Fatigue Properties

There are a few fatigue properties relevant to Seam Weld Fatigue Analysis that allow you to control the fatigue behavior.

Bending Ratio

Experiments show that two types of SN curves are required to perform Seam Weld Fatigue Analysis based on a method suggested by M. Fermér, M Andréasson, and B Frodin. Based on lab tests, two SN curves were plotted (Figure 1). The upper curve is obtained in tests where the maximum stress is dominated by Bending Moment (

m_{2 Y}

) and the lower curve is obtained from tests where membrane force (

f_{2 X}

) dominates the maximum stress.

Figure 1. Example Stress Amplitude vs Fatigue Life log(N) for Bending dominated vs Membrane Dominated Structure

The upper and lower curves are referred to as Bending SN curve and Membrane SN curve, respectively. It is recommended that membrane SN curve should be used when membrane stress dominates in an element, and bending SN curve should be used when bending stress dominates. Interpolation between the two curves may be carried out depending on the degree of bending dominance.

Degree of bending dominance can be determined by the average bending ratio (

r_{B}^{A V G}

). First, the bending ratio (

r

) is defined as:(1)

r = \frac{| σ_{B} |}{| σ_{B} | + | σ_{M} |}

Where,

$σ_{B}$: Maximum bending stress equal to $6 \frac{m_{2 Y}}{T^{2}}$
$σ_{M}$: Maximum membrane stress equal to $\frac{f_{2 X}}{T}$

The average bending ratio (

r_{B}^{A V G}

) for an element is defined as:(2)

r_{B}^{A V G} = \frac{\sum_{i = 1}^{n} (r_{i} {[σ_{T O P}^{2}]}_{i})}{\sum_{i = 1}^{n} {[σ_{T O P}^{2}]}_{i}}

Where,

${[σ_{T O P}^{2}]}_{i}$: Square of the maximum stress at the top surface of shell element $i$ at which the damage is calculated (that is, root, toe, or throat shell elements)
$r_{i}$: Bending ratio of shell element $i$ .

is the

An interpolation factor ( $I F$ ) is now defined as:

$I F = 0.0$ when $0.0 \leq r_{B}^{A V G} \leq r_{B}^{C R I T}$

$I F = \frac{r_{B}^{A V G} - r_{B}^{C R I T}}{1 - r_{B}^{C R I T}}$ when $r_{B}^{C R I T} < r_{B}^{A V G} \leq 1.0$

The

r_{B}^{C R I T}

value is defined by the BRATIO field in the Assign Material dialog. It is set to 0.5 by default. If average bending ratio (

r_{B}^{A V G}

) is less than or equal to the critical bending ratio (

r_{B}^{C R I T}

), then the Membrane SN curve is used to assess damage. If average bending ratio is greater than the critical bending ratio, then an SN curve that is interpolated between membrane SN curve and the bending SN curve is used. The value of the interpolation factor (

I F

) is used in the linear interpolation method as illustrated in Figure 2. For example, if you consider the Fatigue Strength coefficient value (SR1_i) for the interpolated curve, the calculation is performed as:(3)

S R 1_i = [S R 1_m + (S R 1_b - S R 1_m) I F]

Figure 2. Example Stress Amplitude vs Fatigue Life log(N) for Bending dominated vs Membrane Dominated structure

Thickness Correction

The thickness correction process is for size effect correction. SN curves are based on test results from a particular size of the specimen. In reality, the stress vs life curve may vary depending on specimen size. Therefore, thickness correction parameters can be used to correct for this effect. It may be applied based on the thickness $T$ of each shell element under consideration for Fatigue calculation (that is, toe, root, or throat element). The calculations are:

If $T \leq T R E F$ , then there is no Thickness Correction.

T > T R E F

, HyperLife increases stress based on HyperLife:(4)

σ_{T O P / B O T T O M} = σ_{T O P / B O T T O M} {(\frac{T}{T R E F})}^{T R E F_N}

This results in fatigue life reduction, making the design more conservative. TREF and TREF_N can be defined via the corresponding fields in the Assign Material dialog. The default values are 25 and 0.2 respectively. The defaults are in mm.

Thickness Correction can be turned on or off using the corresponding Thickness Correction field in the Fatigue Module dialog for Seam Weld Fatigue Analysis.

Mean Stress Correction

FKM mean stress correction is supported for Seam Weld Fatigue. Stress sensitivity can be defined in the Fatigue Module dialog via the FKMMSS field. Mean stress correction for Seam Weld fatigue can be enabled through the Seam Weld Fatigue Module dialog.

For more information on FKM mean stress correction, see the FKM section under Uniaxial S-N Fatigue.