The winLIFE BASIC module covers the essential basics of fatigue life calculation. Typically a winLIFE user starts with this module.
The following extensions are available for the BASIC module:
winLIFE MULTIAXIAL MULTICORE
winLIFE RANDOM FATIGUE
A typical task for industry will consist not only of one single fatigue life calculation. In general, several load cases and variants will need to be calculated.
For this purpose winLIFE offers a highly capable system for project tasks. Up to 1000 projects can be worked on simultaneously. The diagram below shows the user desktop in which 9 projects are defined. The projects can be worked on one at a time or started together and superimposed.
The project generator makes it possible to create projects where the individual parameters can be systematically varied. Since a lot of calculations lead to a longer calculation time, it is important that this occurs automatically. Therefore, the possibility of batch jobs is particularly important. The project files are saved in XML-Format and can also be started from the desktop as a batch.
Obtaining Fatigue Material Data
winLIFE offers several possibilities of obtaining material data for fatigue life calculations. As well as generating based on static material data, the program package also includes a large material data base.
Creating a S-N-Curve based on component properties and static material data
Since there are many winLIFE customers in the fields of wind energy, ship building and general engineering, special entry generators have been created which can generate fatigue life data based on static material characteristics.
- S-N curve generator for welded joints according to Germanischer Lloyd for ship building
- S-N curve generator for welded joints according to Germanischer Lloyd for wind energy
- S-N curve generator for welded joints according to the FKM-guidelines
- S-N curve generator according to Hück, Trainer, Schütz
- Generator according to Uniform Material Law
The following diagram shows an example of an entry mask for a generator according to the FKM guidelines. This has been extensively tried and tested and is established in many fields. An S-N curve can be estimated using static material data such as yield stress and tensile strength together with component information such as surface, notch factor, stress gradient etc.
The figure above shows the mask to create such an S-N-curve. The material data can be partially modified by the user. In many cases the user knows only some data from test results and he wants to create the unknown data. So he can use winLIFE to generate the data and then overwrite the values he knows.
As well as the S-N-curve, winLIFE uses the presentation in Haigh-diagram, because the influence of mean stress can be seen more evidently (see next figure).
If the failure rate for an analysis is of importance, the user can derive from a 50% S-N-curve other failure rates. winLIFE considers different scatter rates in the endurance range and the range of finite life fatigue strength.
S-N curves according to ASME
In the ASME-code and some other regulations the S-N curve is described by single points. In between these points an interpolation is done. The S-N curve in the following figure is defined by 23 single points. The data are an example from ASCII-code.
You should use this description if you want to calculate according to ASME-code and if the data are available. Using these S-N curves the mean stress influence is usually ignored so that the mean stress influence M is set to zero.
In order to get more realistic and less conservative results winLIFE gives the opportunity to use a mean stress influence according to the regulations of FKM.
Creating a S-N-curve for welded structures
You can calculate welded structures as suggested in the FKM-Guidelines. The analysis is carried out in a similar way to the Nominal Stress Method. You need to classify the welded structure and according to the class a catalogue of S-N-curves exists.
The winLIFE S-N-curve generator helps you to establish such a curve.
This is a relatively approximate procedure for a first step. To improve the results you can calculate the welded structure according to the elastic stress or local stress approach using a FEA analysis, where the geometry of the weld has to be modelled in detail.
For the use of the structural concept interfaces respective macros for the data transfer are available in connection with ANSYS and FEMAP.
Creating e-N Curve
If you wish to use the Local Strain Approach, the program provides various possibilities for creating cyclic material data (Uniform Material Law etc.). The next figure shows the mask for entering input data. Only static material data is entered and based on this an estimation of the e-N-curve is created. The surface and – if no FEA is used – the notch effect factor are used.
Using strain life curves
If you wish to work according to the local strain concept, the necessary material data can be obtained by generating based on static material characteristics (Uniform Material Law) or by using the material data base.
