AFGROW

Last updated

AFGROW (Air Force Grow) is a Damage Tolerance Analysis (DTA) computer program that calculates crack initiation, fatigue crack growth, and fracture to predict the life of metallic structures. Originally developed by the Air Force Research Laboratory, AFGROW [1] is mainly used for aerospace applications, but can be applied to any type of metallic structure that experiences fatigue cracking.

Contents

History

AFGROW's history traces back to a crack growth life prediction program (ASDGRO) which was written in BASIC for IBM-PCs by E. 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 [2] 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. A new method was developed by F. Grimsley (AFWAL/FIBEC) to determine stress intensity, which used a 2-D Gaussian integration scheme with Richardson Extrapolation which was optimized by G. Sendeckyj (AFWAL/FIBEC). The resulting program was named MODGRO since it was a modified version of ASDGRO.

Many modifications were made during the late 1980s and early 1990s. The primary modification was changing 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, 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 but there was a lack of demand. An automated capability was added in the form of a Microsoft Component Object Model (COM) interface.

The program is now developed and maintained by LexTech, Inc.

Software architecture

The stress intensity factor library provides models for over 30 different crack geometries (including tension, bending and bearing loading for many cases). In addition, a multiple crack capability allows the analysis of two independent cracks in a plate (including hole effects) and a non-symmetric cracked corner. 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 the analysis of more than one crack growing from a row of fastener holes.

AFGROW implements five different crack growth models (Forman Equation, [3] Walker Equation, [4] Tabular lookup, Harter-T Method and NASGRO Equation [5] ) to determine crack growth per applied cyclic loading. Other user options include five load interaction (retardation) models (closure, [6] [7] Fastran, [8] Hsu, Wheeler, [9] and Generalized Willenborg [10] ), a strain-life based fatigue crack initiation model, and the ability to perform a crack growth analysis with the effect of the bonded repair. The program also includes tools such as: stress intensity solutions, 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 provided), a residual stress analysis capability, cycle counting, and the ability to automatically transfer output data to Microsoft Excel.

AFGROW uses COM (Component Object Model) Automation interfaces that allow the use of scripts in other Windows applications. The program has a plug-in crack geometry interface that interfaces with structural analysis programs 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. This interface also makes it possible for finite element analysis software to provide three-dimensional based stress intensity information throughout the crack life prediction process.

It is possible to select cases with two, independent cracks (with and without holes). A plug-in stress intensity model capability allows the creation of stress intensity solutions in the form of a Windows DLL (dynamic link library). Drawing tools allow solutions to be animated during the analysis. Interactive stress intensity solutions allow the use of an external FEM code to return updated stress intensity solutions.

Related Research Articles

<span class="mw-page-title-main">Fatigue (material)</span> Initiation and propagation of cracks in a material due to cyclic loading

In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts of the fracture surface. The crack will continue to grow until it reaches a critical size, which occurs when the stress intensity factor of the crack exceeds the fracture toughness of the material, producing rapid propagation and typically complete fracture of the structure.

<span class="mw-page-title-main">Stress concentration</span> Location in an object where stress is far greater than the surrounding region

In solid mechanics, a stress concentration is a location in an object where the stress is significantly greater than the surrounding region. Stress concentrations occur when there are irregularities in the geometry or material of a structural component that cause an interruption to the flow of stress. This arises from such details as holes, grooves, notches and fillets. Stress concentrations may also occur from accidental damage such as nicks and scratches.

<span class="mw-page-title-main">Fracture mechanics</span> Field of mechanics that studies the propagation of cracks in materials

Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.

In engineering, damage tolerance is a property of a structure relating to its ability to sustain defects safely until repair can be effected. The approach to engineering design to account for damage tolerance is based on the assumption that flaws can exist in any structure and such flaws propagate with usage. This approach is commonly used in aerospace engineering, mechanical engineering, and civil engineering to manage the extension of cracks in structure through the application of the principles of fracture mechanics. A structure is considered to be damage tolerant if a maintenance program has been implemented that will result in the detection and repair of accidental damage, corrosion and fatigue cracking before such damage reduces the residual strength of the structure below an acceptable limit.

