- Email: [email protected]
Discussion about surface crack growth criteria under cyclic loading
were shown to be less effective than the use of &J as a correlating parameter. None of these factors affected the suitability of &K for characterizing microatructurel effects. Small and long fatigue crack date were comperebie when plotted against calculated plastic zone size. Low-cycle fatigue beheviour was shown to be related to the beheviour of small fatigue cracks and similar microstructurel effects were observed. Total, rather than plastic, strain was identified as the critical parameter when comparing small fatigue crack and LCF beheviour. Small fatigue crack growth bohaviour was shown to be inconsistent with the plastic work accumulation criterion for the exhaustion mechanism of fatigue. Finally, because of low values of &K and the lack of closure, fatigue mechanisms may be easier to study in small cracks than in long cracks.
Micromachenlsms o f quasi-static and fatigue crack g r o w t h in t i t a n i u m aluminidac. Aswath, P.B. Diss. Abst. Int. Feb. 1981 51, (8), 217 pp The rapid development of advanced aerospace technology has placed an increasing demand on the development of a new generation of structural materials for aircraft engines. Intarmetantc materials with low densities and Improved fracture toughness at elevated temperature show promise for replacing widely used Ni-beeed superalloys. TiAI and TiaAI, which are ordered intermetallic compounds of "13and AI, have been the subject of increasing research activity in the recent pest because of their light weight and high strength. An understanding of the resistance to quasi-static end cyclic-load fracture in these intarmatallica at both ambient and elevated temperatures is of paramount importance for damage-tolerant, fatigue-critical structural applications for which the intermatallice are candidate materiels. This thesis explores the effects of microstructure, microstructurel stability, mechanical variables (such as load ratio and fatigue frequency), and temperature on the quasistatic and fatigue crack growth characteristics of Ti-AI end Ti-AI-Nb alloys. With the aid of controlled heat treatments, duplex microstructures comprising either a2 end ~ or ¢12 and 3' phases as well as predominantly cz2 phase microstructurse ere produced. The fatigue crack growth charentedatice are monitored over s range of &K values from threshold to final failure. The effects of controlled microstructures on the overall fatigue end fracture rseistance are examined in the light of TEM observations of creck tip damage, scanning Auger spectroscopy of fracture surface oxidation and scanning electron frectogrephy. The atabillty of thsee microstructures is examined in the light of their response to beth fatigue crack growth end fracture toughnmm. Mic~ostructurel Matures that lead to a betterment of fatigue crack growth end fracture toughness are discussed in the context of the present experimental results.
A new method of faUgue pre=raddng for brittle matm~ls. Zhang, J. Trans. Met. Heat Treat. (China) Mar. 1990 11, (1), 70-74 (in Chinese) A new method of fatigue precrecking for bdttle materials and the corresponding loading device are described. Using this method, it is realized that the stress intensity factor/(max at the crack tip decreases automatically with the increase of crack length. The instability of the crack can be controlled and the crack length present can be obtained by propedy selecting the loading parameters. There are some advantages for the method in that the shape of the sample is simple, the final length of fatigue precrecking can be chosen and can be limited automatically, unstable cracking cannot be generated and the finishing line of the crack is quite clear. The loading device is easy to operate and reliable. This new method opens up a new means of fatigue precrecking for brittle materiels (e.g., the steels TeA, W18Cr4V and AI203). Graphs, photomicrographs. 6 refs. ' 8 u p e r d e m t ' 3,r=dMiCrMoV t u d d n e r o t o r steel: • status report. II. Machan-
k:al propecUes. McNaughton, W.P., Richman, R.H. and Jaffee, R.I. J. Mater. Eng. Mar. 1991 13, (1), 19-28
Recent international research has resulted in the development of superclsen 3.5NiCrMoV steel for low-pressure turbine rotors. Such steel is highly resistant to tamper embrittlement and will thus facilitate increased efficiency in electricity generation through the use of higher operating temperatures and improvements in design. Steels with impurity contents typical of the supercleen specification can be manufactured for production rotors with properties that equal or exceed those for conventional 3.6NiCrMoV rotors in every detail. Of particular interest are the observations that the superclsen steels appear to be virtually immune to temper embrittlement and may be used up to their creep-limited temperature of 500 °C (932 °F). The mechanical properties of superclean 3.SNiCrMoV steel are reviewed: tamper embrittiement susceptibility, uniformity of strength, impact properties, creep rupture properties, fatigue reeistence, fracture toughness and stress corrosion cracking susceptibility. Graphs. 28 refs. Biaxiel fatigue tests on t h i n - w a l l e d tubes at 950°C.
