Design of test specimens from hard foams for the investigation of biaxial tensile strength

At present parts made of hard polymer foams have many application. The design of these components is however based on the strength values gained primarily from tests on specimens under tension, compression, torsion and hydrostatic compression. Further loading conditions are required for the validation of an adjusted strength hypothesis appropriate for the description of the material behavior. This is especially important for multi-axial tension: there are almost no measurements existing that consider balanced biaxial and hydrostatic tensile loads. First, a suitable experimental setup is developed, which can be realized with a conventional tensile testing machine. In addition to the balanced biaxial stress state further biaxial tension stress states can be generated. The simple manner of force distribution requires a special form of the specimen. The following paper describes the design of two different hard foam specimens. The foam is considered as quasihomogeneous isotropic material. Linear elastic behavior of the foams is assumed for the dimensioning of the specimen. The stresses arising in the specimen under loading are assessed with the strength hypothesis of Mohr-Coulomb. The maximal stress occurring in the notches between the locations of force introduction are minimized by the geometry of the specimen, in order to assure failure in the test area. The dimensions of the specimens are adapted to the commercially produced foam blocks. Failure at hydrostatic tension unfortunately can hardly be realized. Based on the experimental results from tension and balanced biaxial tension, the estimation of the hydrostatic stress is proposed and can be is also considered in the dimensioning of the test specimens.