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Author: T. Nicholas Publisher: ISBN: Category : Crack growth Languages : en Pages : 11
Book Description
Crack-growth-rate experiments were conducted on compact tension (CT) specimens of Inconel 718 in air at 649°C. The loading spectrum consisted of a single 1 Hz cycle at R = 0.1, where R is the ratio of minimum to maximum stress-intensity factors, and a hold time which was applied at various levels of the fatigue cycle. The ratio of the hold-time amplitude to the maximum amplitude of the fatigue cycle, Rin, was kept at 1.0, 0.9, 0.8, or 0.5. Hold times were varied from 0 to 200 s. Tests were performed under computer-controlled constant-K conditions using values of the maximum of the fatigue cycle of 40 and 50 MPa m1/2. Data show that for Rin = 1.0, a linear summation model works well, while at Rin = 0.9 there is a measurable retardation effect on the crack growth during the hold time. For values of Rin less than or equal to 0.8, the sustained-load crack growth is almost completely retarded. A simple retardation model is proposed which can fit the experimental data and is based on the concept of an overload plastic zone being produced by the fatigue cycles. It is concluded that hold times do not contribute to crack growth in this material unless their amplitude is at or near the maximum amplitude of the adjacent fatigue cycles.
Author: T. Nicholas Publisher: ISBN: Category : Crack growth Languages : en Pages : 11
Book Description
Crack-growth-rate experiments were conducted on compact tension (CT) specimens of Inconel 718 in air at 649°C. The loading spectrum consisted of a single 1 Hz cycle at R = 0.1, where R is the ratio of minimum to maximum stress-intensity factors, and a hold time which was applied at various levels of the fatigue cycle. The ratio of the hold-time amplitude to the maximum amplitude of the fatigue cycle, Rin, was kept at 1.0, 0.9, 0.8, or 0.5. Hold times were varied from 0 to 200 s. Tests were performed under computer-controlled constant-K conditions using values of the maximum of the fatigue cycle of 40 and 50 MPa m1/2. Data show that for Rin = 1.0, a linear summation model works well, while at Rin = 0.9 there is a measurable retardation effect on the crack growth during the hold time. For values of Rin less than or equal to 0.8, the sustained-load crack growth is almost completely retarded. A simple retardation model is proposed which can fit the experimental data and is based on the concept of an overload plastic zone being produced by the fatigue cycles. It is concluded that hold times do not contribute to crack growth in this material unless their amplitude is at or near the maximum amplitude of the adjacent fatigue cycles.
Author: Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Crack-growth-rate experiments were conducterd on CT specimens of Inconel 718 at 649 C. The loading spectrum consisted of a single 1 Hz cycle at R = 0.1 and a hold time. The ratio of the amplitude of the hold time to the maximum amplitude of the fatigue cycle, R sub in, was 1.0, 0.9, 0.8 and 0.5 with hold times from 0 to 200 s. Tests were performed under computer controlled constant K conditions using values of the maximum of the fatigue cycle of 40 and 50 MPa. the square root of m. Data show that for R sub in = 1.0, a linear summation model works well, while at R sub in = 0.9 there is a measurable retardation effect on the crack growth during the hold time. For values of R sub in less than 0.8, the sustained load crack growth is almost completely retarded. A simple retardation model is proposed which can fit the experimental data and is based on the concept of an overload plastic zone being produced by the fatigue cycles. It is concluded that hold times do not contribute to crack growth in this material unless their amplitude is at or near the maximum amplitude of the adjacent fatigue cycles. (Author).
Author: K. E. Harms Publisher: ISBN: Category : Languages : en Pages : 178
Book Description
This theses investigates the crack growth behavior of Inconel 718 effected by over-loads at elevated temperature. A cumulative damage model was developed to predict the total time-to-failure. Predictions were noted to improve at higher values of stress intensities. All tests were conducted under sustained loading with isothermal conditions of 650 C. Precracked compact tension specimens were used to establish a data base for baseline and overload conditions. The delay time associated with crack growth retardation was determined by comparing the baseline behavior with the results of the overload specimens. The effects of 20-and 50-percent overloads of 1 minute or 1 hour duration were investigated. The shape of the crack tip plastic zone and, hence, the retardation effect was found to be independent of overload duration. The retardation delay time was found to be dependent upon the overload magnitude and the value of K at which the overload was removed. It was also found that a sudden crack advancement occurred when the overload was applied. The above factors were accounted for in the model development. The model was generally capable of predicting the time-to-failure within about 10 percent of the actual material behavior. Originator supplied keywords: Sustained-load crack growth; Crack growth retardation; Elevated temperature; Nickel-based superalloy; Crack growth model; Overloads; Data reduction; Graphs.
Author: Robert L Hastie (Jr) Publisher: ISBN: Category : Languages : en Pages : 140
Book Description
This thesis investigates methods of modeling the effects of overloads on high-temperature sustained-load crack growth. In addition to a model previously developed for this specific problem, a computer program developed for low-temperature, high-frequency cyclic load applications was evaluated. Sustained-load hold times were converted to equivalent fatigue cycles to analyze a load spectrum, consisting of sustained-load with periodic overloads. The CRACKS crack growth program was used with the Wheeler and Willenborg models used to account for crack growth retardation due to overloads. Predictions were compared with experimental test data generated on specimens of Inconel 718 at 650 C with periodic overloads of either 20 or 50 percent. Crack measurements were made using a electric potential system. The application of the electric potential system to crack growth measurement following overloads was extensively evaluated. It was concluded that the system had to be recalibrated after each overload due to a sudden advancement in crack length.