Hanswillemenke, H., Breitling, H. and Dietz, W. Closed Loop 1990 17, (3), 11-18
The g r o w t h o f short fatigue cracks under compressive e n d / o r tensile cyclic loading. McEvily, A.J. and Yang, Z. Metall. Trans. A M a y 1991 22A, (5), 1079-1082 Under cyclic compressive loading, fatigue cracks can initiate and propagate a short distance before being arrested. Recent studies have shown that the process of crack closure is important in the arrest process. An analytical means for dealing with this type of crack propagation in Fe is presented. Calculations based on this method of analysis agree well with experimental results. The method is general in nature and can also deal with short-crack propagation under tension-compression and tension-tension types of loading. Graphs. 17 refs. Thermomachanicel fatigue of a lead alloy. Lawson, L., Fine, M.E. and
Jeannotte, D. Metall. Trans. A M a y 1991 22A, (5), 1059-1070
The effects of superimposing a thermal cycle onto a strain-controlled fatigue cycle have been studied for 96.SPb-3.5Sn. These tests have been conducted at temperatures near half the melting point under conditions of cycling that included strain ranges from 0.3-3% and periods between 184 and 1040 s/cycle. Cracks were seen to initiate rapidly as grooves along grain boundaries and grow inwards until a drop in the Ioedcarrying capacity of the sample was observed when a significant fraction of the crack population exceeded approximately 300 lam in depth. The presence of an in-phase thermal cycle was seen to reduce the number of cycles to failure substantially relative to a reference isothermal test at the highest temperature of the thermomschanical cycle. For a given set of test conditions, the number of cycles to failure increased with the angle of phase lag of the mechanical cycle with respect to the thermal cycle. Decreasing frequency wee found to increase the number of cycles to failure in thermomechanical fatigue (TMF), while the opposite was observed for the isothermal reference tests. The hold time was found to reduce the number of cycles to failure in TMF. Graphs, photomicrographs. 26 refs. M.rrJq~ertsyatems f o r materials-related problems. Hoffelner, W. and Vifins,
Proc. High Temperature Materials for Power Engineering 1990, II, Liege, Belgium, (Kluwer, Dordrecht, The Netherlands, 1990) pp. 1599-1626 Expert systems are discussed as a tool for the solution of materiels-relsted problems. The basic elements and the architecture of such systems ere briefly introduced. Knowledge representation in materials-related expert systems is dealt with in more detail. Specific emphasis is given to the concept of frames, which is particularly useful for handling knowledge about materials. A knowledge base for the solution of meterials-related problems necessarily contains a variety of different types of material data: material properties, microstructurel information, uncertainties, lifeprediction models, etc. The introduction of these items into the knowledge base is elaborated on. The knowledge of experts must be cast into • structured and tight framework, which is sometimes a problem. Examples from MATEXPERT, an expert system for the solution of materials (low-alloy steels) problems in gas turbines (accept/reject decisions for materials deviating from standard, judgement about conditions of compressor blades) are taken to illustrate how expert systems can be employed. Other materials-releted expert systems (e.g., the intelligent processing of materials, material selection, life time prediction, etc.) are described. Current limitations and aspects for future applications are discussed. Graphs, photomicrographs. 34 refs.
T o w a r d s a probabiUstic d a m a g e accumulation model f o r high-strength
components. Bruckner-Foit, A., Jackels, H. and Quadfaeal, U. Proc. High Temperature Materials for Power Engineering 1990, II, Liege, Belgium (Kluwer, Dordrecht, The Netherlands, 1990) pp. 977-986 The lifetime distribution of high-strength components subjected to fatigue loading depends on the probability that a crack is initiated from an inclusion and then propagates stably until it exceeds the critical crack size and causes catastrophic failure. An analysis of crack growth data obtained from fatigue tests with smooth specimens is given, where the crack growth rate is expressed as a function of an effective stress intensity factor. A comparison of the size of the fraction initiating defect and the theoretical initial crack size indicates that there is virtually no initiation phase and that the cyclic crack growth is accelerated for small cracks. (The test material was PM Udimet 700). Graphs. 11 refs.
Meurer, H.,
Both uni- and bi-axial fatigue tests on the Ni-besed alloy NiCr23Co12Mo (similar to Inconel 617) were carried out at 950°C, 1.8 bar in helium and strains between 0.3-1.0% using an MTS computer-controlled servohydreulic test system, which provided independent loading in the axial and torsional directions. The loading methods, test chamber and specimen geometry are detailed including an induction heating system for the specimen with measurements of the temperature distribution in the thin-walled tubular specimens. The fatigue results at 950 °C show no strain hardening and can be described by the van Mices equivalent strain hypothesis. 7 refs. Metallurgical control o f fatigue crack propagation in superelloys. Chang,
K.-M., Henry, M.F. and Benz, M.G. J. Met. Dec. 1990 42, (12), 29-35
The low-cycle fatigue lives of turbine engine disc alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, end a new generation of high-strength Ni-besed superalloys with fatigue crack propagation in high-strength =uperalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the microstructure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high-~' content superalloys, is to modify the grain
504
boundary structure by means of a controlled cooling from a supersolvus solution. The precipitation reaction occurring on the grain boundaries during cooling generates • serrated structure that exhibits a good stress oxidation resistance for fatigue cracking. Graphs, photomicrographs. 34 refs.
The role o f w e l d i n g in c o m p o n e n t integrity. Cerjak, H.H.
Proc. High Temperature Materials for Powder Engineering 1990, I, Liege, Belgium (Kluwer, Dordrecht, The Netherlands, 1990) pp. 445-472
The role of welding in steam turbine fabrication and operation is described. The basic influence of welding, that is fabrication welding of castings end butt welding of similar and dissimilar materials, on the operational beheviour of steam turbine components is discussed. The actual results, obtained within the COST 505 welding group on the HAZ baheviour of 1% CrMoV castings, manufactured in accordance with the DIN and ASTM specifications for stress relief cracking beheviour, toughness, creep and LCF behaviour are reported. Results on similar welds of 1% CrMoV steels regarding mechanical properties, toughness creep and LCF beheviour and on dissimilar welds of 1% CrMoV to 12% CrMoV steels using 12% CrMoV- and Nibased filler metals are discussed. The optimization of the welding procedures end PWHT parameters was the main goal of the investigations. The results of mathematical modelling support the experimentally determined data. Graphs. 37 refs. Discussion about surface crack g r o w t h criteria under cyclic loading.
Varfolomeyev, I.V. and Vainshtok, V.A. int. J. Frect. Nov. 1990 46, (2), R25-R33
The growth of surface cracks in pressure vessel steels (0.2% yield strengths, tensile strength 584 MPa and ultimate tensile strength of 700 MPa) wee studied with twopoint bending specimens. The initial cracks were made by electrosparks. The stresses were 0.38 to 0.90 of the yield strength. The number of cycles to failure was between
Int J Fatigue November 1991
3000 and 10000. The elastic fracture &K (effective) is the best parameter to predict how structures will behave under cyclic loading with surfeae defects. The steel mentioned well 15Kh2MFA. Photomicrographs, graphs. 17 refe. " f l l m e - d h m w 4 k ) l t ~ etrmm i n t m e l t y f a c t o r analysis o f a surlaco crack in s hlgh-q)ued busting. Ballarini, R. and Hsu, Y. Int. J. Fract. Nov. 1990 441, (2), 141-153 The boundary element method is applied to calculate the stress intensity factors of a surfece crack in the rotating inner raceway of a high-speed roller bearing. The three-dlmenelonel model consists of an axially atresoed surfece-creaked plate sob~ectod to e moving Hertzicn contact loading. A muitldomeln formulation and singular crock-tip elements were employed to calculate the stress intensity factors accurately and effldently for a wide range of configuration parameters. The results can provide the basis for creak growth calculations and fatigue life predictions of the high-performance rolling element bearings that are used in aircraft engines. Graphs. 13 refe. t e m lN.Waturo and sbll.n rote s u p p o s i t i o n =.re
®...
properties.of Iow...~e
,. =.
