A technique for thermomechanical fatigue testing of thin-walled tubular specimens involves the application of cyclic axial (tension/compression) and torsional (shear) strains, along with thermal cycling. In this technique, the phase relationships among the two strain waveforms and the temperature waveform are prescribed and are maintained constant throughout a test.
Heretofore, axial/torsional fatigue testing has commonly been limited to isothermal conditions, while thermomechanical fatigue testing has commonly been limited to axial (only) or torsional (only) strain. The present technique for axial/torsional thermomechanical fatigue (AT-TMF) testing makes it possible to acquire materials data on effects of time-varying thermal and multiaxial mechanical loads similar to those experienced by tubular components of engines during cyclic and/or transient operation. The data can be used, along with mathematical models of thermomechanical behavior, to predict the deformations and fatigue lives of such components.
In principle, one could choose among an infinite number of combinations of mechanical-strain and temperature waveforms; in practice, one must limit to the choice to a representative few. Four different combinations of triangular waveforms were chosen for the present AT-TMF testing technique. The waveforms in these combinations are required to be synchronized, variously, with 0°, 90°, and/or 180° phase differences. Figure 1 presents examples of the four combinations of waveforms. The cycle time and the temperature and strain limits in these examples are specific to tubular specimens (22 mm inner diameter, 26 mm outer diameter) of a cobalt-based superalloy; other limits and cycle times could be chosen to suit different specimens.
Figure 2 schematically depicts the apparatus that was used to implement the present AT-TMF testing technique on the specimens mentioned above. An axial/torsional load frame was controlled with two servocontrollers: one for the axial and one for the torsional actuator. The axial and shear strains were measured by a commercially available, water-cooled, axial/torsional extensometer. The specimen was heated by use of induction coils connected to a controllable audio-frequency power-supply unit rated at 50 kW.
The axial and torsional strains and temperature were controlled and test data were acquired by a computer system (equipped with digital-to-analog and analog-to-digital converters) connected to the servocontrollers, a temperature controller, and temperature sensors. The computer generated command waveforms that corresponded to the specified axial-strain, torsional-strain, and temperature waveforms. For each of 1,000 points during a test cycle, the computer acquired data on axial and torsional loads, strains, and strokes and on temperatures at five locations on the specimen. The computer operated with a C-language program that provided a keyboard interruption capability plus a graphical display of axial and shear stresses versus time, temperatures, and test status.
This work was done by Sreeramesh Kalluri and Christopher S. Burke of NYMA, Inc., and Peter J. Bonacuse of the Vehicle Propulsion Directorate of the U. S. Army Research Laboratory for Lewis Research Center.
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