Behavior of Angle-Ply CFRP : The stress relaxation effects at high temperature in the symmetric angle-ply laminates [±30]_<3s>, [±45]_<3s> and [±60]_<3s> consisting of unidirectional CFRP laminae have also been examined. The stress relaxation effects are clearly observed in all angle-ply laminates. The stress rapidly relaxes in a short period just after the total strain is kept constant, and then the relaxation rate tends to vanish, irrespective of the sustained total strain level and of the ply orientation. When the relaxation stress is normalized with respect to the maximum stress at the instant when the total strain is held, the resulting relative stress relaxation curves tend to come close with each other. These characteristics of the stress relaxation effects in the angle-ply laminates are similar to those in the off-axis specimens of unidirectional CFRP.3. Macromechanical Modeling : A multiaxial constitutive model to describe the time- and rate-dependent behavior of unidirectional composites has been developed on the basis of the irreversible thermodynamics with internal variables. This model can be extended to cover a wide range of inelasticity including damage. The proposed model can moderately describe creep and relaxation behaviors of unidirectional CFRP.4. Micromechanical Modeling : A simple micromechanics model to describe the elastic-viscoplastic behavior of unidirectional fiber composites has been developed using the method of cells for composite homogenization and a viscoplastic model for matrix material. This micromechanics model can moderately describe the non-linearity and rate-dependence of the off-axis tensile behavior of unidirectional CFRP at high temperature.5. Application to Multidirectional Laminates : The classical laminated plate theory fails in accurately predicting the deformation of the angle-ply laminates with small ply-orientation angles because of the effect of enhanced interlaminar stresses. To overcome this problem, appropriate combination of a three-dimensional homogenization method and a complete multiaxial constitutive model for constituent plies should be considered. Less