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M superalloy MERL76
interaction. Experiments showed that periodic overloads can be treated as pseudoconstant-amplitude loading in developing models to predict crack growth. Graphs. 7 refs. A fracture domag.le evolution l a w f o r a cylinddcal I p e c i m e n irradiated
elevated temperature is imposed on a component under strain conditions, stress relaxation occurs. Relaxation data can be used, for example, in 318 austanitic steels, in predicting creep stages; however, interpretation of data obtained from such tests could be misleading in assessing damage. An example is given for life prediction on the basis of two selected criteria: the fraction rule and ductility exhaustion. Graphs, photomicrographs. 41 refs.
by laser beam. LI, C., Li, X. and Li, G. Eng. Fract. Mech. 1990 36, (1), 9 - 1 5 The fatigue process is studied by the method of damage mechanics. The equations for the evolution of fatigue damage of one and the same specimen with different irradiation parameters are given, all obtained by experiment. The results show that the study of fatigue damage of a specimen (e.g., steel 45) after irradiation by a laser beam is of definite significance for the selection of the optimum irradiation parameters. Graphs. 8 refs.
High t e m p e r a t u r e crack g r o w t h in copper under cyclic loading conditions. Kim, Y.-S. and Nix, W.D. Scr. Metell. Mater. Jan. 1990 24, (1), 213-218 The fatigue crack growth behaviour of oxygen-free high-conductivity Cu at 773 K has been characterized. The fatigue crack growth data have been analysed using fracture mechanics parameters, and the fatigue fracture surfaces were examined to relate the appearance of the fracture surface to the operative crack propagation mechanism. Graphs, photomicrographs. 14 refs.
Correlation of mechanical test parameters w i t h fatigue crack g r o w t h rates in structural steel. Vol. I and II. (Dissertation). Divsalar, F. Diss. Abstr. Int. Apr. 1990 50, (10), 419 pp The work undertaken investigated the fatigue phenomenon under cyclic loading in the structural steel BS 4360-50D. This material, extensively used in offshore applications, is subject to crack propagation and crack closure effects. The three aspects investigated were cyclic fatigue parameters, crack closure and crack propagation in centre-crecked rectangular plate together with a theortical modelling of crack closure using finite-element analysis. This was executed using a PAFEC package with the "CLOSURE" program developed for elastic-plastic analysis as part of the investigation. The fatigue design parameters were the fatigue ductility exponent, ¢=, the fatigue strength exponent, "t, and the cyclic strain hardening exponent, ~. All these parameters were determined. The fatigue ductility exponent, ~, revealed upper- and lower-bound values of 0.75 and 0.818, respectively. The material displayed a beheviour variation at 0.75 + 0.1% strain where a cross would occur between the monotonic loading and the cyclic loading curves. From the two different stress ratios of R : -1 and R = 0 the effect of the mean stress of crack closure was examined during the crack propagation. Frectography, as well as compliance methods, showed that crack closure is an important factor during the early stages of propagation. It was surmised that during a complete cycle reversal the area close to the crack tip remained open. Fatigue reek;tanco of steels. Boardman, B. Metals Handbook 10th Edn (ASM International, Materials Park, Ohio, 1990) Vol. 1, pp 673-688 Fatigue is the progressive, localized and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that are less than the tensile strength of the materials. Fatigue testing concepts are defined and both stress- and strein-bseed approaches to fatigue are described. The metallurgical variables having the most pronounced effects on the fatigue beheviour of carbon and low-alloy steels are the strength level, ductility, cleanliness of the steel, residual stresses, surface conditions and aggressive environments. The application of fatigue data in engineering design is addressed. Graphs. 21 refs.
Low-cycle fatigue and microstructurei behaviour of the 1 2 C r - M o - V martonsitic stainless steel. Degallaix, G., Vogt, J.-B. and Feet, J. Mem. Etud. Sci. Rev. Metell. Jan. 1990 87, (1), 47-57 (in French) The low-cycle fatigue behaviour of martensitic stainless steels with 12% Cr has been investigated between 20 and 600 °C for various levels of deformation. The evolution of stress, the cyclical cold working and the lifetimes are determined. At 20°C, the role of ferrite embedded in the martensitlc structure is highlighted, particularly for the cylindrical accommodation behaviour and in the formation of extrusions. Deformation microstructurss are examined with TEM. Various ferrite contents are obtained by thermal treatments prior to tempering. The influence of ferrita content on low-cycle fatigue and propagation fatigue behaviour at room temperature is also investigated. Graphs, photomicrographs. 24 refs. Fretting fatigue strength analysis of T i - 6 A I - 4 V in air. Maruyama, N.,
Sumite, M. and Nakazawa, K. Tetsu-to-Hagane (J. Iron Steel InsL Japan) Feb. 1990 76, (2), 262-269 (in Japanese) The influence of fretting damage on fatigue strength has been investigated using Ti-6AI-4V alloy. Axial fatigue loading was applied at frequencies of 20 and 0.5 Hz in a laboratory environment with the fretting applied to the specimen central section through a fretting pad made of the same material as the fatigue specimen. The reduction in fatigue strength owing to the fretting damage depended on the contact pressure, p. When the value of pwss approximately 20 MPa, the fatigue life exhibited a minimum. The fretting fatigue crack initiated at the centre of the fretted surface. Many microcrecks existed prior to main crack initiation. However, when the value of p was greater than 30 MPa, the fretting fatigue crack initiated at the edge of the fretted surface. Few microcracks existed prior to the main crack initiation. Fretting fatigue cycles at more than 90% of the total life in the interrupted fretting fatigue test are required in order to continue crack propagation after the removal of the fretting pad, although cracks were already initiated at cycles less than 50%. Graphs, photomicrographs. 14 refs.
