Characterizing and Modeling the Effects of Mode Mixity and Bond Line Thickness on the Traction-Separation Response of Structural Adhesive Joints

As adhesive bonding continues to become more prevalent in automotive structures, the need for high fidelity characterization and modelling of adhesive joints has gained in importance. In this thesis, new specimen geometry and analysis techniques are presented to characterize the properties necessary to model the traction-separation response in Mode I, Mode II, and mixed mode (MM) loading of adhesive joints using cohesive zone modelling (CZM) techniques. First, the Rigid Double Cantilever Beam (RDCB) geometry and analysis technique was developed to measure the full traction-separation response of a structural adhesive under Mode I loading. This test approach represents a substantial improvement over current test methodologies, where additional tests are required to extract the parameters necessary to construct the full traction-separation response. Next, the Bonded Shear Specimen (BSS) sample geometry was developed to measure the Mode II traction-separation response, using optical methods to measure separation. The BSS test was then adapted to allow for measurement of MM loading, providing the full traction-separation response for a range of mode mixity (ratio of shear to normal separation) and bond line thickness combinations. Statistically significant differences between the parameters necessary to construct the traction-separation response were found for different bond line thicknesses, identifying that bond line thickness should be accounted for in a generalizable CZM definition. With the full traction-separation response characterized for Mode I, Mode II and MM loading of nominal bond line thicknesses ranging from 0.18 mm to 0.64 mm, several deficiencies in current CZM implementations were identified. Critically, the MM traction-separation response is poorly predicted using conventional CZM implementations and the hardening response exhibited by the adhesive during Mode II and MM loading is not represented in current implementations. Validation testing was undertaken using more traditional Tapered Double Cantilever Beam (TDCB) and Single Lap Shear (SLS) test specimens to provide independent test data with which to assess CZM models integrating the characterization data from the RDCB, BSS and MM specimens. These validation tests were selected due to the well controlled nature of the Mode I (TDCB) and MM (SLS) loading. One limitation of these tests, however, is the large levels of elastic and plastic deformation of the adherends, making the measured forc