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Surface treatment and fatigue strength of forged components
On t h e g r o w t h o f small/short f a t i g u e cracks. McEvily, A.J. JSME Int. J. I Apr. 1989 32, (2), 181-191 A review is made of the stress concentration and the stress intensity factor approaches to dealing with fatigue crack growth. For small cracks, me,those which initiate in unnotched specimens, and for short cracks, me,those which initiate at notches, the stress intensity factor approach is not useful because the factor goes to zero for such cracks. On the other hand, in the stress concentration approach the limiting stress is always the applied stress. A modification to include a material constant in the expression for the stress intensity factor is proposed based upon an analogy with the stress concentration approach. The modified stress intensity factor used in the analysis of surface crack growth. The growth of small/short cracks is also analysed with consideration being given to the effects of crack closure, and the observation that at • given ~K level in the short crack range the shorter the crack the higher the rate of growth, is discussed. Comparisons are also made between experimental results for short cracks growing from notches and predictions based upon the proposed method of analysis. 25 refs. Effect= o f small defects end n o n - m a t c l l i c inclusions on t h e f a t i g u e
st~l
of metals. Murakami, Y. t. J. I Apr. 1989 32, (2), 167-180
The types of fatigue beheviour of metals containing small defects or inclusions are so complicated that the prediction of the influence of defects or inclusions is very difficult. However, recent advances in fracture mechanics have led to quantitative treatment of this problem. First, the characteristics of the effects of small defects and inclusions are discussed by reviewing the existing literature, which mostly reports the defect and inclusion problems qualitatively. Next, it is revealed that the clue to solving the problems quantitatively is the concept that defects and inclusions are virtually equivalent to cracks from the viewpoint of fatigue strength. A method of evaluating the fatigue limits of metals containing small defects based on this concept is introduced. Finally, it is shown that the method is useful in predicting the lower limit of fatigue strength of high-strength steels containing non-metallic inclusions which cause a distinct decrease in fatigue strength and a large scatter of fatigue strength. 99 refs. The effect o f specimen constraints o n t h e shape changes and t h e nucleation o f cracks in cyclically deformed niobium crystals. Vaja, J.
coefficient of friction is not constant. This study is a continuation of a previous one in which it wee ess0med that the coefficient of friction was a constant. In this model it is assumed that the coefficient of friction between the rubbing surfaces is initially zero and slowly increases to a constant value during the early cycles of the fatigue life. The consequences of this change are examined end the results from the two models compared, it is observed that the two models give different frictional forces if the deformation of the surfaces is elastic, but are identical when non-lineer mscroslip occurs on both loading end unloading. Graphs. 2 refs.
Low cycle f a t i g u e lives o f notched c o m p o n e n t s . James, M.N., Dimitriou, C. and Chandler, H.D. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 213-225 In low cycle fatigue situations, the plastic beheviour of the material at the root of stress concentrators is of prime importance in determining the cyclic life. However, simple procedures such as Neuber's rule do not adequately describe the development of plastic behaviour at a notched root, while the expense of s finite element analysis is not justified in many instances. A simple, approximate numerical method of calculating plastic notch stresses and strains that would be of use in such situations is described. The usefulness of the technique is demonstrated by comparing low cycle fatigue lives predicted from notch plastic strains with those determined by fatigue testing of smooth specimens (steel) subjected to similar plastic strain ranges. Graphs. 7 refs.
N o t c h size effect in fatigue. Lukas, P., Kunz, L., Weiss, B. and Stickler,
R. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 175--186 The notch size effect lie the decrease of the notched fatigue limit with increasing
notch size for the same stress concentration factor) was quantitatively derived by describing the threshold conditions for the propagation of a short semi-elliptical crack nucleated at the notch root. A close relation between the Kitegewa-Takabashi diagram for the short crack threshold stress and the dependence of the notched fatigue limit on the notch size was shown. The derived relation for the notch size effect was experimentally verified for several specimen/notch geometries in the cases of pressure vessel steel (2.25Cr-lMo) and Cu (99.98%). 14 refs.
and Guiu, F. Philos. Mag. A M a y 1989 59, (5), 1091-1102
Niobium single crystals, with an orientation conducive to deformation by single glide, have been cyclically deformed st 400 K over a wide range of plastic strain amplitudes and cumulative strains. All the Nb crystals exhibited shape changes which were produced by asymmetric glide. The effects of the end constraints on the axial component of the shape changes ere investigated. It is shown that these constraints deform the specimen into an S shape and create regions of tension and compression on the crystal 'side faces'. Fatigue cracks nucleate at the specimen surface st the regions which ere in tension, and these cracks cause the specimen to fail prematurely, whereas fatigue cracks which have initiated at the intrusions and extrusions in the unconstrained middle regions of the gauge length remain relatively insignificant. The implications of this effect of the orientation dependence of the fatigue life of SCC crystals is discussed. 12 refs.
A s t u d y o f t h e interaction end coalescence o f t w o coplaner f a t i g u e cracks in bending. Soboyejo, W.O., Kishimoto, K., Smith, R.A. and Knott,
J.F. Fatigue Fract. Eng. Mater. Struct, 1989 12, (3), 167-174
The results of an experimental and numerical investigation of the interaction and coalescence of two coplanar semi-elliptical cracks are reported. Fatigue crack growth data were obtained from a four-point bend specimen using the direct current potential difference method and a beachmarking technique. A finite element simulation of the experimental conditions was also performed. Crack advance was computed from the Paris equation on a step by step basis. Good agreement between experimental (Q1N) fatigue crack growth data and the finite element predictions is demonstrated. Graphs. 21 refs.
M i x e d m o d e (I + III) f a t i g u e t h r e s h o l d s in a f o r g i n g steel. Yates, J.R.
and Miller, K.J. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 259-270
The results of mixed mode (I + III) fatigue tests on a low-pressure steam turbine rotor steel are presented. Tests were performed on a uniaxial serve-hydraulic fatigue machine fitted with a torsion-bending loading frame. The onset of failure was marked by fatigue facets which grew perpendicular to the maximum principal stress direction. A criterion for mixed mode (I + III) fatigue thresholds based on the orientation and the crack opening displacement of fatigue cracks is proposed. Graphs. Photomicrographs. 25 refs.
A reliability analysis o f f a t i g u e crack growth under random loading. Huang, X. and Hancock, J.W... Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 247-258 A fracture mechanics approach to the growth of fatigue cracks under random loading has been used to predict both the distribution of crack lengths after a given number of cycles, and the distribution of cycles to grow a crack to a given length. The analysis ignores interaction effects between cycles, and its predictions agree closely with the results of Monte Carlo simulations. These results allow a simple method of assessing the reliability of cracked components subjected to narrow or broad band random loading and the introduction of material variability allows a quantitative description of the parameters which affect the reliability of a structural component. Graphs. 12 refs. A crack closure study to predict the threshold behaviour o f small cracks.
Journet, B.G., Lefrancois, A. and Pineau, A. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 237-246
The small crack problem is addressed within the applicability of linear elastic fracture mechanics as the result of the crack closure phenomenon. The variation of crack closure stress intensity factor Kop as a function of crack length, a, was determined in two materials, namely an A508 steel and a 2024 AI alloy. These results were obtained on two-dimensional small cracks (a~>0.1 mm) which were machined from long fatigue cracks. These measurements of Kop in addition to data published in the literature on a nodular cast iron and a 9Cr-lMo steel yield a unique characteristic function: Kop/Ko = 1-exp(-ka) in which k is the only parameter to characterize the small crack effect. A prediction of the behaviour of small and long cracks on A508 steel is made using the results of crack closure measurements. Graphs. Photomicrographs. 16 refs. The effect o f final friction c o o f f l d e n t on f r e t t i n g f a t i g u e w e v e f o r m s .
Rooke, D.P. and Courtney, T.J. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 227-236
The waveforms of the frictional forces and the relationship between the frictional forces and the applied forces are derived for a fretting fatigue model in which the
Int J Fatigue November 1989
Surface t r e a t m e n t and f a t i g u e strength of f o r g e d c o m p o n e n t s . Golze, N. VDI Z. Apr. 1989 131, (4), 108-109 (in German) Increasing importance is attached to the design of components of the lowest possible weight of material leg Ck35, Ck4f,, 37Cr4 and 49CrMnVS3) including manufacturing losses and high component strength. By using the process and material specific reserves, drop forged components can meet this requirement. Special surface post-treatment processes are one way of achieving this. The effects of rolling, induction hardening and of nitrocerburizetion are discussed.
