Pure Fatigue Research Articles

J0310101 オーステナイト系鋳鋼の熱疲労き裂進展と温度条件

Effect of maximum test temperature on crack propagation behavior of an Austenitic cast steel during Thermo-Mechanical Fatigue (TMF) tests was explored under the In-Phase(IP) and Out-of-Phase(OP) conditions. The behavior was comprared with that of isothermal low cycle fatigue. The experimental results clearly showed that additional mechanism(s) mechanism to pure fatigue begun to work, when the maximum temperature get higher enough. It was also shown that not only the mechanical factors but the microstuructural features;e.g., microsegragation around dendritic boundaries, played an important role in the crack progation process.

Combined cycle fatigue testing with ultrasonic frequency component of S350 steel welded joint

A combined cycle fatigue (CCF) testing system with ultrasonic frequency component was developed to evaluate the CCF properties of S350 steel welded joints in this study. The fatigue testing results indicated that the S-N curves of CCF did not have fatigue limit, which agreed with those of pure high frequency fatigue of welded joints. The S-N curves showed that the CCF strength of welded joints dropped greatly with the increasing interaction between high and low frequency fatigue loading. An approximation design method of CCF was presented using amplitude envelope as the stress range.

Biaxial fatigue tests and crack paths for AISI 304L stainless steel

AISI 304L stainless steel specimens have been tested in fatigue. The tests were axial, torsional and in-phase biaxial, all of them under load control and R=-1. The S-N curves were built following the ASTM E739 standard and the method of maximum likelihood proposed by Bettinelli. The fatigue limits of the biaxial tests were represented in axes ?-?. The elliptical quadrant, appropriate for ductile materials, and the elliptical arc, appropriate for fragile materials, were included in the graph. The experimental values were better fitted with an elliptical quadrant, despite the ratio between the pure torsion and tension fatigue limits, ?FL/?FL, is 0.91, close to 1, which is a typical value for fragile materials. The crack direction along the surface has been analyzed by using a microscope, with especial attention to the crack initiation zones. The crack direction during the Stage I has been compared with theoretical models.

A unified equation for creep-fatigue

“Pure fatigue” is a special case of creep-fatigue; and the Coffin–Manson equation, Δεp=CoN-βo, is a special case of the general creep-fatigue equation, which is proposed to take the form: Δεp=Cos(σ)c(T,f)N-βob(T,f). The functions, s(σ),c(T, f) and b(T, f), embody the stress–time–temperature characteristic of creep. At the reference condition when creep is dormant, s(σ)=c(T, f)=b(T, f)=1, the Coffin–Manson equation is recovered. At the extreme condition when c(T, f)=0, creep-rupture occurs without fatigue. In between these two extreme conditions whence 0⩽c(T, f)⩽1, creep-fatigue prevails.

Damage Evolution and Crack Growth in Nickel-Based Alloys during Ultrasonic Fatigue

Experimental results on the fatigue damage of quasi defect-free materials in the VHCF range are presented. For nickel-based superalloys and pure nickel the likelihood of crack initiation at favorable grain morphologies is studied. Slip band and microcrack formation at the surface was observed even in run-out samples. Hence, microcracks were evaluated regarding their propagation capabilities according to grain orientation and barrier function of grain boundaries. In the VHCF regime crack initiation can shift from surface to subsurface, consequently early crack growth has to be studied by means of optical methods and indirect detection techniques or tomographic methods. In the study presented crack initiation and crack growth was monitored through optical observation and quasi 3-D observation by means of synchrotron radiation. For an as-received and a coarse-grained condition of pure nickel Ni201 fatigue crack growth in the VHCF regime occurs at deltaK as low as 3.54 MPam1/2 for a crack growth rate da/dN = 10E-12 m/cycle. The grain size had no effect on the threshold limit but crack growth retardation at grain boundaries and crack path deflection lead to lower crack growth rates for the coarse-grained condition In the nickel-based alloy Nimonic 80A the influence of microstructure on the intercrystalline crack initiation and propagation was confirmed. Here, the combination of the misorientation angle between two adjacent grains and the orientation of their boundary with respect to the external load defines the magnitude of stress concentration at grain boundaries.

