SED Method of Measuring Yield Strength of Adhesives and Other Materials

Abstract

The strain energy density (SED) method of determining the yield strength of adhesives and other materials that is based on the fundamental principles of mechanics is described in this paper. The results from two sets of tests are reported. In the first set, the yield strength of multiple specimens made of the adhesive FM 300K was determined in the dry/cold, moderate, and humid/hot environments; the strain rates were 10−2/s, 10−4/s, and 10−6/s. In the second set, single specimens of five other adhesives were tested in the dry/cold and humid/hot environments; the strain rate was 10−4/s. Fifty-three out of the 60 specimens have yield strengths calculated with the SED method that are equal to or lower than the values obtained with the 0.2% offset method suggested in the ASTM specifications. The SED method is most valuable when the stress-strain curve of the material exhibits a gradual transition from elastic to the inelastic behavior typical of polymers, aluminum, and some very high strength steels. To ensure the safety of bonded structures, the yield strength of adhesives should be determined with the SED method, which is based on the fundamental principles of mechanics, instead of on an arbitrary estimate of strain offset. Although applied in this paper to stress-strain curves of adhesives, the SED method may also discern the onset of yielding and instability in columns and beams. To ensure that the yield strength of adhesives for bonding load-bearing structures is not systematically overestimated, the authors recommend using the SED method in lieu of the 0.2% offset method.