Elevated-temperature properties of feRRic steels. Metals Handbook 10th Edn (ASM International, Materials Park, Ohio, 1990) Vol. 1, pp. 617-662
Fatigue life under vadablo random loading charectedstic=. Kliman, V. Strojnicky Cas. 1990 41, (1), 41-65 (in Slovak)
Eleveted-temparature mechanical and corrosion properties of carbon and low-alloy steels with ferrite-pserlite and ferrite-bainite microstructures for use in boiler tubes, pressure vessels, and steam turbines are described. Mechanical properties discussed include creep strength, stress rupture, relaxation, fatigue, ductility and toughness. Corrosion effects at elevated temperatures often set the maximum allowable service temperature of an alloy. The three common forms of corrosion at elevated temparaturos--oxidetion, sulphidetion and hydrogen attack--are discussed. Factors influencing the mechanical properties such as microstructuro, heat treatment, composition, decerburizetion and carburizstion are addressed. Graphs, photomicrographs. 88 refs.
A method is presented allowing the assessment of operational fatigue variable random loading during operation. The suggested procedure is derived from the correlation theory of approach to life estimation in view of the random character of the load as well as the possible scatter of the material mechanical properties and allows evaluation by the calculating procedure of the residual fatigue life as a function of time in the probabiliefic interpretation with respect to the courses of random load proceeding. The theoretical and experimental fatigue life results are compared and examples of practical applications of this method ere presented. 18 refs. Creep damage estimation in pure copper using ultrasonic waves.
Stress enhancement of fatigue susceptibility of porous coated T i - 6 A I - 4 V Implants: an elastic analysis. Messersmith, P.B. and Cooke, F.W. J. Biomed. Mater. Res. M a y 1990 24, (5), 591-604 An elastic stress analysis of porous-coated Ti-6AI-4V implant surfaces was performed using the finite-element method. 300 p.m diameter metal beads sinter bonded onto an implant surface were modelled with sinter neck radii of 5, 10, 20, and 50 p.m. Smooth-surface, single-bead, single-layer and double-layer systems were analysed, The finite-element models were loaded to simulate bone-bead contact forces end lateral hip implant tensile forces, Results showed that, for a single bead sintar bonded onto an implant surface, concentration of stress occurs either at the base of the sintar neck or within the neck itself, depending on the type of load applied. Under lateral hip implant tensile loads, a maximum stress concentration factor of 1.97 was obtained for a single beam sinter bonded onto an implant surface. Addition of a single layer of beads onto the implant surface resulted in a significant increase in stress at the most proximal and distal ends of the porous layer, with a maximum stress concentration factor of 4.3. The addition of a second layer of beads did not significantly increase the magnitude of the stress concentration occurring at the ends of the porous layer. The results of this study provide stress concentration factors for porous coatings with sinter necks of known dimensions under loading conditions similar to those present along the leteral surface of a hip prosthesis. Graphs. 8 refs. An interpretation o f axial c r e e p - f a t i g u e damage interaction in t y p e 316 s t a i n l m steel. Zamrik, S.Y. J. Pressure Vessel Technol. (Trans. ASME) Feb. 1990 112, (1), 4 - 1 9 Creep-fatigue end its effect on damage of components in service have been a major concern to analysts. To deal with this problem, several criteria have been proposed and used, such as the cycle-time fraction summation rule, strain limit, fracture maps where damage mechanisms are based on crack initiation or propagation and ductility exhaustion. These concepts are reviewed so that one can interpret the damage mechanisms caused by creep and by fatigue. If a long period of dwell-time at
Int J Fatigue January 1991
Men""shits, T., Hired, M. and Fukuoka, H. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 208-213 (in Japanese) An ultrasonic experiment has been done to find the effect of ultrasonic waves on the accumulative creep damage which occurs in polycrystalline pure Cu during hightemperature tensile loading. The longitudinal wave velocity was sensitive to the intergranular creeping process controlled by grain boundary cavitation and subsequent microcrecking. The velocity decreased slowly and linearly up to approximately 60'% of the time to rupture, when the steady creep shifted to accelerated creep. It then decreased with increasing rate until the eventual failure. Measurements of porosity and photomicrographic observation revealed that the first period is associated with the nucleation and growth of cavities and the second corresponds to cracking perpendicular to the stress. A non-destructive technique for predicting the remaining life time of high-temparsture components is also suggested. Graphs, photomicrographs. 16 refs. Effects of defects and microstrocture on the elevated temperature fatigue p r o p e r t y o f P / M superelloy MERL76. Yokomaku, T., Takigawa,
H. and To¥oda, H. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 188-194 (in Japanese) Elevated temperature fatigue tests were conducted on P/M HIP'eq superalloy, MERL 76. The defects, such as precipitates on the prior particle boundaries (PPBs), gas porosities and non-metallic inclusions, were found at the fatigue origins. The relation between defect size, ai, end fatigue life, Nf, was independent of defect type and was formulated as Nf&orneq = ha1i- rn/2 , &Oeq= ~w{1 + [2Eg(n)/(n + 1)](~u~p/&o)}1/2 , where m is the exponent in the crack propagation law, da/dN = C~Km; h is a material constant, E is Young's modulus, gin) is a function of the cyclic strain hardening exponent, n, and &~ is the plastic strain range. The relation between threshold stress, &O'th, and defect size, ai, was also expressed as ~,Kth = (2/~)AOth(~(ai + a0)] tl2 where AKth is the threshold stress intensity factor for long cracks. The value, a0, was about 0.1 mm, which coincided with the size estimated by an experimental relationship between grain size and a0 on steels. The usage of Ar-treated powders and the hot-work after HIP'ing improved fatigue life
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since they prevented PPB failure. Furthermore, the usage of fine powders is preferable because it can reduce the size of pores and inclusions. Photomicrographs, graphs. 11 refs.
Microstructural and mechanical factors affecting creep-fstigue propertie,= of heat resisting cost steels. Yokomaku, T. and Saori, M. J. Soc. Mater. Sci., Japan Feb. 1990 3g, (437), 181-187 (in Japanese) Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) tests under creep conditions were carried out on HK40, HP and their modified alloys containing a small amount of Zr, Nb and/or Ti. The failure mechanism was established based on microscopic observations by the scanning and transmission electron microscopies. Also, the mechanical factors affecting the LCF life are discussed, by considering the crack propagation law. The main results obtained are as follows: (1) The addition of Nb, Ti and/or Zr improves the LCF life and the FCP resistance under creep conditions. In particular, the effect of Zr is marked. (2) Cracks propagate along the intensive sub-boundaries, on which coarse M23Cs carbides precipitate during the LCF test. (3) Zirconium, Nb and Ti form fine and dispersed secondary carbides (MC) in the matrix of material. These fine carbides lead to a uniform dislocation structure without intensive sub*boundaries and then result in the dispersed precipitation of coarse M23C6carbides. Accordingly, the addition of these elements improves the LCF and FCP properties. (4) From a mechanical viewpoint, the LCF life can be determined by the fatigue and creep crack propagation laws and the strain energy densities, which are obtainable based on J-integral analysis of a cracked body. Graphs, photomicrographs. 6 refs.
Acceleration of fatigue crack growth for HTa0 and SUS304 steels in 3% NaCI aqueous solution under very-low-frequency cyclic loading. Shimodaira, M., Matsuoka, S., Masuda, H. andNishijima, S. J. Soc. Mater. Sci., Japan Feb. 1990 38, (437), 1 6 2 - 1 6 8 (in Japanese) Fatigue crack growth properties under freely corroding and cathodic protection in 3% NaCI aqueous solution were studied under very-low-frequency cycling for HTe0 high tensile and SUS304 stainless steels. The crack growth rate in 3% NaCI solution was accelerated as compared with that in air. The acceleration was dependent on material, environment, ~K level and frequency, and the following three types of acceleration were observed. ( 1) The CF-type acceleration was inherent to corrosion fatigue in free corrosion and the rate was accelerated about twice at frequencies less than 0.3 Hz independently of material and ~K level. (2) The HE(IG) type was related to the intergranular cracking due to hydrogen embrittlement and appeared in the range of &K = 4-20 MPa m 1/2 and at frequencies 10-3-102 Hz only for HT60 steel. The maximum acceleration rate became about five times at &K = 10 MPa m 1!2 and f = 0.3 Hz. (3) The HE(TG) type was related to the transgranular and brittle fracture due to H embrittlement and the rate became constant at frequencies less than 0.03 Hz. This type was also dependent on material, environment and ~K level. The maximum acceleration rate was nearly 30 and ten times for HT80 and SUS304 steels, respectively. Graphs, photomicrographs. 9 refs.
Investigations on small crack growth and life property in fatigue of a circumterentiaUy-notched component of pure copper subjected to combined Ioedings. Hoshide, T., Yokota, K. and Inoue, T. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 144-149 (in Japanese) Fatigue tests were conducted using solid cylindrical specimens of pure Cu with circumferential blunt notches subjected to combined axial-torsional Ioadings. The behaviour of crack growth was observed by a plastic replication technique, and the property of fatigue life was investigated. The cracking at notch root was identified as an intergranular type. The dominant morphology of crack growth was found to be the coalescence of distributed small cracks, while some difference was observed depending on the stress multiaxiality. The fatigue life was correlated with the equivalent stress of the Von Mises type and the maximum shear stress. The fatigue life for the same value of each stress parameter became longer with increasing shear component in the stress state at the notch root. An analytical procedure for the crack growth at the notch root was established by using a model based on the competition between the coalescence growth and the propagation of a dominant crack. Characteristics of intergranular cracking at the notch root and its dependence on the stress state were simulated very well by the present analytical model. Fatigue lives in several test conditions were statistically estimated by a simulation of the Monte Carlo type. The prediction with the simulated scatter-bands almost coincided with the experimental results. Graphs. 12 refs.
were as follows: (1) The acceleration behaviour of the crack propagation rate was observed just after changing the stress level from low to high, and the crack propagation rate became two times that under the constant loading. This acceleration behaviour continued for 500-1000 cycles of high-stress loading. (2) The retardation behaviour of the crack propagation rate was observed just after changing the stress level to tow from high. The number of cycles of low-stress loading needed for the crack propagation rate to return to that of constant loading was approximately 10000-70000. (3) It was difficult to elucidate the acceleration and retardation behaviour only by the change of crack opening stress level. (4) It is necessary to consider the acceleration effect of the crack propagation rate for the estimation of fatigue life under service loading conditions when a higher stress is intermittently loaded. Graphs. 14 refs.
Evaluation of actual fatigue stress by using striation spacing and hardness, Ishida, S., Tokumasu, M,, Nakasa, K. and Ishihara, H. J. Soc. Mater. ScL, Japan Jan, 1990 39, (436), 6 3 - 6 9 (in Japanese) The relation between the range of stress intensity factor &K and striation spacing S was obtained by using CT specimens of various kinds of structural steels with different hardnessss, and the actual fatigue stress of square or round bars under repeated three-point bending loads, which was assumed to constitute an actual machine part, was evaluated from the Vickers hardness HV and the striation spacing measured in the bars. The material constants, Cs and ms, obtained from the relation between S (mm) and &K (MPa~/m) for the CT specimens, S = Cs(&K)ms, were able to be approximately expressed as Cs = 1.68 × 10-1° (HV) 18s and ms - 19.1(HV) o,53respectively. The actual fatigue stress of the square bar under three-point bending agreed relatively well with the stress calculated from the striation spacing measured on the bar and the values of Cs and ms, which were obtained from the above equations by substituting the hardness. For the round bar, however, the calculated stress was larger or smaller than the actual fatigue stress when the crack length was short or long, respectively. Photomicrographs, graphs. 12 refs.
Development of two-step varying load rotating-bending fatigue testing machine and some t e s t results. Hoshi, S., Nakamura, H. and Nagai, M. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 5 8 - 6 2 (in Japanese) In order to obtain fatigue strength data in the long life range over 108 stress cycles under multiple two-step varying load conditions with high frequency of Hi/Lo block cycles, an automatic load changing device for a uniform rotating-bending fatigue testing machine was developed. With this device can be conducted the repeated Hi/Lo varying load fatigue tests with a short interval of the higher stress level of approximately 100 cycles. It was confirmed, through experiments done by using JIS $45C round-bar specimens, that the level of performance of the newly developed load changing device was high. Furthermore, the discussion on the results from the viewpoint of the linear cumulative damage laws showed that, in some cases, the effect of understressing on fatigue lives appeared in two opposite manners, that is fatigue lives became considerably longer than estimated values or vice versa, especially when the number of under-stressing cycles was sufficiently large. To reveal such an inconsistent effect of under-stressing, and further, to clarify the fatigue behaviours in the long life range up to 2 × 109 stress cycles, the authors intend to conduct a series of experiments systematically. 18 refs.
Considerations of grain orientation and work hardening on shortfatigue-crack modelling, de los Rios, E.R. and Navarro, A. Philos. Mag. A Mar. 1990 61, (3), 4 3 5 - 4 4 9 Methods that consider the effect of grain orientation and work hardening on shortfatigue*crack growth are examined. Fatigue cracks are assumed to form in the most favourably oriented grains on the surface of the specimen and propagate through less well oriented grains. A method of deriving the sequence of orientation factors as the crack front traverses an increasing number of grains is presented. Grain boundaries act as barriers to plastic flow in the zone ahead of the crack tip. This has the effect of hindering dislocation movement in the plastic zone, which can be expressed in terms of an increase in the frictional stress. A method of calculating this work-hardening rate is proposed. The condition for the transition from short to long fatigue cracks is established. 18 refs.
Fatigue life prediction method for gas turbine rotor disc alloy FV535. Horikawa, T,, Okada, T. and Tsunenarl, T. J. Soc. Mater. ScL, Japan Jan. 1990 39, (436), 8 2 - 8 8 (in Japanese)
Cyclic stress-strain response and low-cycle fatigue life in metallic materiels. Hatanaka, K. JSME Int. J. I Jan. 1990 33, (1), 1 3 - 2 5
A rotor disc of jet engine and/or gas turbine is subjected to intermittent stress waves, that is the repetition of centrifugal stress at the start and stop of the machine with superimposed vibrating stress. The serrated part and bolt hole of the rotor disc are important parts needed for fatigue strength evaluation. A number of studies have been conducted on the fatigue strength under the intermittent stress wave and the fatigue strength estimation of notched specimens. The fatigue strength evaluation method under the intermittent stress proposed by the authors is being used for the fatigue life prediction of the piston crown of a diesel engine. However, the adaptability of the method to the rotor disc has not been verified yet, Verification of the adaptability of the method for a rotor disc made of martensite stainless steel FV535 was therefore studied. The following conclusions were obtained from this experimental study. ( 1) The fatigue crack initiation life of a notched specimen under constant stress amplitude was well predicted by Koe's or Neuber's method. (2) The fatigue life prediction method proposed predicts the fatigue life of a smooth specimen well in the region of stress level higher than the fatigue limit obtained by using the interaction coefficient C of 0.3. However, in the region of stress near the fatigue limit, the method predicted a very conservative life and the coefficient C was greater than 5. Graphs. 12 refs.
The cyclic stress-strain response and the test method-dependent characteristics of metallic materials were discussed from the viewpoint of such metallurgical factors as the motion of screw dislocations, the formation of strain-induced martensite and dynamic strain aging. Furthermore, their low-cycle fatigue life was examined in terms of the total strain and the plastic strain, and the existence of the unified total strain-based life curve common to the various steel materials was suggested. The plastic strain-based low-cycle fatigue life was successfully estimated through surface crack growth analysis, which was made using the elastic-plastic fracture mechanics; the inflection appearing in the log &~p-log Nf relationship of the high-strength-lowductility materials as well as the Coffin-Manson law embodied in the ductile materials was satisfactorily described through the crack growth analysis. Finally, it was shown that the equivalent stress and plastic strains generated at the notch root were the predominant factors in governing the crack initiation life of the grooved cylinder components. 73 refs.
Acceleration and retardation of crack propagation in high-strength steel under variable loading. Takashina, N. and Misawa, H. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 7 0 - 7 5 (in Japanese) To clarify the interaction effect of stress changing on the crack propagation behaviour, fatigue tests in which the stress sequence was high to low and low to high were carried out on the high-strength steel JIS SNCM439. The results obtained
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Room temperature fatigue behaviour of a normalized steel SAE 4140 in torsion. Klumpp, S.,Eifler, D. and Macherauch, E. Mater. Wiss. Werkst. Tech. M a y 1990 21, (5), 2 0 3 - 2 1 3 (in German) Cyclic deformation behsviour of a normalized steel SAE4140 in shear-straincontrolled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed onto an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear-strain*controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains. Graphs. 12 rsfs.
