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Modelling crack growth for creep and fatigue loading
of intergranular failure was much higher in the ferritic microstructure than in the predominantly paariitic microstructure. The maximum extent of intergrsnular failure in both microstructures was observed st lower stress intensity range, ~K levels, and faster crack velocity values than those recorded for steels. This was the result of the presence of graphite nodules which both enhanced intergranular failure and aided the diffusion of hydrogen through the microstructures. Decohesion between the graphite nodules or spheroids and the matrix was commonplace and a higher number of graphite spheroids (up to about three times) were present on the fatigue fracture surfaces of both microstructures than those observed on a metallographic section. This was the result of an increase in the crack tip stress intensity as the extending fatigue crack approached a 'soft" graphite nodule. Two other general fatigue fracture features were commonly observed in both microstructures. They COrrespond to regions of coarse planar slip, with its characteristic crystallographic appearance, that occurred at low to intermediate AK levels, and areas or facets of flat, cleavage failure within which regular crack arrest markings were clearly imprinted. In the latter, the arrest markings had an average spacing of 2-3 ~,m and were insensitive to &K level and it was suggested that such features were the result of periodic relaxation of the growing cleavage crack causing plastic deformation lines; this type of cleavage crack growth was the result of the Fridel-Orlov mechanism. Graphs. photomicrographs. 31 refs.
The effect o f an o x y g e n a t m o s p h e r e on t h e c r e e p - f a t i g u e failure o f i r o n - c h r o m i u m alloys at 600 ~C. Aghion, E., Ferreira, J. and Eliezer, D.
Prec. INFACON 6, Cape Town, South Africa, 1992. Vol. 2, Chromium
Steel and Alloys (The South African Institute o f M i n i n g and Metallurgy, J o h a n n e s b u r g , Republic o f South Africa, 1992) pp 2 4 1 - 2 4 4 A systematic study was conducted on the effect of an oxygen environment on the creep-fetigue failure of Fe-Cr alloys at 600°C. The materials tested were ferrite steels containing up to 18% Cr, low-carbon steel (no Cr), 3CR12 (12% Cr), and 430 stainless steel (18% Cr). The cyclic loading encountered creep tension and plastic compression according to the CP mode of the strain-range partitioning method under low-cycle fatigue conditions. The strain ranges varied from 0.2 to 0.45%. The tested environments consisted of different mixtures of O and Ar, and an air atmosphere. The crack-growth behaviour was studied by scanning electron microscopy, EDAX analysis, and X-ray diffraction. The results showed that when Cr was absent, as in mild steel, the fatigue life was greater in Ar than in air. The effect was reversed when 12% Cr was present, as for 3CR12. The presence of 18% Cr, as with 430 stainless steel, produced similar fatigue lives in Ar and in environments containing up to 20% O. T h e mechanics of crack initiation
M o d e l l i n g crack g r o w t h f o r creep and f a t i g u e loading. Smith, D.J. and
Ellison, E.G. Int. J. Pressure Vessels and Piping 1992 50, ( 1 - 3 ) , 231-241
Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts, in particular, the creep parameter C has become widely used, together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilize experimental data. For high-temperature components, which are subjected to cyclic (fatigue) and creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which pre-existing cracks grow by creep and fatigue is difficult to quantify. The response of Type 316L stainless steel is examined. This material progressively strain-hardens under reversed cyclic loading, and the creep beheviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fetigue crack growth. Using the maps, a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components. Graphs. 8 refs. L i f e t i m e e s t i m a t e s o f cracked h i g h t e m p e r a t u r e c o m p o n e n t s . Webster,
G.A. Int. J. Pressure Vessels and Piping 1992 50, ( 1 - 3 ) , 1 3 3 - 1 4 5 Cracked high-tempereture components subjected to creep-fatigue loading can fail by crack growth, net-section rupture or some combination of both processes. Models for describing this beheviour are presented. The influence of build-up of damage in a process zone at the crack tip, material deterioration in the uncrecked ligament and the importance of tertiary creep on the mode of failure are each considered. It is shown how the models can be employed to predict crack growth and for making residual life assessments in high-temperature equipment. Finally, a simple cumulative damage law is outlined for describing creep-fetigue interaction. 0.5 X CrMoV is discussed. Graphs. 17 refs. Investigation o f f a t i g u e crack g r o w t h a f t r a a single cycle peak o v e r l o a d in IS 1020 steel. Kumar, R. and Singh, S.B. Int. J. Pressure Vessels and Piping 1992 51, (1), 2 5 - 2 5 Crack propagation experiments were conducted on IS 1020 steel for various overload ratios (1:2, 1:3, and 1:4). On the basis of these experiments, one power law is developed to predict the crack propagation delay period. The delay period after application of s single overload was found to increase ee the magnitude of the overload increased. Crack growth also decreased after the application of an overload cycle but, after a certain number of cycles, it tended to return to the crack propagation rate for constant amplitude loading (CAL). Numeric data, graphs. 12 refs.
