Combined analytical/experimental approach for developing structural model of damaged composite structures

Preliminary results of an ongoing experimental program are presented and discussed with regard to implementation in a numerical formulation. The intent of the experimentation is to quantify the effects of fiber breakage, matrix cracking, local buckling and delamination on variations in stiffness and damping characteristics. Both static and dynamic tests are performed in sequence. For example, damage is developed during tension or compression testing in stages, so that, at different damage states the stiffness and dynamic properties can be determined. Edge replications, x-rays, strains and acceleration data are all utilized in obtaining the data needed. In addition to tension and compression testing, Arcan specimens are being tested to relate shear to matrix cracking and, additionally, a sequence of impact experiments are planned in which levels of damage will be related to dynamic characteristics. Experimental data obtained in these efforts can serve to extend an existing finite element based damage model. The intent is to combine stress based failure criteria with an internal state variable representation of damage to identify failure/damage modes on a ply-by-ply basis. An incremental solution procedure is utilized with the material properties modified as damage progresses under predefined loading conditions. Experimental results are needed to improve predictions of changes in material properties. This, in turn, can provide a more accurate definition of the residual properties exhibited as a result of a given set of loads. The numerical results presented herein reflect use of the stress based criteria alone and, therefore, are not yet extended to include the internal state variable damage representation.