The MSC Fatigue analysis module FEVIB performs many different tasks from simple file conversions to full blown vibration fatigue analysis. Module operation of each of these tasks is described in detail in this section.

FEVIB handles all the processing, data file import and export or conversion, the actual S-N fatigue analysis and all post-processing and design optimization activities. In many ways, it is very similar to the standard fatigue analysis module FEFAT described in Total Life and Crack Initiation (Ch. 5).

The operation of FEVIB can be in one of 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 perform the analysis. (In direct mode from a preprocessor such as MSC Patran, these are passed to FEVIB automatically.) FEVIB can be accessed directly from the operating system prompt by typing the symbol fevib. Once FEVIB 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.

A standard S-N or crack initiation fatigue analysis is split into two parts: the pre-processing (cycle counting) and the analysis. For vibration fatigue analysis, these processes are integral. A rainflow cycle count and the actual fatigue life are the results of a vibration fatigue analysis only when the Dirlik + mean method is used. Other methods only produce a fatigue life. Therefore, when running the global multi-node/element analysis, FEVIB will always run the complete analysis without stopping after a preprocessing stage, unlike FEFAT.

When the Global 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:

For an initial global analysis, you will be presented with a tabular listing of the top most damaged locations. For more result and postprocessing options, you can now proceed to Design Optimization, 634 or Results Postprocessing, 652.

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 FEVIB provides a set of semi-automatic tools to assess fatigue design options. It is almost identical in its usage to the standard time-based fatigue module FEFAT. Unlike FEFAT, it does not begin with a rainflow histogram but rather, uses the same input file as the global analysis, the difference being that you are investigating a single location as opposed to the entire model (or a portion thereof). 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.

Having selected the node or element of interest, FEVIB 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.

FEVIB presents its results in the form of analysis summary reports, 3‑dimensional cycle histograms, and fatigue life sensitivity tables.

The operation of FEVIB 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 on entry to the program. (In direct mode from MSC Fatigue Pre & Post or MSC Patran, this is passed to FEVIB automatically.)

The first screen to be presented when the program starts is shown in Figure 8‑16. This screen will be skipped if FEVIB is entered from MSC Patran and the screen in Figure 8‑17 will be presented instead. This is because FEVIB already knows the jobname as specified from the MSC Fatigue menus. The node/element number will also be passed to FEVIB.

The fields on these screens are described below.

When all fields are filled in appropriately, click the OK button. At this stage, FEVIB 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 to the pfatigue.prt file. See Figure 8‑18.

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 other details presented summarize the analysis parameters. These details are specific to the type of job being carried out and not all parameters will always be present.

Some of the parameters are not defined in the global analysis such as other analysis methods, global offset stress, the Miner’s constant, and others. 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.

For the Dirlik + mean analysis option a cycle histogram file is created. The generic histogram name is made up from the jobname and the node or element number. This name is used for the cycles file which has the following naming conventions:

After going through the initial reanalysis of a particular node or element, the main analysis screen is shown in Figure 8‑19. 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 design life. When a “back” or sensitivity analysis is defined, the type of analysis is reported in this area of the screen also.

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 FEVIB’s automatic routines calculate the value of the chosen parameter that will achieve the target life; see Figure 8‑20. There are three fatigue analysis parameters which may be used in this type of calculation. 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, 649 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 8‑21. 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 8‑22 shows the Material optimization form.

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

This design optimization option allows for changing individual parameters or to reset individual parameters back to their original values. This form is shown in Figure 8‑23.

The presentation of the results in both tabular and graphical form is handled from this menu. The options available are shown in Figure 8‑24 and discussed below:

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 8‑17. 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 8‑16). 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 8‑25.

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 FEVIB to return to the main menu, saving the analysis results summaries in the pfatigue.prt file.

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

It allows the output response power spectrum to be output at a node or element to be exported and written to a standard time history .dac format (with a .psd extension). One or more nodes can be processed in a single run of this option resulting in a number of .psd files.

The output power spectrum .psd 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 responses can be used in subsequent single location analyses such as those provide by Frequency Fatigue Life Estimation (MFLF), 656 which may provide even more insight into the results of the analysis.

The option automatically also allows for graphical display of the output response spectrum. You can also use the Graphically Display a PSD, 652 option from the main menu.

Output Power Spectrum creation consists of two forms. First you specify the input jobname.fes file. Then a form displaying the output .psd file names and a number of other inputs is shown. See Figure 8‑26.

The fields are as follows:

Once a power spectrum has been created via the Output Power Spectrum option or created by any other means within PTIME, the time history or FEVIB, it can be graphically displayed. The module MTPD is used to display these plots. The explanation of power spectrum graphical displays is identical to that already discussed in Plot an Entry Option, 217.

This option reviews the fatigue job output file jobname.fef and displays user selected items to a tabular listing using the module PFPOST.

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

For vibration fatigue, the content of the jobname.fef file is different than that of the time-based fatigue result files. It consists of seven columns of data aside from the location (node/element) number itself. The columns of data in this file are:

The actual format of this file is either a MSC Patran nodal or elemental results file described in The Results Files (jobname.fef/fos), 382.

This option consists of a number utilities for converting .dac files between various formats. Once the program has started, the user will be presented with a menu of options as described in Figure 8‑27.

The options menu looks like this:

The utilities are identical to those described in the FEFAT module. See Utilities, 324.

FEVIB analysis can be operated in batch mode with the following keywords:

Design optimization mode is generally not used in batch mode.

Example:

This batch line would open mymodel.fes and perform the vibration fatigue analysis with defaults used for all other parameters. Any existing output files with the same prefix name as the input file would be overwritten.

Example:

fevib /opt=u/uopt=a/inp=mymodel/out=mymodel/ov=y/*=tt

This example selects the utility option in FEVIB and executes the submenu option of creating an ASCII file from a binary file (A), with a jobname of mymodel. It will search for a file called jobname.fes and create an output file called jobname.asc.