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Effect of partial coverage upon the fatigue fracture behaviour of peened components
fhe dispersoid phase can alter the mode of fatigue fracture by its influence on the deformation uniformity of the alloy. The dispersoid itself may dscohere from the dispersoid/matrix interface under cyclic fatigue stress to form small-sized dimples. Photomicrographs. 5 refs. Threshold of short fatigue crack f o r 8090 A I - L i alloy. Chen, Z. Acta Metal/. Sin. (China) Oct. 1990 26, (5), A374-A377 (in Chinese) The threshold of short through fatigue cracks for an AI-Li 8090 alloy was evaluated to be lower than that of long cracks, and to decrease with shortening of the crack length. The correlation between the threshold, crack wake and crack tip shielding of short fatigue cracks is discussed. The critical length and lowest theoretical threshold of short through fatigue cracks are presented. Graphs. 4 refs,
forged and machined. Of the four categories, the cold-forged components are limited in size and overall chemistry. Most of the cold-forged items used are precision hightensile fasteners, falling into a large category with respect to chemical composition, mechanical and structural properties, besides s wide variety of configurations. However, the multiplicity of the grades of steels used for cold-forgad fasteners/components for the same property class continues to be amazingly high and without justification. If cost effectiveness is the prime requirement it has to be ensured that dogmatism on the selection of materials gives way to a pragmatic approach towards finding suitable alternatives without sacrificing either the quality or the endusability. The objective is to stress the fact ~that cost reduction of subcontracted components/high-tensile fasteners can only be achieved through an in-depth analysis of the mutual constraints and the evolving well defined material specifications to the advantage of both, and in this the standardization organizations have a definite role to perform.
Mechanism map of torsional fatigue fracture. Hu, Z., Wu, Y., Cai, H. and
Me, L. Acta Metall. Sin. (China) Oct. 1990 26, (5), A362-A367 (in Chinese)
Studies have been made of the torsional fatigue fracture life of notched specimens, the macroscopic fractogreph and microscopic fracturing mechanism of 40Cr steel as various tempered states under different stresses. As Stress increases the fracture type changes from normal stress fracture to longitudinal shear end then transverse shear. Under the same stress but with an increase of strength, the fracture type transferred from shear to normal stress fracture. The mechanism of normal stress fracture may be: tranegrenular fracture --* striation + intergrenular fracture --* dimple + intergranuler fracture, and of shear fracture may be: trenegrenular frecture--~ shear trace--* dimple. From the experimental results a fracture mechanism map of torsional fatigue may be constructed. Photomicrographs. 5 refe. Effect o f m i c r o a t m c t u r e on tensile and fatigue properties of superplaaticolly f o r m e d A I - U - , C o - M g - Z r 8000 alloy sheet. Shakesheff, A.J.,
McDarmaid, D.S. and Gregson, P.J. Mater. ScL Technol. Mar. 1991 7, (3), 276-281
The effect of superplestic deformation on the microstructure of a low-density AI-Li-Cu-Mg-Zr (8090) alloy has been investigated. Changes in grain structure, grain misodentetion, and texture were observed with increasing superpleatic deformation. These observations are consistent with strein-enhenosd grain growth accompanied by grain boundary sliding and rotation. The tensile and fatigue properties of suparplastically formed (SPF) 8090 depend on soparpleatic strain, postformation heat treatment end Cu content. It is shown that, in the heat-treated condition, the continuous loss in strength with suparplaatic strain can be attributed to grain growth and solute depletion at the surface. The changes in grain structure lead to increased surface roughness end a consequent reduction in fatigue life. In alloys of high Cu content, prscipitetion during age hardening is additionally dependent on the rate of cooling after SPF. Photomicrographs, graphs. 12 rsfs. B e n d i n g f a t i g u e strength of the curburizing steel SS 2506. Preston, S. Mater. ScL Technol. Feb. 1991 7, (2), 105-110 The effect of steel cleanness, core hardenebility, and microstructure after carburizing on the bending fatigue strength is described for the standard steel grade SS 2506. Examination of the fatigue fracture surfaces revealed that initiation had occurred at the surface where an internally oxidized layer was present. Differences in the inclusion content were not a major cause of fatigue failure. The highest fatigue strength was obtained for the material with the smallest grain size but the effect of grain size is small in compadeon with the base composition. An explanation for the variation in fatigue strength between heat treatments is given with reference to the difference in case depth and core hardenabllity, and their effect on the compressive residual stresses within the cast. Optimum fatigue strength was achieved for the shortest ceea depth and a low core herdenability. The results indicate the importance of close control over these two parameters in producing the optimum fatigue strength. Photomicrographs, graphs. 19 refs. The qsplicotion o f t h e consecutive Wohler curve cun~ept t o celoulats t h e n u m b e r o f failure c y d e s o f different load processes w i t h i n t h e l o w cyclic fatigue region. Schott, G. Materialwissen. Werkstofftech. Mar. 1991 22, (3), 99-106 (in German) Until now the consecutive Wohler curve concept has been successfully used in the area of fatigue strength during stress-controlled block progremmes and random loads. This paper deals with the test of the suitability of this concept, contrary to the local concepts, which contain a linear sum of the damage, for the calculation of the number of failure cycles of strain-controlled, different load processes in the LCF area. As the new concept takes into consideration the reel fatigue behaviour during the calculation, it is possible to find the exact numbers of failure cycles in the LCF area as well. Graphs. 15 refs. High-cycle and long-life f a t i g u e of 25CdW104 under multlaxlal load conditions by three alternating stresses. Grun, P., Troost, A., Akin, O.
and Klubberg, F. Materialwissen. Werkstofftech. Mar. 1991 22, (3), 73-80 (in German)
Statistically verified experimental results from high-cycle and long-life fatigue tests (HCF and LLF, respectively) on 537 unnotched solid cylindrical and thin-walled hollow specimens are used to demonstrate the fatigue boheviour (S--Ncharecteristica, scatterband) of 25CrMo4 under uniaxial loading with superimposed static stresses (consideration of the mean stress effect) under biaxial Ioedings in variation of phase differences between the three combined normal and torsional stresses (~x,~y and Txp The fatigue strength commonly decreases with life time in the high-cycle regime until the fatigue endurance limit is reached in the transition range to infinite life. In comparison with the specific case of biaxial combined loading with synchronous amplitudes, the fatigue resistance characteristics are detrimentally influenced by out-of-phase normal stresses (r~ (rv; a phase difference of 180° between the normal stress amplitudes is the most critical state of combination, especially in the lower cycle regime caused by a greater slope coefficient. Graphs, photomicrographs. 40 refs. Standardization o f steel f o r cold f o r g e d auto components. Sinha, G.S.P. Tool andAIIoy Steels Dec. 1990 24, (12), 477-480 In an automobile engine, its drive mechanism, wheel assembly, etc., there are four distinct classes of steel-related large and small components: case, hot-forged, cold.
506
Life predictions by three c r e e p - f a t i g u e interaction models: influence of m u l t l a x l a l i t y end time-variable Ioedings. Bertini, L. Mater. High Temp. Feb. 1991 9, (1), 23-29 The life predictions given by three creep-fatigue damage models, namely the 'continuous damage model', the "damage rate approach', and the 'time and cycle fractions summation' method, were evaluated for more than 200 high-tempareture fatigue tests reported in the literature. These tests, which were conducted on AISI 304 and 316 stainless steals, including several different multiaxial and timevariable loading conditions (in-phase and out-of-phase biaxial stress, various strain weveforms, complex loading histories, etc.) whose influence on the distribution of the ratio between predicted and observed cycles to failure was analysed. Results obtained by the three models are discussed, emphasizing differences in accuracy and cases in which the creep-fatigue behaviour does not appear to be satisfactorily predicted. Graphs. 27 refs. Effects o f grain size on w e d g e - t y p e crack initiation in low-cycle fatigue at high temperature, lizuka, H. and Tanaka, M. Mater. High Temp. Feb. 1991 9, (1), 17-22 The effects of grain size on wedge-typa crack initiation were investigated under high-temparature low-cycle fatigue conditions in an austenitic 304 stainless steel at 973 K. The accumulated grain-boundary sliding (GBSI displacement and the crack initiation life were measured in fatigue tests with slow-slow and slow-fast strain wave shapes. The accumulated GBS displacement per cycle was proportional to the grain diameter, D. Wedge-type cracks were initiated by the accumulated GBS at grain-boundary triple junctions. The crack initiation life, hi, was proportional to D-°.s. The grain-size dependence of nl agreed well with the analytical result that was obtained with the micromechanics model. Graphs, photomicrographs. 16 refs. Analysis o f fatigue life data using the B o x - C o x transformation. Hinkle,
A.J. and Emptage, M.R. Fatigue Fract. Eng. Mater. Struct. 1991 14, (5), 591-600
The fatigue life data customarily analysed using the logarithmic transformation may be analysed better using the Box-Cox transformation. This transformation has the advantages of stabilizing the variance, simplifying the model and transforming to approximate normality, in addition, simple schemes for inputing run-outs can be constructed and the endurance limit can be easily calculated. The use of the transformation on fatigue life data from a thick-plate AI-Zn-Cu-Mg alloy is considered in detail. A simplified method for determining the transformation is presented and then the transformation is used to determine the distinctions in quality between different samples and confidence intervals on the lifetime. Graphs. 1 ref.
