Specimen for biaxial tension test on hard foams

The balanced biaxial tensile loading test on hard foams is significant for the fitting of extended strength hypotheses [1]. This test can be easily realized with a conventional tensile testing machine [2], e. g. ZWICK / Z020. This simple loading scheme requires a special specimen: a cruciform specimen or an octuple notched waisted disc. The minimum thickness of the specimen in the test area should be larger than s = 8 mm to treat the cellular structure of the material as quasi-homogeneous. The diameter of the test area is set equal to d = 5 s. This dimension is sufficient for the recording of the displacements with a CCDcamera during the tests. The regions at the boundary of the test area (Saint-Venant distance) must be excluded from the evaluation. The external dimensions of the specimen is limited by the distance 400 mm between the grips of the testing machine with D = 240 mm and the thickness of the foam block t = 80 mm. For thicknesses in the range t = 40 - 80 mm (range R 20, DIN 323-1, 1974) optimal geometries have been determined for both forms of specimens. The stress relation is taken in the middle of the test area and in the notches between the force applications as the optimization criterion. It has been found that the octuple notched waisted disc provides a 5-7% better stress relation compared to the cruciform specimen. The stress relation for both geometries of more than 1.2 can be achieved if the specimen fails rather ductile. For brittle behavior of foams with the inelastic Poisson's ratio less than 0.1 no optimal shape could be found for the specimen to ensure failure in the test area.