Fracture mechanics and fatigue crack growth analysis. Fatigue lives were calculated using the crack growth relations and. A model for probabilistic growth of complex fatigue crack shapes that will be used to perform sensitivity studies and to optimize inspection intervals for railroad tank cars is presented. In this case, fatigue crack growth analysis is key to safe operation of the system. Fatigue crack growth was transgranular with sem fracture surface morphology dependent upon both. Fatigue failure resulting from variable loading pages 1. The use of these two parameters involves kand k max as the driving forces, which are required simultaneously for fatigue crack growth to occur. Region ii encompasses data where the rate of crack growth changes roughly linearly with a change in stress intensity fluctuation.
This diagram can be constructed by conducting several laboratory tests on standard specimens subjected to low amplitude fluctuating loads simulating the actual environment. Fatigue life prediction based on crack closure and. These fatigue countermeasures can improve flight crew performance and alertness. This behavior is identical for sl nd lt crack orientations. The fatigue crack growth resistance of aerospace alloys is generally reduced by concomitant cyclic plastic deformation and exposure to a wide range of aggressive environments at ambient and elevated temperatures. Region iii is characterized by rapid, unstable crack growth. Introduction one of the most important tasks in fracture control analysis is acquiring valid crack growth properties of the materials.
After 17 hours of wakefulness, performance degradation is equivalent to a blood alcohol concentration of 0. Development of the nasa flagro computer program for analysis. Fatigue crack growth computer program, esacracknasa flagro users manual, theory and operation, esa publications division, estec, the netherlands 1995. Fracture mechanics and fatigue crack growth analysis software tool. Using this gradient, crack growth analysis was performed using nasa flagro nasgro software. Fatigue crack growth durability, damage tolerance, and. Crack closure was observed at threshold and near threshold for both temperatures with r 0. The predecessor of the current nasgro computer program for fatigue crack growth analysis, called nasa flagro 2. This type of growth occurs by fatigue processes during cyclic loading of the structure. Fatigue research has been conducted in a variety of aerospace environments and. These properties usually include values for fracture toughness, fatigue and sustained load crack growth rate, and crack growth. Dtdhandbook afgrow air force growth fracture mechanics. Fatigue crack growth rates for the partthrough cracked specimens are compared with a base curve fitted from. Fatigue crack growth life calculations using nasa flagro were conservative.
Fatigue crack growth of beta21s titanium alloy under. Jul 08, 2015 fatigue crack growth life calculations using nasa flagro were conservative. Fatigue research has been conducted in a variety of aerospace environments and simulations, including the space shuttle. Then, supporting evidence for crack growth obtained from the newer discipline of fracture mechanics is given. Fracture mechanics and fatigue crack growth analysis software. Probabilistic method to determine widespread fatigue. Figure 9 is a plot of the fatigue crack growth rate, dadn, as a function of the stress intensity factor range, ak, for the data at the three stress ratios studied in the cr orientation. Fatigue crack growth rates for the partthroughcracked specimens are compared with a base curve fitted from. Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. Stress intensity factor crack length crack growth rate fatigue crack growth stress ratio these keywords were added by machine and not by the authors. Fatigue crack growth computer program nasa flagro, developed by r. To perform fatigue crack growth testing and research to characterize the inservice performance of aerospace structural materials.
Fatigue crack growth behavior was investigated for ti62222 titanium alloy under constant amplitude r 0. However, in most cases, flaws will becomecritical by growing from smaller flaws or from unflawed areas of stress concentration. Open channel software to pitch nasa software to commercial. Effects of grain size and precipitate size on the fatigue. A fatigue crack is shown to nucleate at the surface and grow across several grains controlled primarily by shear stresses and shear strains, and then grow in a zigzag. Fatigue failure resulting from variable loading 271 the crack is orderly. K, fatigue crack growth behavior for a given r ratio showed. Davidson fullscale testing and analysis of fuselage structure 481.
The nasa ames fatigue countermeasures group studies the effects of sleep loss and jet lag, and conducts training to counter these effects. Fatigue crack initiation in piping and pressure vessel steels. Prediction of fatigue crack growth in airframe structures. The crack growth rate accelerates as the maximum stress intensity factor approaches the fracture toughness of the material. Crack growth equations are used to predict the crack size starting from a given initial flaw and are typically based on experimental data obtained from constant amplitude fatigue tests. The model was then used to calculate small and large crack growth rates, and in some cases total fatigue lives, for several aluminum and titanium alloys under constantamplitude, variableamplitude, and spectrum loading. Dawson, drew, and kathryn reid, fatigue, alcohol and performance impairment, nature. Computer modeling the fatigue crack growth rate behavior of.
Arij started work on the important problem of fatigue crack growth in bolts in the early 1990s. Southwest research institute and nasa have been jointly developing nasgro since version 4 in 2001. Army research laboratory vehicle technology directorate langley research center, hampton, virginia scott c. This indicated that roughnessinduced closure had a minimal influence on the differences that were observed in fatigue crack growth behavior. Experimental observations and numerical predictions of.