Acta Matall. Mater. Apr. 1991 39, (4), 619-626 The date from other investigators are shown to follow a general suporposition property of the Iogsp against IogNf curves generated at ell levels of temperature end atreln rate during isothermal strain-controlled fatigue tests, This property is found in consequence of en energy criterion for fatigue failure, where the material (e.g., low-carbon steel, austenitlc stainless steels) ia described by a mechanical constitutive equation for creep controlled by the thermally assisted motion of jogged screw dislocations. The scaling relationship is shown to be dependent on the ratio between the self-diffusion energy and the energy to form a jog. Graphs. 20 refs. Effect o f surfece roughness on t h e life of continuous cycling w i t h o u t ~ w i t a t i o n d a m a g e in 1 2 C r - M o - V steel. Soo, C.L. and See, W.N. Scr. MetalL Mater. Oct. 1990 24, (10), 1971-1974 Mertensitic m i n i m steel (12% Cr) is used for steam turbine rotors in place of 1Cr-Mo-V steel because of its excellent high-temperature properties. Most of the low-cycle fatigue (LCF) life in t2Cr-Mo-V steel at 298 and 873 K ie spent in the crack propagation proos~. The LCF life in 12Cr-Mo-V steel ia rarely influenced by the surface roughness, regardless of the test temperatures. The fatigue cracks are nucleated on prior austenife grain boundaries and the crack initetion of 12Cr-Mo-V steel takes place early during the LCF deformation for apecimens with both smooth and rough surfaces. 12Cr-Mo-V steel containing 0.17C, 11.2Cr, 0.98Mo, 0.28V, 0.63Me and O.6Nt was used for the fatigue test. The heat treatment prior to tasting consisted of auatenitizing at 1323 K for 22 h, oil quenching, tempering at 933 K for 44 h and air cooling. Before LCF testing, two different types of surface roughness were obtained by mechanically polishing specimens perpendicular to the stress axis, using two different emery papers, #80 and #1200. Total-strain-controlled unlexiel fatigue tests were carried out in the air by an electroservohydraulicelly driven closed-loop INSTRON model 1350 machine. A radiant furnace was used to heat the specimen, and the test temperatures were controlled so as to be maintained at 298 and 873 K, respectively. Graphs, photomicrographs. 10 refs.
An investigation of t h e Influence o f h i g h - l o w - end l o w - h i g h - a m p l i t u d e M o c k luedlnge on f l t l g u e crock g r o w t h . Kumar, R. Eng. Fract. Mech. 1991 38, (2-3), 225-230
Crack propagation experiments were conducted on 6061-T6 AI alloy for high-low and low-high block loads for various combinations of overload ratios (0.0-0.1, 0.0-0,2, 0.0-0.3, 0.0-0.5, 0.2-0.0 and 0.3-0.0). On the basis of these experiments one power law is developed for the high-low case to predict the delay period at a particular block load ratio. The delay period after application of the low-amplitude block ioed is increased as the magnitude of the low block load is decreased. In the low-high sequence, an acceleration is observed in the crack growth rate. For the high-low load sequence, a retardation was observed in the crack growth rate. Graphs. 12 rofs.
Piping d y n a m i c r e l i a b i l i t y and code rule change recommendutiona.
Tagart, S.W., Jr, Tang, Y.K., Guzy, D.J. and Ranganath, S. Nucl. Eng. Design Oct. (11) 1990 123, ( 2 - 3 ) 373-385
The conservative nuclear piping design criteria for seismic and dynamic loads have led to piping systems with excessive numbers of snubbers. To improve this undeeirebie situation, a piping and fitting dynamic reliability program was initiated by the Electric Power Research Institute (EPRI) in 1985 with cooperation from the US Nuclear Regulatory Commission (NRC). The objective is to develop improved, realistic and defensible ASME design rules by taking advantage of the inherent dynamic margins in the nuclear piping system. The research results have demonstreted that piping systems have a large reserve dynamic capacity and the dynamic failure mode arises from fatigue or fatigue-retchating rather than plastic collapse. Based on such physical evidence, a set of code rule change recommendations is suggested, in preliminary form. The piping materials discussed are the steels A106B and 316L. Graphs. 5 refs. Mechanics and mechanisms of c r e e p - f a t i g u e crack g r o w t h in C u -
1 wt.% Sb. Gieseke, B.G. Diss. Abstr. Int. Jan. 1991 51, (7), 191 pp
The mechanisms and mechenica of fatigue crack growth in a model, of Cu-lSb alloy, were investigated at a temperature of 400 °C. Fatigue testa were conducted in ultra-high-purity nitrOgen using triangular and trapezoidal loading wsveforms and loading frequencies ranging from 2000 to 1 cycles/h. Two time-dependent crack growth mechanisms operated, each of which occurred as the result of creep deformation. In triangular waveforms, crack growth occurred by the nucleation, growth and coalescence of grain boundaw creep cavities with the advancing crack front. In one trapezoidal weveform with a 90 s hold time, crack growth occurred by both cavity coalescence and by the diffusive separation of grain boundaries (or peeling). For longer hold times, crack extension appeared to occur by both mechanisms simultaneously. To account for creep deformation at the crack tip, the concepts of time-dependent fracture mechanics were applied to the crack growth date generated using trapezoidal loading weveforms. A unique correlation was
Int J Fatigue November 1991
