Bithermal Fatigue: A Link Between Isothermal and Thermomechanical Fatigue

Abstract

Many technologically important elevated temperature service cycles are non-isothermal. Nevertheless, major design codes rely on the most severe—usually the highest—temperature of an operational cycle as being the pertinent temperature upon which to base a design. Consequently, most high-temperature fatigue data for design have been generated under isothermal conditions. There is a growing awareness of the potential inadequacy of such a simplistic approach since many thermomechanical fatigue results have been found to exhibit considerably lower fatigue lives than would be expected on the basis of isothermal results at the maximum cycle temperature. Yet, variable-temperature, low-cycle fatigue tests are difficult to conduct and to interpret. The considerable gap between isothermal and thermomechanical fatigue technology can be bridged by an approach which retains the simplicity and ease of interpretation of isothermal fatigue, but captures many of the first order effects of the greater complexities involved in thermomechanical fatigue. We have developed a procedure for conducting what has been designated as bithermal fatigue experiments. In this procedure, the tensile and compressive halves of the cycle are conducted isothermally at two significantly different temperatures. The higher temperature is chosen to be in the time-dependent creep and oxidation prone regime and the lower temperature in the regime wherein time dependencies are minimized due to lack of thermal activation. Interestingly, bithermal fatigue tests prior to those performed for this paper have been conducted in conjunction with the evaluation of the isothermal Strainrange Partitioning characteristics of high-temperature alloys, not with thermomechanical behavior per se. Nevertheless, the bithermal fatigue test may well be used as an alternative to thermomechanical cycling. In this paper, we place emphasis on using the bithermal testing concept as a link between isothermal and thermomechanical testing. New bithermal fatigue data for the nickel-base superalloy B1900 + Hf are presented herein.

Bithermal low‐cycle fatigue evaluation of automotive exhaust system alloy SS409

“…The SRP approach currently does not track damage specifically due to oxidation. Nevertheless, this seemingly crucial omission has not rendered the method from being able to correlate and predict high‐temperature creep‐fatigue and thermomechanical fatigue behaviour of a wide range of types of metals and alloys in air and vacuum 4 –9 for a variety of engineering applications.…”

“…Table 1 briefly describes the isothermal (HRSC), the bithermal out‐of‐phase plastic–plastic fatigue (HROP) and the bithermal plastic–creep fatigue (CCOP) tests conducted in this study. See Refs [ 4–6] for more details on bithermal fatigue testing. Tests were conducted at various strain ranges.…”

“…Cyclic stress–strain–temperature–time response as well as cyclic crack initiation resistance is documented and used to evaluate lifing model constants 1 –3 . TMF characteristics are measured in this study using the bithermal testing techniques reported in Refs [ 4] and [ 5]. Low‐cycle TMF data were generated for the ferritic stainless steel exhaust system alloy, SS409.…”

Effect of Cycle Duration and Phasing on Thermomechanical Fatigue of Dog-Bone Specimens Made form Steel

“…In cumulative damage based models, explicit consideration of the different damaging mechanisms is carried out, (Halford et al, 1976;1988;Rees and Dyson, 1986;Neu and Sehitoglu, 1989;Lemaitre and Chaboche, 1990;Sehitoglu, 1992;Kang et al, 2007). In crack growth based models, life is related to local inelastic strains at the crack tip (Newman, 1984;Christ et al, 2003;Djavanroodi, 2008).…”

The Determination of Hysteresis Loops in Thermo‐mechanical Fatigue Using Isothermal Stress‐strain Data

“…It is therefore not surprising that investigators choose the simpler I test, arguing that data produced at the maximum temperature of the service cycle will lead to conservative lifetime predictions. The alternative is computer modelling using visco-plastic relations from I tests at selected temperature intervals [ 12-141, or to compromise experimentally by using unequal strain rates in the I tests ( e g "slow-fast" [lS]) or "bi-thermal" fatigue tests [16]. In the latter, the specimen is held at zero stress during temperature alteration and if the subtracted thermal signal is slightly imperfect, the two halves of the mechanical hysteresis loop are simply displaced by the amount of error.…”