AFGROW History

AFGROW's history traces back to a crack growth life prediction program (ASDGRO) which was written in BASIC for IBM-PCs by Mr. Ed Davidson at ASD/ENSF in the early-mid 1980's. In 1985, ASDGRO was used as the basis for crack growth analysis for the Sikorsky H-53 Helicopter under contract to Warner-Robins ALC. The program was modified to utilize very large load spectra, approximate stress intensity solutions for cracks in arbitrary stress fields, and use a tabular crack growth rate relationship based on the Walker equation on a point-by-point basis (Harter T-Method). The point loaded crack solution from the Tada, Paris, and Irwin Stress Intensity Factor Handbook was originally used to determine K (for arbitrary stress fields) by integration over the crack length using the unflawed stress distribution independently for each crack dimension. After discussions with Dr. Jack Lincoln (ASD/ENSF), a new method was developed by Mr. Frank Grimsley (AFWAL/FIBEC) to determine stress intensity, which used a 2-D Gaussian integration scheme with Richardson Extrapolation which was optimized by Dr. George Sendeckyj (AFWAL/FIBEC). The resulting program was named MODGRO since it was a modified version of ASDGRO. .

Early Years

In 1987, James Harter came to work for the Air Force Wright Aeronautical Laboratories (AFWAL/FIBEC) and rewrote MODGRO, Version 1.X (still in BASIC for PC DOS). Over the next 2 years, a tabular crack growth rate database was added. Decreasing-increasing crack growth rate tests were performed to obtain data below 1.0E-08 inches/cycle for 7075-T651 Aluminum and 4340 Steel. During that period, MODGRO, Version 1.X included part-through flaw solutions from Newman and Raju, and standard closed-form solutions for symmetrical through-cracks (center, single edge, and double edge cracks). These solutions could also be modified for arbitrary stress fields using a Gaussian integration method with a stress distribution defined by the ratio of the unflawed stress field of interest divided by the unflawed stress field for the baseline geometry. The error in this method, of course, increases with crack length, but error in life is minor since the majority of life is consumed while the crack lengths are relatively short.

In 1989, MODGRO, Version 2.X was rewritten in Turbo Pascal for PC-DOS as a move to a more structured computer language. At that time, Dr. George Sendeckyj provided MUCH assistance in de-bugging and optimizing the arithmetic operations. George was also learning the C language and was practicing by translating the BASIC code to Structured BASIC and then C at the same time I was coding it in Turbo Pascal. Runtime comparisons were made in the spirit of friendly competition. Actually, George's C version of MODGRO, Version 1.X was faster. George was the first to have written a version of MODGRO in the C language. Additions to version 2.X of the code included a plasticity based closure model, which was based on work by Erdogan, Irwin, Elber, M. Creager, and Sundar. The model is a variable amplitude closure model and more detail will be available in the on-line help in the code. There is also credit due to Mitch Kaplan because of his good suggestion to only recalculate the beta (or alpha) values at user defined crack growth increments. It was decided to simply use the user-input value for the Vroman integration percentage, which is normally used when analyzing blocked spectra. A real-time crack length plotting capability was also added to the program. The code was totally changed in the process, but the name MODGRO remained.

From 1990-1993 the code changed very little (still released in Turbo Pascal). Small changes/repairs were made based on errors that were discovered. The code was used to help manage the flight test program for the X-29. During high angle-of-attack maneuvers, the vertical tail experienced severe buffeting. MODGRO, Version 2.X was used by NASA/Dryden to estimate the vertical tail life from actual flight test data collected for each flight. The use of the code allowed the Program Managers to assess the effect of various flight maneuvers on the vertical tail, and in some cases, flights were re-arranged to maximize the amount of flight data and minimize tail damage accumulation.

Transition to AFGROW

In 1993, the Navy was interested in using MODGRO to assist in a program to assess the effect of certain (classified) environments on the damage tolerance of aircraft. The Navy wanted to build a user-friendly code to be used in the program and initiated an agreement with WL/FIBEC to develop a state-of-the-art user interface with the added capability to perform life analysis under adverse environments. This effort required additional manpower for software development and baseline crack growth testing. On-site contract support was used to meet this requirement. Work began at that time to convert the MODGRO, Version 3.X to the C language for UNIX to provide performance AND portability to several UNIX Workstations. The workstation platform was chosen to provide additional computational power for MODGRO.

In 1994, a research contract with Analytical Services and Materials was established to provide support for the Navy effort and assist in future research and development requirements of WL/FIBEC. This was when the current UNIX interface was born. In July, 1994, a presentation of the results for the Navy project to the Navy sponsor was given and WL/FIBE management. After the presentation, the WL/FIBE Branch Chief (Mr. Jerome Pearson) requested that the code be renamed AFGROW, Version 3.X. .

A composite repair patch analysis capability was added to the UNIX version of AFGROW. It was a starting point to allow damage tolerance assessments to be made using real flight spectra to determine the life of a cracked metallic structure after a composite repair patch is applied. Other improvements have been made to the closure model, and additional stress intensity solutions have been added. Work on the Windows 95 version of AFGROW was started in October of 1996.

Since 1996, the Windows based version of AFGROW has replaced the UNIX version since the demand for the UNIX version did not justify the cost to maintain it. There was also an experiment to port AFGROW to the Mac OS. The Mac version had the same problem (lack of demand) as the UNIX version. An automated capability was added to AFGROW in the form of a Microsoft Component Object Model (COM) interface. The AFGROW COM interface allows users to use AFGROW as the crack growth analysis engine for any Windows based software.

