OBJECTIVE: Develop an analytical method, failure criteria, and required material test methods to enable accurate predictions of failures on bonded composite structures. DESCRIPTION: Modern research has attempted with some success to develop a mixed mode failure theory for adhesive bonds and ductile metals. The ABAQUS finite element software currently used within the Navy includes a cohesive zone element that represents either a composite layer"s interlaminar strength or an adhesive layer"s stiffness and strength. This element has several ways to combine tension and shear into a failure index, but no way to combine compression and shear or account for the multi-axial directions of these stresses. The Defense Science and Technology Organization in Australia performed some testing and observed the behavior of a structural adhesive under complex loading. The correlation to existing yield criteria was not particularly good but it highlighted the effect of combined stresses and need to account for them in a design. The objective of this topic is to advance the state of the art by identifying a failure theory with a basis in basic material properties that can be employed during structural analysis and will result in quantified margins of safety for bonded joints. The combined failure theory should include, at the minimum, the full range of complex loading, the inclusion of that theory into a finite element analysis application, the test methods required to generate the needed material properties, and finally the validation of the method through the analysis and test of complexly loaded bonded structures. It should include all of the potential failure modes like adhesive failure, cohesive failure, and first-ply delamination of the composite adherands. The method shall be compatible with commercially available finite element analysis software. Identification of material properties needed to support the analysis, and identification of, or development of appropriate test methods to measure those properties is an important part of the research. The research shall include the ability to quantify the effects of moisture and temperature into the analysis. Lastly, the research shall include demonstration of the ability to successfully predict failure of several distinct joint geometries and stress states. PHASE I: Prove feasibility of a bonded joint failure theory for predicting cohesive, adhesive and adherand failures based on material characterization and finite element analysis methods. PHASE II: Develop, demonstrate and verify the failure theory, and its implementation, with several types of adhesives and joint geometries. User defined materials, elements, or other modifications consistent with the finite element software"s standard methods for accommodating user unique requirements are acceptable at this phase. PHASE III: Verify failure theory against a larger variety of joint geometries, materials, and loadings. Implement in a finite element analysis software"s (such as ABAQUS) standard library of options. PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Bonded joints and their analysis are equally important to the department of defense and the private sector. There is also commercial potential for the analytical software companies that implement the method into their finite element analysis products.