Special Problems Associated with Boron-Epoxy Mechanical Test Specimens Available to Purchase

Boron-epoxy composite material specimens were developed and tested to provide basic mechanical property data for design of the Grumman R&D FB-111 boron composite wing box extension. Special problems associated with tension, compression, and shear tests are presented in this paper. Considerable development was required to achieve consistent results even in the apparently simple case of the tension of a unidirectional laminate. Specimen geometry and loading tab configuration have a considerable effect upon the failure stress and the location of the fracture origin, and several designs were tried before consistent failures were produced in the central portions of the specimens. Problems were encountered also in testing compression coupon specimens. “Brooming” failures occurred when the specimen ends were not encapsulated securely. Considerable scatter evident in early tests indicated that compression specimens failed prematurely due to combined flexure-transverse shear and eccentric loading effects. This was confirmed by theoretical analysis. The testing of flat panels in shear by the standard picture frame method presented difficulties. An anisotropic finite element analysis yielded stresses which were in good agreement with strain gage readings but indicated a isevere stress concentration at the corner radius. Additional shear data were obtained from a modified picture frame method, the rail shear test. Experience gained in these programs indicates that the testing of high-strength, high-modulus, anisotropic composites with low normal shear properties and negligible ductility lead to special problems in the design and analysis of specimens.