TWI fatigue life criterion

The TWI fatigue life criterion extends the BWI fatigue life criterion into the low cycle and high cycle fatigue regimes. In the schematic of the TWI S-N curve, the blue curve is the BWI S-N curve, the red curve is the TWI extension into the low cycle regime, and the green curve is the TWI extension into the high cycle regime.

Schematic of the TWI S-N curve

For the TWI life criterion, you need to specify the same parameters that you specified for the BWI life criterion:

• The weld class.

• The number of standard deviations, d, below the mean value of the stress range or the probability of failure.

In addition, you also need to specify the following unique TWI parameters:

• For the extension of the BWI criterion into the low cycle region, you must specify the low cycle stress range cutoff, ΔσI, and its extension, ΔσII, as a proportion of the yield stress.

• For the extension of the BWI criterion into the high cycle region, you must specify the non-propagating stress range, NNP, and its slope change, δ.

Low cycle fatigue in TWI criterion

The limit of the BWI curve in the low cycle region is given by the low cycle stress range cutoff, ΔσI as a proportion of the material yield stress. The other limit is given by the low cycle stress range extension, ΔσII. For the low cycle stress range, ΔσI < Δσ < ΔσII, the curve is a straight line in the log Δσ – log Nf space as shown by the red curve in the schematic of the S-N curve.

Hence, in the ΔσI < Δσ < ΔσII region, the relation between the number of cycles to failure, Nf, and the stress range, Δσ, is given by the following straight line equation:

where log NfI is given by the BWI equation as:

In this equation, m, a, and stdev are parameters dependent on the weld class. They are defined in BWI fatigue life criterion.

High cycle fatigue in TWI criterion

The limit of the BWI curve in the high cycle region is given by the number of cycles to non-propagating range, NNP. For the Nf > NNP region, the curve is extrapolated in a straight line in the log Δσ – log Nf space as shown by the green curve in the schematic of the S-N curve.

Hence, in the Nf > NNP region, the relation between the number of cycles to failure, Nf, and the stress range, Δσ, is given by the following straight line equation:

where log ΔσNP is given by the BWI equation as:

All logarithms are to the base 10.

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TWI fatigue life criterion, Simcenter 3D 2021.1 Series

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