sought between the average rate of crack growth dudng the hold times, (daldt)=vg, end the creak-tip parameters, K and Ca. Since Ct varied within each cycle, it was necessary to USe average values of the parameter C~ (Ct)w0. These values were estimated using an analytical expression for Ct end, in SOme cases, were measured. A unique correlation wee obtained between (daldt)wg end the measured values of (Ct)avgfor hold times of 0.5 and 1 h. These data alSO compared well with the creep crack growth rate data for this alloy. However, because of discrepancies between the measured end the analytically estimated values of (Ct)wg, such a correlation could not be established between (da/dt)~ and analytical estimates of (Ct)~. The data for Cu-Sb were combined with data for steels from other studies to assess the ability of (Ct)~ to correlate the hold-time crack growth rates. For one Cr-Mo steel, the analytical estimates and measured values differed, while these compared well for one Cr-Mo-V steel. The discrepancies between the estimated and measured values of (Ct).vg were attributed to the use of an analytical expression for (Ct)=v0 that ignored the analytical expression for (Ct)~ that ignored the effects of instantaneous crack-tip plasticity. The influence o f p o r o s i t y on low-cycle fatigue. Gerard, D.A. Diss. Abstr. Int. Jan. 1991 51, (7), 180 pp The influence of porosity on low-cycle fatigue has been examined using powderprocessed Ti as a model material. The presence of even low levels of porosity (0.4%) effects the cyclic flow beheviour and is very detrimental to low-cycle fatigue life. An analysis has been performed by considering low-cycle fatigue in terms of the following stages: (1)the initiation of 15 lain microcreaks, (2)the propagation of microcreaks, (3) the linking process to form a macrocreak, and (4) the propagation of a maorocreak to failure. All except the microcreak growth stage are significantly accelerated by isolated porosity and even the growth of microcrecks occurs much more rapidly in materials containing 6% interconnected Porosity. Microcreak initiation is accelerated the most by porosity under ell the conditions examined. Microcreck linking and mearocreak growth are both accelerated in the presence of porosity, the degree of acceleration becoming more severe with increasing porosity level and strain amplitude, and when the porosity is interconnected. The above experimental observations have been modelled both quantitatively and qualitatively. Modelling pores at a free surface as through-thicknees holes in a plastically deforming matrix and utilizing a Coffin-Manson failure criterion on a local scale, the number of cycles required to initiate a microcreak have been predicted theoretically. The results of these predictions agree very well with the expodmentel observations of crack initiation in the porous Ti specimens. The linking of short cracks has been addressed semiquantitetively by considering the growth of a crack of mean length, which depends on both the density and the size distribution of microcrecks. The results for linking indicated that once the mean creak length reaches s critical size (approximately 0.1 of the macrocreak length) there is a very high probability that s macrocreak has formed. The pore-induced acceleration of macrocreak growth can be understood on a qualitative basis by the auperposition of (a) intrinsic, fully plastic crack growth and (b) localized, pore-induced shear instability fracture. BfmeCt o f " compressive hold t i m e on fatigue life and creep-fatigued age in t h e 80014 alloy at 750 °C. Mu, Z., Bothe, K. and Gerold, V. Scr. Metall. Mater. Nov. 1990 24, (11), 2145-2150 The effect of compressive hold time on fatigue life and internal damage for a comparison with creep results was studied using etrese-controlled fatigue tests with a constant plastic strain of 1% applied on low-carbon (0.07%) alloy 800H (31Ni-20Cr-0.3Ti-0.3AI-0.7Mn-0.2Co) at 750=C. Internal cavity damage at grain boundaries occurred for asymmetric fatigue tests and creep tests, but not in symmetric test cycles when cavity shrinkage was noted during the compressive cycle. The degree of fatigue damage is nearly twice that obtained by the creep test at a similar tensile stress level. An interaction of internal damage with the advancing fatigue cracks reduces the cycle lifetime. The fast compressive part of a fatigue cycle produces large athermel plastic compressive deformation and stress, which lead to nucleation of cavities via decohseion at carbide interfaces. Graphs, photomicrographs. 13 refs. Relationship b e t w e e n t h e propagation o f fatigue cracks end the behnviour of plastic f l o w under roBing/sliding c o u t a c t s ~ e f f e c t of slip ratio. Chiou, Y.-C and Hwang, J.-R. Tribol. Int. Apr. 1991 24, (2), 101-107 The relationship between the propagation of fatigue creaks and the beheviour of plastic flow in the surface layer was experimentally investigated under lubricated rolling-sliding contact. The test specimen material was medium carbon steel ($45C) with hardness Hv 187 after annealing. Results show that the displacement of plastic flow in the surface layer of the follower increased with increasing number of revo:utions. Crack propagation occurred in the surface layer of the follower when the displacement of plastic flow on the centre of the contact reached a saturated value. On the contact surface of the follower, the propagating angle of the fatigue crack was found to be correlated with the angle of plastic flow, which was independent of the slip ratio. The rate of plastic flow increased with increasing slip ratio, which resulted in a reduction of fatigue life. The displacement of plastic flow beneath the contact surface could be evaluated from the profile change of the plastic flow on the contact surface. Graphs, photomicrographs. 14 refs. H o w t o increase t h e life of moulds t h a t w o r k at high t e m p e r a t u r e end delay the appearance o f t h e r m a l _fj~.. us. Cugat, M. Deform. Metal. 1990 16, (Special Edit,on 2), 19-24 (in Spanish) The causes of thermal fatigue cracking are related to the various factors that can follow it and the problems that are likely to follow them. Various methods of thermal treatment end cooling are given, largely in graphical form. The effects of such methods are then related, and the effects of the Temple method on microstructure and Charw impact strength are given. In order to avoid the appearance of thermal fatigue it is vital that indust W has adequate equipment and knowledge tO combat these difficulties. The problems are complex and the best microstructures must be obtained, including a complete transformation of austenite into martensite. 14 refs. influence o f c o m p o s i t i o n and dispersoid on fatigue fracture behaviour o f A I - M g - S I eUoy~ Jiang, D., Hong, B. and Lei, T. Acta Matall. Sin. (Chine) Oct. 199026, (5), A388-A390 (in Chinese) A study was made on the fatigue beheviour under different ageing conditions of two AI-Mg-Si alloys with different chemical compositions and dispersoid contents.
505
Micromachenlsms o f quasi-static and fatigue crack g r o w t h in t i t a n i u m aluminidac. Aswath, P.B. Diss. Abst. Int. Feb. 1981 51, (8), 217 pp The rapid development of advanced aerospace technology has placed an increasing demand on the development of a new generation of structural materials for aircraft engines. Intarmetantc materials with low densities and Improved fracture toughness at elevated temperature show promise for replacing widely used Ni-beeed superalloys. TiAI and TiaAI, which are ordered intermetallic compounds of "13and AI, have been the subject of increasing research activity in the recent pest because of their light weight and high strength. An understanding of the resistance to quasi-static end cyclic-load fracture in these intarmatallica at both ambient and elevated temperatures is of paramount importance for damage-tolerant, fatigue-critical structural applications for which the intermatallice are candidate materiels. This thesis explores the effects of microstructure, microstructurel stability, mechanical variables (such as load ratio and fatigue frequency), and temperature on the quasistatic and fatigue crack growth characteristics of Ti-AI end Ti-AI-Nb alloys. With the aid of controlled heat treatments, duplex microstructures comprising either a2 end ~ or ¢12 and 3' phases as well as predominantly cz2 phase microstructurse ere produced. The fatigue crack growth charentedatice are monitored over s range of &K values from threshold to final failure. The effects of controlled microstructures on the overall fatigue end fracture rseistance are examined in the light of TEM observations of creck tip damage, scanning Auger spectroscopy of fracture surface oxidation and scanning electron frectogrephy. The atabillty of thsee microstructures is examined in the light of their response to beth fatigue crack growth end fracture toughnmm. Mic~ostructurel Matures that lead to a betterment of fatigue crack growth end fracture toughness are discussed in the context of the present experimental results.
A new method of faUgue pre=raddng for brittle matm~ls. Zhang, J. Trans. Met. Heat Treat. (China) Mar. 1990 11, (1), 70-74 (in Chinese) A new method of fatigue precrecking for bdttle materials and the corresponding loading device are described. Using this method, it is realized that the stress intensity factor/(max at the crack tip decreases automatically with the increase of crack length. The instability of the crack can be controlled and the crack length present can be obtained by propedy selecting the loading parameters. There are some advantages for the method in that the shape of the sample is simple, the final length of fatigue precrecking can be chosen and can be limited automatically, unstable cracking cannot be generated and the finishing line of the crack is quite clear. The loading device is easy to operate and reliable. This new method opens up a new means of fatigue precrecking for brittle materiels (e.g., the steels TeA, W18Cr4V and AI203). Graphs, photomicrographs. 6 refs. ' 8 u p e r d e m t ' 3,r=dMiCrMoV t u d d n e r o t o r steel: • status report. II. Machan-
k:al propecUes. McNaughton, W.P., Richman, R.H. and Jaffee, R.I. J. Mater. Eng. Mar. 1991 13, (1), 19-28
Recent international research has resulted in the development of superclsen 3.5NiCrMoV steel for low-pressure turbine rotors. Such steel is highly resistant to tamper embrittlement and will thus facilitate increased efficiency in electricity generation through the use of higher operating temperatures and improvements in design. Steels with impurity contents typical of the supercleen specification can be manufactured for production rotors with properties that equal or exceed those for conventional 3.6NiCrMoV rotors in every detail. Of particular interest are the observations that the superclsen steels appear to be virtually immune to temper embrittlement and may be used up to their creep-limited temperature of 500 °C (932 °F). The mechanical properties of superclean 3.SNiCrMoV steel are reviewed: tamper embrittiement susceptibility, uniformity of strength, impact properties, creep rupture properties, fatigue reeistence, fracture toughness and stress corrosion cracking susceptibility. Graphs. 28 refs. Biaxiel fatigue tests on t h i n - w a l l e d tubes at 950°C.
Hanswillemenke, H., Breitling, H. and Dietz, W. Closed Loop 1990 17, (3), 11-18
The g r o w t h o f short fatigue cracks under compressive e n d / o r tensile cyclic loading. McEvily, A.J. and Yang, Z. Metall. Trans. A M a y 1991 22A, (5), 1079-1082 Under cyclic compressive loading, fatigue cracks can initiate and propagate a short distance before being arrested. Recent studies have shown that the process of crack closure is important in the arrest process. An analytical means for dealing with this type of crack propagation in Fe is presented. Calculations based on this method of analysis agree well with experimental results. The method is general in nature and can also deal with short-crack propagation under tension-compression and tension-tension types of loading. Graphs. 17 refs. Thermomachanicel fatigue of a lead alloy. Lawson, L., Fine, M.E. and
Jeannotte, D. Metall. Trans. A M a y 1991 22A, (5), 1059-1070
The effects of superimposing a thermal cycle onto a strain-controlled fatigue cycle have been studied for 96.SPb-3.5Sn. These tests have been conducted at temperatures near half the melting point under conditions of cycling that included strain ranges from 0.3-3% and periods between 184 and 1040 s/cycle. Cracks were seen to initiate rapidly as grooves along grain boundaries and grow inwards until a drop in the Ioedcarrying capacity of the sample was observed when a significant fraction of the crack population exceeded approximately 300 lam in depth. The presence of an in-phase thermal cycle was seen to reduce the number of cycles to failure substantially relative to a reference isothermal test at the highest temperature of the thermomschanical cycle. For a given set of test conditions, the number of cycles to failure increased with the angle of phase lag of the mechanical cycle with respect to the thermal cycle. Decreasing frequency wee found to increase the number of cycles to failure in thermomechanical fatigue (TMF), while the opposite was observed for the isothermal reference tests. The hold time was found to reduce the number of cycles to failure in TMF. Graphs, photomicrographs. 26 refs. M.rrJq~ertsyatems f o r materials-related problems. Hoffelner, W. and Vifins,
Proc. High Temperature Materials for Power Engineering 1990, II, Liege, Belgium, (Kluwer, Dordrecht, The Netherlands, 1990) pp. 1599-1626 Expert systems are discussed as a tool for the solution of materiels-relsted problems. The basic elements and the architecture of such systems ere briefly introduced. Knowledge representation in materials-related expert systems is dealt with in more detail. Specific emphasis is given to the concept of frames, which is particularly useful for handling knowledge about materials. A knowledge base for the solution of meterials-related problems necessarily contains a variety of different types of material data: material properties, microstructurel information, uncertainties, lifeprediction models, etc. The introduction of these items into the knowledge base is elaborated on. The knowledge of experts must be cast into • structured and tight framework, which is sometimes a problem. Examples from MATEXPERT, an expert system for the solution of materials (low-alloy steels) problems in gas turbines (accept/reject decisions for materials deviating from standard, judgement about conditions of compressor blades) are taken to illustrate how expert systems can be employed. Other materials-releted expert systems (e.g., the intelligent processing of materials, material selection, life time prediction, etc.) are described. Current limitations and aspects for future applications are discussed. Graphs, photomicrographs. 34 refs.
