Introduction to PTIME

Fatigue User’s Guide > Loading Management > Introduction to PTIME

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PTIME is a loading (time series, histogram, PSDF) database manager which has been designed to enable the MSC Fatigue user to manipulate and manage time history and other data file types. PTIME provides a secure system for recording information about the loading. By using PTIME, MSC Fatigue jobs may be archived and restored at a later date with no loss of information. This is essential for the successful re-analysis of the component.

The time history and other loading type files themselves are not loaded into the database, but are resident in the local directory together with the ptime.tdb file which contains the associated database data for each loading file. The loading information are held in binary files with default file type filename.dac (.cyh or .psd for matrix files or PSDFs respectively). See for DAC File Format Description, 249.

PTIME may be accessed from the MSC Fatigue main menu under the Loading Information form by depressing the Database Manager button on the top of the form. It can also be run in batch mode by typing ptime in a command window as described on page 162.

Once initiated, PTIME will present a set of screen displays which may be manipulated using the keyboard and mouse. A description of the way to use the screen displays is given in Module Operations (Ch. 17) for the Motif and Mask drivers as well as some of the generic capabilities of these drivers. The Motif driver will be referred to throughout this chapter.

When first invoked, PTIME appears displaying two forms when in the motif driver.

Figure 4‑1 PTIME Utility Form

The top, small form is a generic form and allows for general program control. This is discussed in detail in Module Operations (Ch. 17), using the Motif driver.

The menu structure for PTIME is shown in Figure 4‑2. The top menu contains options to further submenus such as the Change option. This multi-menu layering has been necessary to ensure the legibility and usability of each menu.

If the database is being created for the first time, the first menu to be presented will automatically be the Add submenu.

Figure 4‑2 The PTIME Menu Structure

Using PTIME to Create Loading Inputs

The first step in setting up the loading inputs is to determine what loading inputs are required. These differ for each FE analysis type. These requirements are set out below.


FEA Type	Description
Linear Static	The variation of load with time for every linear static analysis load case must be defined for the MSC Fatigue analysis. The only exception is where the FE analysis models a non time varying stress state such as residual stresses from forming or preload stress. These non-varying loads are referred to as STATIC load cases. The association of the time histories with the individual FE load cases and their subsequent superposition in the case of multiple load cases give rise to the response time history necessary to feed the fatigue analysis.
Transient	The time history for transient FEA is defined at the FE job setup stage and used in the FEA itself. Consequently, the response (stress or strain) time history is embedded in the FE results and no other external time history is required beyond the one used in the FE analysis itself. The use of PTIME is not necessary for these types of FE analyses.
Frequency Response	Also known as steady state dynamics. In a similar way to the linear static analysis, this analysis requires a description of the load input in the form of power spectral density function (PSDF). In the case of multiple loading inputs, the definitions of the cross correlation between inputs using cross spectral density functions (CSDF) is allowed. The relationship between load cases and loading PSDFs/CSDFs is defined using an n x n matrix where n is the number of load inputs.
Random Vibration	Also known as Random Response. Again in a similar way to linear transient, no input PSDFs are required for this analysis type since they are defined in the FE analysis itself and the response PSDFs are created directly from the FE analysis. The use of PTIME is not necessary for these types of FE analyses.

All the functions to add an entry to the PTIME databank or to create a binary loading description file which is then registered in the databank are selected from the “Add an entry…” form. The specific functions required for each analysis type are detailed below.

Linear Static Analysis

The time history data may be in a number of forms. These are described in the table below together with the tools/action path to import or create the required PTIME entries.


Time History Format	PTIME Tool
Standard Time History	Copy it from the central databank (Copy from central) and apply a calibration factor, setting the load type and units to match the FEA loading.
Constant Amplitude	Create using Waveform creation or Block program option with 1 cycle in 1 block.
Block loading	Create using Block program option defining the number of cycles and their size for each block.
.dac Files	Load the file - .dac means the file must be in the MSC Fatigue single parameter binary format.
ASCII File- Value and Time Pairs	Use the ASCII convert + load option. The Sample Rate (in samples/second or Hz) can be any real number. Linear interpolation is used to calculate intermediate values. Finally set Equally Spaced Data to X-y pairs. If the data is arranged in columns and there are 2 columns (a time column and a value column) then Take all Values should be set to Yes; if not select No and refer to the documentation for more information.
ASCII File - Fixed Time Increment	Use the ASCII convert + load option. The Sample Rate (in samples/second or Hz) can be any real number. Linear interpolation is used to calculate intermediate values. Finally set Equally Spaced Data to Y-values only. If the data is arranged in columns and there is just 1 column containing the values, then Take all Values should be set to Yes, if not select No and refer to the documentation for more information.
List of Numbers	Either: Create an ASCII file using a text editor and use ASCII convert + load option, or Use the X-y time series option to enter the values interactively.
Rainflow Matrix Input	In this case, there must be just 1 load case and 1 FEA result. Load the rainflow matrix (must be format .cyh file) or create a Rainflow matrix from a time history using the rainflow Matrix option.

Transient Analysis

No PTIME action is required.

Frequency Response Analysis

The PSDF and CSDF data may be in a number of forms. These are described in the table below together with the tools/action path to import or create the required PTIME entries.


Loading Format	PTIME Tool
Standard Loading PSDF	Copy it from the central databank (Copy from central). Apply a calibration factor and set the load type and units to match the FEA loading using the Change an entry option. Note that the Units of the PSD are (Load)2/Hz - set the Load Type to the correct value of Load, e.g. g2/Hz implies Load Type = g
ASCII File - Values and Frequency Pairs	Use the ASCII convert + load option. The Frequency Rate (points/Hz) can be any real number. Linear interpolation is used to calculate intermediate values. Finally set Equally Spaced Data to X-y pairs. If the data is arranged in columns and there are 2 columns (a frequency column and a value column) then Take all Values should be set to Yes. If not, select No and refer to ASCII Convert + Load, 186.
ASCII File - Fixed Frequency increment	Use the ASCII convert + load option. The Frequency Rate (points/Hz) can be any real number. Linear interpolation is used to calculate intermediate values. Finally set Equally Spaced Data to Y-values only. If the data is arranged in columns and there is just 1 column containing the values, then Take all Values should be set to Yes. If not, select No and refer to the documentation for more information.
List of PSDF/CSDF - Frequency Numbers	Either: Create an ASCII file using a text editor and use ASCII convert + load option, or Use the x-Y psd entry option to enter the values interactively in a spreadsheet.
ASCII File - PSDF Matrix	The PSDF Matrix defines the correlation between PSDFs, CSDFs and load cases. The ASCII file must be formatted in n columns by n rows where n is the number of load cases and is defined on line one. The file type for the ASCII file must be .pmx. The names of the PSDFs must be defined on the leading diagonal, e.g. test101.psd. The cross correlation file names must be defined in the relevant cross matrix positions. If no cross term is required, a zero (0) or the word NONE must be entered. To import an ASCII PSDF Matrix into the PTIME databank, ensure that all PSDF and CSDF files have been defined in the PTIME databank. Select the ASCII convert + load option. Choose the .pmx file, and set Data Type to psd Matrix. Set the Load Type and Units in the same way as for loading PSDFs.
Interactive Creation	Select the Psd matrix option. Define the Load Type and Units in the same way as for loading PSDFs. Set the Matrix Size; normally equal to the number of load cases. Input the PSDF and CSDF file names into the spreadsheet. These file names must be already defined in the PTIME databank.

Random Vibration Analysis

No PTIME action is required or necessary.