Effects of Thickness on Plasticity-Induced Fatigue Crack Closure PDF Download
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Author: KK. Chan Publisher: ISBN: Category : Aluminum forging Languages : en Pages : 19
Book Description
The crack-opening stress was measured using a strain gage technique on 7050 aluminum alloy, under constant amplitude and repeated overload. The behavior of crack-opening stress predicted by Newman's FASTRAN-II is consistent with the experimental results for repeated overload. It is also found that the FASTRAN-II program is capable of predicting crack growth on the 7050-T76 aluminum plate and 7050-T7452 aluminum hand forging under complex simulated flight loading which contains a significant number of compression cycles.
Author: KK. Chan Publisher: ISBN: Category : Aluminum forging Languages : en Pages : 19
Book Description
The crack-opening stress was measured using a strain gage technique on 7050 aluminum alloy, under constant amplitude and repeated overload. The behavior of crack-opening stress predicted by Newman's FASTRAN-II is consistent with the experimental results for repeated overload. It is also found that the FASTRAN-II program is capable of predicting crack growth on the 7050-T76 aluminum plate and 7050-T7452 aluminum hand forging under complex simulated flight loading which contains a significant number of compression cycles.
Author: AJ. McEvily Publisher: ISBN: Category : Crack-filling closure Languages : en Pages : 9
Book Description
A brief review of the various forms of closure is given, and the relative importance of plasticity-induced closure and roughness-induced closure is considered. On the basis of the lack of a thickness effect and an insensitivity to R in the absence of closure it is concluded that plasticity-induced closure plays a relatively minor role in affecting the rate of fatigue crack propagation with the exception of overload effects. A new form of closure, transitional closure is described and is shown to affect importantly the rate of fatigue crack growth upon a change in R ratio. The closure process for a short crack approaching a grain boundary is also discussed.
Author: ME. Pawlik Publisher: ISBN: Category : Closure Languages : en Pages : 13
Book Description
A numerical method for predicting closure and its effects on thermomechanical crack growth has been developed. A finite element model, using linear-elastic fracture mechanics shape functions, is employed to predict crack tip displacements. The effective changes in stress intensity, and therefore crack growth, are obtained from the minimum and maximum crack tip displacement predictions. When a flaw is loaded in Mode I, a ligament of material ahead of the flaw yields, and a maximum crack tip displacement is computed. Upon unloading, plastically deformed material from prior plastic zones acts to limit the minimum displacements of the crack tip. The material is modeled as elastic-perfectly plastic. The yield strength of the material is varied based on the degree of constraint. The upper limit of constraint is a plane strain condition while the lowest constraint is a plane stress condition. The level of constraint is predicted by relating the stress intensity to the thickness of the component. Temperatures also affect yield strength, along with stiffness, and can cause the plastic zone to expand due to creep. During variable-amplitude loadings, and/or temperature changes, the irregular shape of the wake can be accommodated with this numerical procedure. The method has proven to accurately account for load interaction effects such as delayed retardation, crack arrest, initial accelerations following overloads, and the transient growth and stabilization of closure level with number of overloads. This method has been verified against data obtained in the literature, and data collected under the program in which the method was developed, NASA's High Speed Research [1].
Author: DM. Fisher Publisher: ISBN: Category : Crack closure Languages : en Pages : 15
Book Description
The location of crack closure with respect to crack wake and specimen thickness under different loading conditions was determined. The rate of increase of KCL in the crack wake was found to be significantly higher for plasticity induced closure in comparison to roughness induced closure. Roughness induced closure was uniform throughout the thickness of the specimen while plasticity induced closure levels were 50% higher in the near surface region than in the mid-thickness. The influence of state of stress on low-high load interaction effects was also examined. Load interaction effects differed depending upon the state of stress and were explained in terms of ?Keff.
