Viscoelastic adherend effects in adhesively bonded composite-to-composite joints

A complex state of stress is present in adhesive joints. With composite adherends, this can cause a considerable amount of creep even in the absence of an extreme operational environment. The present research is centered around characterizing a composite polymeric adherend material and studying, experimentally and theoretically, its reaction in one adhesive joint configuration. The particular material investigated is a fibrous graphite/epoxy composite material with a rubber-toughened matrix. The composite is nonlinear and viscoelastic at room temperature. The objective of the study is to develop a constitutive equation to account for this behavior and to use it in a finite element analysis of an adhesive joint. Experimental verifications of the constitutive model and adhesive joint are made.;The graphite/epoxy composite is characterized using a viscoelastic model and a work-potential model. Unlike the viscoelastic model, the work-potential model accounts for changes in the microstructure, or damage, as well as viscoelasticity. The quasi-elastic method, in which the actual deformation behavior is approximated by a time-dependent elastic behavior, is used in the second characterization to account for the time dependence of the material. Parameters necessary for each model are determined from creep/recovery and constant load rate data. Single-lap joints made of the same material and a brittle adhesive are then tested at two different load rates to failure. A brittle adhesive is used to isolate most of the viscoelastic response in the adherend material. Strains on the outer surface of the joint are recorded as well as changes in failure patterns with load rate. A commercially available finite element program is used to analyze the strains on the outer boundary. This program is augmented with an incremental constitutive law using the experimentally determined work-potential model and the quasi-elastic method to account for the nonlinearity and viscoelasticity of the adherend material.