Non Contact Mechanical Testing at High Temperature Using Electromagnetic Forces.

Ultra high temperature ceramics (UHTCs) recently captured interest as potential materials for reusable thermal protection systems and other components in future generation supersonic and hypersonic vehicles where temperatures can reach > 2000 °C. A novel method for mechanical testing of UHTCs at such ultra high temperatures is developed utilizing electromagnetic force. Resistively heated and self-supported specimens in thin ribbon geometry under application of a transverse magnetic field undergo flexural stress from the electromagnetic Lorentz forces, which act as a distributed mechanical load and deform the specimen. This non-contact technique, termed Electro-Magnetic Mechanical Apparatus (EMMA), allows performing rapid tests in a low cost table-top apparatus at temperatures, as high as 2200 °C, otherwise impossible to achieve. The flexibility of this method offers ample opportunity to explore a wide range of mechanical properties. For example utilizing a DC current for resistive heating with a DC magnetic field creates constant loads for Creep testing; replacing with a AC current generates cyclic loads for Fatigue testing; larger magnetic fields can be used for Fast – Fracture experiments; and impulse excitation of the magnetic field vibrates the specimens and enables the determination of the material’s Elastic and Loss Modulus. Zirconium Diboride and Silicon Carbide (ZrB2-SiC) is a prominent member of UHTCs. The creep properties of this composite are explored using this technique in the temperature range 1600 – 2200 °C under stress ranging from 20 – 50 MPa in ambient air as well as non-reactive Nitrogen atmosphere. The kinetic parameters of creep, activation energy and stress exponent are established in the testing range. The creep response from the two environments is compared to understand the effect of the concomitant oxidation during high temperature testing in air. Comparison of creep data from conventional 3-pt, 4-pt flexure tests corroborate the results obtained from EMMA and validate the use of the technique to obtain comparable creep rates.Ph.D.Materials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91586/1/sindhu_1.pd