A BASELINE NONLINEAR MATERIAL CHARACTERIZATION FOR PREDICTING THE LONG-TERM DURABILITY OF COMPOSITE STRUCTURES
- Paper ID
IAF-98-W.1.07
- author
- company
T he U niversity of Texas at A ustin, D epartm ent of Aerospace Engineering and Engineering M echanics
- country
U.S.A.
- year
1998
- abstract
A previously-developed constitutive equation for nonlinear viscoelastic materials, which accounts for both viscoelasticity and changes in microstructure, is used to mathematically characterize the ply-level stress-strain behavior of a rubber-toughened carbon/epoxy composite. Experimental techniques are used to derive the initial constant stress rate response of the material in the dry state, at room temperature. With these conditions, a baseline for isolating the effect of damage growth from intrinsic viscoelasticity and for determining temperature and moisture dependence is established. Property dependency on all of these factors will be needed to make long-term durability predictions and to develop accelerated testing methodologies. It is found for a large portion of the loading curve that the shear and transverse compliances can be described by a quasi-elastic model with a single scalar function of stress state, a ratio of compliances, a time or rate exponent, and two elastic terms. This model offers a simple way to incorporate nonlinearity and time dependence in a material model for limited loading conditions. Special considerations for using off-axis coupons are examined and unique tabbing is recommended to derive properties out to high stress levels. Finally, the consistency of the material characterization is checked by comparing experimental stress- strain response to a predicted response based on averaged properties put in the mathematical model.