AFGROW

Information and insight from fracture mechanics experts

New Features/Capabilities in AFGROW Release 5.3

AFGROW, Release 5.3 includes a number of new capabilities that have resulted from User requests and input at our Workshops. While we haven’t been able to include all requests, we are happy to announce the following list of improvements and additions. 64 Bit Executable Code Converting AFGROW to 64 bit solves some compatibility issues with other 64 bit Windows software when using COM automation. It also provides a path forward for future Windows OS upgrades. Advanced Bearing Solution for Through Cracked Holes This solution has been long requested because it allows Advanced Models under combined loading conditions to retain the bearing portion of the loading after transition to single or double through cracks. Since this capability will be added, we are also including an out-of-plane bending option that can be used for Advanced through crack models. Out-of-Plane Bending Option In all previous AFGROW releases, out-of-plane bending solution for Classic through crack models have been approximated as 2/3 of the applied tension classic solution. This was done to allow bending effects to be conservatively approximated after transition where the crack was assumed to be straight through crack (single crack length through-the-thickness). Many Users attending our Workshops felt that this approximation was far too conservative, and a factor of 0.4 was more realistic. After comparing analytical results to our oblique crack solution, it looked like the factor varied between 0.4 and 0.5 as a function of thickness. Since the factor, 2/3, has been used to date, we think it makes sense to add a new user-defined preference with a default value of 2/3, but allow users to make a choice from 0.4 to 2/3. As noted above, this will also be applied for Advanced Models where a bending fraction is used for crack(s) at holes. However, it should also be noted that the correction in these cases will be applied to the corresponding Advanced Model Axial solution. Advanced, Axial Load Solution for a Corner Crack at a C/S Knuckle This solution is based on the excellent work done by Jody Cronenberger at SWRI. Jody’s solution set is for a centered hole. We hope to expand this capability in the future to include hole offset and the bearing load case. User-Defined Beta for Interdependent Through Cracks This was another Workshop request to allow user-defined betas to be input for two crack growth directions where the width and thickness of the model may be different for each crack length. The term, interdependent, is used because for many applications the beta solutions will be dependent on the length of the other crack. This means that the user will have to make an assumption regarding the crack lengths expected during the analysis. Examples where this capability may be helpful are: cracks growing through two legs of a stiffener, or a part-through crack where a user has some expectation of the natural crack shape. The ability to use different thickness values for each length is needed for the former example where stress state will be a function of thickness for each crack length and if the NASGRO growth rate model is used, thickness is required as a parameter. New Weight Function Solution for Part-Through Cracks The new solution was provided by Prof. Glinka for use in cases where the stress distribution is changing in the c-direction. In all earlier releases, part-through weight function solutions were limited to stress fields changing in the a-direction. None of these solutions will allow stress fields to change in both directions simultaneously, but the new solution is far more practical. New Weight Function Solutions for Through Crack in Finite Plates We are adding two new weight function solutions from Prof. Glinka for edge through cracks in finite plates. One solution where in-plane rotation is constrained along the cracked edge, and the other has in-plane rotational constraints imposed on the far edge. Using Different Tabular Material Data for Orthogonal Crack Growth Directions This allows users to input different tabular crack growth rate data for the a-direction and c-direction. Normally, crack growth data for the L-T orientation is used for both crack directions. However, as we try to expand our capabilities to predict actual crack shapes seen in the field (or in laboratory tests), it is helpful to have the option to use different data when they may be available. New Spectrum Format An option for XML format is included to allow spectrum levels to be “tagged” to identify temperature (altitude/humidity) or for use in later releases to track damage for different spectrum stress levels. Using Different Tabular Material Data as a Function of the Loading Spectrum This release will allow the use of different tabular crack growth rate data as a function of temperature tags in an xml-based loading spectra. We anticipate that this capability may be expanded in the future to include other tags. Tabular Crack Growth Rate Data Curve Fits Tabular fits for over 60 materials in our Fracture Mechanics Database will be available for download through our AFMAT database on the AFGROW Web Page. We will continue to expand this database as we add more material data to the database.

Through Cracked Holes - Bi-Axial Loading Conditions

One of the items of interest at our recent European Workshop was the solution for through cracks at holes under bi-axial loading. We have recently done some modeling work to look at this case, and have some results for a few basic cases.   Plate Width: 4 inches Plate Height: 4 inches Diameter: 0.25 inches   The following load cases were considered: Load Case 1: σ1 = 1, σ2 = 0 Load Case 2: σ1 = 0, σ2 = 1 Load Case 3: σ1 = 1, σ2 = 1 Load Case 4: σ1 = 1, σ2 = 0.5  

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.

Annual AFGROW User Workshop

The 3rd Annual AFGROW User Workshop was held at the Davis Conference Center in Layton, UT on 11 - 12 Sep 2012. 54 representatives from different companies and government agencies were in attendance. We would like to thank all of our attendees and speakers who have made the AFGROW User Workshop 2012 such a success. As a result of this year,s Workshop, example plug-ins for a corner cracks at a countersunk hole and multiple straight shank holes will be demonstrated. These plug-ins will use StressCheck ® as the real-time K-solver. The 2012 AFGROW User Workshop proceedings are now available online. 

Removing Bonded Repair Capability

As we approach the first commercial release of AFGROW, it makes sense to do a little "code cleaning." Based on user input (or lack thereof), it appears that the bonded repair analysis capability is not being used. Therefore, we are planning to remove the capability from the new release. We are asking for input from anyone who would be adversly affected by this decision.