1. SN curve
1.2. Basic theory
The basic form of the SN curve given in recommended practice DNVGLRP0005:201406 is as follows:
where

N = predicted number of cycles to failure for stress range \(\Delta\sigma \)

\(\Delta\sigma\) = stress range

m = negative inverse slope of SN curve

\(log\bar{a}\) = intercept of log Naxis by SN curve
Note the following:

The base 10 (or common) logarithm is normally used in standards.

The intercept of the log Naxis depends on the unit in the SN curve. MPa is chosen as unit in the DNV recommended practice, but other units may be used elsewhere.

The program uses the intercept of the yaxis (stress range)as input. An example of how this is calculated is given below.
Thickness may be accounted for by modifying the design SN curve for thickness larger than the reference thickness:
Where:

\(t_{ref}\) = reference thickness

k = thickness exponent on fatigue strength
1.3. Predefined SN curves
If the Use Predefined curve option is selected, an SN curve can be selected from the drop down list. The reference thickness factor is the ratio of base material thickness and reference thickness (\(\frac{t}{t_{ref}}\)). This factor should normally be larger than 1.0. A default thickness exponent is used. To inspect and edit the resulting input, select Use values in editable version.
1.4. Direct input of SN curves
The SN curves are defined using the following input:

Negative inverse slope (of first segment)

Intercept stress, stress range resulting in failure after one cycle. This is the Yintercept of the first segment.

Reference thickness factor, the ratio of thickness and reference thickness (\(\frac{t}{t_{ref}}\))

Thickness exponent, thickness exponent on fatigue strength (k)

Negative inverse slope of next segment (m)

Transition point between given and previous segment, logarithm of cycles at transition.
Note that the \(log\bar{a}\) (Xintercept) is calculated in MPa and shown to the user for reference.
1.5. Calculating intercept stress
The basic form of the SN curve is:
The intercept stress is the stress range resulting in failure after one cycle (logN =0). The intercept stress is then calculated as follows (assuming base 10 logarithm):
For DNV B1 curve in air this results in :
Note the unit of the resulting intercept stress as DNV curves are in MPa. If units in the workspace are set to m/N, the stress must be multiplied by 1e+06 to convert to Pa before entering the value as intercept stress.