Author: RH. Dodds Publisher: ISBN: Category : Contact Languages : en Pages : 19
Book Description
In ductile metals, plasticity-induced closure of fatigue cracks often retards significantly measured crack growth rates in the Paris regime and contributes strongly to the observed R-ratio effect in experimental data. This work describes a similarity scaling relationship based on the 3D small-scale yielding framework wherein the thickness, B, defines the only geometric length-scale of the model. Dimensional analysis suggests a scaling relationship for the crack opening loads relative to the maximum cyclic loads (Kop/Kmax) governed by the non-dimensional load parameter ̄K=Kmax/?0 ?B, i.e., a measure of the in-plane plastic zone size normalized by the thickness. Both Kop and Kmax refer to remotely applied values of the mode I stress-intensity factor. Large-scale, 3D finite element analyses described here demonstrate that Kop/Kmax values vary strongly across the crack front in thin sheets but remain unchanged when Kmax, B, and ?0 vary to maintain ̄K = constant. The paper also includes results to demonstrate that the scaling relationship holds for non-zero values of the T-stress (which affect the Kop/Kmax values) and for an overload interspersed in the otherwise constant amplitude cycles. The present results focus on R = Kmin/Kmax = 0 loading, although the scaling relationship has been demonstrated to hold for other R > 0 loadings as well. The new similarity scaling relationship makes possible more realistic estimates of crack closure loads for a very wide range of practical conditions from just a few analyses of the type described here.
Author: James M. Larsen Publisher: ISBN: Category : Aluminum alloys Languages : en Pages : 286
Book Description
An investigation was performed to study the effects of slip character and crack closure on the propagation of small fatigue cracks in titanium- aluminum alloys. The materials examined were solution-treated Ti-4Al and Ti-8Al, as well as aged Ti-8Al. The propagation of naturally initiated surface cracks of depths as small as 25 micrometers was compared with the behavior of large through-thickness cracks. An extensometer was used to monitor crack closure throughout the large crack tests, and the closure behavior of the small cracks was measured using a computerized laser interferometric displacement gage having a displacement resolution of 0.01 micrometer. The measurements of crack closure were used to compute an effective stress intensity factor range. In all three alloys and for all test conditions, which included a range of stress levels and stress ratios, small cracks propagated faster than large cracks subjected to an equivalent Delta K, and the small cracks propagated under conditions that were significantly below the large-crack threshold, Delta K(th). Although the character and distribution of slip in Ti-Al alloys may have a dramatic influence on fatigue crack initiation and on the propagation of large cracks, this effect was minimal for small cracks.
Author: David L. Davidson Publisher: Society for Mining Metallurgy & Exploration ISBN: 9780895204752 Category : Technology & Engineering Languages : en Pages : 584
Author: DL. Davidson Publisher: ISBN: Category : ASTM compliance offset method Languages : en Pages : 22
Book Description
An elastic-plastic finite element model and high resolution experimental measurements of growing fatigue cracks are employed in partnership to investigate the relationship between crack closure, near-tip deformation behavior, and remote load-displacement measurements. The model focuses on plasticity-induced closure effects for fatigue cracks in the Paris regime, while the experiments study fatigue cracks in the Paris and near-threshold regimes in a variety of materials. The suitability of the ASTM compliance offset method and the recently-proposed adjusted compliance ratio (ACR) method to characterize near-tip closure effects is evaluated. The experiments and analyses both show that crack-tip strains below the crack-opening stress, Sop, are a relatively small fraction of the total crack-tip strain range for cracks in the Paris regime, and therefore may be insignificant for crack-tip deformation and damage and crack growth. The experiments further indicate that crack-tip strains below Sop are a relatively larger fraction of the total crack-tip strain at lower ?K values, nearer the threshold, where Sop values are higher. The current ASTM compliance offset procedure for determining the crack opening load is shown to provide accurate information about true crack-tip opening loads in theory. However, due to the limited sensitivity of the method in practice, and the corresponding need to employ a non-zero compliance offset, the method gives underestimates of opening and closing loads that may be considerably in error relative to the true crack-tip values. The current ACR technique does not appear to do a good job of characterizing the near-tip deformation response for cracks in the Paris regime, because remote displacement measurements do not appear to be adequately sensitive to near-tip strains.
Author: KK. Chan
Author: KK. Chan
Author: AJ. McEvily
Author: ME. Pawlik
Author: Wolf Elber
Author: R. Craig McClung
Author: DM. Fisher
Author: RH. Dodds
Author: James M. Larsen
Author: David L. Davidson
Author: DL. Davidson