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The role of primary carbides in fatigue crack propagation in aeroengine bearing steels
Use o f b e a r i n g steel as a m a t e r i a l f o r safety related c o m p o n e n t s in a u t o m o t i v e applications. Beewick, J.M. Volkmouth, J. and Hengerer, F. Harterei-Technische Mitteilungen Sept.-Oct. 1990 diS, (5), 266--273 (in German) Hub units made from through hardening bearing steel 100 Cr6 (SAE 52 100) have been applied very successfully in millions of automobiles. These units have flanged outer rings in a soft-annealed or tough-tempered condition and selective inductionhardened bearing raceways. The outer rings are regarded as 'safety related components'. Based on the results of microstructurel characterization combined with tensile, fatigue and fracture mechanics teats, this paper shows how this, st first sight, unusual material and heat treatment selection was developed and applied. The advantages of such a combination are weight reduction, and easier mounting maintenance, as compared with conventional bearings. Graphs, maps, photomicrographs. 10 refs. Low-cycle f a t i g u e b e h a v i o u r o f a l o w - a l l o y h i g h - s t r e n g t h steel. Singh,
V., Ralu, P.V.S.S., Namboodhiri, T.K.G. and Rao, P.R. Int. J. Fatigue J u l y 1990 12, (4), 2 8 9 - 2 9 2
The low-cycle fatigue beheviour of a high-strength CrMoV steel has been evaluated in the as-received (AR) end the quenched and tempered (QT) conditions, at room temperature. While the quenching and tempering treatment causes a marked increase in the tensile strength and hardness, it also leads to an impairment of the fracture toughness end the low-cycle fatigue (LCF) resistance of the material. A frectogrephic examination revesls that, while there is a large zone of fatigue crack propagation in the AR condition, there is in sharp contrast a small zone of fatigue crack propagation and a large zone of static fracture in the QT condition. The reduction in fatigue resistance of the QT material is attributed to a loss in fracture toughness and a fall in the values of ~I and ~. Graphs, photomicrographs. 16 refs.
showed the locations of the fatigue crack initiation. The mechanisms of c~rrosion fatigue were studied by performing tests in 1 M NaCI at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be insignificant in the corrosion fatigue of HSLA steel in this study, The 1 M NaCI corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced the fatigue beheviour. A pit depth at which no damage occurred was found.
The s t u d y o f scuffing and p i t t i n g failure o f a c a m - t a p p a t r u b b i n g pair.
Liu, J., Lu, Z. and Chang, Y. Wear Oct. 1990 140, (1), 135-147 The mechanism of scuffing is generally considered to be adhesive wear. However, it has been found that the main mechanism is fatigue wear in the form of dsiamination and that plastic deformation plays a very important role in the scuffing formation, which shows an obvious strain fatigue nature. The analyses of wear debris also verify this conclusion. Pitting is the opposite of scuffing and displays a stress fatigue nature. All cracks are formed without detectable plastic deformation. The statistical analysis showed that the majority of pitting cracks initiate on or near the surface. To confirm the viewpoint that scuffing and pitting failure belong to strain and stress fatigue separately, tests simulating wear were conducted using materials (e.g., 3.45-3.65 carbon, 2.1-2.2 Si, 0.6-0.9 Mn, 0.2-0.4 Cr, 0.3-0.5 Mo) with different structures and hardnesses. On the basis of failure analyses and the results of wear tests the mechanism of scuffing and pitting is discussed in detail. Graphs, photomicrographs. 12 refs.
Fatigue d a m a g e accumulation in 2024-1351 aluminium subjected to periodic reversed overloads. Jurcevic, R., DuQuesnay, D.L., Topper, T.H.
A study on variable a m p l i t u d e load f a t i g u e a n d an e x p e r t s y s t e m f o r f a t i g u e assessment o f h i g h w a y steel bridges. (Dissertation}. Melhem, H.G. Dies. Abstr. Int. Sept. 1990 51, (3), 373 p p
The variation in damage accumulation rates that occur when periodic tension-compression and compression-tension overloads are introduced into an otherwise constant-amplitude loading history are examined. Block loading histories consisting of either compression-tension or tension-compression overloads followed by smaller constant-amplitude fully reversed cycles were applied to smooth, axial, 2024-T351 AI specimens to determine the increase in damage accumulation rates for the fully reversed cycles following such overloads. The results show that the interactive damage/cycle remains constant for approximately 450 cycles following an overload and then it decays as s power law function of the number of cycles. The periodic application of a compression-tension or a tension-compression overload contributes significantly to damage accumulation during subsequent small fully reversed cycles, either above or below the constantamplitude fatigue limit. Consequently, cycles with stress amplitudes below the constant-am plitude fatigue limit, which are usually omitted from damage calculations, should be accounted for in fatigue life prediction analyses for variable.amplitude histories. Graphs. 29 refs.
