The MSC Fatigue analysis module FEFAT performs many different tasks from simple file conversions to full blown fatigue analysis. Module operation of each of these tasks is described in detail in this section. The name FEFAT refers to FE‑Fatigue which implies fatigue analysis from finite element (FE) data.

FEFAT handles all the pre-processing, data file import and export, all S-N and crack initiation analysis including factor of safety analysis. The fatigue design optimization analyzer is also available from the FEFAT menu.

The operation of FEFAT can be in two modes: within the MSC Fatigue Pre & Post and MSC Patran environments or in stand alone mode from the system prompt. The only difference is that in stand alone mode, the user must supply the jobname when asked to preform the analysis. (In direct mode from a preprocessor such as MSC Patran, these are passed to FEFAT automatically.) FEFAT can be accessed directly from the operating system prompt by typing the symbol fefat. Once FEFAT has been initiated in either of these modes, two windows will be presented. The top, small form is a generic form and allows for general program control. This is discussed in detail in Module Operations (Ch. 17) for the Motif driver.

The main menu appears as follows. Each item is discussed in this section.

The complete fatigue analysis is actually split into two parts: the pre-processing (cycle counting) and the analysis. This is mainly because it affords more flexibility in the design optimization analysis. When running the global multi-node/element analysis, however, the FEFAT pre-processing option will run the analysis also if requested.

When the pre-processing option has been selected, the user will be presented with a number of questions. The first question asks for the input file name. Click the OK button once a file name (jobname.fes) has been selected. Use the List button to list all available fatigue input files. These files have been created by the PAT3FAT or FATTRANS translator. The default will be the last jobname.fes created. Once a valid file name has been entered, the user will be presented with a summary of the jobname.fes file that has been opened. Each of these parameters can be changed or edited.

The following table explains each entry on the previous form.

If a partial analysis is specified then there is no post-analysis form. In the case of a full analysis then a results form is displayed with the results of the most damaged nodes similar to that presented by the results viewing module PFPOST. See Figure 5‑33.

This option allows a pre-processed job to be analyzed (or reanalyzed). The input file will be an intermediate results file produced within pre-processing and analysis as described in the previous section. The file extension is .fpp.

The Fatigue analysis only option has two principal forms. The first allows the input and output file names to be specified, and offers you the option of editing the preparedness job. Editable parameters are shown on Figure 5‑5. If no parameters are to be edited the second form will not appear.

After an analysis, a results form is displayed with the results of the most damaged nodes similar to that presented by the results viewing module PFPOST. See Figure 5‑33.

The MSC Fatigue factor of safety analysis option in FEFAT provides a set of semiautomatic tools to assess stress factors based on S-N or ε-N fatigue curves. It uses the pre-processed fatigue data from the global multi-node/element analysis (jobname.fpp file) to carry out its analyses. Both total life (S-N) and crack initiation (ε-N) calculation methods are supported by FEFAT.

This option calculates a factor of safety, i.e. over-design, which is reflected in the excess of the estimated life over the target life. Two analysis methods are provided: stress-based and life-based. Only the life-based method is available for crack initiation jobs.

FEFAT presents its results in the form of analysis summary reports. The factors reported are color coded for easy interpretation in PFPOST.

The stress-based method needs a fatigue endurance limit, and calculates the amount by which the stress must be factored to exceed that limit.

Factors less than 1.0 indicate a nonconservative design. For variable amplitude loading the worst scale factor is reported based on an analysis of the largest load cycle.

The life-based method performs a full fatigue back calculation to determine the stress scaling factor.

Analyses may be performed on a single node/element or for the entire data-set and every loading cycle.

The first screen to be presented when the program starts is shown in Figure 5‑7.

The fields on this screen are described below.

The next screen to appear allows for the setup of various parameters dependent on the analysis type selected. For a life-based factor of safety analysis, the form in Figure 5‑8 appears.