Fatigue t h r e s h o l d r e g i m e o f a l o w a l l o y ferritic steel u n d e r rJosurefree t e s t i n g conditions. I. C o m p l i a n c e v a r i a t i o n s in t h e t h r e s h o l d regime.
Vaidya, W. V. ASTM J. Test. Eval. M a y 1992 20, (3), 1 5 7 - 1 6 7 Compliance variations in the threshold regime of a high-strength ferritic steel (ag 21CrMoNiV5 7) tested under closure-free conditions st room temperature and in air are reported. In contrast to the Paris regime, and irrespective of whether the data are considered, during load shedding, at threshold, or after post-threehold load increase, it is found that compliance varies inconsistently in the threshold regime. Therefore a 1:1 correlation between the averaged optical crack length and that inferred from compliance was not observed, Possible reasons for this discrepancy are discussed, and the crack front lag inferred is explained in terms of the constraint. Photomicrographs, graphs. 29 raft;. Fatigue t h r e s l m l d r e g i m e o f • l o w a l l o y farritic mmel u n d e r alosurefree tW~ing ~ U. ~ in n w r - t l m m l m l d f a t f i l u e crack propeEatlon: an e x p e r i m e n t a l assessment, Vaidya, W.V ASTM J. Test. Eval. M a y 1992 20, (3), 1 6 8 - 1 7 9 Near-threshold fatigue crack propagation baheviour of a high-strength steel (21CrMoNiV57) was investigated in laboratory air under closure-free testing conditions at R = 0.7 (= Raft) and at two different K-gradients. Depending on the criterion assumed, the threshold value differed; the criterion of non-propagation gave a lower threshold value than that assumed by the propagation criterion. Nevertheless, the subsequent propagation following s load increase was discontinuous in both cases, and da/dN versus AK curves obtained on the mime specimen during the K-decreasing and the K-increasing test were not necessarily identical in the threshold regime. This behaviour, hysteresis, is analysed mainly from the experimental viewpoint, and it is shown that hysteresis is not an artefact. Photomicrographs, graphs. 45 refs.
344
at hard particles in rolling
line
contacts. Salehizadeh, H. and Sake, N. J. Tribology (Trans. ASME) Apr. 1992 114, (2), 3 4 1 - 3 4 7 The mechanics of crack initiation in rolling contact bearings is investigated by considering the possibility of hard particle debonding from the surrounding softer matrix. The local elaetoplsstic stress and strain fields around a hard cylindrical particle located at the point of maximum shear stress below the Hertz contact are calculated by the finite element method. The calculations show that if the applied Hertz loading is sufficiently high, the matrix contiguous with the particle deforms plastically. Upon unloading, tensile residual stresses will be set up normal to the particle-matrix interface. The particle-metrix interface debonds provided the stored elastic energy in and around the particle is larger than the work of adhesion, and the maximum residual radial stress is greeter than the cohesive strength of the particle-matrix interface. The gradual softening of the martensitic matrix around non-metallic inclusions or carbides, which is known to occur after a large number of stress cycles in bearing steels, could result in a residual radial stress greeter than the interfacial strength and cause particle-matrix debonding. 52100 and M50 steels are discussed. Graphs. 23 refs. S i m p l i f i e d contact f a t i g u e life p r e d i c t i o n model. II. N e w model. Tallian, T.E. J. Tribology (Trans. ASME) Apr. 1992 114, (2), 2 1 4 - 2 2 2 Part I reviews 11 published models as a basis for the construction of a simplified, readily calculated engineering model encompassing the major life variables, belled on explicit assumptions and derivations. Part II presents the new model. It u m the applied alternating shear stress field to define the critical stress as a function of depth, and computes life as crack propagation time through this field. The model defines surface defects as critical defects. Materiel fatigue susceptibility, fatigue limit stress and the defect severity distribution are the main endurance parameters. In addition to Hertz pressure, life-modifying variables are: interface traction, surface microgeomatry and EHD film. The influence of the variables in the new model on life distribution and on load/life law are compared with the reviewed models, and it is shown that the new model is capable of representing the influence of all major consensus parameters on life, by relationships that fall within the bounds of previously published behaviour. Cyclic ¢h.enge in h u m i d i t y o f t h e e n v i r o n m e , t d u r i n g f a t i g u e creck p r o p a g l m o n a n d its effect o n f r ~ t u r e marfece I l p p e e r e n ~ . Dervish, M.