Fracture m e c h a n i s m s in iron and nickel aluminides. Rnal report, 1 March 1984-31 M a y 1 N 8 . Stoloff, N.S. Rensselaer Polytechnic Institute Report No AD-A 199 365/8/GAR 15 Aug. 1988 The high cycle fatigue (HCF) resistance of several boron-doped nickel aluminide alloys has been determined over a range of test temperatures. Fatigue and tensile properties of two Ni-rich ternary alloys were much superior to those of the cast 26% AI alloy or a P/M alloy with 9.3% Mn. Crack paths were transgranular in the Ni-rich alloys and intergranular or interdendritic in Ni-26% AI. HCF lives decreased sharply at temperatures > 500 °C. Crack growth rates increased with temperatures to 600 °C, in spite of a rising yield stress over the same temperature range, perhaps due to oxygen-induced embrittlement. Single crystals of Ni~,l +S displayed a marked flow stress asymmetry in tension and compression. Point defects were observed in large numbers; these condense into voids, thereby contributing to each crack initiation. The high cycle fstigue (HCF) and crack growth resistance of several iron aluminide-type alloys were determined in the temperature range 25-600 °C. Longrange order (DO3-type) was effective in prolonging high cycle fatigue lives in Fe-28.1% AI, but not in Fe-23.7% AI at 25 °C. Crack growth rates were higher in the DO3 condition. Fatigue results on both Ni3AI and Fe3AI alloys are discussed on the basis of microstructure, surface slip band development and internal dislocation, substructures revealed by transmission electron microscopy.
The effect o f biaxial stress state and changes o f state on f a t i g u e crack Q r o w t h behaviour. Yuuki, R., Akita, K. anc[Kishi, N. Fatigue Fract. Eng. Mater. Struct. 1989 12, (2), 93-103 The effect of biaxial loading on the fatigue crack growth properties for stainless steel 304 has been examined. From comprehensive experiments, a significant biaxial stress effect on crack growth was found when the stress level was high and the crack was short. The critical region where the effect of biaxial stress appears was clarified quantitatively, Moreover, the effect of changing the biaxial stress condition on fatigue crack growth behaviour was investigated. Significant acceleration of crack growth was observed just aher the uniaxial or equibiaxial stress condition was changed to the shear stress condition. This acceleration seems to be due to the change of plastic zone shape at the crack tip. 13 refs.
453
st~l
of metals. Murakami, Y. t. J. I Apr. 1989 32, (2), 167-180
The types of fatigue beheviour of metals containing small defects or inclusions are so complicated that the prediction of the influence of defects or inclusions is very difficult. However, recent advances in fracture mechanics have led to quantitative treatment of this problem. First, the characteristics of the effects of small defects and inclusions are discussed by reviewing the existing literature, which mostly reports the defect and inclusion problems qualitatively. Next, it is revealed that the clue to solving the problems quantitatively is the concept that defects and inclusions are virtually equivalent to cracks from the viewpoint of fatigue strength. A method of evaluating the fatigue limits of metals containing small defects based on this concept is introduced. Finally, it is shown that the method is useful in predicting the lower limit of fatigue strength of high-strength steels containing non-metallic inclusions which cause a distinct decrease in fatigue strength and a large scatter of fatigue strength. 99 refs. The effect o f specimen constraints o n t h e shape changes and t h e nucleation o f cracks in cyclically deformed niobium crystals. Vaja, J.
coefficient of friction is not constant. This study is a continuation of a previous one in which it wee ess0med that the coefficient of friction was a constant. In this model it is assumed that the coefficient of friction between the rubbing surfaces is initially zero and slowly increases to a constant value during the early cycles of the fatigue life. The consequences of this change are examined end the results from the two models compared, it is observed that the two models give different frictional forces if the deformation of the surfaces is elastic, but are identical when non-lineer mscroslip occurs on both loading end unloading. Graphs. 2 refs.
Low cycle f a t i g u e lives o f notched c o m p o n e n t s . James, M.N., Dimitriou, C. and Chandler, H.D. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 213-225 In low cycle fatigue situations, the plastic beheviour of the material at the root of stress concentrators is of prime importance in determining the cyclic life. However, simple procedures such as Neuber's rule do not adequately describe the development of plastic behaviour at a notched root, while the expense of s finite element analysis is not justified in many instances. A simple, approximate numerical method of calculating plastic notch stresses and strains that would be of use in such situations is described. The usefulness of the technique is demonstrated by comparing low cycle fatigue lives predicted from notch plastic strains with those determined by fatigue testing of smooth specimens (steel) subjected to similar plastic strain ranges. Graphs. 7 refs.
N o t c h size effect in fatigue. Lukas, P., Kunz, L., Weiss, B. and Stickler,
R. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 175--186 The notch size effect lie the decrease of the notched fatigue limit with increasing
notch size for the same stress concentration factor) was quantitatively derived by describing the threshold conditions for the propagation of a short semi-elliptical crack nucleated at the notch root. A close relation between the Kitegewa-Takabashi diagram for the short crack threshold stress and the dependence of the notched fatigue limit on the notch size was shown. The derived relation for the notch size effect was experimentally verified for several specimen/notch geometries in the cases of pressure vessel steel (2.25Cr-lMo) and Cu (99.98%). 14 refs.
and Guiu, F. Philos. Mag. A M a y 1989 59, (5), 1091-1102
Niobium single crystals, with an orientation conducive to deformation by single glide, have been cyclically deformed st 400 K over a wide range of plastic strain amplitudes and cumulative strains. All the Nb crystals exhibited shape changes which were produced by asymmetric glide. The effects of the end constraints on the axial component of the shape changes ere investigated. It is shown that these constraints deform the specimen into an S shape and create regions of tension and compression on the crystal 'side faces'. Fatigue cracks nucleate at the specimen surface st the regions which ere in tension, and these cracks cause the specimen to fail prematurely, whereas fatigue cracks which have initiated at the intrusions and extrusions in the unconstrained middle regions of the gauge length remain relatively insignificant. The implications of this effect of the orientation dependence of the fatigue life of SCC crystals is discussed. 12 refs.
A s t u d y o f t h e interaction end coalescence o f t w o coplaner f a t i g u e cracks in bending. Soboyejo, W.O., Kishimoto, K., Smith, R.A. and Knott,
J.F. Fatigue Fract. Eng. Mater. Struct, 1989 12, (3), 167-174
The results of an experimental and numerical investigation of the interaction and coalescence of two coplanar semi-elliptical cracks are reported. Fatigue crack growth data were obtained from a four-point bend specimen using the direct current potential difference method and a beachmarking technique. A finite element simulation of the experimental conditions was also performed. Crack advance was computed from the Paris equation on a step by step basis. Good agreement between experimental (Q1N) fatigue crack growth data and the finite element predictions is demonstrated. Graphs. 21 refs.
M i x e d m o d e (I + III) f a t i g u e t h r e s h o l d s in a f o r g i n g steel. Yates, J.R.
and Miller, K.J. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 259-270
The results of mixed mode (I + III) fatigue tests on a low-pressure steam turbine rotor steel are presented. Tests were performed on a uniaxial serve-hydraulic fatigue machine fitted with a torsion-bending loading frame. The onset of failure was marked by fatigue facets which grew perpendicular to the maximum principal stress direction. A criterion for mixed mode (I + III) fatigue thresholds based on the orientation and the crack opening displacement of fatigue cracks is proposed. Graphs. Photomicrographs. 25 refs.
A reliability analysis o f f a t i g u e crack growth under random loading. Huang, X. and Hancock, J.W... Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 247-258 A fracture mechanics approach to the growth of fatigue cracks under random loading has been used to predict both the distribution of crack lengths after a given number of cycles, and the distribution of cycles to grow a crack to a given length. The analysis ignores interaction effects between cycles, and its predictions agree closely with the results of Monte Carlo simulations. These results allow a simple method of assessing the reliability of cracked components subjected to narrow or broad band random loading and the introduction of material variability allows a quantitative description of the parameters which affect the reliability of a structural component. Graphs. 12 refs. A crack closure study to predict the threshold behaviour o f small cracks.