Author: Jonas Saarimäki Publisher: Linköping University Electronic Press ISBN: 9176858715 Category : Languages : en Pages : 49
Book Description
Gas turbines are widely used in industry for power generation and as a power source at "hard to reach" locations where other possibilities for electrical supply are insufficient. There is a strong need for greener energy, considering the effect that pollution has had on global warming, and we need to come up with ways of producing cleaner electricity. A way to achieve this is by increasing the combustion temperature in gas turbines. This increases the demand on the high temperature performance of the materials used e.g. superalloys in the turbine. These high combustion temperatures can lead to detrimental degradation of critical components. These components are commonly subjected to cyclic loading of different types e.g. combined with dwell-times and overloads at elevated temperatures, which influence the crack growth. Dwell-times have shown to accelerate crack growth and change the cracking behaviour in both Inconel 718 and Haynes 282. Overloads at the beginning of the dwell-time cycle have shown to retard the dwell time effect on crack growth in Inconel 718. To understand these effects more microstructural investigations are needed. The work presented in this licentiate thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", concentrating on fatigue crack growth mechanisms in superalloys during dwell-times, which have shown to have a devastating effect on the crack propagation behaviour. Mechanical testing was performed under operation-like conditions in order to achieve representative microstructures and material data for the subsequent microstructural work. The microstructures were microscopically investigated in a scanning electron microscope (SEM) using electron channeling contrast imaging (ECCI) as well as using light optical microscopy. The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at the maximum load (0% overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. When an overload is applied prior to the dwell-time, the crack growth rate decreases with increasing overload levels in Inconel 718. At high temperature crack growth in Inconel 718 took place as intergranular crack growth along grain boundaries due to oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along phase boundaries with subsequent severe oxidation of the phase. This thesis comprises two parts. The first giving an introduction to the field of superalloys and the acting microstructural mechanisms that influence fatigue during dwell times. The second part consists of two appended papers, which report the work completed so far in the project.
Author: Jonas Saarimäki Publisher: Linköping University Electronic Press ISBN: 9176853853 Category : Languages : en Pages : 63
Book Description
Gas turbines are widely used in industry for power generation and as a power source at hard to reach locations where other possibilities for electrical power supplies are insufficient. New ways of producing greener energy is needed to reduce emission levels. This can be achieved by increasing the combustion temperature of gas turbines. High combustion temperatures can be detrimental and degrade critical components. This raises the demands on the high temperature performance of the superalloys used in gas turbine components. These components are frequently subjected to different cyclic loads combined with for example dwell-times and overloads at elevated temperatures, which can influence the crack growth. Dwell-times have been shown to accelerate crack growth and change cracking behaviour in both Inconel 718, Haynes 282 and Hastelloy X. On the other hand, overloads at the beginning of a dwell-time cycle have been shown to retard the dwell-time effect on crack growth in Inconel 718. More experiments and microstructural investigations are needed to better understand these effects. The work presented in this thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", where I have mainly looked at fatigue crack growth mechanisms in superalloys subjected to dwell-fatigue, which can have a devastating effect on crack propagation behaviour. Mechanical testing was performed under operation-like cycles in order to achieve representative microstructures and material data for the subsequent microstructural work. Microstructures were investigated using light optical microscopy and scanning electron microscopy (SEM) techniques such as electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at maximum load (0 % overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. The crack growth rate decreases with increasing overload levels in Inconel 718 when an overload is applied prior to the dwell-time. At high temperature, intergranular crack growth was observed in Inconel 718 as a result of oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along ?-phase boundaries with subsequent oxidation of the ?-phase. This thesis comprises two parts. Part I gives an introduction to the field of superalloys and the acting microstructural mechanisms related to fatigue and crack propagation. Part II consists of five appended papers, which report the work completed as part of the project.
Author: National Aeronautics and Space Adm Nasa Publisher: Independently Published ISBN: 9781792718212 Category : Languages : en Pages : 28
Book Description
Fatigue crack growth tests were conducted on 0.09 inch thick, 3.0 inch wide middle-crack tension specimens cut from sheets of 2024-T3 aluminum alloy. The tests were conducted using a load sequence that consisted of a single block of 2,500 cycles of constant amplitude loading followed by an overload/underload combination. The largest fatigue crack growth life occurred for the tests with the overload stress equal to 2 times the constant amplitude stress and the underload stress equal to the constant amplitude minimum stress. For the tests with compressive underloads, the fatigue crack growth life decreased with increasing compressive underload stress. Dawicke, David S. Langley Research Center NASA-CR-201668, NAS 1.26:201668 NAS1-96014; RTOP 538-02-10-01...
Author: D. L. Miller Publisher: ISBN: Category : Languages : en Pages : 80
Book Description
This investigation found linear cumulative damage modeling applicable to creep crack growth under non-isothermal conditions. The best results are obtained for high crack growth rates produced either by high temperature (above 593C), or by high stress intensities (K greater than 50 MPA square root of m). Except for one test, the linear model predicts conservative growth rates. Constant temperature data are collected for 537, 593, and 648C and presented as da/dt vs K curves. Center-cracked specimens of Inconel 718 are used. The isothermal baseline data are used to predict crack growth rates for the non-isothermal tests using linear cumulative modeling. The predicted creep crack growth rates were within a factor of two of the actual test data. The time-to-failure, predicted for one test, is 56 percent of the actual time to failure.
Author: T. Nicholas
Author: T. Nicholas
Author: 
Author: K. E. Harms
Author: Robert L Hastie (Jr)
Author: D. T. Read
Author: Jonas Saarimäki
Author: 
Author: Jonas Saarimäki
Author: National Aeronautics and Space Adm Nasa
Author: D. L. Miller