Elevated Temperature Axial and Torsional Fatigue Behavior of Haynes 188

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Abstract
The results are reported for high-temperature axial and torsional low-cycle fatigue experiments performed at 760° C in air on thin-walled tubular specimens of Haynes 188, a wrought cobalt-base superalloy. Data are also presented for mean coefficient of thermal expansion, elastic modulus, and shear modulus at various temperatures from room to 1000° C, and monotonic and cyclic stress-strain curves in tension and in shear at 760° C. This data set is used to evaluate several multiaxial fatigue life models (most were originally developed for room temperature multiaxial life prediction) including von Mises equivalent strain range (ASME Boiler and Pressure Vessel Code), Manson-Halford, Modified Multiaxiality Factor (proposed in this paper). Modified Smith-Watson-Topper, and Fatemi-Socie-Kurath. At von Mises equivalent strain ranges (the torsional strain range divided by 3, taking the Poisson’s ratio to be 0.5), torsionally strained specimens lasted, on average, factors of 2 to 3 times longer than axially strained specimens. The Modified Multiaxiality Factor approach shows promise as a useful method of estimating torsional fatigue life from axial fatigue data at high temperatures. Several difficulties arose with the specimen geometry and extensometry used in these experiments. Cracking at extensometer probe indentations was a problem at smaller strain ranges. Also, as the largest axial and torsional strain range fatigue tests neared completion, a small amount of specimen buckling was observed.