T o w a r d s a probabiUstic d a m a g e accumulation model f o r high-strength
components. Bruckner-Foit, A., Jackels, H. and Quadfaeal, U. Proc. High Temperature Materials for Power Engineering 1990, II, Liege, Belgium (Kluwer, Dordrecht, The Netherlands, 1990) pp. 977-986 The lifetime distribution of high-strength components subjected to fatigue loading depends on the probability that a crack is initiated from an inclusion and then propagates stably until it exceeds the critical crack size and causes catastrophic failure. An analysis of crack growth data obtained from fatigue tests with smooth specimens is given, where the crack growth rate is expressed as a function of an effective stress intensity factor. A comparison of the size of the fraction initiating defect and the theoretical initial crack size indicates that there is virtually no initiation phase and that the cyclic crack growth is accelerated for small cracks. (The test material was PM Udimet 700). Graphs. 11 refs.
Meurer, H.,
Both uni- and bi-axial fatigue tests on the Ni-besed alloy NiCr23Co12Mo (similar to Inconel 617) were carried out at 950°C, 1.8 bar in helium and strains between 0.3-1.0% using an MTS computer-controlled servohydreulic test system, which provided independent loading in the axial and torsional directions. The loading methods, test chamber and specimen geometry are detailed including an induction heating system for the specimen with measurements of the temperature distribution in the thin-walled tubular specimens. The fatigue results at 950 °C show no strain hardening and can be described by the van Mices equivalent strain hypothesis. 7 refs. Metallurgical control o f fatigue crack propagation in superelloys. Chang,
K.-M., Henry, M.F. and Benz, M.G. J. Met. Dec. 1990 42, (12), 29-35
The low-cycle fatigue lives of turbine engine disc alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, end a new generation of high-strength Ni-besed superalloys with fatigue crack propagation in high-strength =uperalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the microstructure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high-~' content superalloys, is to modify the grain
504
boundary structure by means of a controlled cooling from a supersolvus solution. The precipitation reaction occurring on the grain boundaries during cooling generates • serrated structure that exhibits a good stress oxidation resistance for fatigue cracking. Graphs, photomicrographs. 34 refs.
The role o f w e l d i n g in c o m p o n e n t integrity. Cerjak, H.H.
Proc. High Temperature Materials for Powder Engineering 1990, I, Liege, Belgium (Kluwer, Dordrecht, The Netherlands, 1990) pp. 445-472
The role of welding in steam turbine fabrication and operation is described. The basic influence of welding, that is fabrication welding of castings end butt welding of similar and dissimilar materials, on the operational beheviour of steam turbine components is discussed. The actual results, obtained within the COST 505 welding group on the HAZ baheviour of 1% CrMoV castings, manufactured in accordance with the DIN and ASTM specifications for stress relief cracking beheviour, toughness, creep and LCF behaviour are reported. Results on similar welds of 1% CrMoV steels regarding mechanical properties, toughness creep and LCF beheviour and on dissimilar welds of 1% CrMoV to 12% CrMoV steels using 12% CrMoV- and Nibased filler metals are discussed. The optimization of the welding procedures end PWHT parameters was the main goal of the investigations. The results of mathematical modelling support the experimentally determined data. Graphs. 37 refs. Discussion about surface crack g r o w t h criteria under cyclic loading.
Varfolomeyev, I.V. and Vainshtok, V.A. int. J. Frect. Nov. 1990 46, (2), R25-R33
The growth of surface cracks in pressure vessel steels (0.2% yield strengths, tensile strength 584 MPa and ultimate tensile strength of 700 MPa) wee studied with twopoint bending specimens. The initial cracks were made by electrosparks. The stresses were 0.38 to 0.90 of the yield strength. The number of cycles to failure was between
Int J Fatigue November 1991
3000 and 10000. The elastic fracture &K (effective) is the best parameter to predict how structures will behave under cyclic loading with surfeae defects. The steel mentioned well 15Kh2MFA. Photomicrographs, graphs. 17 refe. " f l l m e - d h m w 4 k ) l t ~ etrmm i n t m e l t y f a c t o r analysis o f a surlaco crack in s hlgh-q)ued busting. Ballarini, R. and Hsu, Y. Int. J. Fract. Nov. 1990 441, (2), 141-153 The boundary element method is applied to calculate the stress intensity factors of a surfece crack in the rotating inner raceway of a high-speed roller bearing. The three-dlmenelonel model consists of an axially atresoed surfece-creaked plate sob~ectod to e moving Hertzicn contact loading. A muitldomeln formulation and singular crock-tip elements were employed to calculate the stress intensity factors accurately and effldently for a wide range of configuration parameters. The results can provide the basis for creak growth calculations and fatigue life predictions of the high-performance rolling element bearings that are used in aircraft engines. Graphs. 13 refe. t e m lN.Waturo and sbll.n rote s u p p o s i t i o n =.re
®...
properties.of Iow...~e
,. =.
