Technically, this is the phenomenon of destruction that occurs under the effect of the repeated application of a charge. It is a local mechanism. Each cycle of stress variation creates damage to the material. The repetition of the cycles leads to the initiation of a crack, a propagation in the thickness and finally a leak or a rupture.
A large number of parameters are involved in the appearance of the phenomenon. These include shape changes and induced stress concentrations, high thicknesses, surface condition, temperature and loading history.
These phenomena have been theorized (Miner’s rule: linear accumulation of damage) and tests on representative welded joints have allowed the determination of fatigue curves (International Institute of Welding) giving their endurance.
Pressure equipment fatigue
Complementary work has been carried out specifically for the Pressure Equipment in order to propose design methods to determine the service lifetime of an equipment.
These procedures propose an analysis by “critical zone”. These are mainly the
welded joins, but also geometrical changes such as an opening, the peripheral area of a bottom or the connection of the plate and the welding neck of a flange.
Each zone is characterized by the parameters influencing its fatigue strength such as materials, thicknesses, imperfections and defects of form (offset of
medium or angular fiber, ovalization).
Critical zones are then studied according to the equipment’s duty cycles to determine the stresses. Thanks to the fatigue curves and the accumulation law, the damage rate will then be evaluated to allow the overall performance of the equipment to be checked.
Fatigue Design with AUXeCAP
The latest version of our boilermaking calculation software AUXeCAP now allows the fatigue study of equipment in response to pressure, temperature and mechanical stress variations. The repeated application of these stresses are at the origin of destruction such as: material fatigue.
Codes propose different approaches to this phenomenon and allow either to conclude that an equipment will not undergo these “exempt” damage phenomena or they determine the number of admissible cycles and the damage rate. Two methods are usually proposed:
- a so-called simplified method which covers mainly cases where the only variable stress is pressure and essentially for equipment dimensioned using analytical formulas,
- so-called detailed method which requires preliminary calculations by the finite element method.
Two steps will be necessary to check in FATIGUE from an equipment already dimensioned in pressure:
- Use case
- Critical areas description
AUXeCAP assists you in these two preliminary tasks for the FATIGUE evaluation by :
- determination of the elementary study cycles: reservoir method,
- Intelligent generation of critical areas.
In the end, AUXeCAP delivers a detailed analysis for each zone and each elementary cycle and a summary report with damage rates and allowable numbers of cycles. Thanks to these study results, you will of course be able to justify the operational performance of the equipment, but also to consider material savings and become more competitive.
If you do not know our solution yet, ask for your free online presentation: Demo request. Only one hour is necessary to give you a good view of the tool.