S-N Curves for the nominal stress method can be created for steel materials,
aluminum alloys and cast iron.
If you wish to use the localconcept, the program provides various possibilities for creating cyclic material data (Uniform Material Law etc.).
The data required for a fatigue life calculation is stored in a data base. This data base can be extended by the user.
The program can generate standard spectra.
Figure 1: Progressive spectra with standard Gauss divisions, created by the collectives generator
This is a support for the user should he not have his own measured data. NEiFatigue is available for Windows 95, 98 ME,NT4, W2K and XP.
A close interaction with FEMAP has been achieved.
1. Fatigue Life Calculation without FEM
A fatigue life calculation can be carried out by the classic methods - nominal stress concept and local concept.
As well as the other material data required for each type of concept, i.e. for the nominal stress concept: stress S-N curves and mean stress sensitivity,
and for the local concept: cyclic stress-strain curves, strain e-N curves and damage parameter curves according to Smith, Watson and Topper,
the component loading is provided. This is made possible because loads are fixed by progressive spectra (meanload, load amplitude and number).
These fixed loads can be either entered manually or taken from the program’s generator.
Another technique is to use a loading history (stress, moment, force), which is normally obtained from a measurement (Figure 2),
in special cases the user can type the data in manually. A data-import from other programs is included.
The length of time history is only limited by the disk space.
Figure 2: Load history from a measurement
Interactive data input and data modification for load spectra, load histories and rainflow-matrix is
possible by marking the range in the graphic and modifying simply by use of mouse or keyboard.
Figure 3: Rainflow matrix of the stress-time history
The NEiFatigue Program uses the Rainflow method as is usual when calculating fatigue life.
The rainflow matrix only contains the signal parts relevant to the damage (Figure 3).
NEiFatigue shows the results of the damage calculation in the rainflow matrix in colour.
The critical signal parts are therefore immediately obvious.
If the local concept is used, the stress-strain paths from the rainflow matrix are calculated and shown in a graph (Figure 4). The total damage is obtained by adding together the damage proportions (linear damage
accumulation hypothesis).
Figure 4: Stress-strain path from the rainflow matrix.
With the nominal stress method, various hypotheses can be used to consider the fatigue limit (original,
modified according to Haibach, elementary, Liu and Zenner). Figure 5 shows these possible hypotheses.
Figure 5: Possible hypotheses available in NEiFatigue
If the calculation is done according to the local concept, the cyclic stress-strain-path and the material memory (masing effect)
will be taken into consideration (Figure 6). Therefore the stress reduction due to plastic changing shape and hysteresis
loops which damage the material are included.
Figure 6: Cyclic stress-strain curve and masing pattern
Figure 7: Cyclic stabilized material properties
The damage calculation is assisted by a damage parameter curve as suggested by Smith, Watson and Topper (Figure 7).
2. Fatigue Life Calculation Combined with FEM
The fatigue life calculation is carried out in a similar way to that described above.
One important difference however, is that a static FE calculation is used to ascertain the stress within the component.
For this purpose the state of stress is calculated in a structure which is subjected to a standard load Fo.
The direction of the standard load must correspond to the actual force F(t).
The elastic stress for each load F(t) can then be calculated in a linear way corresponding to the quotients F(t)/Fo.
If, for example, a force F(t) exists as shown in Figure 1, then the stress within a component can be calculated for any required moment.
In the case of local concept, there is an actual stress-strain curve. This means that the actual stress path for each cycle number node,
including plastic deformation can be calculated using Neuber’s rule.
Figure 8: Result of a fatigue life calculation using NEiFatigue
Figure 9:Damage distribution for a truck beam of a suspension with a prestressed rubber element in the bearing
Figures 8 and 9 show examples of components for which fatigue life calculations were carried out.
Because, generally speaking, not all the nodes of a structure are endangered - the damage usually begins on the surface -
the user can select nodes for the fatigue life calculation according to various criteria. By doing this, the number of nodes
to be examined can be substantially reduced and the calculation time
shortened accordingly.
Macros are supplied with NEiFatigue so that data can be transferred from NE/Nastran via FEMAP.
The results from NEiFatigue, i.e. the damage for each individual node, can be shown by FEMAP in colour. A separate colour is
used for each different range of damage (Figures 8 and 9).
3. Material Data
If you wish to carry out a fatigue life calculation, you will require suitable S-N curves or data describing the damage.
NEiFatigue comes with a data base containing this information for some of the engineering materials commonly used.
This data base is constructed so that the user can add his own data to it. We are always willing to give further assistance
if particular material data is required which is not included in the data base (Figure 10).
Figure 10: Structure of the NEiFatigue data base
In many cases it helps if the fatigue life data is created according to static characteristic material values.
NEiFatigue offers several possibilities for the local concept - as well as the widely accepted Uniform Material Law.
Figure 11 shows which possibilities are available.
For the Nominal stress method a S-N generator according to Hueck, Thrainer, Schütz is available too. This generates S-N curves based on static material data and information on the component.
