Information and insight from fracture mechanics experts

2014 European AFGROW Workshop Summary

Predict, Preferences Issues/Suggestions Output intervals printed in "hours" if the option to display life in hours is selected in the Output Intervals tab Also, the crack length plots should be converted to hours In the Propagation Limit tab, the option to stop at a cycle limit should be automatically switched to hours if the User has elected to display the life in hours. For the Lug Boundary Conditions tab, it is currently not possible to use the bearing B.C. for through cracks - this needs to be fixed ASAP. Add some type of warning in the lug dialog box to let User's know which B.C.s are being used - or at least notify them that the B.C.s are set in the Predict, Preferences dialog. Consider adding a failure criterion based on R-curve data Issues/Suggestions/Action Items Related to K-Solutions Consider adding an option to control the % of the axial load solution that is used to approximate the out-of-plane bending solution for straight through-the-thickness cracks. Action item for Jim: Compare the current oblique crack solution for the through crack at a hole to the results of differing % of the axial solution for a straight crack at a hole (in terms of the predicted life). Add a notification in the weight function dialog box to explain the limitations of the stress distributions for part-thru cracks. Add the capability to use the current 2-D User-Defined Beta model for 2, interdependent through cracks that can be assigned different plate thickness values. This is needed to accommodate the NASGRO crack growth rate model that is a function of thickness, but would also allow the local stress state to be estimated independently for each crack. It would also be a quick and easy way to solve additional cases without developing a Plug-In module. An additional crack growth model could be added to the Classic Interface showing an image of an "L" section with a crack defined along each leg with 2 thickness parameters. Action for Jim: Compare the results of using the beta correction capability in two separate runs (one for a part-thru crack to transition, and one for the thru crack portion) to a single run using my "optimized" transition method (part-thru --> thru crack). The purpose is to see if this will eliminate the error caused by the addition of integration points to avoid high slope changes when the normalized stress distribution is transitioned to 1.0 in the a-direction for radial distances > the plate thickness. Restore the beta correction capability for advanced through cracks at holes. It was disabled for the current release because of problems caused when two through cracks merge and the number of crack tips change. We will have to also come up with a workable method to address this issue. Note: The ability to use beta corrections for each loading case independently for both compression and tension (6, independent corrections as a function of crack length and a few other secondary parameters was presented. This capability is accessed from the Stress Intensity Filters menu. This is why the output has been expanded to display beta values for both tension and compression. Action item for Jim: Update the User's Guide to make it clear that the current offset correction for the bearing load case assumes the plate is constrained to prevent in-plane bending. General Interest Item Consider adding more warning messages to Users about input parameters than could result in poor life predictions. The concern here is for more novice Users who may be tempted to think that AFGROW's User Friendly Interface is a substitute for a good working knowledge of Fracture Mechanics. This will have to be a continuing effort since it is difficult to foresee all of the possibilities. New Spectrum Tool Add the ability to randomize load levels within a given sub-spectrum. Carefully consider issues related to cycle counting 3-channel load data. The goal of this new capability is obviously to allow the option for the axial, bending, & bearing stress fractions to change for each stress level as may be expected in reality. Northrop as been asking for this capability for some time, and they tell us that they have data ready for use. However, for general use, there are many ways to manage 3-channel spectra. One possible way would be to use a single channel spectrum that has been cycle counted directly from flight test data, and break out the 3 load cases based on some estimate of the load fractions for each level based on the users best information. This would be very tedious, but the use of the XML file format could be used to identify load cases and make it possible to help to automate the process. This type of improvement will have to continue to be a work in progress, and I'm sure there will be other good ideas. In any case, the new tool will be very useful for development of standard, single channel spectra. The XML format will also allow us to post-process AFGROW output files to do things like quantify which sub-spectra are most damaging.

ESRD User Workshop

Alex Litvinov (LexTech) gave a presentation at the ESRD (developers of StressCheck) User Workshop, May 15-16 in St. Louis, MO. To view his and other participants presentations, please visit the ESRD Workshop webpage. You will need to log-in first in order to download any of those presentations. 

Changing Your Log-In Password

In case you've forgotten/misplaced your login password and don't like the new one that's been assigned, here are the directions to change your password: 1.      Login with the generated password;2.      After you login in, navigate to following page. It is not on the menu, so you have to copy and paste the url:      Click edit, go to the right top part of the edit profile page, type your old password, new password twice, and click the “reset password” button.  