<span class="mw-page-title-main">Stress intensity factor</span> Quantity in fracture mechanics; predicts stress intensity near a cracks tip

In fracture mechanics, the stress intensity factor is used to predict the stress state near the tip of a crack or notch caused by a remote load or residual stresses. It is a theoretical construct usually applied to a homogeneous, linear elastic material and is useful for providing a failure criterion for brittle materials, and is a critical technique in the discipline of damage tolerance. The concept can also be applied to materials that exhibit small-scale yielding at a crack tip.

<span class="mw-page-title-main">Fracture toughness</span> Stress intensity factor at which a cracks propagation increases drastically

In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a crack with thin components having plane stress conditions and thick components having plane strain conditions. Plane strain conditions give the lowest fracture toughness value which is a material property. The critical value of stress intensity factor in mode I loading measured under plane strain conditions is known as the plane strain fracture toughness, denoted . When a test fails to meet the thickness and other test requirements that are in place to ensure plane strain conditions, the fracture toughness value produced is given the designation . Fracture toughness is a quantitative way of expressing a material's resistance to crack propagation and standard values for a given material are generally available.

Corrosion fatigue is fatigue in a corrosive environment. It is the mechanical degradation of a material under the joint action of corrosion and cyclic loading. Nearly all engineering structures experience some form of alternating stress, and are exposed to harmful environments during their service life. The environment plays a significant role in the fatigue of high-strength structural materials like steel, aluminum alloys and titanium alloys. Materials with high specific strength are being developed to meet the requirements of advancing technology. However, their usefulness depends to a large extent on the degree to which they resist corrosion fatigue.

<span class="mw-page-title-main">Paris' law</span>

Paris' law is a crack growth equation that gives the rate of growth of a fatigue crack. The stress intensity factor characterises the load around a crack tip and the rate of crack growth is experimentally shown to be a function of the range of stress intensity seen in a loading cycle. The Paris equation is

<span class="mw-page-title-main">Fractography</span> Study of the fracture surfaces of materials

Fractography is the study of the fracture surfaces of materials. Fractographic methods are routinely used to determine the cause of failure in engineering structures, especially in product failure and the practice of forensic engineering or failure analysis. In material science research, fractography is used to develop and evaluate theoretical models of crack growth behavior.

Material failure theory is an interdisciplinary field of materials science and solid mechanics which attempts to predict the conditions under which solid materials fail under the action of external loads. The failure of a material is usually classified into brittle failure (fracture) or ductile failure (yield). Depending on the conditions most materials can fail in a brittle or ductile manner or both. However, for most practical situations, a material may be classified as either brittle or ductile.

<span class="mw-page-title-main">Compact tension specimen</span>

A compact tension specimen (CT) is a type of standard notched specimen in accordance with ASTM and ISO standards. Compact tension specimens are used extensively in the area of fracture mechanics and corrosion testing, in order to establish fracture toughness and fatigue crack growth data for a material.

James C. Newman is an American engineer and materials scientist known for his work on fracture and fatigue for aerospace vehicles. NASA has listed him as a "Superstar of Modern Aeronautics".

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise. Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

Crack closure is a phenomenon in fatigue loading, where the opposing faces of a crack remain in contact even with an external load acting on the material. As the load is increased, a critical value will be reached at which time the crack becomes open. Crack closure occurs from the presence of material propping open the crack faces and can arise from many sources including plastic deformation or phase transformation during crack propagation, corrosion of crack surfaces, presence of fluids in the crack, or roughness at cracked surfaces.

<span class="mw-page-title-main">Cascade chart (NDI interval reliability)</span>

A cascade chart is tool that can be used in damage tolerance analysis to determine the proper inspection interval, based on reliability analysis, considering all the context uncertainties. The chart is called a "cascade chart" because the scatter of data points and downward curvature resembles a waterfall or cascade. This name was first introduced by Dr. Alberto W Mello in his work "Reliability prediction for structures under cyclic loads and recurring inspections". Materials subject to cyclic loads, as shown in the graph on the right, may form and propagate cracks over time due to fatigue. Therefore, it is essential to determine a reliable inspection interval. There are numerous factors that must be considered to determine this inspection interval. The non-destructive inspection (NDI) technique must have a high probability of detecting a crack in the material. If missed, a crack may lead the structure to a catastrophic failure before the next inspection. On the other hand, the inspection interval cannot be too frequent that the structure's maintenance is no longer profitable.

