Effect of environment on thermomechanical fatigue life
Introduction
Cyclic loading of metallic engineering materials at high temperature is known to cause a complex evolution of damage which can hardly be described in an unique, simple and straightforward way. This statement holds true even more strictly in the case that a component is subjected to thermomechanical fatigue (TMF) loading conditions which often result in service from a combination of thermal transients during startup and shutdown with mechanical strain cycles. Thermomechanical fatigue (TMF) is considered to be the primary life-limiting factor for engineering components in many high-temperature applications. Depending on the location of the volume element considered and the cooling situation, various types of strain–temperature phasing can arise. In laboratory testing, most commonly the two extreme cases, in-phase (IP) and out-of-phase (OP) TMF, are studied.
Due to the possibly reduced TMF life as compared to isothermal low-cycle fatigue (LCF) life, it is important to investigate the TMF behaviour of engineering materials in order to predict thermomechanical fatigue life correctly. On the other hand, extensive LCF data, which is traditionally used for design purposes, has been generated on several high-temperature materials, and thus, it is tempting to try to predict TMF life based mainly on isothermal LCF data. However, it is well known that depending on the actual loading conditions damage can evolve differently [1], [2], and life prediction may well be non-conservative if the relevant damage mechanisms are not captured accurately. Creep, fatigue and environmental effects (often simply termed oxidation) are considered to be the main damage contributions. The individual extents of these damage constituents and their mutual and synergistic interactions depend very strongly on the loading conditions applied and the material considered.
A well-known and commonly accepted classification of the TMF life behaviour of metallic engineering materials was proposed by Nitta and Kuwabara [3] and is illustrated in Fig. 1. In case of Type I behaviour, creep damage dominates. Therefore, the combined effect of high temperature and tensile stresses during IP loading is more detrimental as compared to OP, where high temperature occurs in compression. Environmental effects may lead to an embrittlement of the surface or the surface-near area giving rise to an early crack initiation if high tensile loads coincide with low temperatures. Consequently, cyclic life under OP conditions may drastically be reduced (Type O behaviour). Finally, if neither environmental effects nor creep damage dominate, IP and OP loading should roughly lead to similar life (Type E behaviour).
The intention of this article is to shed light on the influence of environmental effects on the cyclic life under TMF conditions. For this purpose results are reported from studies, which have been performed in the author's laboratory [4], [5], [6], on three materials which show typical, specific and different behavioural patterns. For the characterization and quantification of environmental effects mainly two methods were employed. The first method is to carry out comparative isothermal and thermomechanical fatigue tests both in laboratory air and high vacuum. The second concept is to predict TMF life on the basis of isothermal results applying suitable fatigue life assessment models, which can be expanded to non-isothermal conditions. The advantage of the combination of these two methods is that those environmental effects can be distinguished which (i) occur during isothermal fatigue, (ii) take additionally place under TMF condition and (iii) act very sensitively to temperature–strain phasing (OP or IP).
Section snippets
Environmental effects and TMF life prediction
Despite the fact that a variety of life prediction models have already been proposed for TMF (e.g. [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]), environmental effects are mostly not explicitly included. Rather, the damage resulting from the reaction with the surrounding gas phase is treated in a global way as part of the time-dependent damage. Hence, a simple possibility to consider environmental effects is already given by the frequency
Environmental effects on TMF behaviour of AISI304L
The isothermal fatigue behaviour of the austenitic stainless steel AISI304L is characterized by an unusual course of the saturation stress amplitude with temperature for tests at constant plastic strain amplitude. While at low temperature (up to 150 °C) the cyclic stress–strain response is determined by a deformation-induced transformation of austenite into martensite [31], the intermediate temperature range (250 °C < T < 525 °C) is characterized by the process of dynamic strain aging (DSA) [32]. DSA
Conclusions
In order to illustrate the variety of environmental influences, which can take place during thermomechanical fatigue of metallic engineering materials, and the resulting effects on TMF life, three different materials were considered. The austenitic stainless steel AISI304L serves as an example for moderate significance of environmental effects. The main damage contribution through the interaction with the surrounding air as compared to vacuum is the preferential grain boundary oxidation leading
Acknowledgement
The author is very grateful to Deutsche Forschungsgemeinschaft for the financial support of most of the studies which this article is based upon.
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