Acta Matall. Mater. Apr. 1991 39, (4), 619-626 The date from other investigators are shown to follow a general suporposition property of the Iogsp against IogNf curves generated at ell levels of temperature end atreln rate during isothermal strain-controlled fatigue tests, This property is found in consequence of en energy criterion for fatigue failure, where the material (e.g., low-carbon steel, austenitlc stainless steels) ia described by a mechanical constitutive equation for creep controlled by the thermally assisted motion of jogged screw dislocations. The scaling relationship is shown to be dependent on the ratio between the self-diffusion energy and the energy to form a jog. Graphs. 20 refs. Effect o f surfece roughness on t h e life of continuous cycling w i t h o u t ~ w i t a t i o n d a m a g e in 1 2 C r - M o - V steel. Soo, C.L. and See, W.N. Scr. MetalL Mater. Oct. 1990 24, (10), 1971-1974 Mertensitic m i n i m steel (12% Cr) is used for steam turbine rotors in place of 1Cr-Mo-V steel because of its excellent high-temperature properties. Most of the low-cycle fatigue (LCF) life in t2Cr-Mo-V steel at 298 and 873 K ie spent in the crack propagation proos~. The LCF life in 12Cr-Mo-V steel ia rarely influenced by the surface roughness, regardless of the test temperatures. The fatigue cracks are nucleated on prior austenife grain boundaries and the crack initetion of 12Cr-Mo-V steel takes place early during the LCF deformation for apecimens with both smooth and rough surfaces. 12Cr-Mo-V steel containing 0.17C, 11.2Cr, 0.98Mo, 0.28V, 0.63Me and O.6Nt was used for the fatigue test. The heat treatment prior to tasting consisted of auatenitizing at 1323 K for 22 h, oil quenching, tempering at 933 K for 44 h and air cooling. Before LCF testing, two different types of surface roughness were obtained by mechanically polishing specimens perpendicular to the stress axis, using two different emery papers, #80 and #1200. Total-strain-controlled unlexiel fatigue tests were carried out in the air by an electroservohydraulicelly driven closed-loop INSTRON model 1350 machine. A radiant furnace was used to heat the specimen, and the test temperatures were controlled so as to be maintained at 298 and 873 K, respectively. Graphs, photomicrographs. 10 refs.
An investigation of t h e Influence o f h i g h - l o w - end l o w - h i g h - a m p l i t u d e M o c k luedlnge on f l t l g u e crock g r o w t h . Kumar, R. Eng. Fract. Mech. 1991 38, (2-3), 225-230
Crack propagation experiments were conducted on 6061-T6 AI alloy for high-low and low-high block loads for various combinations of overload ratios (0.0-0.1, 0.0-0,2, 0.0-0.3, 0.0-0.5, 0.2-0.0 and 0.3-0.0). On the basis of these experiments one power law is developed for the high-low case to predict the delay period at a particular block load ratio. The delay period after application of the low-amplitude block ioed is increased as the magnitude of the low block load is decreased. In the low-high sequence, an acceleration is observed in the crack growth rate. For the high-low load sequence, a retardation was observed in the crack growth rate. Graphs. 12 rofs.
Piping d y n a m i c r e l i a b i l i t y and code rule change recommendutiona.
Tagart, S.W., Jr, Tang, Y.K., Guzy, D.J. and Ranganath, S. Nucl. Eng. Design Oct. (11) 1990 123, ( 2 - 3 ) 373-385
The conservative nuclear piping design criteria for seismic and dynamic loads have led to piping systems with excessive numbers of snubbers. To improve this undeeirebie situation, a piping and fitting dynamic reliability program was initiated by the Electric Power Research Institute (EPRI) in 1985 with cooperation from the US Nuclear Regulatory Commission (NRC). The objective is to develop improved, realistic and defensible ASME design rules by taking advantage of the inherent dynamic margins in the nuclear piping system. The research results have demonstreted that piping systems have a large reserve dynamic capacity and the dynamic failure mode arises from fatigue or fatigue-retchating rather than plastic collapse. Based on such physical evidence, a set of code rule change recommendations is suggested, in preliminary form. The piping materials discussed are the steels A106B and 316L. Graphs. 5 refs. Mechanics and mechanisms of c r e e p - f a t i g u e crack g r o w t h in C u -
1 wt.% Sb. Gieseke, B.G. Diss. Abstr. Int. Jan. 1991 51, (7), 191 pp
The mechanisms and mechenica of fatigue crack growth in a model, of Cu-lSb alloy, were investigated at a temperature of 400 °C. Fatigue testa were conducted in ultra-high-purity nitrOgen using triangular and trapezoidal loading wsveforms and loading frequencies ranging from 2000 to 1 cycles/h. Two time-dependent crack growth mechanisms operated, each of which occurred as the result of creep deformation. In triangular waveforms, crack growth occurred by the nucleation, growth and coalescence of grain boundaw creep cavities with the advancing crack front. In one trapezoidal weveform with a 90 s hold time, crack growth occurred by both cavity coalescence and by the diffusive separation of grain boundaries (or peeling). For longer hold times, crack extension appeared to occur by both mechanisms simultaneously. To account for creep deformation at the crack tip, the concepts of time-dependent fracture mechanics were applied to the crack growth date generated using trapezoidal loading weveforms. A unique correlation was
Int J Fatigue November 1991
