Specimen size effects in the tensile failure strain of an epoxy adhesive

Abstract

The tensile failure strain of a proprietary single-part aerospace epoxy adhesive (3M EC3448) has been investigated. Three different specimen sizes were tested, a large specimen of 1840 mm3, a medium specimen of 125 mm3 and a small specimen of 1.41 mm3. A novel form of video extensometry was utilized to measure the tensile strain and Poisson's ratio of the specimens. It was seen that the stress–strain curve was common between the specimen sizes, with the strain at failure increasing as specimen size reduced. Analysis of the defects causing failure indicated a correlation between the size of the defect and the failure strain. The data from each specimen size were fitted to a two-parameter Weibull distribution with good correlation. A simple scaling chart using the characteristic failure strains from the three specimen sizes was formulated which indicated an equivalent Weibull shape factor of 7.9. © 1998 Kluwer Academic Publishers