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Numerical simulation is more and more used in automotive industry to reduce design time and cost. Moreover, for high performance engines, the development of cylinder heads requires well known and adapted materials to ensure their reliability. The estimation of adapted fatigue criteria based on few experiments is thus an important challenge: to obtain more predictive model on more complex mechanical phenomena. This paper describes the methodology adopted by Renault to estimate fatigue life of a new aluminum-silicon alloy. A low copper aluminum-silicon alloy has been developed for a new generation of cylinder heads. Those kinds of alloys provide a good compromise between the two main failure modes encountered for cylinder heads: high cycle fatigue of water jacket and low cycle fatigue of fire deck. However, those materials are subjected to thermal ageing that can affect mechanical behavior, such as yield stress and hardening, but also fatigue mechanisms at higher temperatures. This article presents first the selected constitutive equations that introduce thermal ageing effect in a cyclic elasto-viscoplastic model. The low cycle fatigue criterion is then calibrated on the relevant database, which includes isothermal and non-isothermal tests. The fatigue criterion is a specific version of a classical model previously developed at Onera. The critical variables for the fatigue part are the stress amplitude and the mean stress, both of them being normalized by an ultimate stress which depends on temperature and ageing state. Finally, the constitutive and damage models have been applied on 3D cylinder head numerical analysis. The paper proposes a brief comparison of those results with more classical simulations made on cylinder heads and highlights the perspectives offered by this new aluminum-silicon alloy. [ABSTRACT FROM AUTHOR]

Dynamic strain ageing (DSA) behavior of modified 9Cr-1Mo steel under uniaxial tensile and constant strain cyclic loading is presented in this paper. Tensile tests are carried out at different strain rates from 10 -3 to 10 -5 s -1 in the temperature range from room temperature to 600 °C. DSA has been established in the temperature range between 250 and 400 °C in terms of peak in tensile strength, minima in ductility, serrated plastic flow and negative strain rate sensitivity. In the region of DSA dislocation substructure revealed high density of dislocations and features like kinks and bowing of dislocations are observed. Low cycle fatigue behavior was studied at 300 °C, over a wide range of strain amplitudes at different strain rates. An inverse effect of strain rate was observed on cyclic stress response and fatigue life at 300 °C due to DSA. [ABSTRACT FROM AUTHOR]

304L austenitic stainless steel is notably used to manufacture pipes in primary cooling circuits of nuclear power plants operating at a temperature close to 300°C in a specific aqueous environment. Customarily, pipes are roughly polished, however, after some interventions, a few zones might be ground. Start-ups, shutdowns and operating transients of the reactor produce Low Cycle Fatigue (LCF) loadings with different strain rates. These solicitations and the modified surface finish create specific conditions whose effects must be taken into account. In order to understand if there is a joint influence of the PWR environment and surface roughness on the successive stages of fatigue damage leading to failure, fatigue tests were conducted at 300°C under a total strain amplitude of ± 0.6%. Two environments were used, laboratory air and simulated PWR primary aqueous environment. Two surface finishes were investigated, namely fine mechanical polishing and grinding (Rt of about 40μm). The analysis of the damage process is based on data concerning crack density, crack length and striation spacing. The integration of the relation between the striation spacing and the strain intensity factor ΔK ɛ , provides an estimation of crack propagation. It is confirmed that in PWR environment one striation can be considered as the crack advance during one fatigue cycle. Moreover, the specific crack front geometry observed in ground specimens and resulting from successive microcrack coalescences explains the reduction in fatigue life induced by grinding. [ABSTRACT FROM AUTHOR]

Low cycle fatigue behavior of single phase polycrystalline aluminium and copper has been investigated under completely reversed strain-control condition. In both cases two different grain sizes have been considered. It is found that over the range of strain amplitude of 0.20 to 1.0 pct the cyclic life is low in case of coarse grained aluminium as compared to fine grained condition. However, the cyclic stress response of coarse grained aluminium is completely different from that of fine grained condition. In contrast to fine grained aluminium cyclic hardening of coarse grained aluminium is found to occur in different stages for all strain amplitudes. The initial hardening is first stabilized and then increases by substantial amount. On the other hand, although the nature of variation of cyclic stress response with number of cycles is almost similar for both fine and coarse grained copper, the magnitude of hardening all through the cyclic life is more in case of coarse grained copper. This is attributed to higher twin density in case of coarse grained copper. However, the cyclic life in coarse grained copper is less as compared to fine grained copper. Irrespective of cyclic hardening behavior and stacking fault energy it is found that cyclic life is more in case of fine grained materials. [ABSTRACT FROM AUTHOR]

Each type of testing machine has its own characteristics, but most of multiaxial fatigue studies have been performed by a push-pull and reversed torsion loading testing machine. This type of testing machine can perform multiaxial fatigue tests under non-proportional loading in which principal directions of stress and strain are changed in a cycle. However, a principal strain ratio range performable in this testing method is −1≤ ϕ ≤−ν, where ϕ and ν are the principal strain ratio and the Poisson's ratio. This study discusses about stress and strain states and multiaxial fatigue testing method. In order to perform fatigue test in −1≤ϕ≤+1 under non-proportional loading, a multiaxial fatigue testing machine of combining the push-pull and reversed torsion loading and cyclic inner and outer pressures is also developed. Then, this paper presents experimental results under several multiaxial loadings carried by the testing machine, too. [ABSTRACT FROM AUTHOR]