The next figure shows an example calculation where the cyclic data was obtained by generating from static characteristics. For this component the diametric quotient and the surface roughness in the notch were given. The stress-strain-path of the component in the notch was calculated from the rainflow-matrix. It is also possible to calculate the path without the rainflow, but using the material memory.
Material Data Bases
The data base has been greatly improved in winLIFE 3.3. The complete FKM guidelines material data base has been integrated and is available on the winLIFE CD. Furthermore, the material data base according to the local concept with more than 1400 material data sets has been added to the winLIFE CD. Until now, this was only partly available on the internet. The user now has immediate access to all material data.
Material data base for local strain approach
Material data base according to FKM
User data base
The data of the user`s activities which include information and data of the component are stored in a user data base.
The user has access to his data and can use if for other projects if he wishes.
To perform a fatigue calculation a loading is needed. This can be a measured load versus time or a load spectrum.
Defining a load spectrum
A load spectrum consist of cycles, which must be defined by
- mean load
- load amplitude
- and number of cycles.
The experiences in various areas of science show that there are different types of load spectra, which can be simply defined. Publications in automotive industry, aerospace or wind energy systems show what kind of load spectrum is acting in a problem such as this. If you have this information the winLIFE spectrum generator allows you to create very fast load spectra. The figure below shows three basic types of load spectra.
Using Load histories
Another way is to use a loading history (stress, moment, force) which is normally obtained from a measurement, in special cases the user can type the data in manually. A data-import from other programs is usually possible without any problems. The length of time history is only limited by the disk space.
Loading manually entered
The load – e.g. forces as a function of time – are entered in the entry mask and then saved. Because of the time involved, you will only want to enter short load sequences using this method.
Force Generator (Sinus)
It is often required to generate simple load time function paths quickly and easily. This is possible using the sinus generator described below.
Import of Measured Load Time Functions
Generally very detailed measured data is used and the length can be several gigabytes.
Interactive data input and data modification for load spectra, load histories and rainflow-matrix is possible by marking the range in the graphic account and modifying comfortably by use of mouse or Keyboard.
The winLIFE Program uses the Rainflow method as is usual when calculating fatigue life. The rainflow matrix only contains the signal parts relevant to the damage.
winLIFE 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. The total damage is obtained by adding together the damage proportions (linear damage accumulation hypothesis).
Frequency load description
In the case of systems which are exited in the range of their eigenfrequencies it is usual to define the acting acceleration by a PSD (Power spectral density). This often is done using an accelerated solid test rig where the component to be tested is fixed. This procedure is described later on in the chapter winLIFE RANDOM.
Fatigue Life Calculation
winLIFE can import data from FEA and use it for the fatigue calculation. In this case the local stresses and further conditions of each node are considered.
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 winLIFE so that data can be transferred from FEA-Programs. The results from winLIFE, i.e. the damage for each individual node, can be shown by the FEA-program in colour. A separate colour is used for each different range of damage. The FEMAP user-interface is available for all data transactions between winLIFE and NASTRAN Users will therefore have no difficulties using this program.
winLIFE is used in collaboration with the following FEA-codes:
- FEMAP-based programs like MSC.NASTRAN, NEiNASTRAN and NX.NASTRAN, WTP2000
The interface to FEA is described in detail and many users wrote their own interface. Successful links to MEDINA, ANSYS and other programs were achieved.
Short description of the calculation procedure for an example
In a short description the fatigue calculation procedure is shown. The following figure shows a part of a truck suspension which is loaded dynamically by a force (see over next figure). Because of the symmetry only half of the part is analysed.
In the first step a static FEA 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 acting 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 the next figure, 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 node, including plastic deformation can be calculated using Neuber's rule.
The loading was got from measurement on a test drive. winLIFE makes a rainflow count of the loading and the result is shown in the next figure.
The Material data for fatigue analysis are the cyclic properties (see figure) and the damage parameter life curve (see figure)
From the rainflow count and the cyclic properties the stress-strain-path is calculated (see figure). Based on the counting and the damage parameter life curve (see figure) a damage sum is calculated for each node of the structure. The results are shown as a map on the structure.