The fields to the life-based form are described here. See Fatigue Theory (Ch. 15) on the formulation of the factor of safety equation.

For a stress-based factor of safety analysis the form appears as in Figure 5‑9.

The stress-based parameters are described below. See Fatigue Theory (Ch. 15) on the formulation of the factor of safety equation

The safety factors are in a simple tabular format as shown below.

Results of unity suggest that the target life is exactly achieved and there is no safety factor margin. Results less than unity should be looked into for possible design changes immediately.

Having completed a global multi-node or multi-element analysis, the user will have identified an area of the structure that is either liable to fail at a life less than the design life or has such a long life that he wishes to explore manufacturing options using more cost effective methods and materials which will still achieve the target life. Alternatively, the user may already have some options for material selection or geometry detail which he wants to assess in terms of their effect on fatigue life.

The MSC Fatigue design optimization FEFAT provides a set of semi-automatic tools to assess fatigue design options. It uses the pre-processed fatigue data from the global multi-node/element analysis (jobname.fpp) file to carry out its analyses quickly. It supports a number of options including back calculation of parameter values which meet a target life, sensitivity studies on critical parameters, and an automatic material selection option based on fatigue criteria. Both total life and crack initiation calculation methods are supported by FEFAT including material and component S-N analyses.

Having selected the node or element of interest, FEFAT will carry out a single fatigue calculation based on the default parameters from the global multi‑node/element analysis and present the results in more comprehensive form than that available in the global analysis. The design optimization analysis options are then presented on a main analysis page from which the user can set up the optimization calculations.

FEFAT presents its results in the form of analysis summary reports, 3‑dimensional cycle or damage histograms, fatigue life sensitivity tables, and life versus parameter plots.

The operation of FEFAT from within the MSC Patran environment is almost identical to its operation in stand alone mode. The only difference is that in stand alone mode, the user must supply the jobname. (In direct mode from MSC Fatigue Pre & Post or MSC Patran, this is passed to FEFAT automatically.)

The first screen to be presented when the program starts is shown in Figure 5‑11. This screen will be skipped if FEFAT is entered from MSC Patran and the screen in Figure 5‑12 will be presented instead. This is because FEFAT already knows the jobname as specified from the MSC Fatigue menus. The node/element number will also be passed to FEFAT (see next page).

The fields on this screen are described below.

When all fields are filled in appropriately, click the OK button. At this stage, FEFAT carries out an initial analysis using the original fatigue analysis parameters defined when the fatigue job was set up. The life computed from this “stage 1" analysis is used as a benchmark against which all subsequent optimization calculations can be judged.

The results from this analysis are presented in a summary table on the screen and also written, with additional information, to the pfatigue.prt file. See Figure 5‑13.

The fatigue life is reported as a mean since the fatigue analysis is based on a rainflow cycle histogram where the damage for each range-mean bin in the histogram can lie between a minimum and maximum value. In practice, for real variable amplitude random loading, the mean damage is almost identical to the absolute damage (i.e., the damage calculated from the exact peak-valley data).

If a design life has been defined, a message will be written under the life result indicating whether the design life has been met or not. The three possible messages are:

The screen also shows the jobname and the node/element selected. Addition details are presented in the pfatigue.pat file. These details summarize the analysis parameters and will be specific to the type of job being carried out. Not all parameters will always be present.

Some of the parameters are not defined in the global analysis such as the stress concentration factor, residual stress, and the Miner’s constant. While these parameters are not available for editing at this stage, they are provided as analysis options in the design optimization input screens described later.

The stress concentration factor allows an additional stress raiser to be defined even though it is not modeled in the FEA and the residual stress allows for the local modeling of manufacturing or assembly stresses. Miners’ constant is the value at which failure occurs when the sum of the damage fractions from all the cycles equals this value. Some applications require this value to be different from one (see later). The generic histogram name is made up from the jobname and the node or element number. This name is used for the cycles and damage files which have the following naming conventions:

On the bottom of the screen is a distribution of damage summary. This feature helps to establish the nature of the fatigue problem and possible solutions. For a discussion of low- and high-cycle fatigue, see High Cycle versus Low Cycle Fatigue, 1308.