Nowadays high strength steels are gained increasing popularity for use in steel constructions and machinery components. Their fatigue properties, however, are not yet well characterized, especially in comparison with widespread conventional structural steel S355J2. In this study were conducted the assessment of use HSS steel in extremely loaded components with existing plastic strains. In order to obtain the necessary characteristics and fatigue properties, materials were tested under LCF strain-based mode with strain ratio R ɛ = 0,1 and strain rate of 10 -2 1/s. Because of its higher strength, S960QL steel exhibits a slightly higher fatigue resistant under the total strain amplitude lower than 0,5% compared to mild steel and similar under higher strains. Obtained results show that investigated HSS steel can successfully supersede presently used conventional mild steel in heavily loaded components simultaneously decreasing dead weight of the structure. [ABSTRACT FROM AUTHOR]

The article presents the research results of a D16ChATV aluminum alloy in preliminary combined loading conditions. Examination of samples was carried out by imposing additional force impulse at the monotonic tension at the stage of elastic deformation. The material modified in this way, was also subjected to fatigue testing in a low cycle fatigue loading conditions. Loading applied during the tests was oscillating sinusoidal, with the strain ratio of R=-0.2 and controlled strain amplitude ɛ ac =const. As a result of studies it was found, that in preliminary combined loading conditions in the test samples a processes occur related to the rapid dissipation of energy, leading to the modification of the mechanical properties of the test material. During imposing of the additional force impulse a single pulsed loading are formed the plastic zones of varying scope in the specimen made from aluminum alloy. The strength of D16ChATV aluminum alloy under a low cycle fatigue loading of the preliminary combined loading is characterized by a small increase or remains at basic level. [ABSTRACT FROM AUTHOR]

In this work, the possibility using two different technologies to repair industrial flat components made of Waspaloy superalloy was investigated. Specimens present a V notch in the central zone of the gage length, which has been refilled by MicroPlasma and Electro Spark Deposition in order to recover the original thickness of the material. These specimens have been used for a complete mechanical characterization, carrying out static, low-cycle fatigue and creep tests. Finally, results have been compared to base material. Static and low-cycle fatigue test have been carried out at Room Temperature and 538 °C, while creep test considered the temperature of 704 °C. Results of uniaxial tensile tests showed that the two repair process have a different effect on the mechanical properties. While MicroPlasma produced a reduced yield stress and tensile strength but a good performance whit respect to the elongation to failure, Electro Spark Deposition assured a better mechanical strength but a reduced elongation to failure. Low-cycle fatigue properties have been determined carrying out tests at different temperature (Room Temperature and 538 °C). Repaired material showed lower fatigue strength and an increase of the data scatter, especially ESD at 538 °C. Finally, creep test carried out on a limited number of specimens allowed establishing some changes about the creep rate and time to failure. MP behaviour was more similar to base material, while ESD show the presence of a marked tertiary creep. [ABSTRACT FROM AUTHOR]

Within the scope of the investigation of welded high-strength steels for application in crane structures, a Gaussian-like test spectrum is derived from an analysis of recorded load time histories. In addition to stress-controlled fatigue tests under constant amplitude loading, the test spectrum is used for the experimental investigation of MAG-welded butt joints and tubular sample components under variable amplitude loading. A linear damage accumulation using Palmgren-Miner-Elementary is conservative for a damage sum of D = 0.5. Application of the theoretical damage sum D th = 1 results in a closer approximation of the Gaßner-curve. For further improvement of this approximation, a rotation of the calculated Gaßner-curve, i.e. a variable damage sum, is suggested for both butt joints and sample components. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the low-cycle fatigue (LCF) behaviour of API 5L X65 pipeline steel was investigated under fully-reversed strain- controlled conditions at room temperature. The companion test method was used to obtain the cyclic stress-strain response curve with strain amplitudes ranging between 0.3% and 1.2%. Results showed that the steel undergoes both cyclic hardening and softening behaviour depending on the strain amplitude. At relatively lower strain amplitudes (0.3 - 0.8%), only cyclic softening was observed whereas at higher strain amplitudes, softening behaviour was preceded by significant hardening in the first few cycles. Based on the cyclic half-life results, the LCF life decreased while stress amplitude and plastic strain amplitude increased with total strain amplitude. Furthermore, analysis of stabilised hysteresis loops showed that the steel exhibits non-Masing behaviour. Complimentary scanning electron microscopy examinations were also carried out on fracture surfaces to reveal dominant damage mechanisms during crack initiation, propagation and fracture. [Copyright &y& Elsevier]

Abstract: Austenitic stainless steel of type X6CrNiNb18-10 is a commonly used material for pipelines of power plants. The fatigue behavior of these components is often operationally determined by thermo-mechanical stresses. Welded joints are subjected to various geometries and micro structural inhomogeneities, which lead to complex stresses in the component. This is the reason for a reduced fatigue life, in contrast to material's life itself, is observed. A parametric finite element model is developed, to investigate the influence parameters. In order to take into account the non- uniform hardness distribution in the region of the weld seam, hardness measurements are the base for the consideration of a material mismatch in plastic behavior. The material parameters for the used Chaboche material model must be modified individually, depending on the mismatch. In addition, the consideration of imperfections of the welding geometry is an important factor, too. Using a local approach by using measured geometry-parameters of the weld seam, acceptable accordance between experimentally and numerically calculated fatigue lives can be gained. The observed phenomena in experimental investigations that fatigue failure occurs either at the base metal or at the weld toe (depending on the loading conditions) can be reproduced by numerical simulations. This knowledge is used to develop an approach for lifetime estimation. [Copyright &y& Elsevier]

Abstract: Transmission (TEM) and scanning electron microscopy (SEM) in combination with automated electron back-scattered diffraction (EBSD) techniques is used to study the surface damage as well as the initiation and propagation of short fatigue cracks in LDSS AL 2003 in two different thermal conditions. Whereas in the as received (AR) condition the initial deformation lines appear in the ferrite phase, in the heat treated (HT) condition the deformation lines appear in both phases at the same time. On the other hand, in both thermal conditions short cracks nucleate along intrusion/extrusions in the ferrite phase. Additionally, applying a numerical model it was found that the path of short cracks not only depends on the Schmid Factor but it is also conditioned by the angles of the crack-plane deflection at a grain boundary. [Copyright &y& Elsevier]

Abstract: Cyclic strain behaviour of metastable austenitic 301LN steel with different grain sizes – coarse (14μm) and ultrafine (1.4μm) grained – produced by the special thermo-mechanical treatment based on the martensite-to-austenite reversion was investigated. Low-cycle fatigue (LCF) tests were conducted on flat specimens at room temperature in fully push-pull mode under strain control with constant strain rate of 2×10–3 s–1 and constant total strain amplitude ranging from 0.4% to 0.9%. LCF behaviour was characterised by cyclic hardening/softening curves and fatigue life curves. After completion of fatigue tests surface fatigue damage was studied using scanning electron microscopy (SEM). Ferritoscope was adopted to reveal the destabilization of austenitic structure and evaluate the extent of deformation induced martensitic transformation. [Copyright &y& Elsevier]

Abstract: A subject of the authors’ research were the fragments of steel structures (of the puddled steel) coming from the turn of the 19th and 20th centuries. The material gained for the research comes from monumental (and still operated) Wroclaw bridges and the hall of the Main Wroclaw Railway Station. The works were focused on the phenomenon of microstructural degradation of those steels and its potential influence on their strength properties. Analysis of the achieved results indicated that those long operated steels are a subject for microstructure degradation processes consisting mainly in: precipitation of carbides and nitrides inside ferrite grains, precipitation of carbides at ferrite grain boundaries and degeneration of pearlite areas. The microstructure degradation processes negatively influence fatigue properties. The results of low cycle fatigue test for the puddled steel (from the Main Railway Station) in the after-operation state indicate for worse plastic properties of that steel in comparison to the normalised state. The results of fatigue crack growth test indicate for worsening of fatigue crack propagation resistance of the material depending on the degree of degradation processes intensity in the puddled steels. A new kinetic equation has been proposed for description of the fatigue of crack growth rate due to the Dimensional Analysis approach. From the utility point of view, the achieved results acquire fundamental importance because of the fact of continuous operation of a high number of the bridges aged more than 50 years (about 68% for Europe). It is worth noticing that almost 28% of them are objects aged over 100 years. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to give a better understanding of damage mechanisms that control lifetime of austenitic stainless steel nuclear components under cyclic loading. The acoustic emission signals were analyzed, in order to identify the acoustic signatures corresponding to a specific damage mode. An unsupervised classification method allows differentiating signals resulting from the plastic deformation or fatigue crack growth. Both phenomena are the two main sources of acoustic emissions in isotropic materials. The main results are the classification of acoustic signals by multivariate statistical methods in different classes. A relation can be established between each class and the present deformation mechanisms and damage, and their order of appearance according to the loading amplitude. [Copyright &y& Elsevier]

Abstract: This paper presents results of an experimental study carried out for developing a model to predict the low-cycle fatigue life of bolted connection. The test specimens are cantilever beams with one end fixed by a bolted connection and the other end free. Each cantilever beam is subjected to a mass at the free end. Different levels of cyclic loading are applied to the beam system by means of a vibration shaker. Moreover, a numerical analysis is performed to study the uncertainty of the experimental model and to predict the lifetime scatter of the bolted connection. Monte Carlo simulations are adopted to include uncertainties by generating samples of the different random variables. The effects of these variables on the fatigue life of the bolted connection are identified. [Copyright &y& Elsevier]

The fatigue design of nuclear components is based on an S-N curve established in air at 20°C. To this curve, different penalty factors, representative of various effects (scale effect, material variability, surface finish, environmental effect, etc), are then applied. This method considers that these effects are independent from each other. This paper is focused on the interactions between surface finish effects and environmental effects in the Low Cycle Fatigue (LCF) domain of an austenitic stainless steel type 304L. The aim of this work is to determine whether the effects of these two parameters are really independent or coupled. To better understand the environmental influence, high vacuum is used as reference. Tests were realized with a triangular waveform signal at a total strain amplitude of ± 0.6% in different environments: vacuum, air and simulated Pressurized Water Reactor (PWR) water, at 300°C. Two surface finishes were considered: ground and polished. Experimental results obtained on ground samples are here compared with results obtained on polished samples in a previous study. It appears that, while the cyclic stress response of the material is not affected by the surface finish, a ground surface finish decreases the fatigue life. Moreover, this surface finish effect seems to be less pronounced when the environmental effect is important, which indicates the presence of a coupling between surface finish influence and environmental effects. [ABSTRACT FROM AUTHOR]

Abstract: This contribution deals with the low cycle fatigue behaviour of butt-welded joints of austenitic stainless steel (Type 347). The influence of the geometrical notch of a butt weld on the fatigue life in LCF regime is of particular interest in this context. For the purpose of comparison different weld geometries – as-welded and machined – are investigated. Fatigue tests reveal that the weld notch does not always constitute the failure location in the LCF regime. Accompanying numerical simulations are based on 3D scans with a resolution of 30μm in order to receive accurate local strain amplitudes at the weld notch or rather the failure location. The combination of experiments and numerical simulations results in local strain life curves. [Copyright &y& Elsevier]