Int J Fatigue January 1991
elevated temperature is imposed on a component under strain conditions, stress relaxation occurs. Relaxation data can be used, for example, in 318 austanitic steels, in predicting creep stages; however, interpretation of data obtained from such tests could be misleading in assessing damage. An example is given for life prediction on the basis of two selected criteria: the fraction rule and ductility exhaustion. Graphs, photomicrographs. 41 refs.
by laser beam. LI, C., Li, X. and Li, G. Eng. Fract. Mech. 1990 36, (1), 9 - 1 5 The fatigue process is studied by the method of damage mechanics. The equations for the evolution of fatigue damage of one and the same specimen with different irradiation parameters are given, all obtained by experiment. The results show that the study of fatigue damage of a specimen (e.g., steel 45) after irradiation by a laser beam is of definite significance for the selection of the optimum irradiation parameters. Graphs. 8 refs.
High t e m p e r a t u r e crack g r o w t h in copper under cyclic loading conditions. Kim, Y.-S. and Nix, W.D. Scr. Metell. Mater. Jan. 1990 24, (1), 213-218 The fatigue crack growth behaviour of oxygen-free high-conductivity Cu at 773 K has been characterized. The fatigue crack growth data have been analysed using fracture mechanics parameters, and the fatigue fracture surfaces were examined to relate the appearance of the fracture surface to the operative crack propagation mechanism. Graphs, photomicrographs. 14 refs.
Correlation of mechanical test parameters w i t h fatigue crack g r o w t h rates in structural steel. Vol. I and II. (Dissertation). Divsalar, F. Diss. Abstr. Int. Apr. 1990 50, (10), 419 pp The work undertaken investigated the fatigue phenomenon under cyclic loading in the structural steel BS 4360-50D. This material, extensively used in offshore applications, is subject to crack propagation and crack closure effects. The three aspects investigated were cyclic fatigue parameters, crack closure and crack propagation in centre-crecked rectangular plate together with a theortical modelling of crack closure using finite-element analysis. This was executed using a PAFEC package with the "CLOSURE" program developed for elastic-plastic analysis as part of the investigation. The fatigue design parameters were the fatigue ductility exponent, ¢=, the fatigue strength exponent, "t, and the cyclic strain hardening exponent, ~. All these parameters were determined. The fatigue ductility exponent, ~, revealed upper- and lower-bound values of 0.75 and 0.818, respectively. The material displayed a beheviour variation at 0.75 + 0.1% strain where a cross would occur between the monotonic loading and the cyclic loading curves. From the two different stress ratios of R : -1 and R = 0 the effect of the mean stress of crack closure was examined during the crack propagation. Frectography, as well as compliance methods, showed that crack closure is an important factor during the early stages of propagation. It was surmised that during a complete cycle reversal the area close to the crack tip remained open. Fatigue reek;tanco of steels. Boardman, B. Metals Handbook 10th Edn (ASM International, Materials Park, Ohio, 1990) Vol. 1, pp 673-688 Fatigue is the progressive, localized and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that are less than the tensile strength of the materials. Fatigue testing concepts are defined and both stress- and strein-bseed approaches to fatigue are described. The metallurgical variables having the most pronounced effects on the fatigue beheviour of carbon and low-alloy steels are the strength level, ductility, cleanliness of the steel, residual stresses, surface conditions and aggressive environments. The application of fatigue data in engineering design is addressed. Graphs. 21 refs.
Low-cycle fatigue and microstructurei behaviour of the 1 2 C r - M o - V martonsitic stainless steel. Degallaix, G., Vogt, J.-B. and Feet, J. Mem. Etud. Sci. Rev. Metell. Jan. 1990 87, (1), 47-57 (in French) The low-cycle fatigue behaviour of martensitic stainless steels with 12% Cr has been investigated between 20 and 600 °C for various levels of deformation. The evolution of stress, the cyclical cold working and the lifetimes are determined. At 20°C, the role of ferrite embedded in the martensitlc structure is highlighted, particularly for the cylindrical accommodation behaviour and in the formation of extrusions. Deformation microstructurss are examined with TEM. Various ferrite contents are obtained by thermal treatments prior to tempering. The influence of ferrita content on low-cycle fatigue and propagation fatigue behaviour at room temperature is also investigated. Graphs, photomicrographs. 24 refs. Fretting fatigue strength analysis of T i - 6 A I - 4 V in air. Maruyama, N.,
Sumite, M. and Nakazawa, K. Tetsu-to-Hagane (J. Iron Steel InsL Japan) Feb. 1990 76, (2), 262-269 (in Japanese) The influence of fretting damage on fatigue strength has been investigated using Ti-6AI-4V alloy. Axial fatigue loading was applied at frequencies of 20 and 0.5 Hz in a laboratory environment with the fretting applied to the specimen central section through a fretting pad made of the same material as the fatigue specimen. The reduction in fatigue strength owing to the fretting damage depended on the contact pressure, p. When the value of pwss approximately 20 MPa, the fatigue life exhibited a minimum. The fretting fatigue crack initiated at the centre of the fretted surface. Many microcrecks existed prior to main crack initiation. However, when the value of p was greater than 30 MPa, the fretting fatigue crack initiated at the edge of the fretted surface. Few microcracks existed prior to the main crack initiation. Fretting fatigue cycles at more than 90% of the total life in the interrupted fretting fatigue test are required in order to continue crack propagation after the removal of the fretting pad, although cracks were already initiated at cycles less than 50%. Graphs, photomicrographs. 14 refs.
