The MSC.Fatigue analysis module SPOTW performs a number of tasks including Global Analysis, Results Listing and Design optimization. Module operation of each of these tasks is described in detail in this section.

The operation of SPOTW can be in three modes: by spawning from the MSC.Fatigue Pre & Post or MSC.Patran environment, in stand alone mode by typing spotw at the system prompt or in batch mode. The only difference is that in stand alone mode, the user must supply the jobname. (In direct mode from MSC.Patran, these are passed to SPOTW automatically). Once SPOTW has been initiated in either of these modes, two windows will be presented with the Motif driver. 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.

Note that the fatigue analysis may take some time. It may be worth considering operating SPOTW in batch. Batch operation is discussed in SPOTW Batch Operation, 782.

When the “Estimate Fatigue Life” 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 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 that has been opened. Each of these parameters may be changed or edited.

The following table explains each entry on the previous form.

When this form is complete, select OK and the analysis will run. While the analysis is running, a form appears which indicates (spotwelds calculated/total number of weld spots), current element, current node (0 indicates the weld nugget is being calculated) and the current angle.

If you wish to stop the analysis, select Pause. You will be prompted to quit or continue the analysis.

When the analysis is complete, a results page appears (see Figure 9‑10). This provides a summary of the results for the ten worst spot welds. The information provided is Damage, Life in Repeats, Life in Equivalent Units. If More is selected, more information is provided about the same list of elements. This form is shown in Figure 9‑11.

The information on this form for each element is, the results location (Sheet1, Sheet2 or the Nugget), the angle at which most damage is predicted, the node number (0 if failure is at the nugget) and the maximum Force through the spot weld.

Having completed a global multi-element analysis, the user will have identified an area of the structure which is of particular interest, for instance, it may be likely to fail before or close to the design life, or it may comfortably exceed the design life, offering opportunity to reduce cost by reducing the strength.

The design optimization option within SPOTW provides a set of semi-automatic tools to assess fatigue design options. It makes calculations for single elements based on the information in the jobname.fes and jobname.fef files. It supports a number of options including back calculation of parameter values which meet a target life, sensitivity studies on critical parameters and a material selection option.

Having selected the node or element of interest, SPOTW will carry out a single fatigue calculation based on the parameters from the global multi-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.

SPOTW presents its results in the form of analysis summary reports, 3 dimensional cycle or damage histograms, fatigue life sensitivity tables, life versus parameter plots, polar damage versus angle plots, and stress history plots.

The operation of SPOTW from within the MSC.Patran environment is identical to its operation in stand-alone mode.

The first screen to be presented when the program starts is shown in Figure 9‑12. Certain information about the analysis that has been carried out is stored in the results (.fef) file. For this reason, it is necessary to specify the results file as well as the input file name.

When this form is accepted the form illustrated in Figure 9‑13 is seen.

The fields on these screens are described below.

The calculated fatigue life is reported, and underneath this a message is written which indicates whether the design life has been met or not. The three possible messages are:

The other details summarize the analysis parameters and certain other results information. The jobname is given, together with the selected Element ID. The node ID helps to identify which end of the spot weld is more likely to fail. This will be set to 0 if the weld spot is predicted to fail through the nugget. Further information is provided about the location of the failure by telling whether the weld is more likely to fail in Sheet 1, Sheet 2 or the Nugget, and indicating at which angle (relative to the gauge coordinate system) the gauge is most likely to crack. The rest of the information gives the dimensions of the spot weld and the sheet metal, the material datasets used in the analysis, and the design criterion.

After going through the initial reanalysis of a particular node or element, the main analysis screen appears as shown in Figure 9‑12. From this menu, all the analysis options are available. The current jobname and element identity is shown at the top of the screen together with the design life. When a back or sensitivity calculation is defined, the type of analysis is reported in this area of the screen also. Menu options which are followed by 3 dots indicate a submenu hangs from that option.

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 SPOTW's automatic routines calculate the value of the chosen parameter that will achieve the target life; see Figure 9‑12.

There are four fatigue analysis parameters which may be used in Parameter Optimization. The parameters on which back calculation may be carried out are:

A sensitivity analysis allows the effect of variation in any of the input parameters on fatigue life to be explored, see Figure 9‑16.

To select one of these types of analysis, simply select the option which will then present the user with a data input form. 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 form allows for changing to a different material dataset for either of the sheets, or for the weld nugget. This facility may help the optimization of the design through selection of a better (or worse and less expensive) material. Note that the S-N curves required for this analysis are Spot Weld S-N curves, and may have no obvious relationship to the parent plate from which the spot welds are formed. Figure 9‑17 shows the Material Optimization Form.

This design optimization option allows for changing individual parameters or to reset individual parameters back to their original values. This submenu us shown in Figure 9‑18.