Abstract: The detection of small fatigue cracks is of particular interest because it is a key element of remaining life prediction. In the present study low cycle fatigue crack initiation in austenitic stainless steel (SUS316NG) subjected to plane bending is observed and compared the change of back-reflection intensity of ultrasonic wave using the scanning acoustic microscope. Ultrasonic data is recorded as a function of the number of cycles and compared with slip band length measured optically. Surface observation using optical and scanning acoustic microscopy revealed that with increasing the number of fatigue cycle the slip band density increased and the amplitude of back reflection intensity gradually decreased before the slip band length increases. Due to the cyclic loading, dislocation density along persistent slip bands increase. The attenuation of ultrasonic back reflection intensity is due to the dislocation damping and this mechanism was considered. [Copyright &y& Elsevier]

Abstract: Grade 91 steel is a heat treatable steel and hence its microstructure is very sensitive to temperature. The weld joint consisting of a heterogeneous microstructure exhibits a lower fatigue life than that of the base metal. This is due to the presence of soft zone in the heat affected zone (HAZ). The failure location in the weld joint is very sensitive to the test parameters such as temperature and strain amplitude and application of hold. At high temperatures and under hold application, strain localization occurs in the soft intercritical HAZ (ICHAZ) which results in the failure in the HAZ. Sub- surface creep cavity formation in the soft region and their linkage cause enhanced crack propagation and that translates into lower fatigue life of the weld joint at high temperatures. Occurrence of compression dwell sensitivity in the material is attributed to the presence of surface oxides. [Copyright &y& Elsevier]

Abstract: Low and high cycle fatigue behaviors of Alloy 690 have been investigated at temperatures up to 330°C and number of cycles up to 2×109. Two interesting phenomena were observed. At high temperature, the alloy shows a secondary strain hardening in the cyclic stress-strain response. Formation of nano-twins and interactions between moving dislocations and stacking faults or interstitial atoms could contribute to this secondary strain hardening. For very high cycle fatigue, subsurface fatigue crack initiation at grain boundaries has been observed. EBSD investigation shows that strain accumulation is much localized. The fatigue damage is a localised plasticity exhaustion process. [Copyright &y& Elsevier]

Abstract: Gas turbine engines are widely used in both Military and Civil Aircrafts. The power generated at the turbine is transmitted to compressor through Engine main shafts. These shafts are classified as Class-I critical components of the engine. During the flight operating conditions engine main shafts are subjected to complex loading conditions, such as Torsional, Centrifugal, Thermal, Gyroscopic etc… The combination of these loads lead to multimode failure mechanisms, such as Low cycle Fatigue (LCF), High cycle Fatigue (HCF) and Ductile overload failures in shafts. Designing of shafts for structural integrity is very critical in single engine aircrafts, as the failure of any one shaft may result in the failure of engine, which in turn may lead to the catastrophic failure of the entire aircraft. The complex geometry of the shafts calls for combination of design tools for stress and life estimation of these parts (both analytical and finite element method (FEM)) backed up by extensive material and component test programs. In the present paper structural analysis and life estimation is carried out on Low Pressure Spool and Spline Coupling under various engine operating conditions. The design analysis cycle consists of several phases, such as Heat Transfer Analysis, Structural Analysis, Optimization and Fatigue Lifing. Detailed modeling and stress analysis carried out to evaluate the strength of the splines is also presented. This paper highlights the methodology of design subjected to clearance of stipulated MIL 5007 D/E specifications to ensure the structural integrity. Based on the stress analysis, a Stress Test Schedule is prepared to carry-out the testing at shaft fatigue rig facility. In the testing facility, mechanical testing of the shaft assembly is carried out to establish strength and fatigue life of the shafts. Brief results on the material tests carried out and fatigue life testing of full scale component are also presented. [Copyright &y& Elsevier]

Abstract: This study discusses multiaxial low cycle fatigue damage of Mod.9Cr-1Mo steel under proportional and non-proportional loadings at room and high (823K) temperatures. Strain-controlled multiaxial low cycle fatigue tests were carried out using a hollow cylinder specimen including interruption tests. Strain paths employed were a push-pull straining, a reversed torsion straining and a circle straining. Behaviors of cyclic deformations and failure lives and an evaluation of the failure life are discussed. Surface cracks and microstructures in fatigued specimens are also observed by a digital microscope and a transmission electron microscope (TEM) in order to investigate mechanisms of cyclic deformation, failure and reduction in failure life due to non-proportional loading. [Copyright &y& Elsevier]

Abstract: Development of materials plays a crucial role in the economic feasibility of fast nuclear fission and fusion power plant. In order to meet this objective, one of the methods is to extend the fuel burnup and decreasing doubling time. The burnup is largely limited by the void swelling and creep resistances of the fuel cladding and wrapping materials. India's 500 MWe Prototype Fast Breeder Reactor (PFBR) is in advanced stage of construction. The major structural materials chosen for PFBR with MOX fuel are alloy D9 as fuel clad and wrapper material, 316LN austenitic stainless steel for reactor components and piping and modified 9Cr-1Mo steel for steam generator. In order to improve the burnup further, titanium, phosphorous and silicon contents in alloy D9 have been optimized for better swelling and creep resistances to develop modified version of alloy D9 as IFAC-1. Creep resistance of inherently void swelling resistance 9Cr-ferritic steel has been improved with the dispersion of nano-size yttria to develop oxide dispersion strengthened (ODS) steel clad tube with long- term creep strength, similar to D9, for increasing the fuel burnup. Development of modified 9Cr-1Mo steel clad tube and 9Cr-1Mo steel wrapper for future metallic fuel reactors being developed for reducing the doubling time are in progress. Extensive studies on resistance of this new generation core materials to void swelling are also under progress along with material development. Improved versions of 316LN stainless steel with nitrogen content of about 0.14 wt.% having higher creep strength to increase the life of fast reactor and modified 9Cr-1Mo steel with reduced nitrogen content and controlled addition of boron to improve type IV cracking resistance for steam generator are other developments. India's participation in ITER programme necessitates the development of India-specific RAFM steel for Test Blanket Module (TBM). A comprehensive research programme is being carried out to develop India-specific 9Cr-W-Ta RAFM steel with the optimization of tungsten and tantalum contents for better combination of strength and toughness. Based of the extensive mechanical tests including impact, tensile, creep and fatigue on four heats of RAFM steels having tungsten in the range 1 – 2 wt. % and tantalum in the range 0.06 -.014 wt., the RAFM steel having 1.4 wt. % tungsten with 0.06 wt. % tantalum is found to possess better combination of strength and toughness. This steel is considered as India-specific RAFM steel and TBM is being manufactured by this RAFM steel. To limit the emission of green house gases, a research and development programme has been initiated to develop advanced ultra super critical fossil fuel fired thermal power plants working at temperature of around 973 K and pressure of 300 bar. High temperature creep strength super 304H austenitic steel and Inconel 617 superalloy tubes are indigenously developed for this purpose. [Copyright &y& Elsevier]

