| The objective of this research is to develop an approximate theoretical model of the time dependent deformation and failure in graphite fiber composites. To this end, Classical Laminate Theory (CLT) in conjunction with a nonlinear viscoelastic (VE) relation will be employed to model time-dependent deformation of a composite specimen. The nonlinear viscoelastic constitutive relation is given in terms of creep-compliance. A primary advantage of using a creep compliance relation, as compared to stress relaxation relation, is that the creep/recovery tests are simpler to perform. The material properties and nonlinear parameters of the constitutive relation are experimentally determined using both constant load rate and creep/recovery tests at various levels of constant stress on uni-directional [0°]10s, [90°]10s, and off-axis [10°]10s specimens. Since CLT requires stiffness, the creep-compliance relation will then be employed in a numerical integration scheme for determining stress-relaxation. Since preliminary experimental evidence seems to indicate a unique critical failure strain in the material direction for various specimen types, the maximum strain criterion will be used to determine time dependent failure of lamina. The implementation of the utilized to predict the response of [+/-55°]5s and [+/-35°]5s flat laminate specimens under uni-axial loading. The theoretical results are compared to corresponding experimental results to demonstrate the viability of the modeling approach. |
