Possibilities and Limits
Verification Types: Static / Fatigue Limit / Fatigue Strength / Crack Propagation
There are several steps in the development of a component. The first step is dimensioning based on the (rarely) occurring static maximum loads and, if applicable, a safety factor. The proof that the component will not fail under static load is a prerequisite for all further verifications.
If the component is also subjected to dynamic loads, it must also be verified that these loads will not lead to failure. The assessment of the fatigue limit is relatively easy to carry out, as only a few details are required. If it can be shown with sufficient certainty that the stresses are below the fatigue limit under worst case assumptions, this result is initially sufficient.
At a later stage, when it is time to optimise the structure and more detailed information is available, the fatigue strength verification may be useful or even mandatory. The need for it depends very much on the consequences of a component failure.
A crack propagation calculation can also be useful to determine inspection intervals.
winLIFE offers suitable calculation modules for all the scenarios mentioned. These are:
| Task | Result | Procedure | winLIFE Module | Comments |
| Static proof for welded and non-welded components | Safety against failure at maximum load | Utilisation ratio of the statically permissible limits is calculated | FKM QUICKCHECK |
According to FKM guideline |
| Fatigue strength assessment for welded and non-welded components | Safety against failure under dynamic load by load spectra | Utilisation ratio of the dynamically permissible limit values is calculated | FKM QUICKCHECK | According to FKM guideline |
| Assessment of the fatigue limit for welded and non-welded components | Safety against fatigue limit even with non-proportional load | Worst case scenario |
FKM QUICKCHECK |
Not according to FKM guideline |
| Certification of fatigue strength under variable amplitude |
Fatigue life (in hours, km, number of repetitions, etc.
|
Analysis of real stresses, their interaction and superposition |
winLIFE BASIC | Uniaxial / Biaxial Proportional |
| winLIFE MULTIAXIAL (Extension for winLIFE BASIC) |
Rotating principal stresses | |||
| winLIFE GEARWHEEL&BEARING (Extension for winLIFE BASIC) |
Tooth root (fracture) / tooth flank (pitting) | |||
| Crack growth | Crack growth from initial crack to fracture | winLIFE CRACKGROWTH (Extension for winLIFE BASIC) |
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| Analysis in the frequency spectrum |
Fatigue life (in hours, km, number of repetitions, etc. |
For a given PSD, the damage is calculated for each node | winLIFE RANDOM FATIGUE | If the largest exciting frequency is greater than 1/3 of the smallest characteristic frequency |
STATIC Proof ACCORDING TO FKM
In the static proof according to the FKM guideline, the characteristic material limits Rm, Re are taken into account. In addition, the ductility of the material is also considered, as exceeding the yield strength is tolerated as long as the permissible elongation is not exceeded.
FATIGUE STRENGTH ASSESSMENT ACCORDING TO FKM
The stress capacity is represented by an S-N curve. In the fatigue strength assessment with local stresses according to FKM, the degrees of utilisation of the individual components are determined for a verification point on the basis of the stress components present there.. A weighted summation of the individual degrees of utilisation is combined into an overall result.
The load can be a single step load or a collective of up to 6 steps.
Assesssment of the fatigue limit
The assesssment of the fatigue limit does not provide a result on the fatigue life, but only a statement as to whether and by how much the most unfavourable stress is below the fatigue limit. If the assesssment of the fatigue limit fails, the much more complex certification of fatigue strength under variable amplitude must be carried out, for which in particular the loads and their temporal relationship must be available.
Certification of fatigue strength under variable amplitude
The certification of fatigue strength under variable amplitude provides the fatigue life in hours, kilometres, cycles, etc. as a result and allows the prediction of the failure locations.
Different design variants can be easily compared in terms of expected fatigue life and critical points can be identified at an early stage. This information is very useful and in some cases can significantly shorten the design process by eliminating design loops.
However, mathematical fatigue life predictions are not yet accurate enough to replace tests on safety-relevant components to prove fatigue strength. For safety reasons, it will generally be necessary to additionally test the components under realistic load conditions.
Many authors have determined the deviations between calculated and actual fatigue life for a large number of fatigue life calculations. It has been shown that even when the methods are correctly applied, the results can vary by a factor of 10 up or down. This shows that a purely mathematical prediction of absolute fatigue life can be too inaccurate. However, if statistically validated experimental results are available in parallel with a fatigue life calculation, an absolute fatigue life prediction can be made. For this purpose, a correction factor is determined from the comparison of the calculation results with the test results, which can then be used to give an absolute fatigue life based on the calculation results.
Crack Growth
In certain areas, such as aircraft construction, crack growth analysis is important. This is because the aluminium alloy-based structures used in these applications have a relatively long crack growth phase, and their use is essential. Most structures, on the other hand, have a relatively short crack growth phase, so there would be little advantage in using them.
Random Analysis
Irregular loads occur in many areas of engineering, but they can be described using statistical analysis because certain regularities occur.
For example, studies of aeroplanes, ships or road vehicles have shown that the excitation at certain frequencies has particular intensities and can be described by the power density spectrum. Using the finite element method, it is comparatively easy to determine the behaviour of structures under excitation using a power density spectrum.
This provides a comparatively simple structural analysis, which has the decisive advantage over static analysis that the dynamic system properties are taken into account in relation to the exciting frequencies.