Dwell Effects on High Temperature Fatigue Behavior: Part I

Effects of dwell times on creep–fatigue behavior of the following materials: (a) pure metals; (b) solder alloys; (c) copper alloys; (d) low-alloy steels; (e) stainless steels; (f) titanium alloys; (g) tantalum alloys; and (h) superalloys have been examined from the published data. The melting temperatures of these materials varied from low (over room temperature) to very high temperatures (above 1600°C). The temperature at which creep–fatigue tests were conducted varied from 0.5 to as high as 0.8 of the homologous temperature. Within this temperature range creep and fatigue processes interact and failure occurs in the low-cycle regime. The creep–fatigue data were compiled from the published literature. Dwell sensitivity, the beneficial or detrimental effect of dwell cycles, can be presented in a dwell-sensitivity map. Normalized cycle ratio (NCR), and total strain range were used to construct such maps. A high NCR (≥1) at a given strain range demonstrates high fatigue resistance and vice versa. Generally, the NCR increased with an increase in strain range, dwell time and test temperature. However, as the strain range decreased and dwell time increased, the NCR decreased. Mean stress evolution in either tension or compression directions was correlated with dwell-sensitivity behavior. Materials were sensitive to a particular test condition depending on temperature, strain range, strain rate, as well as time and direction in which a dwell was applied. In general, dwell cycles decreased the cyclic life. However, a beneficial effect was observed for a few copper alloys and superalloys. The mechanisms controlling deformation and failure for all these materials are discussed in an accompanying paper (Paper II) separately