The fatigue beheviour of highway steel bridges, that is theories, hypotheses and specifications attempting to prevent fatigue failures, is studied, especially as regards variable-amplitude random-sequence loads. The study is part of s research project currently being conducted at the University of Pittsburgh under a multiyesr FHWA contract entitled 'Variable Amplitude Load Fatigue'. The extensive literature survey conducted for this research is reviewed and the major findings affecting the initial project working plan are summarized. The preparatory work necessary to generate new fatigue data for full-scale beams is also presented. Fifteen 101.6 cm deep beams with overall lengths ranging from 3.96 to 6.7 m have been fabricated and are being tested under the variable-amplitude random-sequence loading most representative of the traffic loads applied on actual bridges. The stress spectrum was developed from the latest nationwide weigh-in-motion study, which is considered to be the most appropriate source of data for defining actual traffic characteristics. The stress spectrum is composed of 10000 different cycles arranged in a random sequence. It is applied repeatedly until failure occurs, or 100 million cycles are reached, whichever occurs first. Testing is still in progress and the completion date for this project is far beyond the time limits of this dissertation. However, the preliminary results that were obtained are discussed. The groundwork for developing a surrogate prototype expert system for steel bridge inspection was completed. The prototype (pilot) system has been developed using the professionsi version of the expert system shell (building tool) 'EXSYS'. The system can help the bridge engineer in planning the fatigue inspection as far as: where to look, what to look for, and how to look; and in assessing the criticality of the bridge and its different components with respect to fracture and fatigue. After a bridge inspection has been performed, the system can be used to analyse the results of the inspection, suggest remedial actions, and calculate the approximate remaining fatigue life of the details under consideration.
and Pompetzki, M.A. Int. J. Fatigue J u l y 1990 12, (4), 2 5 9 - 2 6 6
The role o f p r i m a r y carbides in f a t i g u e crack p r o p a g a t i o n in aeroengine b e a r i n g s t e e l s . Iqbal, A. and King, J.E. Int. J. Fatigue J u l y 1990 12, (4), 2 3 4 - 2 4 4 Fatigue crack propagation, tensile and fracture toughness data for four aeroengine bearing steels are reported. The steels involved are the through-hardened tool steels 18-4-1 (T1) and M50, and two similar carburized steels, RBD and Volvic. Crack growth data have been obtained at 20 and 280°C to cover the range of oil temperatures experienced in aeroengine bearing operations. At 20 °C, threshold &K values (~Kth) ranged between 3.5-4.5 MPa ~/m with Paris exponents (m) of between 2.0-3.5. The lowest m-values were seen in the csrburizing steels, which also exhibited lower Paris regime crack growth rates than MS0 and 18-4-1. For all the steels, growth rates were higher at 280 °C than 20 °C, although there was a slight tendency for AKth to increase, probably associated with oxide-induced closure at 280 °C. The effects of primary carbides, strength and toughness on fatigue crack growth behaviour are discussed, in relation to the importance of static-mode cracking. Graphs, photomicrographs. 23 refs. T h e effect o f S-phese d i s t r i b u t i o n o n f a t i g u e crack g r o w t h in peak-aged AI-U-Mg-Cu-Zr alloys. Dudgeon, H.D. and Martin, J.W. Int. J. Fatigue J u l y 1990 12, (4), 2 5 3 - 2 5 8 Curves of da/dN against AK have been compared in CT specimens of two AI-Li-Mg-Cu-Zr alloys of differing Cu content aged to peak hardness at 170 °C. Changes in the S-phase distribution caused by stretching or duplex ageing have been employed as a means of influencing the slip distribution, and fatigue crack growth (FCG) curves at R-values of 0.1 and 0.7 have been compared to evaluate crack closure effects. At R = 0.7, crack closure appears to be negligible and stretched materials show a lower threshold than unstratched ones, owing to slip homogenization. The alloy with higher Cu content exhibited lower thresholds than the other alloy, since the associated higher volume fraction of the S phase gave greater slip homogenization. Duplex-aged (DA) alloys show the greatest crack roughness and the highest thresholds; it is suggested that their high planarity of slip arises from the absence of a dislocation substructure and from the shearing of the finer S-phase particles. At R = 0.1, roughness- and debris-induced crack closure contribute to produce higher thresholds in all specimens. When FCG curves are expressed in terms of the effective stress intensity range (by allowing for crack closure) the data obtained st R = 0.1 and R = 0.7 are comparable. Graphs, photomicrographs. 7 refs. The influence o f environment on t h e f a t i g u e b e h a v i o u r o f HF-80 HSLA steel. (Dissmtation). Ahn, S.-H. Diss. Abstr. Int. Sept. 1990 51, (3), 91 pp Single-pitted specimens of HF-80, a HSLA steel, were tested in laboratory air and in a 1 M NeCI solution to study the influence of s corrosive environment on its fatigue life. The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM
Int J Fatigue May 1991
Fatigue reliability and fracture b e h a v i o u r o f h i g h w a y steel bridges. (Dissertation). Michael, A. Y. Diss. Abstr. InL Sept. 1990 81, (3), 271 pp Structural reliability, in general, can be defined as the probability that a structural component will not fail within its design life. Thus, the fatigue reliability of a highway bridge is the probability that the bridge can have • satisfactory performance under the repetitive traffic loading during its lifetime. As a measure of the reliability of a structure the reliability index, ~, is used. Fatigue requirements have become the controlling factor for the design of steel bridges in all the contemporary design codes. These fatigue requirements are based on the allowable S-N lines, which were produced from fatigue tests and relate stress range, S, to number of cycles to failure, N. Based on the allowable S-N lines a reliability index, 6, format is derived, which is used for the AASHTO fatigue design criteria. The value of the reliability index for all fatigue detail categories specified by AASHTO as well as its uniformity for all details are examined. The same reliability index format is used for the reliability analysis of the fatigue design criteria of the Ontario Highway Bridge Design Code, and the results ere compared between the two codes. The sensitivity of this reliability index to the parameters that affect the fatigue behaviour of a bridge is also investigated. Knowing how each of these parameters affect the reliability index value, recommendations can be made on how to improve the design criteria, targeting a desirable I~-value. Finally, the fatigue reliability of highway steel bridges using the fracture mechanics approach is studied, with the aid of a reliability index format associated with this kind of process.
E s t i m a t i o n o f residual life expectancy. Orna, M. Skoda Rev. 1990 (2), 1 8 - 2 3 Estimation of the residual life of rotors in power turbines is critical to the prevention of expensive failures. Three areas of concern include creep, brittle failure and fatigue. Inspection of components for deformation and cracks are critical to proper estimates of the residual life. The use of fracture mechanics enables better use of material properties end the application of inspection results. Corrosion is another complicating effect on service life end can considerably increase the detrimental effect of fatigue. Non-destructive inspection combined with operating experience allows good success in the prediction of remaining life and affords operating lives of 150-200000 h. CrMoV steels are mentioned.
285
V., Ralu, P.V.S.S., Namboodhiri, T.K.G. and Rao, P.R. Int. J. Fatigue J u l y 1990 12, (4), 2 8 9 - 2 9 2
The low-cycle fatigue beheviour of a high-strength CrMoV steel has been evaluated in the as-received (AR) end the quenched and tempered (QT) conditions, at room temperature. While the quenching and tempering treatment causes a marked increase in the tensile strength and hardness, it also leads to an impairment of the fracture toughness end the low-cycle fatigue (LCF) resistance of the material. A frectogrephic examination revesls that, while there is a large zone of fatigue crack propagation in the AR condition, there is in sharp contrast a small zone of fatigue crack propagation and a large zone of static fracture in the QT condition. The reduction in fatigue resistance of the QT material is attributed to a loss in fracture toughness and a fall in the values of ~I and ~. Graphs, photomicrographs. 16 refs.
showed the locations of the fatigue crack initiation. The mechanisms of c~rrosion fatigue were studied by performing tests in 1 M NaCI at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be insignificant in the corrosion fatigue of HSLA steel in this study, The 1 M NaCI corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced the fatigue beheviour. A pit depth at which no damage occurred was found.
The s t u d y o f scuffing and p i t t i n g failure o f a c a m - t a p p a t r u b b i n g pair.