Beside the Local Strain Approach shown here, it is possible to use stress based methods using S-N curves. As well as the local concept, a stress based concept can also be used for calculation. Here S-N curves can be used based on stresses.
The results of a fatigue life calculation, the damage sum, damage equivalent amplitude. The number of load cycles until failure is available in an Export-File where the FE-Program can access them and show as in the diagram below. For FEMAP an interface is delivered which is automatically installed onto the FEMAP user desktop.
If no FEA is used the calculation is done only for one point – typically the notch. The information about the geometry and the relation between stress and load now must come from the user. This means he has to enter the notch factor, stress gradient, surface, etc. A fatigue life calculation without FEA can be carried out by the two classic methods - Nominal Stress Method and Local Strain Approach.
With the Nominal Stress Method, various hypotheses can be used to consider the fatigue limit (original, modified according to Haibach, elementary, Liu and Zenner). The following figure shows these possible hypotheses.
If the calculation is done according to the Local Strain Approach, this is done similarly to the procedure using FEA.
Addition of different calculation results
Assume you are developing a car which runs on different road types. You have measurements of each road and fatigue calculations for each road for the measured length. Customers use on the road types will be different in length from that which you have measured. You can now transform the fatigue results from your measured road length to the other length of customers use and you can add the damage sum for the total length of all road types. By adding a weight factor, winLIFE adds together the results of different calculations for each node.
Superposition und Extrapolation
Fatigue Life Calculations are generally carried out for a specified loading. The following test scenario of a test drive with a test vehicle on various types of road surface is an example. Usually there is only one measurement for each part of the route so that the results of the measured journey have to be extrapolated for the required time or route and added together for all parts of the route.
A factor can be calculated from the target and actual values. The results of the test route then have to be multiplied by this factor. This process is called extrapolation. The user can calculate and set the factor or he can enter the target and actual values and the program will calculate the factor automatically.
The adding together of the results of the individual parts of the route is called superposition.
Table: Data of a test journey with target and actual values and the factor calculated for the extrapolation.
trial route [s]
test route [km]
20/6,3 = 3,17
The entry mask shown below enables you to ascertain the statistical reliability. The results from the fatigue life calculation previously carried out and the data from the S-N curve used are taken over.
Data Handling and Correction
If measured data is used in many cases a data correction is needed. A lot of faults occur and in the first step the user has to check the data. winLIFE enables to check and correct the data simply in an interactive manner.
You can modify load histories and load spectra in the following way:
- find and remove Spikes
- select data and multiplying, adding or overwriting them
Additionally you can modify the number of cycles in a rainflow matrix. This helps to check the results too.
Presenting and analysing the results
All common graphics in result presenting are available, such as:
- Range Mean Pair Count
- Level Crossings
- S-N-Curve together with the load spectrum and percentage damage
- Haigh-diagram including the loads as points
- Protocol file including the results for each node
The report generator enables the user to print without individual steps such as opening the graphics. The report generator makes it easier to document a project using only a few work steps. For each method an individual collection of graphics can be selected as shown in the following mask for the Nominal Stress Method. Only one mouse click is needed to create a printout or a PDF-file.
Individual Scaling and designing of Graphics
For each diagram special settings of colour and line type are possible. Clicking on the object shows a window with the properties which can be changed. An example of an individual layout shows the next picture. For each graphic you can select the
- Line colour
- Line type
- range of axis
The unit for stresses can be defined in different ways. The internal calculation in winLIFE is done in N/mm2. If another unit for the stress is wished then you can select from the unit-list. Mostly PSI is used and therefore it is prepared.
If you want to define your own unit for stress you can create it by entering the name of the unit and the multiply factor. The other sizes used in winLIFE have the following standard units which cannot be changed.
ISO-Units as ‚Default’
multiply factor 1
multiply factor 145,04
‚Double Default’ (range)
multiply factor 2