Elevated-temperature properties of feRRic steels. Metals Handbook 10th Edn (ASM International, Materials Park, Ohio, 1990) Vol. 1, pp. 617-662
Fatigue life under vadablo random loading charectedstic=. Kliman, V. Strojnicky Cas. 1990 41, (1), 41-65 (in Slovak)
Eleveted-temparature mechanical and corrosion properties of carbon and low-alloy steels with ferrite-pserlite and ferrite-bainite microstructures for use in boiler tubes, pressure vessels, and steam turbines are described. Mechanical properties discussed include creep strength, stress rupture, relaxation, fatigue, ductility and toughness. Corrosion effects at elevated temperatures often set the maximum allowable service temperature of an alloy. The three common forms of corrosion at elevated temparaturos--oxidetion, sulphidetion and hydrogen attack--are discussed. Factors influencing the mechanical properties such as microstructuro, heat treatment, composition, decerburizetion and carburizstion are addressed. Graphs, photomicrographs. 88 refs.
A method is presented allowing the assessment of operational fatigue variable random loading during operation. The suggested procedure is derived from the correlation theory of approach to life estimation in view of the random character of the load as well as the possible scatter of the material mechanical properties and allows evaluation by the calculating procedure of the residual fatigue life as a function of time in the probabiliefic interpretation with respect to the courses of random load proceeding. The theoretical and experimental fatigue life results are compared and examples of practical applications of this method ere presented. 18 refs. Creep damage estimation in pure copper using ultrasonic waves.
Stress enhancement of fatigue susceptibility of porous coated T i - 6 A I - 4 V Implants: an elastic analysis. Messersmith, P.B. and Cooke, F.W. J. Biomed. Mater. Res. M a y 1990 24, (5), 591-604 An elastic stress analysis of porous-coated Ti-6AI-4V implant surfaces was performed using the finite-element method. 300 p.m diameter metal beads sinter bonded onto an implant surface were modelled with sinter neck radii of 5, 10, 20, and 50 p.m. Smooth-surface, single-bead, single-layer and double-layer systems were analysed, The finite-element models were loaded to simulate bone-bead contact forces end lateral hip implant tensile forces, Results showed that, for a single bead sintar bonded onto an implant surface, concentration of stress occurs either at the base of the sintar neck or within the neck itself, depending on the type of load applied. Under lateral hip implant tensile loads, a maximum stress concentration factor of 1.97 was obtained for a single beam sinter bonded onto an implant surface. Addition of a single layer of beads onto the implant surface resulted in a significant increase in stress at the most proximal and distal ends of the porous layer, with a maximum stress concentration factor of 4.3. The addition of a second layer of beads did not significantly increase the magnitude of the stress concentration occurring at the ends of the porous layer. The results of this study provide stress concentration factors for porous coatings with sinter necks of known dimensions under loading conditions similar to those present along the leteral surface of a hip prosthesis. Graphs. 8 refs. An interpretation o f axial c r e e p - f a t i g u e damage interaction in t y p e 316 s t a i n l m steel. Zamrik, S.Y. J. Pressure Vessel Technol. (Trans. ASME) Feb. 1990 112, (1), 4 - 1 9 Creep-fatigue end its effect on damage of components in service have been a major concern to analysts. To deal with this problem, several criteria have been proposed and used, such as the cycle-time fraction summation rule, strain limit, fracture maps where damage mechanisms are based on crack initiation or propagation and ductility exhaustion. These concepts are reviewed so that one can interpret the damage mechanisms caused by creep and by fatigue. If a long period of dwell-time at
Int J Fatigue January 1991
Men""shits, T., Hired, M. and Fukuoka, H. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 208-213 (in Japanese) An ultrasonic experiment has been done to find the effect of ultrasonic waves on the accumulative creep damage which occurs in polycrystalline pure Cu during hightemperature tensile loading. The longitudinal wave velocity was sensitive to the intergranular creeping process controlled by grain boundary cavitation and subsequent microcrecking. The velocity decreased slowly and linearly up to approximately 60'% of the time to rupture, when the steady creep shifted to accelerated creep. It then decreased with increasing rate until the eventual failure. Measurements of porosity and photomicrographic observation revealed that the first period is associated with the nucleation and growth of cavities and the second corresponds to cracking perpendicular to the stress. A non-destructive technique for predicting the remaining life time of high-temparsture components is also suggested. Graphs, photomicrographs. 16 refs. Effects of defects and microstrocture on the elevated temperature fatigue p r o p e r t y o f P / M superelloy MERL76. Yokomaku, T., Takigawa,
H. and To¥oda, H. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 188-194 (in Japanese) Elevated temperature fatigue tests were conducted on P/M HIP'eq superalloy, MERL 76. The defects, such as precipitates on the prior particle boundaries (PPBs), gas porosities and non-metallic inclusions, were found at the fatigue origins. The relation between defect size, ai, end fatigue life, Nf, was independent of defect type and was formulated as Nf&orneq = ha1i- rn/2 , &Oeq= ~w{1 + [2Eg(n)/(n + 1)](~u~p/&o)}1/2 , where m is the exponent in the crack propagation law, da/dN = C~Km; h is a material constant, E is Young's modulus, gin) is a function of the cyclic strain hardening exponent, n, and &~ is the plastic strain range. The relation between threshold stress, &O'th, and defect size, ai, was also expressed as ~,Kth = (2/~)AOth(~(ai + a0)] tl2 where AKth is the threshold stress intensity factor for long cracks. The value, a0, was about 0.1 mm, which coincided with the size estimated by an experimental relationship between grain size and a0 on steels. The usage of Ar-treated powders and the hot-work after HIP'ing improved fatigue life
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since they prevented PPB failure. Furthermore, the usage of fine powders is preferable because it can reduce the size of pores and inclusions. Photomicrographs, graphs. 11 refs.
