Techniques to characterize the mechanical properties of adhesives in a bonded geometry are important because the behavior can differ from that of bulk adhesive samples. Although there are a number of such tests, the simplicity of the sandwich beam specimen makes it attractive. This test involves three-point bending of a beam made by bonding together two metal strips with the adhesive. For linear elastic materials, the shear modulus of the adhesive can be calculated from the bending stiffness of the beam with a number of published analyses. This paper conducts experiments to examine the sandwich test and the potential to extend it to viscoelastic adhesives since standard viscoelastic test equipment can easily measure three-point bending. The results show that the stiffness of the sandwich beam is sensitive to the presence of the adhesive throughout most of the interesting range for adhesive properties. Second, when tested as a function temperature and time (or frequency), the stiffness of the beam behaves like a classic viscoelastic property. Finally, in the rubbery, elastic range, the shear moduli calculated from beam tests with thick bonds agreed with those obtained from tests on bulk samples for adhesives. In the temperature range where the adhesive is hard (glassy behavior), however, problems were observed for the geometries tested here. Some of these problems may be attributable to assumptions made in the analyses used, and this suggests the need for a new analysis that addresses these limiting assumptions and extends the test to viscoelastic materials.
- Shear modulus