# Eurocode 3 Fatigue

Eurocode 3 - a fatigue check according to the Eurocode 3: Design of steel structures - Part 1-9: Fatigue (January 2006).

To add Eurocode 3 standard execute

- from the main menu. Alternatively, use the context menu:To perform the fatigue check FAT Class, Reduced Range, Consequence of Failure and Assessment Methods should be defined.

FAT Class - the fatigue strength for nominal stress ranges is represented by a series of curves and curves (S-N-curves), which correspond to typical detail categories. Each detail category is designated by a number which represents, in N/mm^{2}, the reference value and for the fatigue strength at 2 million cycles.

Fatigue strength curves for the direct stress ranges:

Fatigue strength curves for the shear stress ranges:

Press

to set the Weld Type (FAT Class)In non-welded details or stress-relieved welded details the effective stress range may be calculated by adding the tensile portion of the stress range and 60% of the magnitude of the compressive portion of the stress range:

By default, delta stress is set to Not Reduced. Press

to specify the selection which should use reduced stress range:__Safety factor Gamma_Mf__ depends on the Assessment method and the Consequence of failure:

Assessment method | Consequence of failure | |

Low consequence | High consequence | |

Damage tolerant | 1,00 | 1.15 |

Safe life | 1.15 | 1.35 |

The unit system is used for conversion units in check calculations. By default the MKS (Meter/Kilogram/Second) system is used.

A conversion is used in Allowable Stress and Fatigue Strength at 2 million cycles calculations:

Press to change the unit system.

Stress Ranges should not exceed the limits defined by the Formula 8.1. Yield Stress (fy) is used in the calculation of the allowable limits.

It is important to set this value for each material. Standard checks if all Yield Stress is not equal to zero:

If count > 0, then press to edit Yield Stress and Tensile Strength:

#### Calculation approach

To verify a model according to the Eurocode3 fatigue 2 checks should be performed: Nominal Check and Fatigue Check.

The Nominal Check requires to verify if delta stress ranges are below the following limits:

The Fatigue Check is more complex than theNominal Check. It requires to take into account the stress history (the stress ranges and their number of cycles). For this purpose Fatigue Group and Fatigue Summation Check were implementedin SDC Verifier.

Fatigue Group allows to take into account the stress history. Fatigue Summation Check runs the calculation for every Fatigue Group item (Load Group) and in the end makes a summation of fatigue damage (≤1).

Fatigue check requires to verify 2 conditions.

For normal and shear directions:

And combination rule:

- delta stress range with partial factor;

- allowable stress range;

- safety factor.

Example: Fatigue Group with total amount of cycles 4 million:

**Calculation procedure:**

- Calculate Effective delta stress range.
- Calculates Fatigue damage for each Load Group based on stress range, fatigue strength and item amount of cycles
- Perform summation of fatigue damage for items (Load Groups):

*Summed Damage = Fatigue Damage.* - Calculate Equivalent Damage (8.3):
- Overall Fatigue Damage. If overall fatigue damage < 1.0 then fatigue check is OK:

The following example demonstrates the extreme table for the Fatigue Group with 4 items.

Summed Damage = 0.17+5.66+0.69+0.19=6.71

Note: Delta Stress, Fatigue Strength and Fatigue Damage are calculated for items (Load Groups). Summed Damage, Damage root^{3} and Damage root^{5} are calculated only for Fatigue Group

Damage root^{3} and Damage root^{5} should be used to evaluate an approximate overshoot for the damage.

The check "Fatigue Stresses Overshoots" should be used. The Allowable Stress is calculated for the Load Groups only using the stress history and the FAT class:

And the combination rule:

E.g. stress overshoots in X direction: