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Thermal fatigue of a 12% chromium martensitic stainless steel
306
Fatigue Abstracts
Mixed-Mode I and II fatigue threshold and crack closure in dual-phase steels.
Zheng, Y.S., Wang, Z.G. and Ai, S.H. Metall. Mater. Trans. A (Aug. 1994) 25A (8), 1713-1723 Fatigue threshold under mixed-Mode 1 and II loading and elastic plane-strain conditions has been studied in dual-phase steels (DPS) of two types of volume fraction of martensite (Vm) in laboratory air at room temperature. Near-threshold mixed-mode (I and 1I) fatigue crack growth occurs mainly by two mechanisms: shear mode and tensile mode. Particular emphasis was placed on the influence of the Mode II component. The mixed-mode threshold is controlled not only by Mode 1 displacement but also by the Mode It component. Apparent-and effective-bound curves (corrected closure) are obtained for the threshold condition and discussed in terms of the shape and size of the plastic region of the crack tip; crack surface rubbing; and especially, roughness-induced closure and shear resistance of crack surface, which resulted in an extremely high extrinsic-toughening contribution to the mixedmode fatigue threshold values. The ratio of the threshold value of pure Mode II to that of pure Mode I (AgthAl/AKth.l reached 1.9; the maximum hoop direction stress-intensity factor range of a pure Mode II branch crack tip is 2.2 times that of pure Mode I. Obviously, the resistance of pure Mode I1 crack growth here is far larger than that of pure Mode I; the former is just to introduce the shear resistance of crack surface, the latter to reduce the driving force of the crack tip for crack closure. It is proposed that the apparent- and effective-bound curves are non-conservatively risky and too conservative for design purposes, respectively. Therefore the threshold data should be obtained under the specific conditions found by concrete mechanical, microstructural, and environmental factors. Graphs, photomicrographs, 42 ref. The new implant for the femoral shaft fracture after hip arthroplasty. Li, K.-
C., Chen, C.-M. and Lo, W.-H. Biomed. Eng. Appl. Basis Commun. (25 Apr. 1994) 6 (2), 179-184 A new implant was designed, named "femur reconstruction plate' (FRP), for use when femoral shaft fracture occurs after hip arthroplasty. This plate was characterized by a threaded screw hole, trochanteric hooks, proximal trochanteric screws, and anterior bowing curve to fit the femur shaft. Howmedic's Dall-Miles cable/crimp cerelage system (1.6 mm briaded CoCr alloy) was used to augment the proximal fragments. The cable grooves were designed on the plate to prevent the slippage of cables. In synthetic foam femur bone, which mimics severe osteoporosis, after the cemented stem was applied and Johansson's type II fracture was mimicked, compression tests showed that FRP fixation was 50% stronger than the AO-DCP. In the cadaver femur after implantation with the cemented prosthetic stem, the fixation strength of FRP was 100% superior to the traditional DCP. The fatigue test showed that the new implant-post-THR cadaver femur with fracture can tolerate four times body weight over 100000 cycles. The FRP is designed to fix such special femur fractures, which can allow early partial weight bearing. 19 ref. Creep-fatigue crack growth behavior of a Cr steel. Yoon, K.B., Baek, U.B.
and Lee, ll.M. J. Korean Inst. Met. Mater. (April 1994) 32 (4), 496-502 (in Korean) In starting up turbines of power plants, the procedures recommended by the manufacturer shoffld be carefully observed. However, many utilities often start turbines so fast as to cause accelerated crack growth. Hence, to predict accurately the crack growth life of turbine rotors, a crack growth prediction model is needed in which crack growth rate change due to variation of load increasing time. This is considered by using a typical rotor steel (1Cr). Fatigue testing with a discrete-time stochastic process. Svensson, 12 Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 727-736
A computer program that creates time histories for use in fatigue tests in servohydraulic machines is presented. The time histories can be tailored from certain user specifications, namely the irregularity factor and the level-crossing spectrum. The program is based on the theory of stochastic time series, and the formulae needed for the program are derived in the paper. Problems of optimality are treated both with respect to fulfilment of the specifications and with respect to machine performance. Examples from two fatigue-testing projects, where the program has been applied, are presented. A mechanism for plasticity-induced crack closure under plane strain conditions.
Pippan, R., Kolednik, O. and Lang, M. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 721-726 Plasticity-induced crack closure is associated with a wedge of material in the wake of a fatigue crack. A model is proposed to explain the development of this wedge under plane strain conditions. This model is based on the nonhnearity of the crack path. If the plastic zone is larger than the characteristic length of a crack deflection by an area of magnitude or more, then the plastic deformation changes the shape of both crack flanks in different ways. This causes a residual displacement on the crack flanks. Different examples for the development of such plasticity-induced wedges are shown. 4340 is mentioned. Photomicrographs, 21 ref. Elevated temperature low-cycle fatigue behavior of HK40 alloy. Konosu, S. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 683-693
By conducting creep tests and creep-fatigue tests at temperatures of 800, 900 and 1000°(2 on centrifugally cast HK40 alloy (which is used for example for the steam reformer tubes of fuel cell plants), and varying the strain rate
during the loading and unloading process, the influence of strain history on the life of the material has been clarified. Furthermore, the data obtained from these tests were subjected to analyses by means of life evaluation methods utilizing the life fraction rule, strain range partitioning method, and damage rate approach, and comparative studies were conducted on each life evaluation method. As a result, problematic points concerning the application of the life fraction rule, strain range partitioning method, and damage rate approach have been clarified, and data have been obtained which would be useful in establishing design guidelines for structures to be used under conditions such as those involving the interaction of creep and fatigue. Photomicrographs, graphs, 7 ref. Thermal fatigue of a 12% chromium martensitic stainless steel. Alvarez-
Armas, l., Armas, A.F. and Petersen, C. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 671-681 The cyclic mechanical behaviour during thermal fatigue of a martensitic stainless steel (MANET l) is investigated and correlated with its microstructure. The tests were performed in air by allowing the sample to serve as its own heater and converting any longitudinal thermal deformation of the specimen into elastic or inelastic deformation. The effects of different thermal cycling ranges and the imposition of several hold-time conditions on the mechanical behaviour and the accompanying microstructural changes in the specimen were evaluated. A continuous softening is the characteristic feature observed in all thermal fatigue tests. Higher mean temperatures, larger temperature changes and the application of temperature-hold periods produce an accelerated softening process. All temperature hold-time conditions influence the mechanical behaviour, reducing the number of cycles to failure. The initial tempered martensite lath structure evolves to a mixed structure of expanded laths and subgrains or a fully subgrain structure, depending on the mean temperature, the temperature change and the temperature at which the hold period has been applied. Tests with temperature holds at higher temperatures show more deleterious effects than tests with hold-times at lower temperatures. Photomicrographs, graphs, 11 ref. Fretting patterns in a conductor-clamp contact zone. Zhou, Z.R., C~trdou. A., Goudreau, S. and Fiset, M. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 661-669
Cyclic bending fatigue tests have been performed on a typical electrical conductor held by a suspension clamp. Surface damage of the aluminium wire has been induced during short-duration tests. Various degradation zones have been characterized between wire layers and the clamp. Metallographic examination has shown that two process zones are critical with respect to fretting crack nucleation. Fatigue tests at different bending amplitudes have shown that the wire breaks in these critical zones. Photomicrographs, graphs. 11 ref. An elastic-plastic model for fatigue crack growth at notches. Ahmad, H.Y. and }rates, J.R. Fatigue Fruct. Eng. Mater. Struct. (June 1994) 17 (6), 651-661)
The development of an elastic-plastic ff,3cture mechanics model for predicting fatigue crack growth rates in notched specimens is presented in this paper. The model is based on experimental short and long crack growth results from unnotched specimens, and the distribution of strain ahead of the notch root. This approach may be applied to the assessment of the lifetime of many high technology engineering components. Graphs, 20 ref. Statistical investigation of the behaviour of small cracks and fatigue life in carbon steels with different ferrite grain sizes. Goto, M. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 635-649
In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propogation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks. Graphs, photomicrographs, 32 ref. Prediction of frequency effect in high temperature fatigue crack growth using damage factors. Skelton, R.P. and Byrne, J. Mater. High Temp. (1994) 12 (1), 67-74
An earlier modification of the Paris law for the growth of deep cracks in the linear elastic fracture mechanics regime is extended to include a term enabling the prediction of cyclic crack growth rates at low frequencies. The relation requires (i) a reference growth law under continuous cycling at the appropriate elevated temperature and (ii) a specified, dimensionless degradation term, defined as De, the creep/oxidation damage per cycle, which increases as the applied frequency decreases or as the dwell time at peak load is prolonged. The relationship is validated against data from the previous analysis on low alloy ferritic and austenitic steels in the range 538-650°C and against further
Fatigue Abstracts
Mixed-Mode I and II fatigue threshold and crack closure in dual-phase steels.
Zheng, Y.S., Wang, Z.G. and Ai, S.H. Metall. Mater. Trans. A (Aug. 1994) 25A (8), 1713-1723 Fatigue threshold under mixed-Mode 1 and II loading and elastic plane-strain conditions has been studied in dual-phase steels (DPS) of two types of volume fraction of martensite (Vm) in laboratory air at room temperature. Near-threshold mixed-mode (I and 1I) fatigue crack growth occurs mainly by two mechanisms: shear mode and tensile mode. Particular emphasis was placed on the influence of the Mode II component. The mixed-mode threshold is controlled not only by Mode 1 displacement but also by the Mode It component. Apparent-and effective-bound curves (corrected closure) are obtained for the threshold condition and discussed in terms of the shape and size of the plastic region of the crack tip; crack surface rubbing; and especially, roughness-induced closure and shear resistance of crack surface, which resulted in an extremely high extrinsic-toughening contribution to the mixedmode fatigue threshold values. The ratio of the threshold value of pure Mode II to that of pure Mode I (AgthAl/AKth.l reached 1.9; the maximum hoop direction stress-intensity factor range of a pure Mode II branch crack tip is 2.2 times that of pure Mode I. Obviously, the resistance of pure Mode I1 crack growth here is far larger than that of pure Mode I; the former is just to introduce the shear resistance of crack surface, the latter to reduce the driving force of the crack tip for crack closure. It is proposed that the apparent- and effective-bound curves are non-conservatively risky and too conservative for design purposes, respectively. Therefore the threshold data should be obtained under the specific conditions found by concrete mechanical, microstructural, and environmental factors. Graphs, photomicrographs, 42 ref. The new implant for the femoral shaft fracture after hip arthroplasty. Li, K.-
C., Chen, C.-M. and Lo, W.-H. Biomed. Eng. Appl. Basis Commun. (25 Apr. 1994) 6 (2), 179-184 A new implant was designed, named "femur reconstruction plate' (FRP), for use when femoral shaft fracture occurs after hip arthroplasty. This plate was characterized by a threaded screw hole, trochanteric hooks, proximal trochanteric screws, and anterior bowing curve to fit the femur shaft. Howmedic's Dall-Miles cable/crimp cerelage system (1.6 mm briaded CoCr alloy) was used to augment the proximal fragments. The cable grooves were designed on the plate to prevent the slippage of cables. In synthetic foam femur bone, which mimics severe osteoporosis, after the cemented stem was applied and Johansson's type II fracture was mimicked, compression tests showed that FRP fixation was 50% stronger than the AO-DCP. In the cadaver femur after implantation with the cemented prosthetic stem, the fixation strength of FRP was 100% superior to the traditional DCP. The fatigue test showed that the new implant-post-THR cadaver femur with fracture can tolerate four times body weight over 100000 cycles. The FRP is designed to fix such special femur fractures, which can allow early partial weight bearing. 19 ref. Creep-fatigue crack growth behavior of a Cr steel. Yoon, K.B., Baek, U.B.
and Lee, ll.M. J. Korean Inst. Met. Mater. (April 1994) 32 (4), 496-502 (in Korean) In starting up turbines of power plants, the procedures recommended by the manufacturer shoffld be carefully observed. However, many utilities often start turbines so fast as to cause accelerated crack growth. Hence, to predict accurately the crack growth life of turbine rotors, a crack growth prediction model is needed in which crack growth rate change due to variation of load increasing time. This is considered by using a typical rotor steel (1Cr). Fatigue testing with a discrete-time stochastic process. Svensson, 12 Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 727-736
A computer program that creates time histories for use in fatigue tests in servohydraulic machines is presented. The time histories can be tailored from certain user specifications, namely the irregularity factor and the level-crossing spectrum. The program is based on the theory of stochastic time series, and the formulae needed for the program are derived in the paper. Problems of optimality are treated both with respect to fulfilment of the specifications and with respect to machine performance. Examples from two fatigue-testing projects, where the program has been applied, are presented. A mechanism for plasticity-induced crack closure under plane strain conditions.
