It is possible to simulate the repetition of the load sequence in TransMAX by definition of “Number of Calculation Sequences” greater than 1. If a factor of 100 is given for 10 time points, it corresponds to a total number of 1,000 time points. The increase in this factor is useful for damage calculations, in order to reduce the effect of the Rainflow residuum on the result (not necessary for the Default Rainflow Counting Method).
Caution: When calculating the endurance safety factor, the factor does not affect the result and should remain unchanged (default value 1)!
This repetition factor can be changed as required without necessity to recreate the scratch files, because the duplication of the time points only takes place during the calculation.
As the number of time points is directly corresponding to the calculation time, the repetition should be selected with caution, e.g. in the 10-100 (Fig. 1).
In order to save considerable computation time it is possible to compress the load-time histories imported into ChannelMAX. After pressing the "Compress load-time history" button intermediate points that generally exercise no influence on the Rainflow counting can be filtered out on the one hand ("peak slicing"); on the other hand, cycles with small amplitudes can be ignored ("cycle omission"). Three different options are available for defining the threshold value of these partial cycles, whereby one of the options takes the component stresses into consideration (see figure below).
The largest occurring stress for the entire loading history of the current analysis group is adopted for filtering. In conjunction with the user-defined filter limit (default 5%) this results in a stress that filters the small cycles of all loading channels.
This leads to a number of advantages:
• Stress directions to which the structure's reaction is highly unresponsive (cf. beam under tension/bending) are heavily filtered.
• Unit differences between loading channels are no longer important (e.g. channels defined in [kN] are not filtered preferentially to channels in [N]).
• Various load types such as force, moment, pressure, etc., are correctly evaluated and compressed in relation to one another.
Fig. 2: 1000 samples (original load list), 211 samples (compressed list)
This method is especially efficient if only small areas of the structure are to be analyzed: because many stress directions often only cause very minor stresses at the respective location, these channels can be highly compressed or even deleted, resulting in a substantial reduction in computation time.
The import of the load-time histories and the compression are both very fast. This procedure can therefore be easily repeated (for several critical locations).
ChannelMAX also enables users to carry out fatigue analyses on non-linear results from calculations- e.g. due to contact problems. Such nonlinearities must be handled differently in ChannelMAX, in contrast to TransMAX where they are already taken into account when calculating the stress sequence. A good example here would be a wheel mount for which a contact condition applies between the green and orange-coloured component. A lateral force is introduced as a load at the upper end of the wheel mount. It is not simply a question of combining the non-linear stress results with the stress history shown below (in black), which contains both positive and negative values. Doing so would completely misrepresent the contact relationships.
In order to continue being able to perform a FEMFAT ChanneIMAX calculation, the following two steps are required:
1. The stress history is divided into a positive component and a negative component:
2. In addition, characteristic working points (=load levels) need to be identified from the load history with damage-relevant contents. This must be done for the positive and negative area. In this example, this part is approx. 75% of the maximum loading, i.e. approx. 700N for the positive and 400N for the negative component. These two characteristic loads are now used to perform the non-linear FE-analyses. The stress results obtained from this calculation are imported into FEMFAT as “new” unit load cases for the associated partitioned loading histories in ChannelMAX. It is important to ensure that the factor for the stress results is entered as 1/700 for the first channel and 1/400 for the second channel to achieve the correct stress level once again.
After the FEMFAT analysis, the user finally comes out with a damage distribution taking account as far as possible of non-linear contact effects.
This method can also be used with multi-axial loads. However, in this case, contact states may appear less accurate due to the non-linear transfer effects of multiple channels. This effect is rare however since in most cases only a few directions of the applied force dominate at a particular moment.
In any case, what matters is the correct working point, which is located close to the existing peaks in cases of practical relevance (the reason for this is simply because the largest peaks in load usually cause the highest partial damages).
You should be careful with initial pre-stresses: such stresses should be deducted from the operating load states and an additional constant channel defined.
