SHEAR LAP-JOINT TESTING WITH RUBBER SUBSTRATE

A test method has been developed and demonstrated for the determinatio n of shear strength in a lap joint with rubber-like material as a subs trate. A testing fixture used a stiffened aluminum substrate with a lo ad-transferring lip, combined with a clamp, that restrained the latera l movement of the rubber. This fixture successfully produced shear-typ e failures on two back-to-back single lap-joint specimens. The design of the fixture was based on the near-incompressible nature of the rubb er materials. The experimental part of the task included two phases: a pathfinder test series; and the actual data generation test series. T he former phase involved testing concept development, test fixture des ign 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 a t The Aerospace Corporation. In the second phase, actual shear failure data were generated using specimens that were machined from panels fa bricated by the vendor, Elkton Division, Tactical Operations of the Th iokol Corporation. Linear elastic finite element analyses were perform ed to assist in the optimization of test parameters. The results ident ified optimal dimensions for lap-joint length, transverse groove width , and bracket lip width for the specimen and fixture configuration. Si xteen tests were done on specimens machined from vendor-supplied carbo n phenolic/Epon EA 934 NA/ethylene propylene diene monomer (EPDM) pane ls. 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 th e adhesive whereas other failures occurred between the adhesive and th e substrate. This test method should be considered as a viable techniq ue in determining shear strength of bond line with rubber-like materia l as a substrate. More refinement, however, is needed before an optima l test fixture design can be reached.