Journet, B.G., Lefrancois, A. and Pineau, A. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 237-246
The small crack problem is addressed within the applicability of linear elastic fracture mechanics as the result of the crack closure phenomenon. The variation of crack closure stress intensity factor Kop as a function of crack length, a, was determined in two materials, namely an A508 steel and a 2024 AI alloy. These results were obtained on two-dimensional small cracks (a~>0.1 mm) which were machined from long fatigue cracks. These measurements of Kop in addition to data published in the literature on a nodular cast iron and a 9Cr-lMo steel yield a unique characteristic function: Kop/Ko = 1-exp(-ka) in which k is the only parameter to characterize the small crack effect. A prediction of the behaviour of small and long cracks on A508 steel is made using the results of crack closure measurements. Graphs. Photomicrographs. 16 refs. The effect o f final friction c o o f f l d e n t on f r e t t i n g f a t i g u e w e v e f o r m s .
Rooke, D.P. and Courtney, T.J. Fatigue Fract. Eng. Mater. Struct. 1989 12, (3), 227-236
The waveforms of the frictional forces and the relationship between the frictional forces and the applied forces are derived for a fretting fatigue model in which the
Int J Fatigue November 1989
Surface t r e a t m e n t and f a t i g u e strength of f o r g e d c o m p o n e n t s . Golze, N. VDI Z. Apr. 1989 131, (4), 108-109 (in German) Increasing importance is attached to the design of components of the lowest possible weight of material leg Ck35, Ck4f,, 37Cr4 and 49CrMnVS3) including manufacturing losses and high component strength. By using the process and material specific reserves, drop forged components can meet this requirement. Special surface post-treatment processes are one way of achieving this. The effects of rolling, induction hardening and of nitrocerburizetion are discussed.
Fracture m e c h a n i s m s in iron and nickel aluminides. Rnal report, 1 March 1984-31 M a y 1 N 8 . Stoloff, N.S. Rensselaer Polytechnic Institute Report No AD-A 199 365/8/GAR 15 Aug. 1988 The high cycle fatigue (HCF) resistance of several boron-doped nickel aluminide alloys has been determined over a range of test temperatures. Fatigue and tensile properties of two Ni-rich ternary alloys were much superior to those of the cast 26% AI alloy or a P/M alloy with 9.3% Mn. Crack paths were transgranular in the Ni-rich alloys and intergranular or interdendritic in Ni-26% AI. HCF lives decreased sharply at temperatures > 500 °C. Crack growth rates increased with temperatures to 600 °C, in spite of a rising yield stress over the same temperature range, perhaps due to oxygen-induced embrittlement. Single crystals of Ni~,l +S displayed a marked flow stress asymmetry in tension and compression. Point defects were observed in large numbers; these condense into voids, thereby contributing to each crack initiation. The high cycle fstigue (HCF) and crack growth resistance of several iron aluminide-type alloys were determined in the temperature range 25-600 °C. Longrange order (DO3-type) was effective in prolonging high cycle fatigue lives in Fe-28.1% AI, but not in Fe-23.7% AI at 25 °C. Crack growth rates were higher in the DO3 condition. Fatigue results on both Ni3AI and Fe3AI alloys are discussed on the basis of microstructure, surface slip band development and internal dislocation, substructures revealed by transmission electron microscopy.
The effect o f biaxial stress state and changes o f state on f a t i g u e crack Q r o w t h behaviour. Yuuki, R., Akita, K. anc[Kishi, N. Fatigue Fract. Eng. Mater. Struct. 1989 12, (2), 93-103 The effect of biaxial loading on the fatigue crack growth properties for stainless steel 304 has been examined. From comprehensive experiments, a significant biaxial stress effect on crack growth was found when the stress level was high and the crack was short. The critical region where the effect of biaxial stress appears was clarified quantitatively, Moreover, the effect of changing the biaxial stress condition on fatigue crack growth behaviour was investigated. Significant acceleration of crack growth was observed just aher the uniaxial or equibiaxial stress condition was changed to the shear stress condition. This acceleration seems to be due to the change of plastic zone shape at the crack tip. 13 refs.
453