Evaluation of a linear cumulative damage failure model for an epoxy adhesive

EVALUATION OF A LINEAR CUMULATIVE DAMAGE FAILUREMODEL FOR AN EPOXY ADHESIVEDavid E. Richardson, Alicia Batista-Rodriguez, David Macon, and Peter TotmanThiokol Corporation of Cordant Technologies Inc., Brigham City, UT 84302-0707ABSTRACTRecently a significant amount of work has beenconducted to provide more complex and accuratematerial models for use in the evaluation of adhesivebondlines. Some of this has been prompted by recentstudies into the effects of residual stresses on the integrityof bondlines. Several techniques have been developedfor the analysis of bondline residual stresses. Key tothese analyses is the criterion that is used for predictingfailure. Residual stress loading of an adhesive bondlinecan occur over the life of the component. For manybonded systems, this can be several years. It isimpractical to directly characterize failure of adhesivebondlines under a constant load for several years.Therefore, alternative approaches for predictions ofbondline failures are required. In the past, cumulativedamage failure models have been developed. Thesemodels have ranged from very. simple to very complex.This paper documents the generation and evaluation ofsome of the most simple linear damage accumulationtensile failure models for an epoxy adhesive. This papershows how several variations on the failure model weregenerated and presents an evaluation of the accuracy ofthese failure models in predicting creep failure of theadhesive, The paper shows that a simple failure modelcan be generated from short-term failure data for accuratepredictions of long-term adhesive performance.INTRODUCTIONRecently a significant amount of work has beenconducted to better characterize the material properties ofadhesives. The programs that are conducting thesestudies recognize the need for more complex andaccurate material properties for use in predicting thestructural integrity of several critical bonded systems.For many adhesives, both the constitutive and failureproperties are very much time and temperaturedependent. _'-''3Simple linear constitutive models andsingle valued failure models are not appropriate for allapplications. For example, evaluations of bondlineresidual stresses can be complex. Several techniqueshave been developed for the analysis of bondline residual