The algorithm for calculating the high- and low-cycle fatigue contributions is based on a consideration of the magnitude of each cycle and the part of the life curve used to calculate damage for that cycle. The reasoning behind this logic is that structures experiencing low-cycle fatigue problems require more ductile materials to extend their lives whereas high-cycle fatigue problems are solved by increasing the strength of the material and by carrying out surface treatments. An example of this dependence on the type of fatigue problem is shown in Table 5‑1 (Note that the magnitude of the percentage drop in life is dependent on other fatigue analysis parameters).

After going through the initial reanalysis of a particular node or element, the main analysis screen is shown in Figure 5‑14. From this menu, all the analysis options are available. The current jobname and node or element identity is shown at the top of the screen together with the Analysis and design life. The type of analysis that is reported on the screen depends on what action was last performed. Menu options which are followed by three dots indicate the existence of another form to be filled out.

To use this menu, choose the required option, set up the analysis parameters, and finally, when ready, select the Recalculate option to submit the analysis. A percentage complete message will inform the user of the progress of the calculations. A description of each menu pick follows.

This option is the back calculation facility where a design life is supplied and FEFAT’s automatic routines calculate the value of the chosen parameter that will achieve the target life; see Figure 5‑15. There are five fatigue analysis parameters which may be used in this type of calculation though not all parameters are available for all analysis types (material S-N, component S-N, and crack initiation). The parameters on which back calculation may be carried out are:

Having set up one of the optimization calculations, it is necessary to unset it in order to carry out any other kind of analysis. The easiest way to do this is to select the Original parameters option from the main menu. The other way is to select Change parameters followed by the parameter that was last set to back. The original default value will be offered and if accepted, the back calculation facility will be turned off. See also the section under User Preferences, 311 on Back calculation accuracy.

A sensitivity analysis allows the effect of variation in any of the input parameters on fatigue life to be explored, see Figure 5‑16. For some parameters, all possible values are used in the sensitivity analysis. Parameters which fall into this category are:

To use one of these types of analysis, simply select the option followed by Recalculate on the main menu. For other parameters which are specified in a numerical form, the user is requested to enter a range of values for the chosen parameter. Parameters which fall into this category are:

To select one of these types of analysis, simply select the option which will then present the user with a data input form at the bottom of the screen. In the box on this form, the user will be asked to provide a range of numbers for the parameter. Having done this, it is necessary to select the Recalculate option on the main menu

The material optimization allows for changing to a different material, editing the parameters associated with the current material dataset, and searching for a better or worse material. These tools facilitate the optimization of the materials selection in terms of fatigue performance. For example, a new material may be found that offers the same fatigue performance but has a lower raw material cost and is easier to work with in the manufacturing process. Alternatively, a possible fatigue failure could be designed out of the product by switching material and these tools would give a selection of alternative materials based on a fatigue selection criterion. Figure 5‑17 shows the Material optimization form.

The fields on this screen are defined below.

When the material choice has been optimized you the user will be returned to the main Design Optimization menu.

In the case of an analysis of a B57608 steel or aluminum weld, the material optimization route would follow that described in Material Management (Ch. 3).

This design optimization option allows for changing individual parameters or to reset individual parameters back to their original values. The Change Parameters form is shown in Figure 5‑18.

The fields on this screen are defined below.

The presentation of the results in both tabular and graphical form is handled from this menu. The options available are shown in Figure 5‑19 and discussed below:

Many of the options available from the top pull-down menus are generic to the MSC Fatigue modules and are described fully in Module Operations (Ch. 17) along with the commands that are applicable in the Command databox. Those specific to this display in the MTPD graphical module are described here.