Liu, J., Lu, Z. and Chang, Y. Wear Oct. 1990 140, (1), 135-147 The mechanism of scuffing is generally considered to be adhesive wear. However, it has been found that the main mechanism is fatigue wear in the form of dsiamination and that plastic deformation plays a very important role in the scuffing formation, which shows an obvious strain fatigue nature. The analyses of wear debris also verify this conclusion. Pitting is the opposite of scuffing and displays a stress fatigue nature. All cracks are formed without detectable plastic deformation. The statistical analysis showed that the majority of pitting cracks initiate on or near the surface. To confirm the viewpoint that scuffing and pitting failure belong to strain and stress fatigue separately, tests simulating wear were conducted using materials (e.g., 3.45-3.65 carbon, 2.1-2.2 Si, 0.6-0.9 Mn, 0.2-0.4 Cr, 0.3-0.5 Mo) with different structures and hardnesses. On the basis of failure analyses and the results of wear tests the mechanism of scuffing and pitting is discussed in detail. Graphs, photomicrographs. 12 refs.
Fatigue d a m a g e accumulation in 2024-1351 aluminium subjected to periodic reversed overloads. Jurcevic, R., DuQuesnay, D.L., Topper, T.H.
A study on variable a m p l i t u d e load f a t i g u e a n d an e x p e r t s y s t e m f o r f a t i g u e assessment o f h i g h w a y steel bridges. (Dissertation}. Melhem, H.G. Dies. Abstr. Int. Sept. 1990 51, (3), 373 p p
The variation in damage accumulation rates that occur when periodic tension-compression and compression-tension overloads are introduced into an otherwise constant-amplitude loading history are examined. Block loading histories consisting of either compression-tension or tension-compression overloads followed by smaller constant-amplitude fully reversed cycles were applied to smooth, axial, 2024-T351 AI specimens to determine the increase in damage accumulation rates for the fully reversed cycles following such overloads. The results show that the interactive damage/cycle remains constant for approximately 450 cycles following an overload and then it decays as s power law function of the number of cycles. The periodic application of a compression-tension or a tension-compression overload contributes significantly to damage accumulation during subsequent small fully reversed cycles, either above or below the constantamplitude fatigue limit. Consequently, cycles with stress amplitudes below the constant-am plitude fatigue limit, which are usually omitted from damage calculations, should be accounted for in fatigue life prediction analyses for variable.amplitude histories. Graphs. 29 refs.
The fatigue beheviour of highway steel bridges, that is theories, hypotheses and specifications attempting to prevent fatigue failures, is studied, especially as regards variable-amplitude random-sequence loads. The study is part of s research project currently being conducted at the University of Pittsburgh under a multiyesr FHWA contract entitled 'Variable Amplitude Load Fatigue'. The extensive literature survey conducted for this research is reviewed and the major findings affecting the initial project working plan are summarized. The preparatory work necessary to generate new fatigue data for full-scale beams is also presented. Fifteen 101.6 cm deep beams with overall lengths ranging from 3.96 to 6.7 m have been fabricated and are being tested under the variable-amplitude random-sequence loading most representative of the traffic loads applied on actual bridges. The stress spectrum was developed from the latest nationwide weigh-in-motion study, which is considered to be the most appropriate source of data for defining actual traffic characteristics. The stress spectrum is composed of 10000 different cycles arranged in a random sequence. It is applied repeatedly until failure occurs, or 100 million cycles are reached, whichever occurs first. Testing is still in progress and the completion date for this project is far beyond the time limits of this dissertation. However, the preliminary results that were obtained are discussed. The groundwork for developing a surrogate prototype expert system for steel bridge inspection was completed. The prototype (pilot) system has been developed using the professionsi version of the expert system shell (building tool) 'EXSYS'. The system can help the bridge engineer in planning the fatigue inspection as far as: where to look, what to look for, and how to look; and in assessing the criticality of the bridge and its different components with respect to fracture and fatigue. After a bridge inspection has been performed, the system can be used to analyse the results of the inspection, suggest remedial actions, and calculate the approximate remaining fatigue life of the details under consideration.
