AFGROW

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.

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