In summary, what we have here is a practical method which offers sufficient accuracy in most cases, and which offers enormous time benefits (up to several orders of magnitude) compared over analyzing of many non-linear stress states together with a TransMAX analysis.
Simple load signals (constant Signal, cosine or triangle signal) can be definied directly in ChannelMAX.
An extension of the channel definition for simple generation of signals was created (as a supplement to the diverse interfaces).
The cosine and triangle Signal allow specifying the number of sampling points per wave, the amplitude, the mean leveland a phase shift.
The total number of samples is not queried until the load histories are scratched, if no load data are imported from interfaces.
Typical applications for constant signals include bolt pre-stresses. Cosine signals can be used for rotating loads for example (2 load channels with 90% phase displacement).
In some loading situations, e.g. in crankshafts subject to combined bending/torsional loads, a local change or rotation in the directions of the principal stresses may occur with time.
Tests using combined bending/torsional alternating loads and 90 degree phase shift have shown that for ductile materials (tempered steel) the critical cutting plane method overestimates the lifetime (e.g. see FKM report "Multiaxial Fatigue Analysis, 2002"). In FEMFAT MAX it is possible to correlate the lifetime using "Influence of rotating principal stresses".
The local S/N curve is reduced as a function of a statistical degree of multiaxiality lying between 0 (= proportional load with constant direction of principal stresses) and 1 (= heavily nonproportional load with directions of principal stresses changeable with time).
The influence thus results in a reduction in lifetime in ductile materials.
No impact is defined for brittle cast materials (gray cast iron, cast Al, cast Mg).
Rotating bending with torsion
We recommend activating the influence of rotating principal stresses. However, in certain cases, e.g. where high constant stresses are involved (bolt pre-stresses, residual stresses), the results may be conservative.
When evaluating analysis results for components with multi-axial loadings one question is always what channels (loading directions or modes) are relevant to the damage result for a selected node. In MAX three options are available:
Option 1 – Check input data
Using this command in the analysis start window gives the user (following the scratch process where necessary) a list of the highest stresses per channel (Fig. 1). It can be quickly seen which channels cause high stresses in the analysis groups.
Using the modal method the highest stress falls noticeably after a certain mode number. This is an indication that the excitation spectrum now only has minor influences on the structure.
The disadvantage of this method is that one has no idea of the phase relationship and is thus in danger of eliminating channels with low stresses, which could result in relevant damage.
Fig. 1: Max arising channel-stresses
Option 2 – Compress load history
One procedure for eliminating less relevant channels is the loadtime data reduction provided by ChannelMAX.
To do this, the settings “for the most critical channel” and “with consideration of channel stress” must be selected in the compression menu (see Fig. 2 top).
The advantage here is that channels with small stress portions are completely deleted. In addition, data points between the minimum and the maximum are eliminated, as well as small amplitudes below a user-defined limit which only cause a minor damage share (Fig. 2 bottom).
Beside the advantage of taking phase shifts into account, a considerable reduction in computation time results.
Fig. 2: Load history compression
Option 3 - VISUALIZER
The FEMFAT VISUALIZER provides an option for also visualizing the corresponding load factors multiplied by the stresses from the unit load case for individual nodes (from the “Detailed Result Group”), in addition to the equivalent stress history, damage history and partial damages. After clicking any point in time with a large damage increase, for example, a diagram containing the corresponding stress values per channel is displayed. They are sorted according to size for clarity. From this, in turn, it is possible to derive which load channels contain a high damage component.
Fig. 4 (cut-out fig. 3)
Fig. 5 (result of red area)
Fig. 6 (result for blue area)
For example, two zones were found in the total damage history (Fig. 3) where a substantial increase in damage occurs. To precisely identify the corresponding data points (Fig. 4) the partial damages were also considered. By clicking the “time points” with high partial damages the user is shown the corresponding output for the unit stresses multiplied by the load factor per channel (Figs. 5 and 6). In addition, other diagram options are available, for example with unit stresses only or load factors only.