Microstructural and mechanical factors affecting creep-fstigue propertie,= of heat resisting cost steels. Yokomaku, T. and Saori, M. J. Soc. Mater. Sci., Japan Feb. 1990 3g, (437), 181-187 (in Japanese) Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) tests under creep conditions were carried out on HK40, HP and their modified alloys containing a small amount of Zr, Nb and/or Ti. The failure mechanism was established based on microscopic observations by the scanning and transmission electron microscopies. Also, the mechanical factors affecting the LCF life are discussed, by considering the crack propagation law. The main results obtained are as follows: (1) The addition of Nb, Ti and/or Zr improves the LCF life and the FCP resistance under creep conditions. In particular, the effect of Zr is marked. (2) Cracks propagate along the intensive sub-boundaries, on which coarse M23Cs carbides precipitate during the LCF test. (3) Zirconium, Nb and Ti form fine and dispersed secondary carbides (MC) in the matrix of material. These fine carbides lead to a uniform dislocation structure without intensive sub*boundaries and then result in the dispersed precipitation of coarse M23C6carbides. Accordingly, the addition of these elements improves the LCF and FCP properties. (4) From a mechanical viewpoint, the LCF life can be determined by the fatigue and creep crack propagation laws and the strain energy densities, which are obtainable based on J-integral analysis of a cracked body. Graphs, photomicrographs. 6 refs.
Acceleration of fatigue crack growth for HTa0 and SUS304 steels in 3% NaCI aqueous solution under very-low-frequency cyclic loading. Shimodaira, M., Matsuoka, S., Masuda, H. andNishijima, S. J. Soc. Mater. Sci., Japan Feb. 1990 38, (437), 1 6 2 - 1 6 8 (in Japanese) Fatigue crack growth properties under freely corroding and cathodic protection in 3% NaCI aqueous solution were studied under very-low-frequency cycling for HTe0 high tensile and SUS304 stainless steels. The crack growth rate in 3% NaCI solution was accelerated as compared with that in air. The acceleration was dependent on material, environment, ~K level and frequency, and the following three types of acceleration were observed. ( 1) The CF-type acceleration was inherent to corrosion fatigue in free corrosion and the rate was accelerated about twice at frequencies less than 0.3 Hz independently of material and ~K level. (2) The HE(IG) type was related to the intergranular cracking due to hydrogen embrittlement and appeared in the range of &K = 4-20 MPa m 1/2 and at frequencies 10-3-102 Hz only for HT60 steel. The maximum acceleration rate became about five times at &K = 10 MPa m 1!2 and f = 0.3 Hz. (3) The HE(TG) type was related to the transgranular and brittle fracture due to H embrittlement and the rate became constant at frequencies less than 0.03 Hz. This type was also dependent on material, environment and ~K level. The maximum acceleration rate was nearly 30 and ten times for HT80 and SUS304 steels, respectively. Graphs, photomicrographs. 9 refs.
Investigations on small crack growth and life property in fatigue of a circumterentiaUy-notched component of pure copper subjected to combined Ioedings. Hoshide, T., Yokota, K. and Inoue, T. J. Soc. Mater. Sci., Japan Feb. 1990 39, (437), 144-149 (in Japanese) Fatigue tests were conducted using solid cylindrical specimens of pure Cu with circumferential blunt notches subjected to combined axial-torsional Ioadings. The behaviour of crack growth was observed by a plastic replication technique, and the property of fatigue life was investigated. The cracking at notch root was identified as an intergranular type. The dominant morphology of crack growth was found to be the coalescence of distributed small cracks, while some difference was observed depending on the stress multiaxiality. The fatigue life was correlated with the equivalent stress of the Von Mises type and the maximum shear stress. The fatigue life for the same value of each stress parameter became longer with increasing shear component in the stress state at the notch root. An analytical procedure for the crack growth at the notch root was established by using a model based on the competition between the coalescence growth and the propagation of a dominant crack. Characteristics of intergranular cracking at the notch root and its dependence on the stress state were simulated very well by the present analytical model. Fatigue lives in several test conditions were statistically estimated by a simulation of the Monte Carlo type. The prediction with the simulated scatter-bands almost coincided with the experimental results. Graphs. 12 refs.
were as follows: (1) The acceleration behaviour of the crack propagation rate was observed just after changing the stress level from low to high, and the crack propagation rate became two times that under the constant loading. This acceleration behaviour continued for 500-1000 cycles of high-stress loading. (2) The retardation behaviour of the crack propagation rate was observed just after changing the stress level to tow from high. The number of cycles of low-stress loading needed for the crack propagation rate to return to that of constant loading was approximately 10000-70000. (3) It was difficult to elucidate the acceleration and retardation behaviour only by the change of crack opening stress level. (4) It is necessary to consider the acceleration effect of the crack propagation rate for the estimation of fatigue life under service loading conditions when a higher stress is intermittently loaded. Graphs. 14 refs.
Evaluation of actual fatigue stress by using striation spacing and hardness, Ishida, S., Tokumasu, M,, Nakasa, K. and Ishihara, H. J. Soc. Mater. ScL, Japan Jan, 1990 39, (436), 6 3 - 6 9 (in Japanese) The relation between the range of stress intensity factor &K and striation spacing S was obtained by using CT specimens of various kinds of structural steels with different hardnessss, and the actual fatigue stress of square or round bars under repeated three-point bending loads, which was assumed to constitute an actual machine part, was evaluated from the Vickers hardness HV and the striation spacing measured in the bars. The material constants, Cs and ms, obtained from the relation between S (mm) and &K (MPa~/m) for the CT specimens, S = Cs(&K)ms, were able to be approximately expressed as Cs = 1.68 × 10-1° (HV) 18s and ms - 19.1(HV) o,53respectively. The actual fatigue stress of the square bar under three-point bending agreed relatively well with the stress calculated from the striation spacing measured on the bar and the values of Cs and ms, which were obtained from the above equations by substituting the hardness. For the round bar, however, the calculated stress was larger or smaller than the actual fatigue stress when the crack length was short or long, respectively. Photomicrographs, graphs. 12 refs.