Fatigue crack growth behaviour and life prediction for small crack in aluminium alloy a. Forth langley research center, hampton, virginia richard a. Msc fatigue incorporates the three major principles of fatigue life estimation. Environmental effect on fatigue crack initiation in piping. The fatigue crack propagation behavior of many materials can be divided into three regions as shown in the image. Nasa flagro v2 fatigue crack growth computer program developed as an aid in predicting the growth of preexisting flaws and cracks in structural components using a twodimensional model which predicts growth independently in two directions based on the calculation of stress intensity factors. Fatigue crack growth of ti62222 titanium alloy under. Interaction effects due to overloads and underloads on fatigue crack growth p. Zanganeh1,b 1nasa johnson space center, houston, texas, usa a mohammad.
Results of an experimental investigation of the fatigue growth of small comer cracks emanating from. Crack growth as an interpretation of static fatigue nasaads. Crack growth analysis main cargo door primary and continuing damage 3. Region i is the fatigue threshold region where the dk is too low to propagate a crack. Msc fatigue strain gauge virtual strain gauges pasted on an fe model for fatigue testanalysis correlation. Nov 20, 2001 nasa flagro v2 fatigue crack growth computer program nastran nasa structural analysis system nbod2 program to derive and solve equations of motion for coupled nbody systems nets a neural network development tool, version 3. The chemical composition of the cracktip solution is shown to play an important role in controlling crack growth. A fracture mechanics analysis of fatigue crack growth in a complex cross section a fracture mechanics analysis of fatigue crack growth in a complex cross section rifani, a. The crack propagates across the section at those points of greatest tensile stress. Nasatm2002211673 arltr2725 evaluation of fatigue crack growth and fracture properties of cryogenic model materials john a. Fatigue crack growth behavior in the threshold region r. Evaluation of fatigue crack growth and fracture properties. Fatigue crack growth and applications springerlink.
Fatigue crack growth results raw data from the 23 fatigue crack growth tests may be found in the appendix of this report. Computer modeling the fatigue crack growth rate behavior. Andrew veit, ms, pe director of nads minisim principal. In the current version, the software was enhanced to suit the needs of the aircraft industry. Analysis of small crack behavior for airframe applications 463 r. Under flight spectra conditions, fatigue crack growth life at 175c was 40 to 90 percent longer than at 25c. Widespread fatigue damage wfd issue is a major threat to the structural integrity of the aging aircrafts, so wfd assessment is a key measurement and necessary task to ensure the continuing airworthiness. Nasa contractor report 194982 revised copy computer modeling the fatigue crack growth rate behavior of metals in corrosive environments edward richey iii, allen w. One of the earliest crack growth equations based on the stress intensity factor range of a load cycle is the pariserdogan equation. Probabilistic growth of complex fatigue crack shapes. Johnson space center, houston, tx, august 1986, revised march 1989. Stress corrosion cracking and corrosion fatigue experiments were conducted with the susceptible sl orientation of aa7075t651, immersed in acidified and inhibited nacl solution, to provide a basis for incorporating environmental effects into fatigue crack propagation life prediction codes such as nasa flagro. Under constant amplitude loading, a single variable k or kmax are required in crack growth relationships.
Nasa technical memorandum 109044 fatigue life and crack. Fatigue crack growth rates for the partthrough cracked specimens are compared with a base curve fitted from the data obtained using standard specimens. Successive combinations of modeling and test verifications led to damage tolerance descriptions of 3d fatigue crack growth in boltnut assemblies. Cast aluminum alloy along with the crack growth data.
Fatigue growth of small corner cracks in aluminum 6061t651. Fatigue crack growth rates for the partthroughcracked specimens are compared with a base curve fitted from the data obtained using standard specimens. National laboratory on the fatigue of piping and pressure vessel steels in lwr coolant environments. It was not easy to fit this data with the usual values of q a much larger value of 2. A methodology using probabilistic framework to assess wfd was developed. Interaction effects due to overloads and underloads on. The fatigue crack growth computer program nasa flagro 2. N asgro is a suite of computer programs used to analyze fracture and fatigue crack growth fcg in structures and mechanical components. The sample ruptures by ductile failure when the crack reduces the effective cross section to a size that cannot sustain the applied loads.
Probabilistic method to determine widespread fatigue damage. Nasa flagro v2 fatigue crack growth computer program developed as an aid in predicting the growth of preexisting flaws and cracks in structural components using a twodimensional model which predicts growth independently in two directions based. For each fcl, stress gradients were extracted ahead of the weld toe. In this paper, a general methodology is proposed to predict fatigue life of smooth and circularhole specimens, in which the crack closure model and equivalent initial flaw size eifs concept are employed. This material exhibits a rough fatigue surface unlike most materials. The software is developed jointly by southwest research institute swri and the national aeronautics and space administration nasa. Behavior of surface and corner cracks subjected to tensile and bending loads in a ti6al4v alloy. Flight spectra life calculations using nasa flagro based upon constant amplitude fatigue crack growth data, were primarily conservative but in good agreement with experimental data. Reaching total fracture under cyclic loading involves both fatiguecrack initiation and propagation. Fea model of the tank car was created with mesh refinements at the identified fatigue critical locations fcl. Final fracture occurs during stage iii fatigue, although fatigue is not involved. Therefore, it can be said that fatigue failure occurs in three stages crack initiation. Nasa flagro v2 fatigue crack growth computer program nastran nasa structural analysis system nbod2 program to derive and solve equations of motion for coupled nbody systems nets a neural network development tool, version 3. Perform fatigue crack growth and fracture mechanics research on aerospace alloys, simultaneously mimicking service loads and environments.