The presentation of the results in both tabular and graphical form is handled from this menu. The options available are shown in Figure 9‑19 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 are described here.

Often, design optimization will be carried out on the element that has the shortest life. However, the lives at other elements may also be of concern. When this option is selected, a new element entry screen is presented with the same select options used on the main input screen such as already shown in Figure 9‑15. Having selected a new element, the user will be returned to the Design Optimization Analysis menu.

This option returns the user to the first input screen where the job names are requested. See Figure 9‑15. The current job names are presented as defaults.

The preferences that can be set here are the back calculation accuracy and the Miners Damage Sum. See Figure 9‑22.

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 reanalysis.

Picking this option causes SPOTW to return to the main menu.

List Global results provides a simple way of viewing a summary of a results file (.fef file). When this option is selected, the form illustrated in Figure 9‑23 appears. Use the list button to select the jobname.fef file required.

When this form is accepted, a summary of fatigue results appears as illustrated in Figure 9‑10 and Figure 9‑11.

The List .spt File allows the user to list results in a more detailed way for individual spot welds. When this option is selected, the form illustrated in Figure 9‑24 appears.

When this form has been accepted, a results form similar to that shown in Figure 9‑25 appears. This form has a listing of the parameter selected, for Sheet 1, Sheet 2, and the Nugget (if calculated), for all the angles calculated.

This option allows the user to create a polar plot of Damage, Life, Maximum Effective Stress Range, Maximum Effective Stress or Minimum effective stress as a function of angle around the spot weld. The angle is defined as the angle around the axis of the spot weld with respect to the MSC.Fatigue spot weld x-axis.

When this option is selected a form appears as shown in Figure 9‑26.

An example of this type of display is shown in Figure 9‑21. Typically 3 datasets are plotted - for Sheet 1, Sheet 2, and the Nugget (unless of course the Nugget has not been calculated). This functionality is duplicated in Design Optimization, 758.

The current techniques incorporated into the spot weld analyzer have been developed to make calculations for cases where two sheets are spot welded together. In general, joints with three or more sheets spot welded together are more difficult to make efficiently and undesirable from a durability point. They should be avoided as much as possible in design. Sometimes they may be unavoidable, or alternative designs may be uneconomic.

There is no reason why you should not analyze cases where three sheets are spotwelded together by treating them as two separate spot welds, but the analysis methods currently used are not validated for these cases. This problem is the subject of a current research project. Until a validated solution to this problem has been found, a temporary fix has been provided, called the “3 sheet correction.”

If you model the three sheet spot welds as two spot welds, you very often get failures predicted on the middle sheet, which rarely occur in practice. For this reason, a simple fix has been implemented. First, run a spot weld analysis, treating three sheet spot welds as two normal spot welds joined together. Selecting the “3 sheet correction” option, and the jobname.spt result file just created will detect three sheet spot welds in the .spt file and will create a new jobname.fef results file in which failure at the middle sheet (the common node between the two spot welds) is ignored. The worst result for the remaining calculation points is written to both spot weld elements in the resulting .fef file.

This option makes post-processing the results easier by eliminating spurious predicted failures at the middle sheet. Note that if the middle sheet really would fail, this won't be predicted either! However this does not appear to happen much in practice.

For descriptions of the Job Information File (jobname.fin), the Submit Script (FatigueSubmit), and Fatigue Input File (jobname.fes), see Description of Files, 336 onwards.

The main differences in terms of file formats between Spot Weld Analysis and other global analyses are:

The results from a Spot Weld analysis are written to an elemental results file, which is a standard MSC.Patran results text file. An example of a Spot Weld Results file is shown in Figure 9‑27. The format of this file type is described in The Results Files (jobname.fef/fos), 382.

The header of the file contains the following information.

Then for each element there are 10 columns of information

The flat results file is an ASCII file which stores information about the damage, maximum stress and minimum stress for every calculation point. It also stores that maximum force through each element. A typical example of a .spt file is shown in Figure 9‑28. The first line in Figure 9‑28 consist of: the element number, the node number (0 indicates this block of data represents the weld nugget), the maximum force, the angle of worst damage (0 may mean no damage) and the worst damage.

The next 36 lines have 4 columns - angle, damage, maximum stress, minimum stress.

Following this block the next two blocks contain the same data but for Sheet 1 and Sheet 2.

Batch operation has the following batch keywords:

spotw	/opt=e/inp=myjob/ov=y/*=tt

This batch line would run a spot weld analysis in batch where the first option is called out (Estimate Fatigue Life) and the jobname is myjob. All the defaults are used for the rest of the inputs such as matrix size and Miners sum. The results files will be called jobname.fef and jobname.spt by default.