Normally, design optimization will be carried out on the node or element which has the shortest life based on the assumption that the lives at all other nodes and elements will show at least the same change in life as the critical node/element. However, the lives at other nodes or elements will need checking especially where the surface parameters or additional local effects such as mean stress or stress concentration are different from those at the critical node or element. When this option is selected, a new node or element entry screen is presented with the same select options used on the main input screen such as already shown in Figure 5‑12. Having selected a new node or element, the user will be returned to the Design Optimization Analysis menu.

This option returns the user to the first input screen where the jobname is requested (see Figure 5‑11). The current jobname is presented as a default.

The preferences that may be set here are generally items which are not normally changed for every analysis (i.e., they are not job specific). Each parameter is described in Figure 5‑21.

If at any stage in the design optimization, the user wants to recall the original analysis parameters as defined in the global analysis, then this option will do this. This facility is particularly useful for turning off a previously defined back or sensitivity analysis setup. If the user only wants to reset certain parameters, then he should use the Change Parameters main menu pick.

Once the new analysis parameters have been defined, it is necessary to pick this option to start the re-analysis. Once this option has been selected, a message will appear to tell the user that the analysis parameters are being written to the pfatigue.prt file. A “fatigue analysis xx complete” message is used to report the stage of the analysis, where “xx” is a number between 0 and 100.

Picking this option causes FEFAT to return to the main menu, saving the analysis results summaries in the pfatigue.prt file.

This option allows for a multiaxial assessment of the stress state in your FE model subject to the service loadings. Various parameters can be investigated to give a good understanding of the stress state, whether it be in a uniaxial, proportional or non-proportional loading situation. Much discussion is given in Crack Initiation Solution Parameters, 30 and Multiaxial Fatigue (Ch. 6), particularly Multiaxial Fatigue Theory, 450 as to the usefulness of these parameters.

Briefly these parameters are the biaxiality ratio, ae which is defined as σ2/σ1 where σ1 is the largest in-plane absolute magnitude principal stress and σ2 is the other in-plane principal of a state of plane stress on the surface of the model, the out-of-plane principal being zero, and φ, being the angle that σ1 makes with the local x-axis. These parameters tell us the following:

Before resorting to a multiaxial fatigue analysis, an assessment should be made first. By selecting this option you are presented with this form.

The fields are:

A summary screen will be presented once the assessment is complete which lists the following information:

Once you have accepted the summary page you are presented with a menu for various plotting options.

The relevance of these plots is:

Output Time history provides an important link between test and analysis in the integrated durability management of which MSC Fatigue is a core tool.

It allows the fully pre-processed time history at a node or element to be exported and written to a standard time history .dac format. One or more nodes can be processed in a single run of this option. The local stress or strain histogram is produced by cycle counting the time history .dac file.

The time history .dac file is extremely useful to the understanding of local stress or strain response when working with strain measurements taken from a fatigue test component. These combined time history responses can be used in subsequent, more detailed, single location analyses such as those provide by Advanced Fatigue Utilities, 1033.

The option automatically plots single time histories graphically. For multiple time history creations use the ‘Graphically display a time history’ option on the main menu.

Time History Creation consists of a form on which you specify the input jobname.fes file and output .dac file names, and optionally edit the load cases that the new .dac file creation is based upon. See Figure 5‑24.

The fields are as follows:

If Edit Load Cases is set to Yes the edit form will appear. See Figure 5‑25.

Various operations can be carried out on the time history. They are applied according to the following formula:

[(Time history*scale factor) +offset]/Applied load

Its fields are as follows:

When the time histories have been created a page of results will be displayed. The results will be maximum and minimum stresses per node/element.

For details about time history plotting and plot manipulation see the next section.

