PSI - Issue 7

L.L. Liu et al. / Procedia Structural Integrity 7 (2017) 174–181

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L. L. Liu et al. / Structural Integrity Procedia 00 (2017) 000–000

Fig.4 SEM observations of fracture surfaces of specimens.

The Smith-Watson-Topper (SWT) model is widely used to account for the mean stress effects in fatigue lifetime prediction. The SWT model considers the maximum stress based on the Manson-Coffin formula as (1) shown, in which the σ max is added to correct the influence of the mean stress [20] . For a given fatigue lifetime, the maximum stress σ max and strain amplitude ε a change, while their product σ max ε a (damage parameter) remain unchanged which more suitable for tensile fracture [21,22] For tensile failure, the SWT model regard the maximum principal strain plane as critical plane and it is unnecessary to consider the correction of mean stress. In this paper, the SWT model is adopted to investigate the relationship between low cycle fatigue lifetime and grain size. ( ) ( ) ( ) ' 2 2 ' ' max 2 2 2 b b c f f f f f N N E σ ε σ σ ε + ∆ = + (1) where σ max is the maximal principal stresses in the maximal main strain range, Δ ε is maximum main strain range, N f is fatigue lifetime, ' f σ is fatigue strength coefficient, ' f ε is fatigue ductility coefficient, b is fatigue strength exponent, c is fatigue ductility exponent. It is shown from Fig.3, the low cycle fatigue lifetime of GH4169 specimens is highly correlated with the grain size. As we all know, organizations determine the performance. Usually, the smaller the grain size of the alloy, the longer the fatigue lifetime of the alloys. However, traditional SWT prediction model does