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The magnitude of temperature gradients and their variation with temperature and time was investigated for a thermomechanical fatigue (TMF) cycle between 400°C and 1100°C under a heating rate of 10°C/s. The measurements of the temperature distribution in the gauge section of a flat rectangular specimen of the Nickel base superalloy CMSX-6 were carried out by means of thermocouples located at various positions on the surface and in the volume, respectively. As a reference the temperature distribution was calculated by finite element analysis based on the temperature signal of the control thermocouple. Additionally, the surface temperature distribution was studied by thermographic measurements.
The results reveal that thermal gradients establish in longitudinal and in transverse direction of the gauge section. Although the temperatures in the bulk material exceed the reference given by the control signal during the entire cycle, this effect is most pronounced (+40°C) during cooling by forced air. In comparison, the predictive results of the finite element calculation lead to slightly enhanced surface temperatures during heating (+8°C) and to reduced surface temperatures during cooling (-5°C). Therefore, it is concluded that heat transport in the bulk material is sufficiently fast to guarantee almost homogeneous temperature distribution. The significantly enhanced volume temperatures in the experiment were found to result from influences of the experimental set-up, and from superimposed effects due to forced air cooling and heat flow through the wires of the spot welded thermocouple.
thermo-mechanical fatigue testing, nickel-base superalloy, finite element modeling, temperature measurement, thermographic analysis
Dr. rer. nat., MTU Aero Engines GmbH, Munich,
Professor Dr.-Ing., University of Applied Sciences, Regensburg,
EADS Airbus GmbH, Bremen,
materials engineer, MTU Aero Engines GmbH, Munich,