XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Software Strain Gauge Module (SSG)
This section explains the operation of SSG to extract stress or strain histories once soft gauges have been created, the FE results extracted and an appropriate fatigue input file (jobname.fes) has been created.
The program SSG (Software Strain-Gauge) can be started by clicking on the “SSG Analysis” item on the Soft S/G menu, or from the system prompt (by typing ssg) or by including a suitable line into a batch script.
Interactive Operation
When run interactively, SSG may be used with a Motif or Mask interface, according to user preference. The figures used here depict the Motif interface. This interface works according to the standard of all other MSC.Fatigue modules as explained in
Module Operations (Ch. 17). When the program is started, the screens appears as shown below for the Motif interface.
Figure 11‑10 The SSG Utilities Menu
Figure 11‑11 Software Strain Gauge (SSG) Main Form
Enter the name of the input (.
fes) file, or select one using the List option. Once an input file is selected in
Figure 11‑11 the rest of the form becomes active.
The rest of the fields are described below:
Field | Description |
Gauge Name | Any one gauge can be selected, or the default, which is ALL. If the default is selected, all the gauges included in the .fes file will be processed, according to the information set up on the Strain Gauge Creation and MSC.Fatigue forms. The default analysis route is to calculate the strain histories for all the gauges, giving either elastic histories (el) or elastic-plastic histories (ep) estimated using the notch correction procedure specified in the MSC.Fatigue job file. Each gauge will produce strain histories with names: <jobname>xxxyy.dac where xxx is the gauge number, and yy is the channel number, referring to the gauge element (001, 002 or 003). |
Gauge Type | Alternatively, the user may select only one of the gauges from the toggle list. There are then two possibilities: • If the strain gauge is specified as an elastic gauge, i.e. the gauge name includes “el”, for example “dms_t_1_el_057”, the gauge type will be displayed, and the user is prompted to ask for either elastic stress or elastic strain. • If the strain gauge is specified as an elastic-plastic gauge, i.e. the gauge name includes “ep”, for example “dms_t_3_ep_135” the gauge type will be displayed, and the user is prompted to ask for either stress or strain, followed by all the other available elastic-plastic options. |
Output Type | Select either stress or strain, according to whether you need stress output files or strain output files. |
Apply Hoffmann-Seeger | If Hoffmann Seeger is set to “No”, each strain gauge leg will be treated as if it is in a uniaxial stress field, i.e. no correction will be made for the state of biaxiality. If “Yes” is selected, the Hoffmann-Seeger method will be applied, treating each gauge leg independently, and assuming proportional loading, i.e. that the biaxiality ratio is constant (the mean value is used) and the orientation of principal stress axes is fixed (orientation of absolute maximum principal is taken from the most popular bin). |
E-P Correction | If either the Mertens-Dittmann or the Seeger Beste method is selected, the user is prompted for a shape factor which will be applied to all three gauge legs. |
Shape Factor | The shape factor to be applied for E-P Correction. |
The result of running the software will be a number of .dac files with the naming convention “jobnameyyyzz.dac” where yyy=gauge i.d. and zz=gauge leg (e.g. 01, 02, and 03 for a rosette). These can be post-processed using any appropriate MSC.Fatigue module.
Technical Details
Elastic Strain and Stress Calculation
The elastic strains and elastic stresses output from the strain gauge are the direct strains and stresses parallel to the axes of gauges 1-3, calculated using the full 3D stress tensor, the Young’s modulus E and the Poisson’s ration
.
Elastic-plastic Strain and Stress Calculation
Elastic-plastic strains and stresses are calculated using either the Neuber method or a modified version of the Hoffmann-Seeger method. Both these methods can also be combined with the Mertens-Dittmann or Seeger-Beste methods.
If the Neuber method is chosen (i.e. Hoffmann-Seeger = No), the elastic-plastic stresses and strains will be calculated using the Neuber method, based on the elastic value of the direct strain, and taking no account of the multiaxial state of stress - i.e. using the uniaxial material properties.
If the Hoffmann-Seeger method is selected, the mean value of the biaxiality ratio and the most popular orientation of the absolute maximum principal stress are calculated for each gauge in exactly the same way as they are in MSC.Fatigue biaxiality analysis. An equivalent (von Mises) elastic strain history is then calculated on the basis of the direct strain along the axis of the gauge, the mean biaxiality ratio and the most popular angle, assuming that the ratio and angle are constant, i.e. proportional loading.
The Neuber correction is then carried out on the equivalent strain, and the angle and biaxiality ratio are used again to predict the direct stress and strain in the gauge. This process is only really valid for proportional loadings, but the implementation does not preclude its use for non-proportional loadings. There are a number of philosophical problems with using this method for non-proportional loadings, e.g. that the equivalent stress may be different for each gauge, and also be a function of gauge orientation. In the near future a non-proportional notch correction procedure will be implemented in this program, but this is not sufficiently well validated for inclusion at this stage.
Correlation with Test
Once the strain histories have been generated from the FE model, they may readily be compared with the corresponding measured strains from the real component. Here are a few possible methods for comparing them:
• Using the Multi-File Display module MMFD to overlay or cross-plot the data
• By comparing the statistics of the signals, max, min, RMS etc.
• Using the strain gauge rosette analysis option in MSSA
Correlation is a very important aspect of reliable durability calculations. If a correlation exercise indicates that there is poor qualitative and quantitative correlation between predicted and measured strain histories, any fatigue calculations are also likely to give poor results. Likely causes of poor correlation are:
1. Errors in setting up the MSC.Fatigue job, particularly in matching the correct channels to the correct load cases with the correct scaling factors
2. Errors in calculating the loading histories
3. Poor definition of the loads and boundary conditions, or missing loads
4. Inadequate meshing
5. Inaccurate strain gauge placement
6. Inappropriate analysis (e.g. quasi-static when the problem is dynamic)
7. Poor materials
8. Non-Proportional Loadings together with high levels of plasticity.
SSG Batch Operation
The program can also be operated in batch mode, in the same way as any other MSC.Fatigue modules such as FEFAT. The syntax required of each batch line is:
<program name> /<keyword>=<value>/<keyword>=.......... etc.
or
<program name> @<filename> /<keyword>=<value> etc.
where <filename> is the name of a file containing keyword/value pairs, with one pair to a line:
/<keyword>=<value>
/<keyword>=<value>
etc.
If a keyword/value pair is not specified, the default value for that keyword will be used. A typical batch line might be:
ssg /inp=test.fes/gauge=dms_t_1_ep_003/hofseg=y/ov=y/\*=tt
meaning run SSG with an input file = test.fes, use gauge dms_1_ep_003, use the Hoffmann-Seeger method, overwrite any existing files, echo output to the screen and default the rest of the questions.
The batch keywords, meanings, and possible values relevant to the Software Strain Gauge are (defaults in bold):
/INP | Input filename: jobname.fes |
/GAUGE | Software strain gauge i.d.: e.g. dms_t_3_ep_047 or “all” |
/OTYPe | Output data type: Stress or Strain |
/HOFSEG | Use Hoffmann-Seeger method: Y, N |
/EPTYPE | Elastic-Plastic Correction: Neuber, Mertens-Dittmann, Seeger-Beste |
/SHAPe | Shape factor, a number - 0 or 1 < n < * |
/OV | Overwrite: Y,N |
<*> | Output to tt (screen), <filename>, none |