sought between the average rate of crack growth dudng the hold times, (daldt)=vg, end the creak-tip parameters, K and Ca. Since Ct varied within each cycle, it was necessary to USe average values of the parameter C~ (Ct)w0. These values were estimated using an analytical expression for Ct end, in SOme cases, were measured. A unique correlation wee obtained between (daldt)wg end the measured values of (Ct)avgfor hold times of 0.5 and 1 h. These data alSO compared well with the creep crack growth rate data for this alloy. However, because of discrepancies between the measured end the analytically estimated values of (Ct)wg, such a correlation could not be established between (da/dt)~ and analytical estimates of (Ct)~. The data for Cu-Sb were combined with data for steels from other studies to assess the ability of (Ct)~ to correlate the hold-time crack growth rates. For one Cr-Mo steel, the analytical estimates and measured values differed, while these compared well for one Cr-Mo-V steel. The discrepancies between the estimated and measured values of (Ct).vg were attributed to the use of an analytical expression for (Ct)=v0 that ignored the analytical expression for (Ct)~ that ignored the effects of instantaneous crack-tip plasticity. The influence o f p o r o s i t y on low-cycle fatigue. Gerard, D.A. Diss. Abstr. Int. Jan. 1991 51, (7), 180 pp The influence of porosity on low-cycle fatigue has been examined using powderprocessed Ti as a model material. The presence of even low levels of porosity (0.4%) effects the cyclic flow beheviour and is very detrimental to low-cycle fatigue life. An analysis has been performed by considering low-cycle fatigue in terms of the following stages: (1)the initiation of 15 lain microcreaks, (2)the propagation of microcreaks, (3) the linking process to form a macrocreak, and (4) the propagation of a maorocreak to failure. All except the microcreak growth stage are significantly accelerated by isolated porosity and even the growth of microcrecks occurs much more rapidly in materials containing 6% interconnected Porosity. Microcreak initiation is accelerated the most by porosity under ell the conditions examined. Microcreck linking and mearocreak growth are both accelerated in the presence of porosity, the degree of acceleration becoming more severe with increasing porosity level and strain amplitude, and when the porosity is interconnected. The above experimental observations have been modelled both quantitatively and qualitatively. Modelling pores at a free surface as through-thicknees holes in a plastically deforming matrix and utilizing a Coffin-Manson failure criterion on a local scale, the number of cycles required to initiate a microcreak have been predicted theoretically. The results of these predictions agree very well with the expodmentel observations of crack initiation in the porous Ti specimens. The linking of short cracks has been addressed semiquantitetively by considering the growth of a crack of mean length, which depends on both the density and the size distribution of microcrecks. The results for linking indicated that once the mean creak length reaches s critical size (approximately 0.1 of the macrocreak length) there is a very high probability that s macrocreak has formed. The pore-induced acceleration of macrocreak growth can be understood on a qualitative basis by the auperposition of (a) intrinsic, fully plastic crack growth and (b) localized, pore-induced shear instability fracture. BfmeCt o f " compressive hold t i m e on fatigue life and creep-fatigued age in t h e 80014 alloy at 750 °C. Mu, Z., Bothe, K. and Gerold, V. Scr. Metall. Mater. Nov. 1990 24, (11), 2145-2150 The effect of compressive hold time on fatigue life and internal damage for a comparison with creep results was studied using etrese-controlled fatigue tests with a constant plastic strain of 1% applied on low-carbon (0.07%) alloy 800H (31Ni-20Cr-0.3Ti-0.3AI-0.7Mn-0.2Co) at 750=C. Internal cavity damage at grain boundaries occurred for asymmetric fatigue tests and creep tests, but not in symmetric test cycles when cavity shrinkage was noted during the compressive cycle. The degree of fatigue damage is nearly twice that obtained by the creep test at a similar tensile stress level. An interaction of internal damage with the advancing fatigue cracks reduces the cycle lifetime. The fast compressive part of a fatigue cycle produces large athermel plastic compressive deformation and stress, which lead to nucleation of cavities via decohseion at carbide interfaces. Graphs, photomicrographs. 13 refs. Relationship b e t w e e n t h e propagation o f fatigue cracks end the behnviour of plastic f l o w under roBing/sliding c o u t a c t s ~ e f f e c t of slip ratio. Chiou, Y.-C and Hwang, J.-R. Tribol. Int. Apr. 1991 24, (2), 101-107 The relationship between the propagation of fatigue creaks and the beheviour of plastic flow in the surface layer was experimentally investigated under lubricated rolling-sliding contact. The test specimen material was medium carbon steel ($45C) with hardness Hv 187 after annealing. Results show that the displacement of plastic flow in the surface layer of the follower increased with increasing number of revo:utions. Crack propagation occurred in the surface layer of the follower when the displacement of plastic flow on the centre of the contact reached a saturated value. On the contact surface of the follower, the propagating angle of the fatigue crack was found to be correlated with the angle of plastic flow, which was independent of the slip ratio. The rate of plastic flow increased with increasing slip ratio, which resulted in a reduction of fatigue life. The displacement of plastic flow beneath the contact surface could be evaluated from the profile change of the plastic flow on the contact surface. Graphs, photomicrographs. 14 refs. H o w t o increase t h e life of moulds t h a t w o r k at high t e m p e r a t u r e end delay the appearance o f t h e r m a l _fj~.. us. Cugat, M. Deform. Metal. 1990 16, (Special Edit,on 2), 19-24 (in Spanish) The causes of thermal fatigue cracking are related to the various factors that can follow it and the problems that are likely to follow them. Various methods of thermal treatment end cooling are given, largely in graphical form. The effects of such methods are then related, and the effects of the Temple method on microstructure and Charw impact strength are given. In order to avoid the appearance of thermal fatigue it is vital that indust W has adequate equipment and knowledge tO combat these difficulties. The problems are complex and the best microstructures must be obtained, including a complete transformation of austenite into martensite. 14 refs. influence o f c o m p o s i t i o n and dispersoid on fatigue fracture behaviour o f A I - M g - S I eUoy~ Jiang, D., Hong, B. and Lei, T. Acta Matall. Sin. (Chine) Oct. 199026, (5), A388-A390 (in Chinese) A study was made on the fatigue beheviour under different ageing conditions of two AI-Mg-Si alloys with different chemical compositions and dispersoid contents.
505