The aim of the present paper is to study the effect of strain rate on the low cycle fatigue (LCF) behavior of indigenously developed 316LN stainless steel weld joint (0.14N wt. % base metal welded with 0.1wt. % N electrodes). Fully reversed total axial strain-controlled LCF tests were conducted at 823K using a servohydraulic machine, equipped with a resistance heating furnace. Tests were conducted in air at ±0.6% strain amplitude employing strain rates 3×10−3s−1, 3×10−4s−1 and 3×10−5s−1 to investigate the effect of strain rate in the dynamic strain aging regime (DSA). Initial brief hardening, negative strain rate stress response and lowering of fatigue life with decreasing strain rate have been correlated to DSA operating at 823K. The LCF behaviour of weld joints are correlated to the microstructural changes during fatigue cycling in order to explain the underlying deformation mechanism and fracture behaviour. [ABSTRACT FROM AUTHOR]

Cu-Cr-Zr-Ti alloy (Cr: 0.5-0.7, Zr: 0.02-0.05, Ti: 0.02-0.05, Fe: 0.015 and balance Cu) is a candidate material for the inner wall of the double-walled and regeneratively cooled combustion chamber of cryogenic rocket engine. Low cycle fatigue (LCF) behaviour of the alloy was investigated in the solution annealed (1253K for 10min. followed by air cooling) condition. Fully reversed, strain controlled LCF tests were performed in air at total strain amplitudes varying from±0.25% to±1.0%. Different temperatures over 300 to 900K range were employed for the tests. Additionally, a few tests with fully compressive strain cycling were performed, employing strain amplitudes ranging from -0.5 to -2.0% at a constant temperature of 900K. All tests were carried out using a fixed strain rate of 3×10−3 s−1. The stress response behaviour under fully reversed and mean compressive cycling conditions was observed to be similar for a given total strain range and was seen to be strongly dependent on the temperature of testing. Mean compressive cycling yielded marginally better lives compared to fully reversed cycling. A pronounced initial cyclic hardening followed by a regime of saturation characterised the behaviour at room temperature. However, in the temperature range of 673-773K, a secondary hardening stage following a saturated stress response was observed. The extent of secondary hardening was seen to be maximum at 673K. The magnitude of the initial hardening was seen to decrease with increasing temperature. A continuous softening upto failure characterised the stress response behaviour at 900K. The results are explained in the light of detailed optical and SEM investigations. [ABSTRACT FROM AUTHOR]

Low cycle fatigue tests were carried out on 316 L(N) stainless steel at room temperature employing strain amplitudes ranging from ± 0.3 to ±1.0% and a strain rate of 3×10−3s−1. The material showed initial hardening for a few cycles followed by prolonged softening, saturation and final failure. The fatigue life was found to decrease with increase in strain amplitude. The analysis of stable hysteresis loops showed non-Masing behaviour for this material. The elasto-plastic response of the material under cyclic loading was characterized taking into account isotropic and kinematic hardening occurring during cycling. The material parameters were obtained from the experimental hysteresis loops and cyclic stress response of the material. Finite element analysis of elasto-plastic deformation was carried out to obtain the stabilized hysteresis loop and cyclic stress response of the material. The predicted hysteresis loops showed good agreement with experimental results. The low cycle fatigue life prediction was carried out based on plastic strain energy dissipation with cycling. [ABSTRACT FROM AUTHOR]

In the present paper, low cycle fatigue (LCF) behaviour of modified 9Cr-1Mo steel (P91 steel) and tungsten allo ed 9Cr steel (P92 steel) is presented. Total axial strain controlled fatigue tests were performed in air employing a constant strain rate 3×10-3s-1 with strain amplitudes in the range ±0.25% to ±0.6% in the temperature range 823 – 873K. Both steels exhibited a continuous softening behaviour before the final load drop that occurred due to crack propagation. In general, P92 steel exhibited a lower stress response compared to P91 steel. At lower strain amplitudes, P92 steel showed a higher stress response than P91 steel. The softening rate of P92 steel is found to increase with increase in strain amplitude whereas the softening rate of P91 steel has remained constant with strain amplitude. Comparison of fatigue lives of the steels exhibited a mixed behaviour. Fatigue life of P92 was found to be higher at higher strain amplitude and lower at lower strain amplitudes than P91 steel. [ABSTRACT FROM AUTHOR]

Abstract: This investigation deals with the study of a compressor disc made of α-β titanium alloy of a typical fighter aero-engine having a TBO (Time Between Overhaul) of 550hours and TTL (total technical life) of 1500hours. In general, the failure of most of compressor discs is attributed to low cycle fatigue (LCF) due to the presence of residual tensile stresses at the dovetail slots. In the present study, shot peening was carried out to introduce compressive residual stress at the dove tail slots in order to enhance fatigue life of the disc. The local stress at the longitudinal and transverse edge of the slot generally increases upto 450MPa and 230MPa, respectively, during service. In the present study, the introduced residual compressive stresses measured by XRD technique on the slot and face of the disc has been found to be of the order 650MPa & 770MPa, respectively. Stress controlled LCF tests (R=0.1, maximum stress = 85% Yield Strength, frequency = 0.33Hz) have been conducted at room temperature. The LCF life of shotpeened disc was observed to be nearly three times as compared to that of unpeened disc. Following this surface treatment, life of the disc has been revised from 800 to 1500hours, thereby eliminating the need of disc replacement at operating units. This has improved the reliability and serviceability of the fleet. [Copyright &y& Elsevier]

Abstract: Low cycle fatigue and long-term creep properties, as well as compatibility with liquid sodium coolant, govern the choice of materials for out-of-core structural and steam generator components of sodium cooled fast reactors (SFRs). 316L(N) SS is preferred for all high temperature structural components while modified 9Cr-1Mo steel is preferred for the steam generator components for which resistance to stress corrosion cracking in steam is an additional requirement. A thorough understanding of the deformation, damage and fracture behavior of these materials helps to improve confidence in the design of the components and performance in service, and also to develop improved materials. The cyclic and creep deformation behavior have to be understood under complex stress conditions and in the liquid sodium, o erating environment. This paper highlights the creep, low cycle fatigue and creep-fatigue properties of current SFR materials in air and sodium environments, and discusses the studies underway to develop improved materials for future SFRs for longer design life. [Copyright &y& Elsevier]

Abstract: The article deals with determining of fatigue lifetime of structural materials during by multiaxial cyclic loading. The theoretical part focuses on fatigue and criterions for evaluation of the multiaxial fatigue lifetime. The experimental part deals with modeling of combined bending -torsion loading and determining the number of cycles to fracture in region low-cycle fatigue and also during of the loading with the sinusoidal wave form under in phase φ = 0°. Based on the experimental results the fatigue design curves are compared to fatigue data from base metal and weldments. [Copyright &y& Elsevier]

Abstract: This paper presents a new fatigue model to predict life of steel bridges considering the effect of high amplitude loading. It consists of a modified strain-life curve and a new strain based damage index. Modified strain-life curve consists of Coffin-Manson relation in low cycle fatigue region and a new strain-life curve in high cycle fatigue region. The damage variable is based on a modified von Mises equivalent strain to account for the effects of loading nonproportionality and strain path orientation in multiaxial stress state. The proposed model was verified with experimental test results of two materials available on the literature. Then, it was illustrated with an old riveted wrought iron railway bridge. The obtained results verify the effectiveness of the proposed model over commonly used Miner''s rule based life prediction of steel bridges. [Copyright &y& Elsevier]

Abstract: The development of cohesive zone models in the finite element framework dates back some 30 years, and cohesive interface elements are nowadays employed as a standard tool in scientific and engineering communities. They have been successfully applied to a broad variety of different materials and loading scenarios. However, many of such constitutive models are simply based on traction-separation relations without deducing them from energy potentials. By way of contrast, a thermodynamically consistent cohesive zone model suitable for the analysis of low cycle fatigue is elaborated in the present contribution. For that purpose, a plasticity-based cohesive law including isotropic hardening/softening is supplemented by a damage model. First results of this new approach to cyclic loading will be presented illustrating the applicability to low cycle fatigue. [Copyright &y& Elsevier]

Abstract: The deformation behaviour of an Al(6061)/Al2O3 metal matrix composite was experimentally and numerically investigated under low cycle fatigue. The deformation behaviour,fatigue damage initiation and development are studied on the specimen surface and in the bulk material. The three-dimensional FE model based on tomographic data has been created and loaded step by step up to 100 cycles.For the simulation of the Al(6061) matrix behaviour aChaboche-type model has been used. The numerical results indicate that stress and strain concentrations in the microstructure can be responsible for the formation of extrusions and intrusions and subsequently for the crack formation. [Copyright &y& Elsevier]

Abstract: Cyclic strain localization and observation of surface relief in nearly lamellar cast γ-TiAl based alloy Ti–48Al–2Nb–2Cr–0.82B (all in at. %) cycled in strain control were experimentally studied. Hysteresis loop area, the cyclic hardening curves, the cyclic stress-strain curve and V H parameter were measured. Surface relief of cyclically strained specimens was observed using scanning electron microscopy. The variations of the V H parameter indicate the cyclic slip localization. The localization has a pronounced effect on surface relief formation. Persistent slip markings are formed on the surface along interlamellar interfaces γ/α2 and γ/γ. These locations were nuclei for fatigue cracks in the interior of grains. Experimental results concerning the loop shape parameter and surface relief are used to discuss the role of cyclic slip localization on the cyclic stress-strain response and fatigue crack initiation. [Copyright &y& Elsevier]

Abstract: A more accurate life prediction for gas turbine blade takes into account the material behaviour under the Low Cycle Fatigue cycles normally encountered in turbine operation. In this study, low cycle fatigue tests are performed as the variables of total strain range and temperatures. The relations between plastic and total strain energy densities and number of cycles to failure are examined in order to predict the low cycle fatigue life of superalloy at different temperatures. The fatigue lives that are predicted by Coffin-Manson method and strain energy methods are compared with the measured fatigue lives at different temperatures. [Copyright &y& Elsevier]