Modelling residual stresses and fatigue crack g r o w t h at cold-expanded
festonur holes. Clark, G. Fatigue Fract. Eng. Mater. Struct. 1991 14, (5), 579-589
A model developed for predicting residual stresses and crack growth in residual stress fields, and the application of the model to crack growth from cold-worked fastener holes in thick-section aircraft components are described. Comparison with experimental I'esuits demonstrates that the model can provide useful predictions of critical crack length, and a capability for correctly predicting the maxima and minima in the crack growth rate for cracks from cold-expanded holes. It also permits the observed asymmetry in cracking from cold-worked fastener holes to be better understood. Graphs, photomicrographs. 26 rsfs.
Effect o f partial coverage upon t h e fatigue fracture beheviour o f peened
components. Meguid, S.A. Fatigue Fract. Eng. Mater. Struct. 1991 14, (5), 515-530
The achievement of full coverage during the application of shot peening to critical components represents a major concern in the aerospace end power generation industries. It has been accepted for many years that full COverageof such components is needed to attain the beneficial fetigue-life effects of the treatment. It has also been proposed by a number of industrialists that partial coverage may apparently shorten the fatigue life of the component because of the presumed presence of tensile residual stresses in the uncovered areas. Three aspects of the investigation were accordingly examined. The first deals with the accurate measurement of peening coverage using a three-dimensional surface profilomater arrangement. The second deals with the development of fatigue crack growth data for fully- and partially-pesned components using an instrumented rotating-bend fatigue rig. The third deals with monitoring the residual stress field associated with different peening coverage using the dissection method. To highlight the effect of the treatment upon the fatigue beheviour of the components tested, both theoretical and experimental techniques were considered. In the theoretical work, thres-dimeneionel finite-element analysis of cimumferentielly cracked, notched and unnotched cylindrical components was considered in the development of the corresponding atrees-intenaity factor (/~) under bending loads. The stress-intensity factor against crack length relationship was subsequently used in the experimental determination of fatigue crack growth rate data at the different strees levels examined. Accordingly, room temperature rotating-bend fatigue tests were conducted on apacimens made from medium carbon steel 060 M40 (En 8) and AI alloy (7075-T6). Surprisingly, ~:he results of both materials reveal that a life improvement of up to 50% can be "attained at 35% coverage level. Graphs. 16 rsfs.
Int J Fatigue November 1991
forged and machined. Of the four categories, the cold-forged components are limited in size and overall chemistry. Most of the cold-forged items used are precision hightensile fasteners, falling into a large category with respect to chemical composition, mechanical and structural properties, besides s wide variety of configurations. However, the multiplicity of the grades of steels used for cold-forgad fasteners/components for the same property class continues to be amazingly high and without justification. If cost effectiveness is the prime requirement it has to be ensured that dogmatism on the selection of materials gives way to a pragmatic approach towards finding suitable alternatives without sacrificing either the quality or the endusability. The objective is to stress the fact ~that cost reduction of subcontracted components/high-tensile fasteners can only be achieved through an in-depth analysis of the mutual constraints and the evolving well defined material specifications to the advantage of both, and in this the standardization organizations have a definite role to perform.
