Elasticity Measurement at High Temperatures

Abstract

A magnetostrictive delay line technique is used to excite the resonant modes of vibration of thin discs. The temperature variations of Poisson's Ratio and Young's modulus are measured using these modes. The elastic constants of a variety of grades of graphite, including pyrolytic graphite (which was found to have a negative Poisson's Ratio parallel to the basal plane) have been measured up to a temperature of 1000°C. Tables of a nomialised frequency parameter for thin discs are given covering the ranges of Poisson's Ratio -0.5 to +0.5 in steps of 0.01 to enable the calculation of Poisson's Ratio and Young's modulus from the thin disc resonant frequencies. The delay line technique was found to be applicable to the excitation of end resonances in cylindrical solids. Experimental evidence is given of end resonant modes having 2 or more nodal diameters. Comparisons are made between the end resonant frequencies and those of the corresponding thin disc modes. It was found that in all these cases the end resonant frequency was below the cut-off frequency. A technique of elastic constant measurement at high temperatures, complementary to the resonant thin disc method is given. This consists of a double pulse, time of flight method which does not require a high material Q and easily lends itself to automation. A design for an instrument is presented which automatically tracks the variation in time of flight resulting from the temperature change of the material. The method is demonstrated by measuring the variation of the Young's modulus of Thoriated Tungsten up to 1800°c.