The Challenges of Testing Adhesives - Effects of imperfections, grip pressure, specimen shape and frequency in quasi-static and fatigue loading

Parameters influencing tensile testing of bulk structural adhesives have been investigated to better understand tab failures, fracture shapes and high-frequency fatigue testing. Dumbbell molded structural adhesive specimens with tapered aluminum tabs have been subjected to quasi-static loading at various grip pressures as well as to fatigue testing at a combination of loads and frequencies. The experimental results were capture by the force and displacement sensors of the tensile testing machine, digital image correlation plus a high-speed camera to get insight of the fracture dynamic. A finite elements model was created to validate and complement the experimental measures. Quasi-static testing showed that the occurrence of tab failure is linked to both grip pressure and tab tilt. Voids in-between the adhesive and the tabs also showed a link to tab failure. The experimental measures and finite element calculation at the edge of the narrows section both showed non-uniform axial stresses across the cross-section of the specimen. High-speed footage of the crack showed a cyclic behavior after failure resulting in the hammering of the fracture line multiple times. Finally, S-N curves obtained for 25Hz in fatigue testing did not show any significant deviation from the reference 10Hz curve. 40Hz testing had too many tab failures to draw a regression curve.