Mechanism map of torsional fatigue fracture. Hu, Z., Wu, Y., Cai, H. and
Me, L. Acta Metall. Sin. (China) Oct. 1990 26, (5), A362-A367 (in Chinese)
Studies have been made of the torsional fatigue fracture life of notched specimens, the macroscopic fractogreph and microscopic fracturing mechanism of 40Cr steel as various tempered states under different stresses. As Stress increases the fracture type changes from normal stress fracture to longitudinal shear end then transverse shear. Under the same stress but with an increase of strength, the fracture type transferred from shear to normal stress fracture. The mechanism of normal stress fracture may be: tranegrenular fracture --* striation + intergrenular fracture --* dimple + intergranuler fracture, and of shear fracture may be: trenegrenular frecture--~ shear trace--* dimple. From the experimental results a fracture mechanism map of torsional fatigue may be constructed. Photomicrographs. 5 refe. Effect o f m i c r o a t m c t u r e on tensile and fatigue properties of superplaaticolly f o r m e d A I - U - , C o - M g - Z r 8000 alloy sheet. Shakesheff, A.J.,
McDarmaid, D.S. and Gregson, P.J. Mater. ScL Technol. Mar. 1991 7, (3), 276-281
The effect of superplestic deformation on the microstructure of a low-density AI-Li-Cu-Mg-Zr (8090) alloy has been investigated. Changes in grain structure, grain misodentetion, and texture were observed with increasing superpleatic deformation. These observations are consistent with strein-enhenosd grain growth accompanied by grain boundary sliding and rotation. The tensile and fatigue properties of suparplastically formed (SPF) 8090 depend on soparpleatic strain, postformation heat treatment end Cu content. It is shown that, in the heat-treated condition, the continuous loss in strength with suparplaatic strain can be attributed to grain growth and solute depletion at the surface. The changes in grain structure lead to increased surface roughness end a consequent reduction in fatigue life. In alloys of high Cu content, prscipitetion during age hardening is additionally dependent on the rate of cooling after SPF. Photomicrographs, graphs. 12 rsfs. B e n d i n g f a t i g u e strength of the curburizing steel SS 2506. Preston, S. Mater. ScL Technol. Feb. 1991 7, (2), 105-110 The effect of steel cleanness, core hardenebility, and microstructure after carburizing on the bending fatigue strength is described for the standard steel grade SS 2506. Examination of the fatigue fracture surfaces revealed that initiation had occurred at the surface where an internally oxidized layer was present. Differences in the inclusion content were not a major cause of fatigue failure. The highest fatigue strength was obtained for the material with the smallest grain size but the effect of grain size is small in compadeon with the base composition. An explanation for the variation in fatigue strength between heat treatments is given with reference to the difference in case depth and core hardenabllity, and their effect on the compressive residual stresses within the cast. Optimum fatigue strength was achieved for the shortest ceea depth and a low core herdenability. The results indicate the importance of close control over these two parameters in producing the optimum fatigue strength. Photomicrographs, graphs. 19 refs. The qsplicotion o f t h e consecutive Wohler curve cun~ept t o celoulats t h e n u m b e r o f failure c y d e s o f different load processes w i t h i n t h e l o w cyclic fatigue region. Schott, G. Materialwissen. Werkstofftech. Mar. 1991 22, (3), 99-106 (in German) Until now the consecutive Wohler curve concept has been successfully used in the area of fatigue strength during stress-controlled block progremmes and random loads. This paper deals with the test of the suitability of this concept, contrary to the local concepts, which contain a linear sum of the damage, for the calculation of the number of failure cycles of strain-controlled, different load processes in the LCF area. As the new concept takes into consideration the reel fatigue behaviour during the calculation, it is possible to find the exact numbers of failure cycles in the LCF area as well. Graphs. 15 refs. High-cycle and long-life f a t i g u e of 25CdW104 under multlaxlal load conditions by three alternating stresses. Grun, P., Troost, A., Akin, O.
and Klubberg, F. Materialwissen. Werkstofftech. Mar. 1991 22, (3), 73-80 (in German)
Statistically verified experimental results from high-cycle and long-life fatigue tests (HCF and LLF, respectively) on 537 unnotched solid cylindrical and thin-walled hollow specimens are used to demonstrate the fatigue boheviour (S--Ncharecteristica, scatterband) of 25CrMo4 under uniaxial loading with superimposed static stresses (consideration of the mean stress effect) under biaxial Ioedings in variation of phase differences between the three combined normal and torsional stresses (~x,~y and Txp The fatigue strength commonly decreases with life time in the high-cycle regime until the fatigue endurance limit is reached in the transition range to infinite life. In comparison with the specific case of biaxial combined loading with synchronous amplitudes, the fatigue resistance characteristics are detrimentally influenced by out-of-phase normal stresses (r~ (rv; a phase difference of 180° between the normal stress amplitudes is the most critical state of combination, especially in the lower cycle regime caused by a greater slope coefficient. Graphs, photomicrographs. 40 refs. Standardization o f steel f o r cold f o r g e d auto components. Sinha, G.S.P. Tool andAIIoy Steels Dec. 1990 24, (12), 477-480 In an automobile engine, its drive mechanism, wheel assembly, etc., there are four distinct classes of steel-related large and small components: case, hot-forged, cold.