Development of two-step varying load rotating-bending fatigue testing machine and some t e s t results. Hoshi, S., Nakamura, H. and Nagai, M. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 5 8 - 6 2 (in Japanese) In order to obtain fatigue strength data in the long life range over 108 stress cycles under multiple two-step varying load conditions with high frequency of Hi/Lo block cycles, an automatic load changing device for a uniform rotating-bending fatigue testing machine was developed. With this device can be conducted the repeated Hi/Lo varying load fatigue tests with a short interval of the higher stress level of approximately 100 cycles. It was confirmed, through experiments done by using JIS $45C round-bar specimens, that the level of performance of the newly developed load changing device was high. Furthermore, the discussion on the results from the viewpoint of the linear cumulative damage laws showed that, in some cases, the effect of understressing on fatigue lives appeared in two opposite manners, that is fatigue lives became considerably longer than estimated values or vice versa, especially when the number of under-stressing cycles was sufficiently large. To reveal such an inconsistent effect of under-stressing, and further, to clarify the fatigue behaviours in the long life range up to 2 × 109 stress cycles, the authors intend to conduct a series of experiments systematically. 18 refs.
Considerations of grain orientation and work hardening on shortfatigue-crack modelling, de los Rios, E.R. and Navarro, A. Philos. Mag. A Mar. 1990 61, (3), 4 3 5 - 4 4 9 Methods that consider the effect of grain orientation and work hardening on shortfatigue*crack growth are examined. Fatigue cracks are assumed to form in the most favourably oriented grains on the surface of the specimen and propagate through less well oriented grains. A method of deriving the sequence of orientation factors as the crack front traverses an increasing number of grains is presented. Grain boundaries act as barriers to plastic flow in the zone ahead of the crack tip. This has the effect of hindering dislocation movement in the plastic zone, which can be expressed in terms of an increase in the frictional stress. A method of calculating this work-hardening rate is proposed. The condition for the transition from short to long fatigue cracks is established. 18 refs.
Fatigue life prediction method for gas turbine rotor disc alloy FV535. Horikawa, T,, Okada, T. and Tsunenarl, T. J. Soc. Mater. ScL, Japan Jan. 1990 39, (436), 8 2 - 8 8 (in Japanese)
Cyclic stress-strain response and low-cycle fatigue life in metallic materiels. Hatanaka, K. JSME Int. J. I Jan. 1990 33, (1), 1 3 - 2 5
A rotor disc of jet engine and/or gas turbine is subjected to intermittent stress waves, that is the repetition of centrifugal stress at the start and stop of the machine with superimposed vibrating stress. The serrated part and bolt hole of the rotor disc are important parts needed for fatigue strength evaluation. A number of studies have been conducted on the fatigue strength under the intermittent stress wave and the fatigue strength estimation of notched specimens. The fatigue strength evaluation method under the intermittent stress proposed by the authors is being used for the fatigue life prediction of the piston crown of a diesel engine. However, the adaptability of the method to the rotor disc has not been verified yet, Verification of the adaptability of the method for a rotor disc made of martensite stainless steel FV535 was therefore studied. The following conclusions were obtained from this experimental study. ( 1) The fatigue crack initiation life of a notched specimen under constant stress amplitude was well predicted by Koe's or Neuber's method. (2) The fatigue life prediction method proposed predicts the fatigue life of a smooth specimen well in the region of stress level higher than the fatigue limit obtained by using the interaction coefficient C of 0.3. However, in the region of stress near the fatigue limit, the method predicted a very conservative life and the coefficient C was greater than 5. Graphs. 12 refs.
The cyclic stress-strain response and the test method-dependent characteristics of metallic materials were discussed from the viewpoint of such metallurgical factors as the motion of screw dislocations, the formation of strain-induced martensite and dynamic strain aging. Furthermore, their low-cycle fatigue life was examined in terms of the total strain and the plastic strain, and the existence of the unified total strain-based life curve common to the various steel materials was suggested. The plastic strain-based low-cycle fatigue life was successfully estimated through surface crack growth analysis, which was made using the elastic-plastic fracture mechanics; the inflection appearing in the log &~p-log Nf relationship of the high-strength-lowductility materials as well as the Coffin-Manson law embodied in the ductile materials was satisfactorily described through the crack growth analysis. Finally, it was shown that the equivalent stress and plastic strains generated at the notch root were the predominant factors in governing the crack initiation life of the grooved cylinder components. 73 refs.
Acceleration and retardation of crack propagation in high-strength steel under variable loading. Takashina, N. and Misawa, H. J. Soc. Mater. Sci., Japan Jan. 1990 39, (436), 7 0 - 7 5 (in Japanese) To clarify the interaction effect of stress changing on the crack propagation behaviour, fatigue tests in which the stress sequence was high to low and low to high were carried out on the high-strength steel JIS SNCM439. The results obtained
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Room temperature fatigue behaviour of a normalized steel SAE 4140 in torsion. Klumpp, S.,Eifler, D. and Macherauch, E. Mater. Wiss. Werkst. Tech. M a y 1990 21, (5), 2 0 3 - 2 1 3 (in German) Cyclic deformation behsviour of a normalized steel SAE4140 in shear-straincontrolled torsion is characterized by cyclic softening and cyclic hardening. If mean shear stresses are superimposed onto an alternating shear stress, cycle-dependent creep occurs, and the number of cycles to failure decreases. In shear-strain*controlled torsional loading, mean stresses are observed to relax nearly to zero within a few cycles. Fatigue life is not influenced by mean shear strains. Graphs. 12 rsfs.
Int J Fatigue January 1991