Effects of multiaxial stress state and saltwater on fatigue damage and failure of glass/epoxy composite

Fiber composites used in advanced mechanical systems are generally subjected to cyclic loading with multiaxial stress state. Thus multiaxial fatigue is a subject of major importance in design and evaluation of long-term performance of composite structures and components.; Theoretical work was initially performed to address the fundamental issues of damage evolution and material degradation in glass/epoxy composites subject to multiaxial cyclic stresses. A continuum damage mechanics formulation based on irreversible thermodynamics was used to construct multiaxial cyclic constitutive equations of the composite. With coupling of damage anisotropy and multiaxial stress state and using Gibb's free energy function, damage driving force was determined. Based on the second principle of thermodynamics, evolution equations were also obtained to describe the change of the damage state in the composite under multiaxial fatigue loading.; A newly introduced failure-life theory was employed for the fiber composite, based on anisotropic nature of composite strength property and distinct mechanisms of microcrack formation and growth under different multiaxial cyclic stresses. A driving force for fatigue failure was formulated with a governing function, involving combined cyclic shear and transverse tensile stresses along the weakest principal material plane of the composite. Multiaxial fatigue life of the composite was related to the forcing function using a power-law.; A formulation of thermomechanical response of a fiber composite based on the general thermodynamics theory was proposed to establish a model that can couple water sorption, applied stress and damage in the composite. Starting from general Gibbs free energy, constitutive equations, which describe the relationships between state variables and corresponding driving forces, were derived. The evolution laws that describe relationships of damage driving force-damage evolution and chemical potential gradient-water flux were also given.; Critical multiaxial fatigue experiments were conducted on filament-wound glass/epoxy composite tubular specimens to characterize the damage initiation and evolution. At the same time, experiment data were also been used to determine the material and multiaxial stress parameters of the fatigue-life laws for the composite and to demonstrate the validity of the theory.