Abstract: A modeling approach that captures the plastic strain amplitude dependence of strain induced martensite fraction evolution during low cycle fatigue (LCF) loading of metastable austenitic steels has been developed. The model is based on a modified version of the Olson-Cohen kinetic equation that retains connection to the underlying mechanisms of the transformation by relating cyclic austenite stability to monotonic stability. The kinetic model is then input into an evolving composite mechanical model that relies on a simple rule of mixtures formulation to predict cyclic stress-strain behavior, including increases in stress amplitude due to martensitic transformation. Predictions from the modeling approach compare well to fatigue data in the literature for AISI 304 stainless steel. [Copyright &y& Elsevier]

Abstract: Generally fretting fatigue S-N curve have two stages, one is high cycle (low stress) region and second is low cycle (high stress) region. In previous paper we introduced the fretting fatigue life estimation methods in high cycle region by considering the wear process. And in this estimation method the fretting fatigue limit can be estimated as the crack initiation limit at contact edge. In this paper we estimated the low cycle fretting fatigue life based on new critical distance theory, which is modified for high stress region using ultimate tensile strength σB, and fracture toughness KIC. Firstly the critical distance for estimating low cycle fatigue strength was calculated by interpolation of critical distance on fatigue limit (estimated from σw0 and ΔKth) with critical distance on static strength (estimated from σB and KIC). The validity of this method is confirmed by the V notch specimens. And then we applied this method on estimation of low cycle fretting fatigue strength and life. By unifying these low cycle fretting fatigue life estimation method with high cycle fretting fatigue life estimation method which was presented previous paper we can estimate the total fretting life easily. And to confirm the availability of this estimation method we perform the fretting fatigue test using Ni-Mo-V steel. [Copyright &y& Elsevier]

Abstract: The life assessment for a component subjected to high strain concentration in critical regions can be treated as a crack growth estimation starting from the first cycle of the component life. This approach assumes that the presence of same small surface defects or slip bands causes a quickly crack nucleation in critical regions where Low Cycle Fatigue (LCF) design is adopted. The work is focused on the analysis of LCF tests carried out by the aim of smooth cylindrical specimens and specimens with semi-circular notches made of a quenched and tempered steel. The crack growth has been monitored using a thin acetate-foil based replica technique. Two different approaches have been analyzed in order to define a model able to predict the crack growth rate for high constant strain amplitudes. Such a tool is useful to assess the residual life of a component in presence of a crack in critical regions where high stress concentrations cause cyclic yielding of the material. Finally a new analytical model able to predict the crack growth rates is presented. [Copyright &y& Elsevier]

Abstract: Cylindrical specimens of Inconel 713LC in untreated condition and surface treated with Al and Al-Si diffusion coating were cyclically strained under strain control at 800 ̊C in air. The structural and hardness characteristics of the hardened surface layer are presented. Cyclic hardening/softening curves, cyclic stress-strain curves, and fatigue life curves are obtained. The sections parallel to the specimen axis have been examined to study fatigue damage mechanisms. The effect of Al coating on stress response and fatigue life slightly differs from that of Al-Si coating. [Copyright &y& Elsevier]

Abstract: This paper presents tensile and low cycle fatigue properties of copper thin films used in electronic devices. Copper films tested were rolled copper, electrolytic copper, direct current plated copper and pulse plated copper films. Tensile tests and strain controlled 4-point bending low cycle fatigue tests were carried out and cycles to crack initiation and those for propagation were obtained. The direct current plated copper showed the largest Young''s modulus among the copper films. The electronic copper showed the largest yield stress and ultimate tensile strength, and the pulse plated copper the largest elongation. The strongest resistance to crack initiation was found in the rolled copper but there was no large difference in the cracking resistance among the other copper films. Cracks of the direct current and pulse plated coppers propagated faster than those of the other copper films. [Copyright &y& Elsevier]

Abstract: This paper focuses on the effect of grain size on the damage mechanism of WaspaloyTM related to the localization of plastic deformation by fatigue. Such effect is examined from nano- to meso-scale by investigating the cyclic behavior of damaged specimens related to the irreversibility of plastic strain. The evolution of the average extrusion height as a function of number of cycles could be divided into three parts, which could be related to the internal stresses evolution. Damage occurrence is observed only when a saturation state, which depends on grain size is observed in terms of stress and height of extrusion. [Copyright &y& Elsevier]

Abstract: The cyclic deformation behaviour of an austenitic stainless steel and its links with microstructure are examined by analysing changes in the stress-strain hysteresis loop characteristics and dislocation condition with increasing cycle number. In terms of peak tensile stress, AISI 316L exhibits a hardening stage followed by a softening stage, and finally a stable response stage, during strain-controlled cyclic loading. During the hardening stage, dislocation density increases, mainly in the form of planar structures, i.e. stacking faults, pile-ups. Dislocations then rearrange to form walls and channels during the softening stage, which further develop to become a cellular structure later in this stage and during the final stable response stage. An analysis of the change in hysteresis loop shape during cyclic loading shows that the increase in dislocation density is responsible for an increase in effective stress. In addition, the long range internal stress (back stress), which is strongly sensitive to dislocation distribution and dislocation structure, is mainly responsible for the cyclic deformation response. The loop shape parameter and other quantities derived from the stress-strain hysteresis loop also evolve along with the development of the dislocation condition. [Copyright &y& Elsevier]

Abstract: The high-strength aluminium alloy AA2618 is a typical material to manufacture centrifugal compressor wheels of exhaust turbochargers. In general these components are manufactured from heat treated forged slugs. Depending on the size of the slugs during the heat treatment different microstructure may arise. Especially for large slugs the emerging microstructure will not be uniform within the manufactured component. For this reason, the mechanical properties of the material will depend on the location within the component. To investigate this effect a T6 heat treated forged slug measuring 530mm in diameter and 200mm in high was investigated. From the metallographic investigations and from measurements of the hardness three different regions could be defined. Specimens were taken from different locations and their behaviour in tensile tests and low cycle fatigue tests were determined. It turns out that the yield strength depends on the grain size. This behaviour can be described using a Hall-Petch relation. The low cycle fatigue behaviour, the lifetime and the development of fatigue damage are influenced by the local microstructure. The reduction in the lifetime correlates with the number of subgrains. [Copyright &y& Elsevier]

