PSI - Issue 57

Giovanni M. Teixeira et al. / Procedia Structural Integrity 57 (2024) 670–691 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The adjustable parameter  is related to the temperature independent constant A in Arrhenius Equation (equation 41). The original idea was that  could be extracted directly from this factor. However, as the viscous behaviors various significantly for every material,  can’t always be determined directly from the creep tests. Hence this parameter is usually calibrated with the aim of optimizing the fit of the D TMF Life model. The exponent n is indeed directly extracted from the Arrhenius Equation. It is recommended to adopt an initial value for  =A and check how close ( and satisfactory ) the life predictions are. In case the predictions are not good enough, then an optimization procedure should be performed to improve the fit. ̇ = ⋅ ⋅ (− ) (42)

It is convenient to define a parameter ( ′ ) this way: ′ = −1

Fig. 10. Secondary creep included in the D TMF method.

(43)

So that the its dimension reduces to 1 ⁄ and consequently the equation for the creep factor F cr becomes: =(1+ ′ ∫ ( ) −2 0 − ) 1

(44)

The integration of equation 18 results in: = 1− − 01− 1− 1 ( ′ TMF ) −

(45)

N f is the number of cycles that corresponds to the growth of a microcrack of size a 0 to the size a f . Such micro-crack propagation can be also termed the fatigue initiation of a macro-crack of size a 0 . = 1− − 01− 1− 1 ⏟ ( ′ TMF ) − (46)