Review of the literature shows that varying degrees of complexity need to be incorporated in constitutive models, depending on the composite system studied. In some studies only the addition of viscoplastic strain was needed to make good predictions of material behavior. In others, damage only appears to affect the elastic strain. For certain materials only linear viscoelasticty, where all nonlinearity came from damage, was needed.
When approaching a new material, one frequently generates stressstrain data from constant load-rate or strain-rate tests, cyclic load/unload tests and ramp to failure experiments. A proposed constitutive model is applied which captures the effects seen for some limited amount of data. Complexity is built in as necessary to explain results from all of the experiments. Finally, some ‘validation’ experiments are run where the loading history, or perhaps in the case of composites, a different laminate is tested to justify the
constitutive model used.
In terms of durability, material behavior over long times or many fatigue cycles is needed. Certain material behavior, such as viscoelasticity, may seem negligible over typical time-frames used for tests in the laboratory if standard rate-type loadings are used. However, in ten or fifteen years neglected strains may become significant. The time-dependent microcracking detected in 90o material, discussed in Chapter 7, is a good example of where rate-loadings do not give any indication of time-dependent effects.
Here we take the opposite approach and leave as much material complexity in place as possible so that testing methodologies will have the widest applicability. Experiments are used that emphasize the time-effect, although the change in creep strains measured over the short time-frame of testing is small. These methods are evaluated using a composite which displays all of the mentioned complexities.
This work uses the theory previously established by Schapery (1999) to develop experimental and data analysis methods for isolating the softening effects of nonlinear elasticity, nonlinear viscoelasticity, viscoplasticity and damage. Damage enters through internal state variables. If all these mechanisms
are significant, the author is not aware of any existing method to extract both the damage evolution and to differentiate its effect on the material parameters from that of stress based solely on stress-strain information. A direct measure of microcracking is needed to help separate these effects. A major focus, therefore, was to develop relatively short-term experimental and data analysis methods for determining which material complexities have a significant effect on material behavior. The major difficulty is separating
the intrinsic effect of stress from that of damage on the nonlinear viscoelastic (NLVE) behavior. This problem was addressed with three concurrent
- Develop experiments and numerical data analysis methods to fit strain data which are affected by hereditary damage effects.
- Conduct a Damage Effect Study to identify which nonlinear material parameters are affected by damage.
- Develop a real-time nondestructive method to monitor damage growth.
The focus of the first effort was to assess the effect of damage on each material parameter, in particular the parameter which causes hereditary damage effects. The material displays a fading memory of the loading path with which it arrived at a given damage state. Testing methodology and methods of data analysis were devised without removing any material complexity. The Damage Effect Study was designed to determine which material parameters are affected by damage. This information can then be used to simplify the
analysis. Acoustic emission monitoring was used in the third effort to track how damage evolves with different loading histories and load combinations. With this knowledge, the state of damage in the material will be known for any loading and can be used to tie together the first two efforts.
Related Publications: Bocchieri, R.T. and Schapery, R.A., “Nonlinear Viscoelastic Constitutive Equations for Carbon/Epoxy Composites and their Correlation through Micromechanics,” Proc. 2nd Int. Conf. on Composite Materials for Offshore Operations, Houston, pp. 513-536, American Bureau of Shipping, 1999.
Bocchieri, R.T. and R.A. Schapery, “Nonlinear Viscoelastic Behavior of Rubber-Toughened Carbon and Glass/Epoxy Composites,” Time Dependent and Nonlinear Effects in Polymers and Composites, ASTM STP 1357, 2000, pp. 238-265.