506
Life predictions by three c r e e p - f a t i g u e interaction models: influence of m u l t l a x l a l i t y end time-variable Ioedings. Bertini, L. Mater. High Temp. Feb. 1991 9, (1), 23-29 The life predictions given by three creep-fatigue damage models, namely the 'continuous damage model', the "damage rate approach', and the 'time and cycle fractions summation' method, were evaluated for more than 200 high-tempareture fatigue tests reported in the literature. These tests, which were conducted on AISI 304 and 316 stainless steals, including several different multiaxial and timevariable loading conditions (in-phase and out-of-phase biaxial stress, various strain weveforms, complex loading histories, etc.) whose influence on the distribution of the ratio between predicted and observed cycles to failure was analysed. Results obtained by the three models are discussed, emphasizing differences in accuracy and cases in which the creep-fatigue behaviour does not appear to be satisfactorily predicted. Graphs. 27 refs. Effects o f grain size on w e d g e - t y p e crack initiation in low-cycle fatigue at high temperature, lizuka, H. and Tanaka, M. Mater. High Temp. Feb. 1991 9, (1), 17-22 The effects of grain size on wedge-typa crack initiation were investigated under high-temparature low-cycle fatigue conditions in an austenitic 304 stainless steel at 973 K. The accumulated grain-boundary sliding (GBSI displacement and the crack initiation life were measured in fatigue tests with slow-slow and slow-fast strain wave shapes. The accumulated GBS displacement per cycle was proportional to the grain diameter, D. Wedge-type cracks were initiated by the accumulated GBS at grain-boundary triple junctions. The crack initiation life, hi, was proportional to D-°.s. The grain-size dependence of nl agreed well with the analytical result that was obtained with the micromechanics model. Graphs, photomicrographs. 16 refs. Analysis o f fatigue life data using the B o x - C o x transformation. Hinkle,
A.J. and Emptage, M.R. Fatigue Fract. Eng. Mater. Struct. 1991 14, (5), 591-600
The fatigue life data customarily analysed using the logarithmic transformation may be analysed better using the Box-Cox transformation. This transformation has the advantages of stabilizing the variance, simplifying the model and transforming to approximate normality, in addition, simple schemes for inputing run-outs can be constructed and the endurance limit can be easily calculated. The use of the transformation on fatigue life data from a thick-plate AI-Zn-Cu-Mg alloy is considered in detail. A simplified method for determining the transformation is presented and then the transformation is used to determine the distinctions in quality between different samples and confidence intervals on the lifetime. Graphs. 1 ref.
Modelling residual stresses and fatigue crack g r o w t h at cold-expanded
festonur holes. Clark, G. Fatigue Fract. Eng. Mater. Struct. 1991 14, (5), 579-589
A model developed for predicting residual stresses and crack growth in residual stress fields, and the application of the model to crack growth from cold-worked fastener holes in thick-section aircraft components are described. Comparison with experimental I'esuits demonstrates that the model can provide useful predictions of critical crack length, and a capability for correctly predicting the maxima and minima in the crack growth rate for cracks from cold-expanded holes. It also permits the observed asymmetry in cracking from cold-worked fastener holes to be better understood. Graphs, photomicrographs. 26 rsfs.
Effect o f partial coverage upon t h e fatigue fracture beheviour o f peened
components. Meguid, S.A. Fatigue Fract. Eng. Mater. Struct. 1991 14, (5), 515-530
The achievement of full coverage during the application of shot peening to critical components represents a major concern in the aerospace end power generation industries. It has been accepted for many years that full COverageof such components is needed to attain the beneficial fetigue-life effects of the treatment. It has also been proposed by a number of industrialists that partial coverage may apparently shorten the fatigue life of the component because of the presumed presence of tensile residual stresses in the uncovered areas. Three aspects of the investigation were accordingly examined. The first deals with the accurate measurement of peening coverage using a three-dimensional surface profilomater arrangement. The second deals with the development of fatigue crack growth data for fully- and partially-pesned components using an instrumented rotating-bend fatigue rig. The third deals with monitoring the residual stress field associated with different peening coverage using the dissection method. To highlight the effect of the treatment upon the fatigue beheviour of the components tested, both theoretical and experimental techniques were considered. In the theoretical work, thres-dimeneionel finite-element analysis of cimumferentielly cracked, notched and unnotched cylindrical components was considered in the development of the corresponding atrees-intenaity factor (/~) under bending loads. The stress-intensity factor against crack length relationship was subsequently used in the experimental determination of fatigue crack growth rate data at the different strees levels examined. Accordingly, room temperature rotating-bend fatigue tests were conducted on apacimens made from medium carbon steel 060 M40 (En 8) and AI alloy (7075-T6). Surprisingly, ~:he results of both materials reveal that a life improvement of up to 50% can be "attained at 35% coverage level. Graphs. 16 rsfs.
Int J Fatigue November 1991