2D-tension loading on hard foams and ceramics

Parts made of hard foams have found wide application in lightweight constructions. The design of these components is based on the strength values gained primarily from tests on specimens under tension, compression, torsion and hydrostatic compression. Beyond this, multi-axial tension tests are required for the validation of an adjusted strength hypothesis [1]. A simple experimental setup for biaxial tension test is developed, which can be realized with a conventional tensile testing machine. The idea of the device is based on the force distribution with girders in three levels [2]. The device allows the introduction of force in the ratios 1/1, 1/2 and 3/4. The simple manner of force distribution requires a specimen in octopus form (Fig. 1). The specimen was designed in such way that the failure develops from the center (test area) to the outside. The radial dependence of the thickness from the test area outwards is C2-steady. The notches between the force insertions are chosen to be ellipses in order to reduce the stress concentrations. A linear elastic behaviour of the foams is assumed for the dimensioning of the specimen. The geometry of the specimen is optimized for the worst case with the elastic Poisson ratio equal to 0 and evaluated using the normal stress hypothesis. The maximal stress occurring in the notches are minimized by the geometry of the specimen. The dimensions of the specimen are adapted to the commercially produced foam blocks. The experimental results from tension and balanced biaxial tension are valuable additional strength information. The experiment can be used to test many different types of materials like glass, ceramics, aerated concrete, sintered materials, granular materials etc.