Abstract: Total strain controlled tests have been performed on cylindrical specimens of austempered ductile cast iron (ADI) and ductile cast iron with the ferritic matrix, both alloyed with 2.75% Ni, with the aim to compare cyclic plasticity and the fatigue life at temperatures 23 °C and–45 °C. Microstructure and its features are documented using light microscope (image analysis) and neutron diffraction. In ADI the measurement of retained austenite by neutron diffraction before and after fatigue tests is used to discuss the cyclic stress-strain response. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were obtained for both temperatures and materials. Cyclic stress-strain curve obtained at–45 °C is shifted to higher stress amplitudes relative to room temperature curve. The fatigue life curves at both temperatures can be well approximated by the Manson-Coffin and Basquin laws. The Manson-Coffin curves are identical for both materials and temperatures while the Basquin curve is shifted to longer fatigue life for ADI. The surface relief developed during fatigue was studied using SEM. Fatigue cracks initiated preferably from persistent slip markings at the interface of graphite nodule. [Copyright &y& Elsevier]

Abstract: Interrupted total strain controlled tests have been performed on cylindrical specimens of austempered ductile cast iron (ADI) with the aim to study fatigue crack initiation and provide the fine numerical simulations at temperatures 23 °C and–45 °C. The systematic observation of the same places on surface specimen by SEM was done in interrupted low cycle fatigue test. Fatigue cracks initiated preferably from persistent slip markings at the interface of graphite nodule and the ausferrite matrix and places without visible graphite nodule. To prove this assumption the numerical simulations were performed in order to study this situation in more details. The three-dimensional (2D) model of the particle in the matrix was created in order to estimate the stress concentration factor caused by the particle. The loading of the model was composed of two steps–the cooling and then applied the tension stress. These steps are identical to the real testing conditions. To study the influence of the particle position near the surface the two-dimensional (2D) numerical analysis was performed where the position of the particle changed its position within the matrix. The highest sensitivity to the cracking of the matrix represented the position of the particle near by the surface of the specimen. The numerical analyses presented here helped to better understanding of the experimentally observed processes in the fatigued ADI. [Copyright &y& Elsevier]

Abstract: The low-cycle fatigue properties of TiAl alloys with 2 at.% and 7 at.% of Nb with nearly lamellar microstructure were studied at room temperature and 750 °C. Cyclic hardening/softening curves, cyclic stress-strain curves (CSSC) and fatigue life curves were obtained at both temperatures as well as monotonic tensile properties. Both materials show stable cyclic response at 750 °C. At room temperature, significant cyclic hardening characteristic for 2 at.%Nb alloy is not observed for 7 at.% Nb alloy. This difference is attributed to higher Nb content which increases the stacking fault energy and thus prevents intensive twinning. Parameters of the CSSC, the Manson-Coffin law and the Basquin law were determined. The cyclic strength of the alloy with 7 at.% Nb is substantially higher in comparison with 2 at.% Nb, probably due to lower thickness of the lamellas. [Copyright &y& Elsevier]

Abstract: Based on in situ observations with scanning electron microscopy (SEM), we investigated the low cycle fatigue (LCF) propagation behaviors of a Ta-rich single crystal nickel-based superalloy at 550oC in vacuum. Specimens in three different orientations, i.e. [010], [011] and [111], were tested in stress-controlled tension-tension LCF tests with R=0.1. Slip traces can be observed ahead of the crack and distinct crystallographic fracture was dominated. By analysis on the surface slip traces and the crack propagation planes, the operated slip systems were examined to be octahedral slip and the active octahedral plane was identified in this context. The whole cracking process of small fatigue cracks were meticulously tracked and recorded. The evaluation of short crack growth rate for specimens in three orientations was carried out by fracture mechanics approach. The influence of crystallographic orientations on fatigue crack growth are analyzed and discussed. Especially, the interaction between the short crack and local microstructure such as the carbides ahead of the crack were analyzed, altering the propagation path as well as influencing the crack growth rate. The parameter of crack opening displacement was examined to correlate well the crack growth data in different orientations. [Copyright &y& Elsevier]

Abstract: The effect of nitrogen alloying (0.078% to 0.22%) in 316 LN stainless steel has been investigated for its Low Cycle Fatigue properties in the temperature range 300 to 873 K. The effect of dynamic strain ageing has been evaluated as a function of nitrogen content, temperature, strain amplitude and strain rate. The temperature range of operation of DSA has been found to be a strong function of the above parameters. Increase in nitrogen content shifted the range of operation of DSA to higher temperatures. For temperatures ≥673 K, fatigue life is found to be maximum for a nitrogen content of 0.14 wt%, while it is found to saturate or increase with increasing nitrogen content at temperatures <673 K. [Copyright &y& Elsevier]

Abstract: Low cycle fatigue (LCF) and fatigue-creep tests were conducted under different dwell conditions at 650 °C on a P/M nickel base superalloy, which was used widely as aero-engine turbine disc. The specimens, taken from a hot isostatic pressing (HIP) disc , were tested under total fully reversed axial strain control. Compared with continuous cycling tests without dwell, the ones with tensile, compressive or tensile-compressive strain dwells showed some shorter fatigue lives. The difference of fatigue lives under different strain dwells reflects the fatigue-creep interactions at elevated temperature. The cyclic deformation and life distribution features were analysed and a lifing model was proposed to evaluate the fatigue life of the alloy. The prediction agrees well with the experimental results. [Copyright &y& Elsevier]