In this research, a triaxial shear test is used as a means to provide yield surface data as well as other strength characteristics for carbon microballoon (CMB) syntactic foam. Additionally, pure shear tests and tensile tests are used to probe areas of this stress space not included in the triaxial shear tests. The data are used to characterize the material's yield strength in stress space. The determined yield surface, the strain and other deformational behavior characteristics provide the necessary information for an accurate model and engineering design. The CMB foam specimens were divided into two sets: one with Thornel pitch-based carbon fibers and one without; both use Kerimid 601 as the binder, The CMB syntactic foam with fibers exhibited lower shear strength than the CMB syntactic foam without fibers. This is evident not only in the determined shear envelopes but also in the values obtained for the hydrostatic yield of both foams. Complementary analysis of the blending process of mixing fibers with CMB has been shown to destroy the microballoons and thus reduce the foams strength. The consequences of incorporating alternative materials can be verified with further testing. (c) 2006 Springer Science + Business Media, Inc.