Bending Bug in Legacy AFGROW Version

While trying to verify some AFGROW solutions for the Single Corner Crack at Hole in bending, I was comparing the Stress Intensity Factors (SIFs) that I compute in a separate Excel spreadsheeet (VBA). My VBA program computes the SIFs from the original Newman Raju curve fits in Chapter 9 of Computational Methods in the Mechanics of Fracture, 1986. I am using version 06/29/2006, which I think is last version before AFGROW got really big (since it didn’t include the Fawaz curve fits for corner crack(s) at a hole)—I am running AFGROW on an old computer which is severely RAM limited compared to 2010 computers.   There are actually 3 references given in the AFGROW help for this crack scenario and the bending load, The first one can be downloaded from the AFGROW site,, the 2nd can be downloaded from a NASA website, and the third is unavailable. 1) Newman, J.C., and Raju, I.S., "Stress Intensity Factor Equations for Cracks in Three-Dimensional Bodies Subjected to Tension and Bending Loads," Chapter 9, Computational Methods in the Mechanics of Fracture, Elsevier Science Publishers B.V., 1986 2) Zhao, W., J. C. Newman, Jr., M. A. Sutton, X. R. Wu, and K. N. Shivakumar, "Analysis of Corner Cracks at Hole by a 3-D Weight Function Method with Stresses from Finite Element Method," NASA Technical Memorandum 110144, July 1995. 3) Zhao, W. and Newman, Jr., J. C., Electronic Communication, Unpublished NASA Langley Research Center Results, 24 February 1998.   I noticed substantial differences between what I was computing with my VBA (20% or more) and what AFGROW was computing, suggesting that whatever equations were in Ref. 2 and 3 were affecting my results:   My configuration was double (n=2) or single corner crack (n=1), so I show results for both:                            VBA          VBA                            AFGROW               AFGROW n                        SIFc           SIFa                                SIFc                         SIFa 1                     3.95091       3.6774                             3.952                        3.678 2                     4.01409       3.73621                           4.112                        3.736   SIFc is the SIF for crack “c” and SIFa applies to crack “a.” VBA are my spreadsheet solutions. Geometry was 2 inches wide, thickness 0.25 inches, hole diameter 0.25 inches, and both crack dimensions are 0.05 inches. Bending stress is 10 ksi, no tension (BSR=1, TSR=0, no bearing). Interesting that all these SIFs are close except for the double corner crack, SIFc.   After double checking all of my equations, I contacted AFGROW support to try to resolve the differences. Turns out there is a 4th reference not mentioned in Help that summarizes the equations used to modify the bending SIFs from the Ref. 1 above that you can download from AFGROW’s site: Modification of Stress Intensity Factor Equation for Corner Cracks From a Hole Under Remote Bending by Zhao and Newman.   Jim Harter gave me this document, which contains modifications to 3 sub-equations in the Newman Raju curve fits, coded those into my VBA; still had a difference. I went to another computer that had a more recent version of AFGROW, 07/26/2010, which gave me the following comparison:                                                                                 AFGROW               AFGROW                          VBA          VBA                               n                        SIFc           SIFa                                SIFc                         SIFa 1                     3.95091       3.6774                             3.9509                      3.6774 2                     4.01409       3.73621                           4.0141                      3.7362    Turns out there is a small undocumented bug in this old version of AFGROW; mystery solved.

Rules of Thumb

Several Users have asked me to post some helpful "rules of thumb" that may be useful when performing life prediction tasks. While I am happy to do it, I must first state that while I have found this information to be useful, each user must take full responsibility for their own life predictions. Neither I nor LexTech take any responsibility for how this information is used. Typical Retardation Parameters Willenborg SOLR Aluminum (2.5 to 3.0) Titanium Alloys (2.2 to 2.7) Steel (1.8 to 2.2) Closure Model Aluminum (3.8 to 4.2) Crack Growth Rate Data Aluminum Alloys Threshold @R=0 (1 to 2) Note, while this is the typical range, I have seen reputable data as high as 3 in a few cases. Always look at data for a similar alloy if there are no reliable data for the alloy of interest. A typical crossing point for most aluminum alloys is approximately 1.0 E-5 in/cycle at Delta K = 10 Ksi-sqrt(in) for R=0 Also, note that a "double-knee" is typical for aluminum alloys tested in air. The first "knee" generally ocurrs at approx. 4.0E-07, and the second around 2.0E-8 in/cycle.
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