Pippan, R., Kolednik, O. and Lang, M. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 721-726 Plasticity-induced crack closure is associated with a wedge of material in the wake of a fatigue crack. A model is proposed to explain the development of this wedge under plane strain conditions. This model is based on the nonhnearity of the crack path. If the plastic zone is larger than the characteristic length of a crack deflection by an area of magnitude or more, then the plastic deformation changes the shape of both crack flanks in different ways. This causes a residual displacement on the crack flanks. Different examples for the development of such plasticity-induced wedges are shown. 4340 is mentioned. Photomicrographs, 21 ref. Elevated temperature low-cycle fatigue behavior of HK40 alloy. Konosu, S. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 683-693
By conducting creep tests and creep-fatigue tests at temperatures of 800, 900 and 1000°(2 on centrifugally cast HK40 alloy (which is used for example for the steam reformer tubes of fuel cell plants), and varying the strain rate
during the loading and unloading process, the influence of strain history on the life of the material has been clarified. Furthermore, the data obtained from these tests were subjected to analyses by means of life evaluation methods utilizing the life fraction rule, strain range partitioning method, and damage rate approach, and comparative studies were conducted on each life evaluation method. As a result, problematic points concerning the application of the life fraction rule, strain range partitioning method, and damage rate approach have been clarified, and data have been obtained which would be useful in establishing design guidelines for structures to be used under conditions such as those involving the interaction of creep and fatigue. Photomicrographs, graphs, 7 ref. Thermal fatigue of a 12% chromium martensitic stainless steel. Alvarez-
Armas, l., Armas, A.F. and Petersen, C. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 671-681 The cyclic mechanical behaviour during thermal fatigue of a martensitic stainless steel (MANET l) is investigated and correlated with its microstructure. The tests were performed in air by allowing the sample to serve as its own heater and converting any longitudinal thermal deformation of the specimen into elastic or inelastic deformation. The effects of different thermal cycling ranges and the imposition of several hold-time conditions on the mechanical behaviour and the accompanying microstructural changes in the specimen were evaluated. A continuous softening is the characteristic feature observed in all thermal fatigue tests. Higher mean temperatures, larger temperature changes and the application of temperature-hold periods produce an accelerated softening process. All temperature hold-time conditions influence the mechanical behaviour, reducing the number of cycles to failure. The initial tempered martensite lath structure evolves to a mixed structure of expanded laths and subgrains or a fully subgrain structure, depending on the mean temperature, the temperature change and the temperature at which the hold period has been applied. Tests with temperature holds at higher temperatures show more deleterious effects than tests with hold-times at lower temperatures. Photomicrographs, graphs, 11 ref. Fretting patterns in a conductor-clamp contact zone. Zhou, Z.R., C~trdou. A., Goudreau, S. and Fiset, M. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 661-669
Cyclic bending fatigue tests have been performed on a typical electrical conductor held by a suspension clamp. Surface damage of the aluminium wire has been induced during short-duration tests. Various degradation zones have been characterized between wire layers and the clamp. Metallographic examination has shown that two process zones are critical with respect to fretting crack nucleation. Fatigue tests at different bending amplitudes have shown that the wire breaks in these critical zones. Photomicrographs, graphs. 11 ref. An elastic-plastic model for fatigue crack growth at notches. Ahmad, H.Y. and }rates, J.R. Fatigue Fruct. Eng. Mater. Struct. (June 1994) 17 (6), 651-661)
The development of an elastic-plastic ff,3cture mechanics model for predicting fatigue crack growth rates in notched specimens is presented in this paper. The model is based on experimental short and long crack growth results from unnotched specimens, and the distribution of strain ahead of the notch root. This approach may be applied to the assessment of the lifetime of many high technology engineering components. Graphs, 20 ref. Statistical investigation of the behaviour of small cracks and fatigue life in carbon steels with different ferrite grain sizes. Goto, M. Fatigue Fract. Eng. Mater. Struct. (June 1994) 17 (6), 635-649
In order to study the relation between the scatter characteristics of small crack growth behaviour and fatigue life, rotatory bending fatigue tests of smooth specimens were carried out using 0.21% carbon steels of different ferrite grain sizes. Fifteen to eighteen specimens were fatigued at each stress amplitude, and the initiation and propagation behaviour of the cracks which led to the final fractures were examined for all the specimens. The physical basis of scatter in fatigue life was investigated, based on the successive observation of fatigue damage on the surface using the plastic replica technique, followed by an analysis of the data assuming a Weibull distribution. A statistical investigation of the physical basis of scatter in relation to the ferrite grain size was performed, i.e. the distributions for crack initiation life, crack propogation life, fatigue life and growth rate of small cracks. Finally, the fluctuation of crack growth rate was studied in relation to the application of a crack growth law for microstructurally small cracks. Graphs, photomicrographs, 32 ref. Prediction of frequency effect in high temperature fatigue crack growth using damage factors. Skelton, R.P. and Byrne, J. Mater. High Temp. (1994) 12 (1), 67-74
An earlier modification of the Paris law for the growth of deep cracks in the linear elastic fracture mechanics regime is extended to include a term enabling the prediction of cyclic crack growth rates at low frequencies. The relation requires (i) a reference growth law under continuous cycling at the appropriate elevated temperature and (ii) a specified, dimensionless degradation term, defined as De, the creep/oxidation damage per cycle, which increases as the applied frequency decreases or as the dwell time at peak load is prolonged. The relationship is validated against data from the previous analysis on low alloy ferritic and austenitic steels in the range 538-650°C and against further