The concepts for assessing fatigue strength and material fatigue have grown historically. With their different approaches, they reflect the existing possibilities that existed at the time they were created. Their continuous further development and adaptation to the capabilities of modern calculation techniques cannot hide this. If one would start today with a white sheet of paper, it would be questionable whether a nominal stress concept could still find such a wide distribution beyond teaching. Since, admittedly rightly, the knowledge base of the industry, the standardisation committees and the approval authorities are happy to fall back on proven data and concepts, several approaches will also exist in parallel in the medium to long term.
Nominal stress concept
In the nominal stress concept, external loads are related to a net cross-sectional area, which deliberately neglects the notch stresses due to cross-sectional changes. Instead, these are taken into account by a notch effect factor. The method is suitable for components with a simple control geometry in which a nominal cross section can be defined and reliable values for the support number, form or notch effect factor are available. The process is based on the assumption of elastic material behaviour and can only be used if this also applies. Sequence influences of the load spectrum are lost in the evaluation. Despite these clear limitations, the nominal stress concept is still very widespread, especially in steel, crane and shipbuilding.
Structure stress concept
The structural stress concept is mainly used in the evaluation of welded joints. In this case, it represents an extension of the nominal stress concept. The fictitious structural stress is determined using an FEM calculation in which only the basic geometry of the design is modeled, but not the local seam geometry. This has the essential advantage that the welding seam can be imaged with relatively large elements. In this way, even more extensive structures can be examined with a justifiable numerical effort. However, this means that only the notch effect of the design and not that of the seam is taken into account in the simulation. A comparison of the structural stress thus obtained with the corresponding structural stress Woehler curve provides the fatigue strength. Analogous to the nominal stress concept, the assumption of elastic material behaviour is also the basis here, with all the associated limitations.
Notch stress concept
In the notch stress concept, the stresses actually present in the notch base are used to evaluate the fatigue behaviour. The basis for this is the exact representation of the real geometry in the FEM simulation. In principle, the stresses determined in this way are the nominal stresses increased by the number of shapes, which is why the further procedure for fatigue evaluation does not differ significantly from the nominal stress concept. This is one of the main advantages of this method. With it, the existing material properties can still be used and at the same time the limitation to the few known notches is omitted. As a result, complex geometries can be examined with this local concept. At least as long as the notch stress concept is based on the assumption of an elastic material load.
Notch strain concept
With the notch concept or better the notch strain concept, the partly unrealistic restriction to a purely elastic material stress is eliminated. The notch strain concept can also be used to evaluate service life or fatigue in cases where alternating plastification takes place. The basis for the comparison are strain Woehler curves determined with uniaxially stressed, unnotched test specimens. The basic notch concept also enables the consideration of sequence influences when accumulating damage.
If a macroscopic crack is already present in a geometry to be examined, the number of load cycles up to a crack is no longer of interest; this is already present, but only the crack propagation and the statement about the possible remaining service life of the component under continuous cyclic loading. The aforementioned concepts cannot provide any statement in this respect. The fracture mechanics or the crack propagation concept provide the answers in this case.