Once a time history has been created from the preprocessed data or via the Output Time Histories option or created by any other means within PTIME, the time history can be graphically displayed. The MQLD module is used to display these plots. The explanation of time history graphical displays is identical to that already discussed in Plot an Entry Option, 217.

This option enables the user to extract and view a rainflow matrix for a particular node or element using both graphical and text based tools. The display option also allows cycle (jobnamenn.cyh) or damage (jobname.dhh) matrix to be plotted.

See Figure 5‑26 for the matrix options form.

The fields are as follows:

Plotting, or listing a matrix are further described below (extraction is dealt with on the form above).

When the user picks the plot Cycles histogram or the plot Damage histogram options from the results Display main menu pick, he is presented with a 2 and 3D graphical histogram presentation tool - the MP3D module. Both cycle and damage histograms produced by the design optimization fatigue analyzers may be viewed. The histograms are presented graphically and may be manipulated in a number of ways.

The functions of the plot pull-down menus are described in the table following Figure 5‑27. Many of the options available from the top pull-down menus are generic to the MSC Fatigue modules and are described fully in Module Operations (Ch. 17) along with the commands that are applicable in the Command databox. Those specific to the form displayed in Figure 5‑27 are described below.

As can be seen from, this option lists the values of all the range and mean bins, and the number of cycles in each bin Figure 5‑28.

Once a fatigue analysis has been completed, the results may be viewed tabularly using a module called PFPOST.

Detailed descriptions of the operation of PFPOST are given in Reviewing Results (PFPOST), 328.

This option consists of 5 utilities for converting .fes fatigue input files between various formats. Once the program has started, the user will be presented with a menu of options as described in Figure 5‑29.

The options menu looks like this:

An explanation of each follows:

In all cases above the user will be presented with a form asking for the appropriate input file and an output file name. An example of this file is shown below.

MSC Fatigue Input file - ASCII format
Job name : mask_ci
File revision=3
#
# ----------------- Job Descriptors ------------------------
#
Crack Initiation of Catcher’s Mask
#
# ----------------- Job Information ------------------------
#
Fatigue code=1 # Local Strain
FE results type=0 # Linear Static
Stress/strain=0 # Stresses
Results location=0 # @Node
Results co-ordinate system=0 # Full 3D
Stress/strain combination=3 # Abs Max principal
Number of load cases=1
Number of materials=2
Number of nodes=5601
#
Stress units=0 # MPa
Certainty of survival=50%
Damage model=0 # Smith-Watson-Topper
Elastic-plastic correction=0 # Neuber
Biaxiality correction=0 # None
#
# ---- Load case information ( 8 lines per load case ) -----
#
#
#Number of load cases=1
#
#
Load case number=1
Description=1.2-Default, Static Subcase
Load type=1
Load units=2
Applied FE load=0.16
Scale factor=1
Offset=0
Time history name=BALL_HITS
#
# --------------- Material/Group Information ---------------
#
#
#Number of materials=1
#
#
Group ID number=1
Material source=0 # Local/central databank
Group description=hex_mesh.25
Database name=
Material name=MANTEN
Sub-name=
Surface finish=12
Surface treatment=1
Stress/strain raiser (multiplier)=1
Stress/strain offset=0
Fatigue limit reduction (Kf) =1
Plastic stress concentration factor (Formzahl)=0 # 0 means Neuber
#
# ------------------- Stress Information -------------------
#
Node number=1 Group ID=1 Multiplier=1 Offset=0 Temp.=0
0.300007E0 -1.57632E1 1.825162E0 -5.864792E0 1.835311E0 0.587022E0
Node number=2 Group ID=1 Multiplier=1 Offset=0 Temp.=0
0.300619E0 -1.128119E1 5.086284E0 0.821963E0 3.335781E0 1.55936E0
Node number=3 Group ID=1 Multiplier=1 Offset=0 Temp.=0
1.967203E0 -3.0951E1 3.67726E0 -7.270879E0 1.536527E0 1.258015E0
Node number=-1