Copy the file simpleSN.op2 to a clean working directory to begin.

A linear static finite element analysis has been performed already with a load magnitude of 10,000 Newtons. To begin, import this model and results information into a new database using MSC Fatigue Pre & Post or use MSC Patran. Open a new database from File | New and call it keyhole. The model was run through a MSC Nastran analysis so keep the Analysis Preference set to MSC Nastran when asked.

Press the Import toggle switch (Analysis in MSC Patran) on the main form. When the form appears, set the Action to Access Results, the Object to Read Output2, and the Method to Both (model and results); then, press the Select Results File button and select the file simpleSN.op2. The model will then appear and you are ready to set up a fatigue analysis.

For all fatigue and fracture analyses thus far, we have been defining the three major inputs: geometry, materials, and loading. This is no different for a Crack Growth analysis except that geometry definition takes on a different form than what we have dealt with to this point. For the Total Life and Crack Initiation methods we have been tightly linked to the FE analysis and the stress/strain distribution. This link to FE analysis is much weaker for the Crack Growth method in that the only information necessary is the remote stress used in the Paris Equation, defined as cyclic range of stress intensity, ΔK,

where

and the definition of stress intensity, defined as driving force, K, the applied stress, σ, the crack size, a

where

The driving force behind a crack that causes it to propagate is not stress or strain but the stress intensity factor, K. (This is not to be confused with stress concentration Kt.) The stress intensity factor accounts for both the stress and the crack size and is a way of describing the stress field around a crack tip independent of the overall geometry. The relationship between stress intensity, stress, and crack length is known as the fracture mechanics triangle. If you know two of the corners you can derive the other.

MSC Fatigue contains a library of standard crack geometries from which you can derive a compliance function. Open the main MSC Fatigue setup form and set the Analysis to Growth. Then open the Solution Params... form. On the top of the form you will see a button called Compliance Generator. Press this button. An MSC Fatigue module called PKSOL will initiate that will give you access to the compliance function library.

The first thing that you are asked for is the units in which to define the geometry. Select 1. Millimeters. Then select option four, 4. Generate a Y function table. Call it keyhole when asked. A file called keyhole.ksn will be created containing the compliance function lookup table.

Two more menus will be presented to you to select the library entry to use. Select option 1. Standard specimens and then option 8. Compact tension specimen (CTS). You will then be presented with a graphic representation of the geometry to which you will specify dimensions. The dimension of our keyhole model are as shown here.

Press the Define button on the top of the form. At this point you will be asked for the dimensions of the geometry, B (thickness) and W (width). Enter 9.525 and 94 mm. Press the carriage return each time and a final carriage return when asked if any changes are necessary. To generate the compliance function, select the Calculate item on top of the form.

At this point you are presented with one final menu selection which allows you to either plot, tabulate or create another compliance function. Select Plot Y function against crack ratio.

This plot gives you a good feel as to how the crack will grow as it gets larger. In this case, as the crack ratio (a/W) increases, the crack growth rate will accelerate whereas, at first, the growth rate will be much slower. When you are done select File | Exit to close the plot and then quit from PKSOL by
selecting eXit.