A test method has been developed and demonstrated for the determination of shear strength in a lap joint with rubber-like material as a substrate. A testing fixture used a stiffened aluminum substrate with a load-transferring lip, combined with a clamp, that restrained the lateral movement of the rubber. This fixture successfully produced shear-type failures on two back-to-back single lap-joint specimens. The design of the fixture was based on the near-incompressible nature of the rubber materials. The experimental part of the task included two phases: a pathfinder test series; and the actual data generation test series. The former phase involved testing concept development, test fixture design and modification, optimization of test specimen configuration, and failure mode demonstration. This phase was conducted using specimens fabricated in-house by the Mechanics and Materials Technology Center at The Aerospace Corporation. In the second phase, actual shear failure data were generated using specimens that were machined from panels fabricated by the vendor, Elkton Division, Tactical Operations of the Thiokol Corporation.
Linear elastic finite element analyses were performed to assist in the optimization of test parameters. The results identified optimal dimensions for lap-joint length, transverse groove width, and bracket lip width for the specimen and fixture configuration. Sixteen tests were done on specimens machined from vendor-supplied carbon phenolic/Epon EA 934 NA/ethylene propylene diene monomer (EPDM) panels. The average failure stresses were 4.46, 3.39, and 3.32 MPa (0.56, 0.43, and 0.42 ksi) for the 7.62, 10.16, and 12.70-mm (0.3, 0.4, and 0.5 in.) long lap-joints. Some of the failures were initiated inside the adhesive whereas other failures occurred between the adhesive and the substrate.
This test method should be considered as a viable technique in determining shear strength of bond line with rubber-like material as a substrate. More refinement, however, is needed before an optimal test fixture design can be reached.