and Pompetzki, M.A. Int. J. Fatigue J u l y 1990 12, (4), 2 5 9 - 2 6 6
The role o f p r i m a r y carbides in f a t i g u e crack p r o p a g a t i o n in aeroengine b e a r i n g s t e e l s . Iqbal, A. and King, J.E. Int. J. Fatigue J u l y 1990 12, (4), 2 3 4 - 2 4 4 Fatigue crack propagation, tensile and fracture toughness data for four aeroengine bearing steels are reported. The steels involved are the through-hardened tool steels 18-4-1 (T1) and M50, and two similar carburized steels, RBD and Volvic. Crack growth data have been obtained at 20 and 280°C to cover the range of oil temperatures experienced in aeroengine bearing operations. At 20 °C, threshold &K values (~Kth) ranged between 3.5-4.5 MPa ~/m with Paris exponents (m) of between 2.0-3.5. The lowest m-values were seen in the csrburizing steels, which also exhibited lower Paris regime crack growth rates than MS0 and 18-4-1. For all the steels, growth rates were higher at 280 °C than 20 °C, although there was a slight tendency for AKth to increase, probably associated with oxide-induced closure at 280 °C. The effects of primary carbides, strength and toughness on fatigue crack growth behaviour are discussed, in relation to the importance of static-mode cracking. Graphs, photomicrographs. 23 refs. T h e effect o f S-phese d i s t r i b u t i o n o n f a t i g u e crack g r o w t h in peak-aged AI-U-Mg-Cu-Zr alloys. Dudgeon, H.D. and Martin, J.W. Int. J. Fatigue J u l y 1990 12, (4), 2 5 3 - 2 5 8 Curves of da/dN against AK have been compared in CT specimens of two AI-Li-Mg-Cu-Zr alloys of differing Cu content aged to peak hardness at 170 °C. Changes in the S-phase distribution caused by stretching or duplex ageing have been employed as a means of influencing the slip distribution, and fatigue crack growth (FCG) curves at R-values of 0.1 and 0.7 have been compared to evaluate crack closure effects. At R = 0.7, crack closure appears to be negligible and stretched materials show a lower threshold than unstratched ones, owing to slip homogenization. The alloy with higher Cu content exhibited lower thresholds than the other alloy, since the associated higher volume fraction of the S phase gave greater slip homogenization. Duplex-aged (DA) alloys show the greatest crack roughness and the highest thresholds; it is suggested that their high planarity of slip arises from the absence of a dislocation substructure and from the shearing of the finer S-phase particles. At R = 0.1, roughness- and debris-induced crack closure contribute to produce higher thresholds in all specimens. When FCG curves are expressed in terms of the effective stress intensity range (by allowing for crack closure) the data obtained st R = 0.1 and R = 0.7 are comparable. Graphs, photomicrographs. 7 refs. The influence o f environment on t h e f a t i g u e b e h a v i o u r o f HF-80 HSLA steel. (Dissmtation). Ahn, S.-H. Diss. Abstr. Int. Sept. 1990 51, (3), 91 pp Single-pitted specimens of HF-80, a HSLA steel, were tested in laboratory air and in a 1 M NeCI solution to study the influence of s corrosive environment on its fatigue life. The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM
Int J Fatigue May 1991
Fatigue reliability and fracture b e h a v i o u r o f h i g h w a y steel bridges. (Dissertation). Michael, A. Y. Diss. Abstr. InL Sept. 1990 81, (3), 271 pp Structural reliability, in general, can be defined as the probability that a structural component will not fail within its design life. Thus, the fatigue reliability of a highway bridge is the probability that the bridge can have • satisfactory performance under the repetitive traffic loading during its lifetime. As a measure of the reliability of a structure the reliability index, ~, is used. Fatigue requirements have become the controlling factor for the design of steel bridges in all the contemporary design codes. These fatigue requirements are based on the allowable S-N lines, which were produced from fatigue tests and relate stress range, S, to number of cycles to failure, N. Based on the allowable S-N lines a reliability index, 6, format is derived, which is used for the AASHTO fatigue design criteria. The value of the reliability index for all fatigue detail categories specified by AASHTO as well as its uniformity for all details are examined. The same reliability index format is used for the reliability analysis of the fatigue design criteria of the Ontario Highway Bridge Design Code, and the results ere compared between the two codes. The sensitivity of this reliability index to the parameters that affect the fatigue behaviour of a bridge is also investigated. Knowing how each of these parameters affect the reliability index value, recommendations can be made on how to improve the design criteria, targeting a desirable I~-value. Finally, the fatigue reliability of highway steel bridges using the fracture mechanics approach is studied, with the aid of a reliability index format associated with this kind of process.
E s t i m a t i o n o f residual life expectancy. Orna, M. Skoda Rev. 1990 (2), 1 8 - 2 3 Estimation of the residual life of rotors in power turbines is critical to the prevention of expensive failures. Three areas of concern include creep, brittle failure and fatigue. Inspection of components for deformation and cracks are critical to proper estimates of the residual life. The use of fracture mechanics enables better use of material properties end the application of inspection results. Corrosion is another complicating effect on service life end can considerably increase the detrimental effect of fatigue. Non-destructive inspection combined with operating experience allows good success in the prediction of remaining life and affords operating lives of 150-200000 h. CrMoV steels are mentioned.
285