A Contribution to Time-Dependent Damage Modeling of Composite Structures
The paper presents a new damage model for predicting stiffness loss due to creep loading and cyclic fatigue. The model, developed within a continuum damage mechanics framework, is based on the idea of a time-dependent damage spectrum, some elements of which occur rapidly and others slowly. The use of this spectrum allows a single damage kinematic to model creep and fatigue damage and to take into account the effect of stress amplitude, R ratio, and frequency. The evolution equations are based on similar equation than the one describing the viscoelasticity model and are relatively easy to implement. The new model is compared to the experimental results on carbon fiber/epoxy tubes. Quasi-static, creep and fatigue tests are performed on filament-wound tubular specimens to characterize the elastic, viscoelastic and plastic behavior of the composite material. Varying amounts of damage are observed and discussed depending on stress level and R ratio. The experimental work aims to develop and validate the damage model for predicting stiffness loss due to creep loading and cyclic fatigue.
Creep Strain and Stress Analysis in Laminated Composite Pres-sure Vessels
This study investigates the time-dependent long-term creep strain in a composite cylinder made of glass/vinylester with a unidirectional ply. The cylinder is subjected to an internal pressure and the boundary condition is free–free and acts as thermal insulation. The classical lamination theory (CLT) is used to derive the governing equations as a second-order equation to determine the radial, circum-ferential, axial, and effective stresses in the cylinder wall. The distribution of the radial and circumferential creep strains is based on the Schapery’s single integral model for nonlinear viscoelastic materials. This study focuses on the effect of the orientation of the fibers on the creep strain distribution in the wall of a cylinder. The results show that the creep strain is lower when  than at . As the angle of the fibers increases, the distribution of the creep strain becomes more uniform.
Geometrical and interfacial non-linearities in the analysis of delamination in composites
The subject of this paper is the study of interactions between delamination and geometrical nonlinearities. This problem generally addressed by means of Fracture Mechanics, is here treated by modelling interfacial degradation through irreversible, softening interface constitutive laws. Geometrical nonlinearities are associated to large displacements of thin layers which are assumed to be hyperelastic. A general formulation is presented and then specialised to the case of rectilinear laminated beams. Numerical aspects concerning finite element implementation are discussed together with a local control algorithm for the treatment of unstable paths. Numerical examples showing the effect of interaction between geometrical and material nonlinearities are presented.