XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX''"> Review the Results
Open the MSC Fatigue Results... form. No color contour plotting is available with Crack Growth.
Tabular Listing
On the MSC Fatigue Results... form, change the Action to List Results and click Apply. This will start the module PCPOST which tabularly lists the fracture analysis results and also plots the final situation.
The main menu of PCPOST includes a host of items, the most useful perhaps, being the Results summary page. Selecting this will reveal that the crack grew to a bit over 10 mm before fracture and took over 400 repeats of the SAETRN time history. The mode of failure is also revealed (stress intensity exceeding the fracture toughness of the material). Click the End button to continue.
Also plot the final a-N curve. This plot will be described in more detail shortly. This reveals how the crack grew over time. Select Return to return to the main menu of PCPOST.
Interpolate Crack Sizes
One of the interesting things you can do with PCPOST is interpolate the life based on different crack sizes. Select Interpolate life. Now you can enter a different initial crack length or a different final crack length or both. It will then, on-the-fly, report back to you the interpolated life. It is very possible that the actual initial crack is much larger than previously thought. With this tool you can quickly assess any deleterious effect this may have on product life. Note that an extra millimeter initial crack length (3.54 mm) will half the life.
Press eXit to leave PCPOST.
Interactive Operation
Back on the MSC Fatigue Results... form, change the Action to Optimize and click Apply. This will invoke the Crack Growth analyzer PCRACK.
By running PCRACK interactively we can rerun the entire Crack Growth analysis and make any changes necessary. You will be presented with a number of setup screens before the job is started.
1. Loading Definition: The first of these is where you can alter the scaling factor or impose a constant residual offset. Accept all the defaults by clicking the OK button.
2. Output Parameters: The second screen is for graphical updates and general output parameters. Change the Results File Output Interval and the Screen Update Interval to 0.25 Repeats. This is done so that the updates do not occur at the end of the signal but somewhere in the middle of the signal where the crack growth rate and stress intensity are more certainly non-zero. This interval is simply a snap-shot at a particular time or location in the signal; so we will get four snap-shots per repeat of the signal. Click OK to continue.
3. Local Geometry Definition: On this page you select the compliance function. Click OK. The form then updates to allow you to modify initial and final crack length specifications and notch dimensions. Accept the defaults and click OK.
4. Material and Environment Selection: The last page allows for selection of material and environment. Accept the defaults and click OK.
The Crack Growth analysis will initiate and you will be presented with a graphical screen that updates as the crack grows. You will see the plot update as well as the numbers on the top of the plot and the modifying effects to the right of the plot. The plot features crack size versus life in cycles. The following explanations are given from left-to-right and top-to-bottom:
1. Repeats: This is self explanatory. This reports the number of repeats of the time history that the component has survived.
2. Size: This reports the length of the crack at the given snap-shot in time.
3. DLKAPP: This is the apparent stress intensity (ΔK) or the apparent crack driving force without accounting for any modifying effects.
4. DLKEFF: This is the effective stress intensity (ΔK) or the actual crack driving force which is based on the apparent ΔK with modifying effects. All modifying effects are listed to the right of the plot. If a modifying effect is highlighted, it was being experienced at that particular snap-shot in time.
5. da/dN: This is the current crack growth rate at the reported time.
6. CLOSURE: This is an effect used to modify the apparent ΔΚ. When this modifying effect is lit, the component is currently experiencing crack closure which necessarily slows down the crack growth.
7. HISTORY: This modifying effect to the apparent ΔΚ is caused by the sequence of cycles. A large cycle followed by many smaller cycles can actually cause a slow down in the crack growth rate due to an extension of the plastic zone around the crack tip. This is called crack retardation. It than takes some time for the driving force to become sufficiently large to overcome that plastic zone and continue the crack propagation.
8. NOTCH: In our example we modeled an additional notch into our compact tension specimen. The influence of a notch is also accounted for as a modifying effect to ΔK apparent.
9. ENVIRON: If we had selected a material and used an alternate environment (a function of the material properties), this modifying effect would be lit.
10. STAT FRAC: Static fracture modes are accounted for also as modifying effects. These occur when the driving force approaches the fracture toughness of the material causing the crack to grow rather rapidly.
All of these effects are discussed in more detail in the MSC Fatigue User’s Guide. When the analysis is done, you are presented with a page describing the final situation just as was done when using the result listing facility, PCPOST. Click the End button when you are satisfied that the same answers are given as the original analysis. The PCRACK main Post Analysis Menu will appear. This is very similar to the Design Optimization main menu of FEFAT for Crack Initiation and Total Life jobs.
Optimization
From this Post Analysis Menu you can do numerous things such as view the final situation graphically or tabularly, interpolate on life as has been discussed already, or change any of the original parameters without redoing the entire job setup.
As an exercise, let us change the material from MANTEN to RQC100 as was done with the original Total Life analysis to see the effects on the propagation life of the component. Select Edit analysis parameters | Select material and environment. When this form appears, change the material to RQC100 and click OK. Press or double-click the Recalculate switch on the main menu. You will be asked to allow overwrite of the output file. Select the Yes button. The job will restart using the new material.
Note that with RQC100, which is a higher strength steel than MANTEN, the propagation life is a little bit longer (closer to 500 Repeats) but the final crack size is a couple of millimeters shorter (~8 mm) before ultimate failure. This would indicate that it is a more brittle material and less resistant to plastic deformation. Click the End button to go back to the main menu and then select final a-N graph. Note the final situation plot has a much flatter, constant slope confirming our suspicion. Select File | Exit and then press or double-click the eXit button when done to quit from PCRACK.
