A failure condition is proposed for the composite material “syntactic foam” based upon an experimental program carried out under combined biaxial and triaxial stress states. The material itself is composed of hollow glass “microspheres” embedded in an epoxy resin matrix. It exhibits finite strength under hydrostatic compression, unlike any other engineering material and thus gives rise to a closed failure surface in principal stress space. In addition to loadings corresponding to radial lines starting from the origin in stress space out to the failure envelope, other load paths were also used. Some effects due to loading and unloading and then reloading to another stress state were also investigated. Under biaxial stress states it was found that the ultimate strength of the material is sensitive to its past stress history to some degree, the reason being attributed to the initially isotropic material developing weak planes perpendicular to the compressive principal stress resulting in a “latent anisotropy.”

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