Comparing In Situ and Bulk Constitutive Properties of a Structural Adhesive

In the continuing quest for more efficient designs, structural adhesives are being used in place of, or with, traditional fastening methods; however designing with adhesives is refined as traditional methods. To obtain the adhesive design properties, tests are often performed on bulk tensile and bonded shear specimens. Questions remain about the relationship between properties obtained from in situ adhesive joints and bulk adhesive specimens. As a result, an experimental plan was developed which characterized both the linear and nonlinear region of bulk and in situ adhesive performance of a two-part acrylic adhesive from Dow Chemical Company. A standard uniaxial tensile test was used for the bulk normal, while an Iosipescu shear test was used to characterize the bulk shear performance. In situ testing was performed on a napkin-ring specimen loaded in both tension and torsion. Stress-strain relationships in both shear and normal were developed and bulk and in situ adhesive performance was compared. Observations from testing were:

1. Bulk shear and in situ shear tests showed similar performance in both the linear and nonlinear regions.
2. Modulus of elasticity in bulk adhesive tests was similar to the effective modulus of elasticity in in situ tests.
3. Prediction of normal yield strengths of the in situ adhesive through simple failure theory models proved to be inaccurate. Stress singularities, loading imperfections, and potentially a hydrostatic sensitivity were considered possible explanations.
4. Adhesive showed sensitivity to voiding and surface flaws when loaded in a tensile configuration, refinement of specimen fabrication minimized these effects.