Safe structural design for fatigue and creep using cyclic yield strength

Gorash, Yevgen and Mackenzie, Donald (2014) Safe structural design for fatigue and creep using cyclic yield strength. In: ECCC - Creep and Fracture Conference 2014, 2014-05-05 - 2014-05-07.

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    Abstract

    This study proposes cyclic yield strength (CYS, σ_cy) as a potential characteristic of safe design for structures operating under fatigue and creep conditions. CYS is defined on a cyclic stress-strain curve (SSC), while monotonic yield strength (MYS, σ_my) is defined on a monotonic SSC. Both values of σ_cy and σ_my are identified using a 2-step fitting procedure of the experimental SSCs using Ramberg-Osgood and Chaboche material models. A typical S-N curve in stress-life approach for fatigue analysis has a distinctive minimum stress lower bound, the fatigue endurance limit (FEL, σ_f-lim). Comparison of σ_cy and σ_f-lim reveals that they are approximately equal. Thus, safe fatigue design is guaranteed in the purely elastic domain defined by the σ_cy. A typical long-term strength (LTS) curve in time-to-failure approach for creep analysis has 2 inflections corresponding to the σ_cy and σ_my. These inflections separate 3 sections on a LTS curve, which are characterised by different creep fracture modes and creep deformation mechanisms. Thus, safe creep design is guaranteed in the linear creep domain with brittle failure mode defined by the σ_cy. These assumptions are confirmed using 3 structural steels for normal and high-temperature applications. The advantage of using σ_cy for characterisation of fatigue and creep strength is a relatively quick experimental identification. The total duration of cyclic tests for a cyclic SSC identification is much less than the typical durations of fatigue and creep rupture tests at the stress levels around the σ_cy.