| Proper determination and modeling of constitutive properties, damage, and strength of fiber-composite structures under multiaxial loading are critical to their design and life prediction. Improved understanding of underlying mechanisms and critical data for multiaxial stress-states are required. The following related issues were studied on E-glass/epoxy composite: (1) interaction of multiaxial stresses on deformation and failure strength of fiber composites, (2) contribution of irreversible stiffness degradation to deformation nonlinearity, (3) physical damage mechanisms leading to property degradation and failure, and (4) continuum mechanics modeling of composite damage under mechanical loading.Constitutive relations and strength under combined shear and normal stresses were studied in experiments with off-axis unidirectional and symmetric angle-ply composites. Initial linear-elastic response was unaffected by stress multiaxiallity, but nonlinear behavior started earlier and was more pronounced than predicted by a commonly used decoupled, nonlinear-elastic shear model.A novel cruciform glass/epoxy specimen with a unidirectional gage section was developed to study the behavior under combined high transverse and longitudinal tensile loads. Previously unavailable data indicate an increase in transverse nonlinearity and a reduction of unidirectional composite strength under high biaxial tensile loading. The interaction was well described by existing failure-mode based strength criteria, accounting for reduction of transverse strength at high longitudinal stress.Microscopic observations of fiber-matrix debonds and fiber filament breaks were made in loaded specimens. Associated permanent stiffness degradation from total stress-strain nonlinearity was discerned in multi-step variable loading experiments with repeated incremental loading blocks. The procedure showed reduction of transverse and shear stiffness at 5% and 10% of their virgin values, respectively, prior to ply failure. Considerably higher degradation obtained with a commonly used procedure may be attributed to the repeated loading effect identified with the current method.A continuum damage mechanics model, based on irreversible thermodynamics, was used to formulate the composites constitutive equations. The model is consistent with in-situ microscopic observations of damage mechanisms. Based on the second principle of thermodynamics, a damage state function for multiaxial static loading was proposed in terms of thermodynamic driving forces derived from Gibb's free energy. |
