A wave propagation technique to measure the dynamic tensile strength of brittle materials

Abstract
Brittle materials, as advanced ceramics, present a weak behaviour under tensile loads. For that reason, determination of the tensile strength in these materials is needed. A testing methodology to measure this property is not well defined, more highlighted at high strain rates, where dynamic loads make testing more complex. Uniaxial tensile tests are very difficult to reach out, and alternative tests have been proposed. Testing at high strain rates with three or four point bending tests or the diametrical compression of short cylindrical specimens are widely used. In those tests the stress state is not uniaxial and if the equilibrium in the specimen is not reached, interpretation of results becomes more complex. For that reason a new testing method has been proposed, the spalling test of long bars. This test can provide the tensile strength of brittle materials at high strain rates in uniaxial conditions. In this work, the experimental set-up developed in the Department of Materials Science of the Polytechnic University of Madrid is presented. The experimental device is based in the wave propagation in long rods and its reflection in a free end. In this paper, the equipment developed and used is explained in detail. With high-speed photography and an analytical analysis, the tensile strength can be measured. The stress wave reflection at the free end of the specimen bar causes the failure. This reflection progress is simulated by an analytical tool, and the stress state at the moment of rupture and the position of the first crack are measured, providing the maximum tensile stress in the material. It is also described the method to determine the tensile strength based on the data obtained from the instrumentation of the device and the measurements in the pictures taken. The results obtained in long rod specimens of alumina are presented.