AFGROW (Air Force Grow) is a Damage Tolerance Analysis (DTA) framework that allows users to analyze crack initiation, fatigue crack growth, and fracture to predict the life of metallic structures. Originally developed by The Air Force Research Laboratory, AFGROW is mainly used for aerospace applications, but can be applied to any type of metallic structure that experiences fatigue cracking. AFGROW is now being independently developed and maintained by LexTech, Inc.
The stress intensity factor library provides models for over 30 different crack geometries (including tension, bending and bearing loading for many cases). In addition, an advanced, multiple crack capability allows AFGROW to analyze two independent cracks in a plate (including hole effects), non-symmetric corner cracked. Finite Element (FE) based solutions are available for two, non-symmetric through cracks at holes as well as cracks growing toward holes. This capability allows AFGROW to handle cases with more than one crack growing from a row of fastener holes.
AFGROW implements five different material models (Forman Equation, Walker Equation, Tabular lookup, Harter-T Method and NASGRO Equation) to determine crack growth per applied cyclic loading. Other AFGROW user options include five load interaction (retardation) models (Closure, FASTRAN, Hsu, Wheeler, and Generalized Willenborg), a strain-life based fatigue crack initiation model, and the ability to perform a crack growth analysis with the effect of the bonded repair. AFGROW also includes useful tools such as: user-defined stress intensity solutions, user-defined beta modification factors (ability to estimate stress intensity factors for cases, which may not be an exact match for one of the stress intensity solutions in the AFGROW library), a residual stress analysis capability, cycle counting, and the ability to automatically transfer output data to Microsoft Excel.
AFGROW provides COM (Component Object Model) Automation interfaces that allow users to build scripts in other Windows applications to perform repetitive tasks or control AFGROW from their applications.
AFGROW also has new plug-in crack geometry interface that allows AFGROW to interface with any structural analysis program capable of calculating stress intensity factors (K) in the Windows environment. Users may create their own stress intensity solutions by writing and compiling dynamic link libraries (DLLs) using relatively simple codes. This includes the ability to animate the crack growth as is done in all other native AFGROW solutions. This interface also makes it possible for FE analysis software (for example, StressCheck) to feed AFGROW three-dimensional based stress intensity information throughout the crack life prediction process, allowing for a tremendous amount of analytical flexibility.
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-1980s. 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.
Many upgrades were made during the late 1980s and early 1990s. The primary improvement was modifying the coding language from BASIC to Turbo Pascal and C. Numerous small changes/repairs were made based on errors that were discovered. During this time period, NASA/Dryden implemented MODGRO in the analysis for the flight test program for the X-29.
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. 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.
In 1994, the results of the Navy project were presented to the Navy sponsor and MODGRO was renamed AFGROW, Version 3.X.
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.
An advanced model feature has been added to allow users to select cases with two, independent cracks (with and without holes). This feature continues to be improved and expanded to cover more combinations of corner and through-the-thickness cracks. 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.
Verification testing is a continuing process to improve AFGROW and expand the available database. There are plans to continue to add new technology and improvements to AFGROW. A Consortium has been started with users in Government and Industry to combine the best fracture mechanics methods available.