Stress Properties |
 
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Stress Properties |
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You have the option of defining effective geometric properties to element sets for use in calculating stresses. These geometric properties include element external and internal diameters, cross-sectional area, and moments of area about the local element axes. Naturally this option is strictly relevant to fatigue analyses only, as general cycle counting analyses do not involve any stress computations. Even for fatigue analyses, it may seem curious that you might want to specify geometric properties, given that these values have already been input to Flexcom. The reason is because Flexcom does not echo this data to timetrace output files, and LifeTime fatigue analyses may be performed solely on the basis Flexcom timetrace output. In fact, earlier versions of Flexcom required that time histories of axial force, Y bending moment and Z bending moment at each hot spot be stored via timetrace output, whereas more recent versions also allow you to store/retrieve this data using the more widely used database output. So it is conceivable that, if you are using timetrace output as the source of axial force and bending moment data, and have not requested any database output at all, you would need to specify relevant stress properties in LifeTime itself.
If the fatigue analysis is based on timetrace output, and if database output (even a minimal content is sufficient, as explained below) is also available, then LifeTime can read the required structural data from that source, thus eliminating the need for a repeat specification of this data. In this case, LifeTime reads the name of each Flexcom analysis and opens the appropriate timetrace output files for that analysis. The program also checks for the existence of a database output file from the analysis. If a database exists, the program retrieves the external and internal diameter values for each hot spot from that file, and then uses these values to calculate cross-section areas and moments of inertia. There are a number of important points to note with regard to this facility:
•Only one database needs to exist, and it can correspond to any analysis in your list.
•LifeTime reads database output once only, regardless of how many database files exist. Once it has retrieved the required structural properties from a database file, the program does not check for the existence of database output from any other referenced dynamic analysis.
•The database file need not contain very much actual results output. The inputs required by LifeTime are written by default to a header block at the start of the database file at the time of the first output to the database. So in fact database output at only one time is sufficient, and the actual amount of output can be minimised accordingly.
•If database output exists but you specify structural properties for some or all hot spot sets, the values you input in LifeTime take precedence over the values in the database.
There are several keywords which may be used to assign effective structural properties to element sets for use in calculating stresses.
•*PROPERTIES in the $MODEL section.
•*PROPERTIES in the $DATABASE POSTPROCESSING section.
•*PROPERTIES in the $LIFETIME FATIGUE section.