and Johansson, S. Scand. J. Metal/. Apr. 1992 21, (2), 6 8 - 7 7
A new method of studying the fatigue crack growth at a near-threshold growth rate is demonstrated, based on alternately exposing the growing crack to the environments of high and low humidity (cyclic condensation). These variations will cause the formation of markings (beach marks) on the fracture surface. Each beach mark is an indication of the crack extension during an environmental cycle (dry + wet). The formation of markings is dependent on both the material and loading conditions. These markings indicate the micromechanisms of crack growth at low growth rate and can also be related to the crack growth rate. The t e r n have been carried out at two stress ratios using two high strength AI alloys, 7075 and 8090, and one martensitic steel, P20. Graphs, photomicrographs. 16 refs. Frecture mechanics a p p r o a c h f o r p r e d i c t i o n
o f f a t i g u e life o f h i g h
strength steel components. Barui, K.L. and Bhettacharya, B. Fatigue Frect. Eng. Mater. Struct. J u n e 1992 15, (6), 6 0 7 - 6 1 6 A fatigue life estimation procedure based on a fracture mechanics approach is analytically developed for components made of a 4140 grade high-strength steel. The maximum allowable flaw sizes for both static and cyclic loads are computed. For different stress levels and shapes of the dofecls, initleliy existing, allowable defect size versus number of operating cycles, and the thickmms versus tolerable defect sizes curves are presented. The maximum allowable operating cycles of components for non-destructively detectable minimum defects are also estimated. From these estimations, a more realistic prediction of the safe service life of components is possible. Graphs. 23 refs. M o d e l l i n g s h o r t crack g r o w t h
b e h o v l o u r in n l c k e l - b a ~
auperalloys.
Grabowski, L. and King, J.E. Fatigue Fract. Eng. Mater. Sfruct. J u n e 1992 15, (6), 5 9 5 - 6 0 6
The features and mechanisms of facated short crack growth in Ni-baee superalloys are deecribed, and existing short crack growth models are reviewed bdefiy in terms of their application to Ni-bese alloys. The concept of 'soft barriers' is introduced to produce • new two-phase model for local microstructural effects on short crack growth in Wespaloy. This is derived from detailed obeervetlons of creck growth through individual grains. The model differs from all previous approaches in highlighting the importance of crack path perturbations within grains. Potential applications of the model in alloy development ere discussed. Photomicrographs, graphs. 19 refs.
Int J Fatigue July 1993
The effect o f an o x y g e n a t m o s p h e r e on t h e c r e e p - f a t i g u e failure o f i r o n - c h r o m i u m alloys at 600 ~C. Aghion, E., Ferreira, J. and Eliezer, D.
Prec. INFACON 6, Cape Town, South Africa, 1992. Vol. 2, Chromium
Steel and Alloys (The South African Institute o f M i n i n g and Metallurgy, J o h a n n e s b u r g , Republic o f South Africa, 1992) pp 2 4 1 - 2 4 4 A systematic study was conducted on the effect of an oxygen environment on the creep-fetigue failure of Fe-Cr alloys at 600°C. The materials tested were ferrite steels containing up to 18% Cr, low-carbon steel (no Cr), 3CR12 (12% Cr), and 430 stainless steel (18% Cr). The cyclic loading encountered creep tension and plastic compression according to the CP mode of the strain-range partitioning method under low-cycle fatigue conditions. The strain ranges varied from 0.2 to 0.45%. The tested environments consisted of different mixtures of O and Ar, and an air atmosphere. The crack-growth behaviour was studied by scanning electron microscopy, EDAX analysis, and X-ray diffraction. The results showed that when Cr was absent, as in mild steel, the fatigue life was greater in Ar than in air. The effect was reversed when 12% Cr was present, as for 3CR12. The presence of 18% Cr, as with 430 stainless steel, produced similar fatigue lives in Ar and in environments containing up to 20% O. T h e mechanics of crack initiation
M o d e l l i n g crack g r o w t h f o r creep and f a t i g u e loading. Smith, D.J. and
Ellison, E.G. Int. J. Pressure Vessels and Piping 1992 50, ( 1 - 3 ) , 231-241
Estimates of creep crack growth in engineering components under steady load conditions are usually based on the application of fracture mechanics concepts, in particular, the creep parameter C has become widely used, together with creep crack growth data obtained from laboratory tests. There are now a number of practical methods to utilize experimental data. For high-temperature components, which are subjected to cyclic (fatigue) and creep loading, the estimation of the fracture mechanics parameters becomes much more difficult, and consequently the extent to which pre-existing cracks grow by creep and fatigue is difficult to quantify. The response of Type 316L stainless steel is examined. This material progressively strain-hardens under reversed cyclic loading, and the creep beheviour also changes. Using uniaxial fatigue and creep results, fracture parameter maps are developed to establish the appropriate regimes for creep-fetigue crack growth. Using the maps, a model is developed which can predict the combined effect of fatigue and creep on crack growth. The implications of the model are discussed in relation to the limitations of obtaining results from laboratory tests at short times, and the assessment of practical engineering components. Graphs. 8 refs. L i f e t i m e e s t i m a t e s o f cracked h i g h t e m p e r a t u r e c o m p o n e n t s . Webster,
G.A. Int. J. Pressure Vessels and Piping 1992 50, ( 1 - 3 ) , 1 3 3 - 1 4 5 Cracked high-tempereture components subjected to creep-fatigue loading can fail by crack growth, net-section rupture or some combination of both processes. Models for describing this beheviour are presented. The influence of build-up of damage in a process zone at the crack tip, material deterioration in the uncrecked ligament and the importance of tertiary creep on the mode of failure are each considered. It is shown how the models can be employed to predict crack growth and for making residual life assessments in high-temperature equipment. Finally, a simple cumulative damage law is outlined for describing creep-fetigue interaction. 0.5 X CrMoV is discussed. Graphs. 17 refs. Investigation o f f a t i g u e crack g r o w t h a f t r a a single cycle peak o v e r l o a d in IS 1020 steel. Kumar, R. and Singh, S.B. Int. J. Pressure Vessels and Piping 1992 51, (1), 2 5 - 2 5 Crack propagation experiments were conducted on IS 1020 steel for various overload ratios (1:2, 1:3, and 1:4). On the basis of these experiments, one power law is developed to predict the crack propagation delay period. The delay period after application of s single overload was found to increase ee the magnitude of the overload increased. Crack growth also decreased after the application of an overload cycle but, after a certain number of cycles, it tended to return to the crack propagation rate for constant amplitude loading (CAL). Numeric data, graphs. 12 refs.
Fatigue t h r e s h o l d r e g i m e o f a l o w a l l o y ferritic steel u n d e r rJosurefree t e s t i n g conditions. I. C o m p l i a n c e v a r i a t i o n s in t h e t h r e s h o l d regime.
Vaidya, W. V. ASTM J. Test. Eval. M a y 1992 20, (3), 1 5 7 - 1 6 7 Compliance variations in the threshold regime of a high-strength ferritic steel (ag 21CrMoNiV5 7) tested under closure-free conditions st room temperature and in air are reported. In contrast to the Paris regime, and irrespective of whether the data are considered, during load shedding, at threshold, or after post-threehold load increase, it is found that compliance varies inconsistently in the threshold regime. Therefore a 1:1 correlation between the averaged optical crack length and that inferred from compliance was not observed, Possible reasons for this discrepancy are discussed, and the crack front lag inferred is explained in terms of the constraint. Photomicrographs, graphs. 29 raft;. Fatigue t h r e s l m l d r e g i m e o f • l o w a l l o y farritic mmel u n d e r alosurefree tW~ing ~ U. ~ in n w r - t l m m l m l d f a t f i l u e crack propeEatlon: an e x p e r i m e n t a l assessment, Vaidya, W.V ASTM J. Test. Eval. M a y 1992 20, (3), 1 6 8 - 1 7 9 Near-threshold fatigue crack propagation baheviour of a high-strength steel (21CrMoNiV57) was investigated in laboratory air under closure-free testing conditions at R = 0.7 (= Raft) and at two different K-gradients. Depending on the criterion assumed, the threshold value differed; the criterion of non-propagation gave a lower threshold value than that assumed by the propagation criterion. Nevertheless, the subsequent propagation following s load increase was discontinuous in both cases, and da/dN versus AK curves obtained on the mime specimen during the K-decreasing and the K-increasing test were not necessarily identical in the threshold regime. This behaviour, hysteresis, is analysed mainly from the experimental viewpoint, and it is shown that hysteresis is not an artefact. Photomicrographs, graphs. 45 refs.