Common case studies of marine structural failures intechopen. Fatigue crack growth behavior in the threshold region nasa. Gangloff university of virginia, charlottesville, virginia nasacr194982rew computer modeling the fatigue crack growth rate behavior of metals in corrosive environments final report. The existing fatigue sn data have been evaluated to establish the effects of. A fracture mechanics analysis of fatigue crack growth in a. Detailed tabulation of the fracture toughness data and results of life prediction using the nasa flagro program are presented. Dotfaaar0515 fatigue crack growth database for damage. This approach to allowing and accounting for a safe level of crack growth during the operation of the system is referred to as damagetolerant design. Flight spectra life calculations using nasa flagro based upon constant amplitude fatigue crack growth data, were primarily conservative but in. Dotfaaar0749 analyses of fatigue crack growth databases. Effect of microstructure on fracture toughness and fatigue.
Experimental observations of threedimensional fatigue crack growth are compared to numerical predictions from three computer codes. The fatigue loading used in the crack growth testing was of constantamplitude sinusoidal type. Fatigue crack growth thresholds, fatigue crack growth testing, fatigue crack bifurcations, fatigue crack roughness. Fatigue life prediction based on crack closure and equivalent. Different effects of crack closure on small crack growth region and long. The fatigue life of a component can be expressed as the number of loading cycles required to initiate a fatigue crack and to propagate the crack to critical size. The transferability of fatigue laws, obtained under constant amplitude loading to variable amplitude fatigue, requires at least an additional variable, whose evolution with crack length accounts for the interactions effects between cycles of different types. Nasfla the main module calculates fatigue crack growth lifetimes. Mechanisms, behavior and analysis mechanical engineering theory and applications ping tang, jm leor zhang on. The frequency response of environmental fatigue in several 7000 series alloys is variable and depends on undefined compositional or microstructural variables.
As discussed in chapter 2, the traditional approach to estimate the total life of a structural part is to use the sn diagram alternating stress, s, versus the total number of cycles to failure, n. The development of this fcgd will begin the process of developing a consistent set of standard fatigue crack growth material properties. Based upon nominal deltak values, constant amplitude fatigue crack growth behavior at 175 c was either similar to r 0. Nasgro is actually a suite of different computer programs for different types of fatigue and fracture mechanics analysis. Study on propagation rules of a close crack in rockmass. Sep 14, 2017 using crack growth equations to solve for cyclic fatigue when stress is alternated in two different ways. For simple geometries contained in standard libraries of some codes, restricting numerically predicted crack shapes to elliptical quadrants does not adversely affect crack growth rate predictions. Behavior of surface and corner cracks subjected to tensile. At high stress intensities, crack growth rates are extremely high and little fatigue life is involved. Finally, the influence of ion exchange and chemical reactions at crack tips is discussed with regard to crack growth. Computer modeling the fatigue crack growth rate behavior of metals in corrosive environments edward richey iii, allen w. The two stages of fatigue crack growth are stage i shear mode and stage ii tensile mode. This process is experimental and the keywords may be updated as the learning algorithm improves. In this model, crack growth is based on results from a deterministic crack growth analysis with additional terms to introduce stochastic behavior.
Fatigue crack growth data of discontinuously reinforced composites published in the literature has been reevaluated using the twoparametric approach developed by the authors. Some systems may be designed to allow for some crack growth before repair and replacement. Different effects of crack closure on small crack growth region and long crack. Provide mechanical test data needed to characterize damage progression, including digital image core, nde results e. Stresslife this is the traditional stress life, or sn method, used for high cycle fatigue applications hcf low load long life that makes no distinction between initiation or growth of a crack, but rather, predicts the. Effect of microstructure on fracture toughness and fatigue crack growth behavior of ti17 alloy article in metals open access metallurgy journal 68. The fatigue crack growth computer program nasa flagro. Analyses of fatigue crack growth databases for use in a damage tolerance approach for aircraft propellers and rotorcraft. In this book, the authors present current research in the study of the mechanisms, behaviour and analysis of fatigue crack growths. Open channel software to pitch nasa software to commercial it. Fatigue crack growth is shown schematically in fig.
Recent advances in the modelling of crack growth under fatigue. Nasgro software overview southwest research institute. Starting at an initial detectable crack size 18, the crack. The major feature of this method is that a stochastic process which is introduced to crack initiation. The mild timedependence is due to cycletimedependent corrosion fatigue growth.