Solder fatigue is the mechanical degradation of solder due to deformation under cyclic loading. This can often occur at stress levels below the yield stress of solder as a result of repeated temperature fluctuations, mechanical vibrations, or mechanical loads. Techniques to evaluate solder fatigue behavior include finite element analysis and semi-analytical closed-form equations.

Fracture of biological materials may occur in biological tissues making up the musculoskeletal system, commonly called orthopedic tissues: bone, cartilage, ligaments, and tendons. Bone and cartilage, as load-bearing biological materials, are of interest to both a medical and academic setting for their propensity to fracture. For example, a large health concern is in preventing bone fractures in an aging population, especially since fracture risk increases ten fold with aging. Cartilage damage and fracture can contribute to osteoarthritis, a joint disease that results in joint stiffness and reduced range of motion.

<span class="mw-page-title-main">Crack growth equation</span>

A crack growth equation is used for calculating the size of a fatigue crack growing from cyclic loads. The growth of fatigue cracks can result in catastrophic failure, particularly in the case of aircraft. A crack growth equation can be used to ensure safety, both in the design phase and during operation, by predicting the size of cracks. In critical structure, loads can be recorded and used to predict the size of cracks to ensure maintenance or retirement occurs prior to any of the cracks failing.

<span class="mw-page-title-main">Fatigue testing</span> Determination of a material or structures resiliency against cyclic loading

Fatigue testing is a specialised form of mechanical testing that is performed by applying cyclic loading to a coupon or structure. These tests are used either to generate fatigue life and crack growth data, identify critical locations or demonstrate the safety of a structure that may be susceptible to fatigue. Fatigue tests are used on a range of components from coupons through to full size test articles such as automobiles and aircraft.

Fastran is a computer program for calculating the rate of fatigue crack growth by combining crack growth equations and a simulation of the plasticity at the crack tip.

References

  1. ↑ Harter, James A. (2003). AFGROW Reference Manual (version 4.0). Wright-Patterson Air Force Base, AFRL/VASM.
  2. ↑ Tada, Hiroshi; Paris, Paul C.; Irwin, George R. (1973). The stress analysis of cracks handbook. Del Research Corporation.
  3. ↑ Forman, R. G.; Hearney, V. E.; Engle, R. M. (1967). "Numerical analysis of crack propagation in cyclic-loaded structures". Journal of Basic Engineering. 89 (3): 459–464. doi:10.1115/1.3609637.
  4. ↑ Walker, K. (1970). "The effect of stress ratio during crack propagation and fatigue for 2024-T3 and 7075-T6 aluminum". Effects of Environment and Complex Load History for Fatigue Life. American Society for Testing and Materials. pp. 1–14.
  5. ↑ NASGRO Fracture Mechanics and Fatigue Crack Growth Analysis Software, Version 4.02. SwRI. 2002.
  6. ↑ Elber, Wolf (1970). "Fatigue crack closure under cyclic tension". Engineering Fracture Mechanics. 2: 37–45.
  7. ↑ Elber, Wolf (1971). The Significance of Fatigue Crack Closure, ASTM STP 486. American Society for Testing and Materials. pp. 230–242.
  8. ↑ Newman, J. C. Jr. (1992). FASTRAN II -- A fatigue crack growth structural analysis program, Technical Memorandum 104159. NASA.
  9. ↑ Wheeler, O. E. (1972). "Spectrum Loading and Crack Growth". Journal of Basic Engineering. 94: 181–186.
  10. ↑ Willenborg, J. D.; Engle, R. M.; Wood, H. A. (1971). "A crack growth retardation model using an effective stress concept". NASA.{{cite journal}}: Cite journal requires |journal= (help)
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.