344
at hard particles in rolling
line
contacts. Salehizadeh, H. and Sake, N. J. Tribology (Trans. ASME) Apr. 1992 114, (2), 3 4 1 - 3 4 7 The mechanics of crack initiation in rolling contact bearings is investigated by considering the possibility of hard particle debonding from the surrounding softer matrix. The local elaetoplsstic stress and strain fields around a hard cylindrical particle located at the point of maximum shear stress below the Hertz contact are calculated by the finite element method. The calculations show that if the applied Hertz loading is sufficiently high, the matrix contiguous with the particle deforms plastically. Upon unloading, tensile residual stresses will be set up normal to the particle-matrix interface. The particle-metrix interface debonds provided the stored elastic energy in and around the particle is larger than the work of adhesion, and the maximum residual radial stress is greeter than the cohesive strength of the particle-matrix interface. The gradual softening of the martensitic matrix around non-metallic inclusions or carbides, which is known to occur after a large number of stress cycles in bearing steels, could result in a residual radial stress greeter than the interfacial strength and cause particle-matrix debonding. 52100 and M50 steels are discussed. Graphs. 23 refs. S i m p l i f i e d contact f a t i g u e life p r e d i c t i o n model. II. N e w model. Tallian, T.E. J. Tribology (Trans. ASME) Apr. 1992 114, (2), 2 1 4 - 2 2 2 Part I reviews 11 published models as a basis for the construction of a simplified, readily calculated engineering model encompassing the major life variables, belled on explicit assumptions and derivations. Part II presents the new model. It u m the applied alternating shear stress field to define the critical stress as a function of depth, and computes life as crack propagation time through this field. The model defines surface defects as critical defects. Materiel fatigue susceptibility, fatigue limit stress and the defect severity distribution are the main endurance parameters. In addition to Hertz pressure, life-modifying variables are: interface traction, surface microgeomatry and EHD film. The influence of the variables in the new model on life distribution and on load/life law are compared with the reviewed models, and it is shown that the new model is capable of representing the influence of all major consensus parameters on life, by relationships that fall within the bounds of previously published behaviour. Cyclic ¢h.enge in h u m i d i t y o f t h e e n v i r o n m e , t d u r i n g f a t i g u e creck p r o p a g l m o n a n d its effect o n f r ~ t u r e marfece I l p p e e r e n ~ . Dervish, M.
and Johansson, S. Scand. J. Metal/. Apr. 1992 21, (2), 6 8 - 7 7
A new method of studying the fatigue crack growth at a near-threshold growth rate is demonstrated, based on alternately exposing the growing crack to the environments of high and low humidity (cyclic condensation). These variations will cause the formation of markings (beach marks) on the fracture surface. Each beach mark is an indication of the crack extension during an environmental cycle (dry + wet). The formation of markings is dependent on both the material and loading conditions. These markings indicate the micromechanisms of crack growth at low growth rate and can also be related to the crack growth rate. The t e r n have been carried out at two stress ratios using two high strength AI alloys, 7075 and 8090, and one martensitic steel, P20. Graphs, photomicrographs. 16 refs. Frecture mechanics a p p r o a c h f o r p r e d i c t i o n
o f f a t i g u e life o f h i g h
strength steel components. Barui, K.L. and Bhettacharya, B. Fatigue Frect. Eng. Mater. Struct. J u n e 1992 15, (6), 6 0 7 - 6 1 6 A fatigue life estimation procedure based on a fracture mechanics approach is analytically developed for components made of a 4140 grade high-strength steel. The maximum allowable flaw sizes for both static and cyclic loads are computed. For different stress levels and shapes of the dofecls, initleliy existing, allowable defect size versus number of operating cycles, and the thickmms versus tolerable defect sizes curves are presented. The maximum allowable operating cycles of components for non-destructively detectable minimum defects are also estimated. From these estimations, a more realistic prediction of the safe service life of components is possible. Graphs. 23 refs. M o d e l l i n g s h o r t crack g r o w t h
b e h o v l o u r in n l c k e l - b a ~
auperalloys.
Grabowski, L. and King, J.E. Fatigue Fract. Eng. Mater. Sfruct. J u n e 1992 15, (6), 5 9 5 - 6 0 6
The features and mechanisms of facated short crack growth in Ni-baee superalloys are deecribed, and existing short crack growth models are reviewed bdefiy in terms of their application to Ni-bese alloys. The concept of 'soft barriers' is introduced to produce • new two-phase model for local microstructural effects on short crack growth in Wespaloy. This is derived from detailed obeervetlons of creck growth through individual grains. The model differs from all previous approaches in highlighting the importance of crack path perturbations within grains. Potential applications of the model in alloy development ere discussed. Photomicrographs, graphs. 19 refs.
Int J Fatigue July 1993