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Author: Mitsuhiro Okayasu Publisher: ISBN: 9780494160251 Category : Languages : en Pages : 282
Book Description
A theoretical and experimental basis regarding the fatigue crack closure model, proposed by measuring the externally applied loads versus crack opening displacement, was established. The obstructions of crack closure were simulated using an artificial asperity. The crack closure model was defined with three distinct stages of the unloading curve: (i) the asperity does not contact the upper crack face, (ii) a process where both the asperity and specimen deform elastically, and (iii) the plastic deformation in the specimen around the asperity occurs. An equation was proposed to estimate the crack opening displacement values, where the effects of deformations in the asperity and specimen were considered. Fundamentals of crack surface contact and crack closure behaviors were examined via two special experiments: the collection of the fracture debris and the direct observation of the contact zones on the fracture surface through an ink dyeing method. To obtain different severity of crack surface contact, the fatigue tests were conducted under different loading conditions. The results of this study showed that the stress amplitude and R-ratio were the major factors that determine the contacting status between the mating fracture surfaces, where the higher stress amplitude and the lower R-ratio make stronger crack surface contact. The contact severity was also attributed to the location of fracture surface, e.g., the severity of fracture surface contact in the area near crack tip was relatively stronger than that in the other area. A crack closure characteristic during fatigue was examined using an artificial asperity. With asperity wedging, the fatigue crack growth rate was obviously reduced. The crack growth rate was then correlated with the effective crack driving force, defined as DeltaPeff = Pmax--Pmin, real, which itself was also correlated to the plastic deformation level in the asperity and specimen. Moreover, the DeltaPeff value was found to change with increasing the crack length. In a short crack range, the value of Delta Peff decreased due to the artificial asperity- and plasticity-induced crack closing behaviors. As the crack length increased, the Delta Peff value increased as well until just prior to the fracture because of the reduction in the closing severity.
Author: Mitsuhiro Okayasu Publisher: ISBN: 9780494160251 Category : Languages : en Pages : 282
Book Description
A theoretical and experimental basis regarding the fatigue crack closure model, proposed by measuring the externally applied loads versus crack opening displacement, was established. The obstructions of crack closure were simulated using an artificial asperity. The crack closure model was defined with three distinct stages of the unloading curve: (i) the asperity does not contact the upper crack face, (ii) a process where both the asperity and specimen deform elastically, and (iii) the plastic deformation in the specimen around the asperity occurs. An equation was proposed to estimate the crack opening displacement values, where the effects of deformations in the asperity and specimen were considered. Fundamentals of crack surface contact and crack closure behaviors were examined via two special experiments: the collection of the fracture debris and the direct observation of the contact zones on the fracture surface through an ink dyeing method. To obtain different severity of crack surface contact, the fatigue tests were conducted under different loading conditions. The results of this study showed that the stress amplitude and R-ratio were the major factors that determine the contacting status between the mating fracture surfaces, where the higher stress amplitude and the lower R-ratio make stronger crack surface contact. The contact severity was also attributed to the location of fracture surface, e.g., the severity of fracture surface contact in the area near crack tip was relatively stronger than that in the other area. A crack closure characteristic during fatigue was examined using an artificial asperity. With asperity wedging, the fatigue crack growth rate was obviously reduced. The crack growth rate was then correlated with the effective crack driving force, defined as DeltaPeff = Pmax--Pmin, real, which itself was also correlated to the plastic deformation level in the asperity and specimen. Moreover, the DeltaPeff value was found to change with increasing the crack length. In a short crack range, the value of Delta Peff decreased due to the artificial asperity- and plasticity-induced crack closing behaviors. As the crack length increased, the Delta Peff value increased as well until just prior to the fracture because of the reduction in the closing severity.
Author: D. Chen Publisher: ISBN: Category : Carbon steels Languages : en Pages : 14
Book Description
Based on the S-shaped unloading hysteresis curve a new method for determining the crack closure has been developed in which the plastic deformation near the crack tip is taken into consideration. The crack growth rate can be expressed uniquely by the effective stress intensity range, ?Keffop = Kmax - Kop or ?Keffcl = Kmax - Kcl, which is independent of the stress ratio and the loading history. The tip of a nonpropagating crack does not open even under the maximum tensile load during one loading cycle. The measured values of the crack opening ratios Uop(?Keffop/Kmax) and Ucl(?Keffcl/Kmax) of a steadily propagating crack are mainly controlled by Kmax and the stress ratio R.
Author: National Aeronautics and Space Administration (NASA) Publisher: Createspace Independent Publishing Platform ISBN: 9781721572359 Category : Languages : en Pages : 32
Book Description
The results of an analytical closure model that considers contributions and interactions between plasticity-, roughness-, and oxide-induced crack closure mechanisms are presented and compared with experimental data. The analytical model is shown to provide a good description of the combined influences of crack roughness, oxide debris, and plasticity in the near-threshold regime. Furthermore, analytical results indicate that closure mechanisms interact in a non-linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism. Newman, John A. and Riddell, William T. and Piascik, Robert S. Langley Research Center NASA/TM-2003-211755, NAS 1.15:211755, L-18207, ARL-TR-2774
Author: JC. Gasco Publisher: ISBN: Category : Aluminum alloys Languages : en Pages : 11
Book Description
Experimental data on fatigue crack closure for two steels which present different mechanical properties are presented. The data are compared with experimental results obtained under similar test conditions on aluminum alloys. On the basis of such a comparison, an attempt is made to identify the most suitable engineering parameters to describe the fatigue crack closure behavior. As a result, it is shown how fatigue crack closure can be related to several basic features of fatigue crack growth.
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: Mitsuhiro Okayasu
Author: Mitsuhiro Okayasu
Author: John A. Newman
Author: D. Chen
Author: National Aeronautics and Space Administration (NASA)
Author: JC. Gasco
Author: Wolf Elber
Author: R. Craig McClung
Author: Anders F. Blom
Author: James M. Larsen
Author: Z. T. Bieniawski