What does fatigue strength mean
Fatigue - durability
Fatigue strength / fatigue strength
Fatigue of the material means failure due to cyclical loads. The load that can be tolerated is lower than the maximum load that can only be tolerated a few times (up to approx. 10,000 cycles).
The relationship between the tolerable stress amplitude and the number of cycles is described by the Wöhler curve.
The fatigue strength is divided into the following 3 areas:
- Short term strength
- Fatigue strength
- Fatigue strength
Range of services
We offer the following services in the area of fatigue strength / operational strength:
- Durable design of components and structures
- Proof of fatigue strength
- Investigations of vibration-loaded components in the short-term strength range
We pay particular attention to the fatigue strength verification of structures with non-linear behavior. These non-linearities are often caused in mechanical engineering by:
- Screw connections
- roller bearing
- Constructions with lifting contacts
For verifications of highly non-linear structures, we use our self-developed software packages. In this way, we can map the often underestimated influence of non-linearity on operational stability with high precision and understand the causes. In this way, both oversized structures and serial damage are avoided.
The damage is calculated depending on the load situation on the basis of:
- One level collective
- Multi-level collective
- Markov matrices
- Time series (calculation with Ansys nCode or in-house program)
Dipl.-Ing. Florian Stache
We like to cooperate with Mr. Hanke and his team for our projects in the field of wind energy. The engineering office Hanke supports us with damage analyzes as well as with the design of new components. The competence in the areas of fatigue strength and screw calculation is outstanding.
Examples of completed customer projects
Rotor shaft of a wind turbine (4 MW)
For the forged rotor shaft of a wind power plant, tests were carried out on the operational strength. The verifications were made on the basis of the FKM guideline. The stress on the rotor shaft was determined using FEM, taking into account the stiffening influence of the rotor hub. The damage calculation was carried out taking into account the influence of the medium voltage. By using the FKM guideline, it was possible to significantly reduce the shaft diameter compared to the usual procedure according to the more conservative GL guideline.
Test stand for blade bearings
The operational stability of a test stand for blade bearings was demonstrated in the project. The test stand construction consists of a welded steel structure. The main focus was on the fatigue strength of the weld seams. A reliable design was not possible due to the high number of intended cycles.
The welds were evaluated according to Eurocode 3 using the structural stress concept.
Rotor hub of a wind turbine - non-linear fatigue calculation
The fatigue strength verification for a rotor hub (GJS 400) was provided. In the fatigue calculation, the non-linear relationship between applied loads and stresses in the hub was taken into account. This is caused by the non-linear load-bearing behavior of the blade bearings modeled with. Both the effects of different directions of action of the leaf loads and the interaction between the leaf bearings were also mapped. The damage was calculated using the stress time series. These were evaluated with our in-house program for the analysis of the fatigue strength using the method of the critical cutting plane.
Proof of fatigue strength
The verification of the fatigue strength is divided into the following three areas:
- Fatigue strength: The maximum occurring oscillation amplitude can be endured infinitely often. The proof of fatigue strength is provided for components that have a very high number of cycles of the collective components with a high vibration amplitude in the collective load. Typical examples are crankshafts of internal combustion engines and wheel tires of locomotives.
- Fatigue strength: It is demonstrated that the load does not lead to a failure of the structure during the period of the intended use. This proof is less conservative than a fatigue strength proof and therefore leads to lighter components. However, the fatigue strength verification is advantageous compared to the fatigue strength verification only with a suitable collective form (low number of cycles with high vibration amplitudes). Typical components that are designed to be operational are the towers of wind turbines (low number of cycles in the case of gusts of wind), bridges and ships.
- Short term strength: The component is fatigued with very high, plastic strain amplitudes of less than approx. 104 Oscillation numbers. The short-term strength is only of subordinate technical interest.
Process of a fatigue strength verification
- Determination of the collective stress. If time series are available, the collectives are generated with the help of a counting process (nowadays usually by rainflow count) and subsequent classification. In the case of load time series, this assumes that only one load is the cause of the stresses. If the voltages are caused by several loads, a voltage time series must first be created from the load time series, which is then converted into a voltage spectrum using the counting process.
- Conversion of the individual collective levels to the stress ratio of the Wöhler curve used in order to take into account the effects of the mean stress influence.
- Determination of a Wöhler curve taking into account the material, the roughness, the notch effect number and the probability of survival.
- Damage accumulation, for example using the miner's rule.
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