Near-Tip and Remote Characterization of Plasticity-Induced Fatigue Crack Closure PDF Download
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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: 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: JE. Allison Publisher: ISBN: Category : Alloys Languages : en Pages : 15
Book Description
Accurate experimental characterization of the crack closure process is important for a number of reasons, the most important of which may be for its use in developing materials and structures that are resistant to the growth of fatigue cracks. This paper describes the results of an investigation aimed at quantifying the point of crack closure. An ?/? titanium alloy, Ti-6242S, was chosen for this investigation because its microstructure can be manipulated to produce large variations in the level of asperity-induced crack closure and is thus an ideal model alloy for understanding this important crack closure mechanism. Comparisons of two "global" compliance techniques (backface strain and crack mouth opening displacement) and a near crack-tip surface technique (two-stage replication) were made. In tests conducted at low stress intensities, all three methods gave nearly identical measurements of crack closure. For the two compliance techniques, different numerical procedures were used to determine the inflection point in the load-displacement curve. The magnitude of and variations in crack closure determined using these compliance methods were found to be strongly dependent on the numerical procedure used. Studies of crack-tip opening showed that in ?/?-processed Ti-6242S the crack tip is tightly closed even at positive loads, indicating that plasticity induced closure is the predominant form of closure. In contrast, measurements in ?-processed material showed that the crack tip is propped open even when no load is applied to the sample, indicating that closure in this microstructure is asperity-induced.
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: James Lankford (Jr) Publisher: ISBN: Category : Languages : en Pages : 21
Book Description
During the past three years, fatigue crack tip plasticity and overload induced fatigue crack retardation have been studied using several complementary techniques, especially selected area electron channeling and high resolution SEM observation of crack tips under various stages of loading. It is found that while some materials exhibit plastic yielding behavior in agreement with existing theory, some materials, especially aluminum alloys, diverge widely both from one another and from theory. In particular, for equivalent stress intensities, overload ratios, and pre-overload crack growth rates, the retardation period (in cycles) for 2024-T3 significantly exceeds that for 6061-T6 and 7075-T6. This behavior is intimately related to the size and shape of the overload plastic zone, variations in which probably are controlled by the plastic flow rules for the alloys.
Author: DL. Davidson
Author: DL. Davidson
Author: R. Craig McClung
Author: JE. Allison
Author: Steven R. Daniewicz
Author: James C. Newman (Jr.)
Author: KK. Chan
Author: I. Milne
Author: James Lankford (Jr)
Author: Wolf Elber
Author: