XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Introduction
The Software Strain Gauge (SSG) is a Test-Analysis Correlation Tool. This is a fatigue specialist tool for correlating measured responses with those extracted from FE analysis. Throughout this chapter, certain knowledge by the user of standard MSC.Fatigue terminology, awareness, and usage is assumed. If you are unfamiliar with MSC.Fatigue, it is suggested that you read chapters 1 through 5 before attempting to use this tool.
The software strain gauge is a software tool which allows a direct correlation to be made between measured strain histories obtained using resistance strain gauges and predicted strain histories from the surface of finite element models. This is achieved by allowing the application of simulated strain gauges to the surface of the FE model in the same positions as real strain gauges on the corresponding component.
The simulated gauges consist of one or more thin shell elements which are fitted to the surface of the FE model. The gauges can then be used to extract the results of previously carried out FE stress/strain analyses at the locations and in the orientations defined. Then, using MSC.Fatigue and one of its modules called SSG it is possible to predict the stress or strain histories from the gauges in a way which is directly comparable with direct strain measurements.
In addition to its use as a correlation tool, the software strain gauge has other uses. For instance, the weld fatigue analysis techniques described in BS7608 require structural stresses on welds to be determined from finite elements by a process of extrapolating the stresses to the weld toe, using stresses up to a certain distance from the weld toe. It can be difficult to obtain this information in an automated way. The problem can be overcome by simulating the recommended experimental technique of placing a strain gauge of 3 to 6 mm in size at 0.1 x (sheet thickness) from the weld toe.
Users may also find the software strain gauge useful simply for obtaining static stress and strain results from particular locations within elements, or in particular directions.
In addition to the software strain gauge tool, this option of MSC.Fatigue also includes a module (MSSA) for processing rosette data and creating outputs suitable for use by either the stress or strain-life fatigue analyzers MCLF or MSLF. It also provides an indication of the state of multiaxiality present and suggests possible processing routes through the fatigue analyzers.
Basic Information
The software strain gauge requires first that a FE stress/strain analysis or analyses be carried out, and that the model and results be read into the MSC.Fatigue Pre & Post or MSC.Patran database. The software strain gauge supports both static and transient results. It is normally expected that the results be elastic.
Note: | You do not have to carry out an elastic-plastic FE analysis. |
Having carried out the FE stress analyses, the following programs are normally used in the course of an analysis:
1. Data Preparation
| Preparation of Material Data |
| Preparation of Time Histories including an ASCII to Binary Convertor |
| A Multi-file Display Program A Text Editor for adding new gauges to a gauge definition file may also necessary. |
2. Strain Gauge Management
| Position, Modification, and Deletion of Gauges and Extraction of Results |
3. Time History Response Creation
| Model database (MSC.Patran) to Fatigue Input Translator |
| New model database (MSC.Patran) to Fatigue Input Translator |
| Creation of Stress or Strain Histories |
| A Multi-file Display for Results Comparison Purposes with Measured Strains |
4. General Utilities
| FES File ASCII/Binary Convertor |
| Terminal Driver |
| Binary to Binary File Convertor |
Analysis Route
The basic steps in a software strain gauge analysis are as follows:
1. First run a finite element analysis. This can be either a transient analysis, or a set of static load cases, such as might be suitable for use in a standard MSC.Fatigue quasi-static analysis. Then read the model and results into the MSC.Patran or MSC.Fatigue Pre & Post database.
2. Set the Analysis Type to Soft S/G for software strain gauges in the main form. Select the Gauge Tool option. Use the Gauge Tool to apply the numbers and types of gauges required. See
Creating and Modifying Gauges, 850.
Gauges are selected from a gauge definition file
gauges.def which is described in
The Gauge Definition File, 846. The Gauge Tool allows gauges to be positioned, modified and deleted. Placement of gauges is described in more detail in the section entitled “Applying and Modifying Gauges and Extracting Results”. Note that if you want the software to calculate elastic-plastic strain histories, you should specify “plastic” on the Gauge Tool form.
3. When all the gauges are positioned as required, return to the Soft S/G menu and start up the Results Extraction form. Use this form to select the results cases to be extracted, and the gauges to which the results will be written. See
Extracting the FE Results, 853.
5. Run SSG either by Pressing on “Rosette Analysis” on the Soft S/G menu, or by typing
ssg at the system prompt using the newly created .
fes fatigue input file. See
Software Strain Gauge Module (SSG), 859.