Final Air Force Development Work

The AFGROW user base continued to grow dramatically in 1998. Air Force Air Logistics Center (ALC) use and strong support for the code was greatly responsible for additional funding, provided in late 1998, for multiple crack and time dependent analysis capabilities. The Air Force Aging Aircraft Office (ASC/SMS) provided these funds. As a result of this funding these new features were added to the code. AFGROW treats each crack tip as a separate object, and is able to accommodate the analysis of a large number of cracks.

One of the biggest challenges to the multiple crack analysis capability was the design of the user interface. A drag and drop design interface was added for the more advanced multiple crack capability.

The advanced model feature was added to allow users to model cases with two, independent cracks (with and without holes). This feature was expanded to cover more combinations of corner and through-the-thickness cracks. Dr. Scott Fawaz provided tabular stress intensity solutions for double, non-symmetric corner cracks under axial, bending, and bearing loading. These solutions included tabular data for 2 points (a and c-dimensions) and a multiple points (11) along each crack front. These solutions and more information are available in the open literature, allow AFGROW to handle cases with more than one crack growing from a row of fastener holes.

The COM capabilities in AFGROW have allowed it to be used with an external K-solver program to communicate with AFGROW to perform real time crack growth analysis for multiple cracks (more than two) and cracks growing in complex and/or unique structure. Additional stress intensity solutions and spectrum load interaction models have been added to AFGROW. Finally, user-defined plug-in modules may now be used by AFGROW to allow users to include proprietary or unique stress intensity solutions.

A user-defined plug-in stress intensity model capability has also been added to AFGROW. This allows users to create their own stress intensity solutions in the form of a Windows DLL (dynamic link library). Drawing tools have been included in AFGROW to allow the user-defined solution to be animated during the analysis. Interactive stress intensity solutions have been demonstrated using AFGROW to perform life predictions while sending geometric data to an external FEM code, which returns updated stress intensity solutions back to AFGROW.

Commercial Development

In 2008, the Air Force Research Laboratory ceased all development, training, and support funding for AFGROW. LexTech, Inc. has taken over these tasks, and Mr. James Harter continues to provide consulting support directly to LexTech. Funding for features of interest to the A-10 and T-38 has been funded directly from these Program Offices. Other development has been funded by LexTech, Inc.

AFGROW, Version 5 was released in 2010 as the first commercial version of AFGROW. New capabilities include: Spectrum Filtering, Stress Intensity Filtering, K-Solution for Continuing Damage, and a Fracture Mechanics Database. A spectrum filtering capability was added to allow spectrum tension and compression values to be modified independently using tabular interpolation or an embedded java script engine. The modification is based primarily on the c-dimension, but allows other parameters to be used with the java script option.

A stress intensity filtering capability allows users to modify beta values for any load case (axial, bending, and/or bearing) to be modified independently for tensile and compressive spectrum loading. The most practical application is the ability to set bearing beta values to zero for compressive spectrum loads.

Advanced, tabular stress intensity solutions for non-symmetric corner cracks at straight shank holes are available for axial, bending, and bearing load cases. Tabular stress intensity solutions are also included for non-symmetric corner cracks at countersunk holes under axial and bending load cases. However, when the first crack transitions to become a through-the-thickness crack, both cracks must transition to a through crack based on the current c-dimension at the time of transition. Also, the load case is changed become 100% axial at transition. There are currently no advanced solutions available for combinations of corner and through cracks, and no bending and bearing load case solutions for advanced, non-symmetrical through cracks.

Continuing damage crack solutions for corner and through-the-thickness cracks under axial loading were added to allow users to continue to predict crack growth for situations in which a crack grows from a hole to the near edge of a plate. A crack on the other side of the hole is modeled as a crack growing from a slotted edge notch with a root radius equal to the hole radius.

An extensive fracture mechanics database with a search engine is available to all users who have purchase a license for AFGROW, Version 5 or later and remain current with yearly software maintenance.

A new Spectrum Management Tool is now available for download for all users with a current AFGROW license. This tool is very user friendly, and will allow many new capabilities to be added in the future.

Future Plans

Work has been completed for an advanced solution for non-symmetric through cracks at holes under bearing loading. Many of our users have been waiting for this capability, and it will be released in version 5.3.

Another important capability to be added in release 5.3 is the ability to use crack growth rate data for different crack growth directions. In addition, the infrastructure is also available to use different crack growth rate data as a function of a given sub-spectra through the use of tags in a new XML based spectrum format that will be supported by version 5.3 and the new Spectrum Management Tool.

The Spectrum Management Tool currently includes an option to create 3 channels of loading for axial, bending, and bearing load cases. This capability will be supported in a future release.

We plan to include advanced tabular stress intensity solutions for non-symmetric corner cracks at countersunk holes under bearing loading in a future release. Solutions for combinations of corner and through cracks will also be added as soon as they are available and have been tested.

We have also been asked to investigate a solution for cracked holes under biaxial loading. Work in this area is currently underway.

The ability to analyze cracks that grow out-of-plane is an area that will require a substantial effort. It is hoped that work can begin as soon as possible in this area as time and funding permit.

As always, the developers of AFGROW will continue to listen to user comments and suggestions to improve the code.

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