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.
Model for prediction of simultaneous time-dependent damage evolution in multiple plies of multidirectional polymer composite laminates and its influence on creep
Abstract A model to predict time-dependent evolution of simultaneous transverse cracking developed in multiple plies during creep loading and its effects on creep of multidirectional polymer matrix composite laminates is presented. The stress states in the intact regions of the plies are determined using the lamination theory during an incremental change in time. The stored elastic energy, determined using this stress state, is compared with a critical stored elastic energy value for damage to determine if a ply would fracture after the increment. If fracture is predicted, variational analysis is used to determine the perturbation in ply stresses due to cracking. This procedure is repeated to determine the crack evolution and creep strain. Model predictions compared well with experimental results for a [±θm/90n]s laminate.
Characterization of viscoelasticity and damage in high temperature polymer matrix composites
Abstract This paper describes a thermodynamics based model for viscoelastic composites with damage and illustrates its use in characterization of viscoelastic response of polymer matrix woven fabric composites subjected to loading at high temperatures. The characterization is conducted by an experimental method aided by finite element (FE) modeling. The experimental characterization is based on creep data obtained under constant loads of different magnitudes and at different temperatures, and on recovery data collected after unloading. A carbon fiber/polyamide resin woven composite with glass transition temperature (Tg) of around 380 °C was used in the experimental program. A FE model was developed to determine the non-linear viscoelastic response by implementing incremental constitutive relations into an ABAQUS® code. The laminate viscoelastic properties were obtained by finite element micromechanics analysis using the neat resin data as input. Comparing its results with creep-recovery test data at different temperature and stress levels validated the FE model. There are several factors affecting the viscoelastic behavior of polymer matrix composites such as temperature, moisture and stress level. Accordingly, a large number of tests need to be performed to characterize the viscoelastic response experimentally for each fiber–matrix combination. For this purpose an efficient and systematic experimental procedure was used to understand the effects of temperature and stress level on the viscoelastic response, to clarify the damage-viscoelasticity coupling and to determine the viscoelastic properties of the material system.