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

Brittle materials, as advanced ceramics, present a weak behaviour under ten sile 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 m ake testing more complex. Uniaxial tensile tests are very difficult to reac h out, and alternative tests have been proposed. Testing at high strain rat es with three or four point bending tests or the diametrical compression of short cylindrical specimens are widely used. In those tests the stress sta te is not uniaxial and if the equilibrium in the specimen is not reached, i nterpretation of results becomes more complex. For that reason a new testin g method has been proposed, the spalling test of long bars. This test can p rovide the tensile strength of brittle materials at high strain rates in un iaxial conditions. In this work, the experimental set-up developed in the D epartment of Materials Science of the Polytechnic University of Madrid is p resented. The experimental device is based in the wave propagation in long rods and its reflection in a free end. In this paper, the equipment develop ed and used is explained in detail. With high-speed photography and an anal ytical analysis, the tensile strength can be measured. The stress wave refl ection at the free end of the specimen bar causes the failure. This reflect ion progress is simulated by an analytical tool, and the stress state at th e moment of rupture and the position of the first crack are measured, provi ding the maximum tensile stress in the material. It is also described the m ethod 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.