AFGROW Users Guide

AFGROW Background:

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 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.  
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 [1] 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 2.0 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 debugging 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 1.0 was faster. George was the first to have written a version of MODGRO in the C language. Additions to version 2.0 of the code included a plasticity based closure model, which was based on work by Erdogan, Irwin, Elber, M. Creager, and Sunder [2, 3, and 4]. The model is a variable amplitude closure model and more detail is contained in this report. There is also credit due to Mitch Kaplan [5] 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 2.0 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.  
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 3.0 to the C language for UNIX to provide performance and portability to several UNIX Workstations [6]. 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 was given to the Navy sponsor and WL/FIBE management. After the presentation, the WL/FIBE Branch Chief (Mr. Jerome Pearson) requested that the code be renamed AFGROW, Version 3.0. Work on the Windows 95 version of AFGROW was started in October of 1996.  
Since work on the Windows95 version of AFGROW commenced in 1996, it has become the main version for new capabilities and enhancements. A composite bonded repair crack growth analysis capability was added during 1996-97. The bonded repair capability was based entirely on work by Dr. Mohan Ratwani [7]. In addition, a strain-life based crack initiation analysis capability was added. The strain-life initiation analysis capability was taken from APES, Inc. [8]. During reorganizations at Wright-Patterson AFB in 1997, it was decided that AFGROW would not receive further research and development funds. As a result, the on-site software development support provided by Analytical Services and Materials was reduced significantly. 
Since the Windows95 version of AFGROW had become most widely used, it was decided to discontinue the UNIX support. Recent advances in windows hardware capability has made it possible for AFGROW to equal and even surpass the performance capabilities of many UNIX systems. The Air Force organization responsible for AFGROW development was changed from WL/FIBEC to AFRL/VASE during the most recent reorganization in 1998.  
The AFGROW user base continued to grow dramatically in 1998. Air Force Air Logistic 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 have been added to the code. AFGROW now treats each crack tip as a separate object, and is in the position to be able to accommodate the analysis of a large number of cracks.