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The Influence of Electric Current on Crack Propagation in Thermal Fatigue Tests.
Final rept. 1 Aug 84-31 Jul 85,
CARNEGIE-MELLON UNIV PITTSBURGH PA DEPT OF MECHANICAL ENGINEERING
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The use of a benchmark or model problem to estimate errors in stress intensity factors and local temperatures in thermal-mechanical fatigue testing which uses large electric currents for heating has shown that this mode of testing results in a more severe stress state than if conventional heating methods are used. The reason for the more severe stress state is that the application of a constant voltage across a cracked specimen results in a singular current and, consequently, a singular heat source. The singular heat source does not result in a temperature singularity, however, so the temperature remains bounded at the crack tip. The relative discrepancy between conventional heating fatigue testing and resistance heating fatigue is quite dependent on crack size and materials properties primarily thermal and electrical conductivity and linear thermal expansion. Materials exhibiting low electrical resistance and high thermal expansion are, in general, more susceptible to this mode of thermal loading. Simple expressions for estimating the contribution of electric current to the stress intensity factor and local temperature field were developed. These expressions are estimated in light of some basic assumptions made during problem formulation. The validity and consequence of some of these assumptions are discussed.
APPROVED FOR PUBLIC RELEASE