Cyclic Deformation Response of Polycrystalline OFHC Copper under Pure Compression Fatigue

Systematic experimental investigation was carried out to examine the cyclic deformation response and study the crack nucleation mechanisms under pure compression fatigue condition using OFHC Cu. Results show that the cyclic stress strain response and micro-structural evolution of copper under pure compression fatigue exhibits rather dissimilar responses compared to those under general push-pull fatigue conditions. Both rapid hardening and the compressive cyclic creep were observed in all tested conditions. Like under all fatigue conditions, surface micro-structural changes were detected by optical, SEM, and specifically AFM. It was revealed that cyclic plastic strain accommodated by the sample was not in any major way through dislocation activities, as there was only moderate slip activities observed on the surface and no PSB features were detected from TEM observations. Instead, cyclic creep was observed to be the major form of plastic strain accommodation. In addition, the observed surface phenomenon was found to have led to the eventual crack nucleation when the applied stress amplitude was high.

Simulation of Fatigue Life Behavior of Circular Cross-Section Bar with Straight-Fronted Crack

In this paper, fatigue life circular cross-section elastic bar under pure fatigue axial loading is studied through principles of linear elastic fracture mechanics (LEFM) coupled with the three-dimensional finite element technique for determination of critical crack size and residual lifetime. Three different initial notch depths are discussed. The relations between aspect ratio (b/c) and relative crack depth (b/D) are obtained, and it is shown that there is great difference in the growth of cracks with different front shapes and initial notch depths.

Continuum damage mechanism-based life prediction for Ni-based superalloy under complex loadings

A continuum damage mechanism (CDM) based life model for directionally solidified Ni-based superalloy under complex cyclic loadings is presented. Complex loadings including pure fatigue, pure creep and fatigue–creep interaction with different hold time and hold types are considered at 850°C and 980°C. Linear combination of fatigue damage developed by Chaboche and creep damage proposed by Kachanov is used to model the CDM method. The damage analyses reveal that there is a critical hold time of 120s. When the hold time is less than this point, there exists a critical stress under which the creep and fatigue damage are the same at 50% each. When the hold time is more than 120s, the creep damage is more than 50% which means that the creep always dominates the failures. However, when the hold times are less than 120s, fatigue damage dominates under most of the stress levels. About 97 fatigue test data under different loading conditions were used to verify the feasible of the method. The results indicate that a good prediction fell within a factor of 2 scatter band is obtained.

Fatigue Crack Prediction of LC9 with Corrosion Damage Considering Effect of Load Frequency

Considering the degradation effect of corrosion damage on fatigue behavior of aero aluminum alloy, the present thesis made a research on corrosion fatigue crack growth rate. Taking into account the effect of load frequency on fatigue crack growth, a concept of corrosion fatigue frequency factor is proposed. Based on the fact that low frequency will lead to high corrosion fatigue crack growth rate, and frequency higher enough will make little difference between corrosion fatigue and pure mechanical fatigue behavior, an exponential expression of corrosion fatigue frequency factor is proposed. The crack growth rate prediction from proposed formula is proved to be in good agreement with experimental results for steadily extended corrosion fatigue crack.

Evaluating the Critical Temperature of Creep-Fatigue Interaction a Nickel-Based Powder Metallurgy Superalloy

For the components working in high temperature and enduring fatigue loading, the fatigue fracture properties will be reduced remarkably when the working temperature is higher than the critical temperature of creep-fatigue interaction Tc of the material. In consequence, the damage mechanism from creep-fatigue interaction becomes more complex. A method is presented in this paper to determine the Tc of a nickel-based powder metallurgy superalloy. Pure fatigue crack growth and creep-fatigue crack growth tests were conducted in several different elevated temperatures. The fracture mechanism was investigated via observing the fractographic characteristics using scanning electron microscope (SEM). The test results show that the Tc of this superalloy is a little bit lower than a half of the melting temperature Tm.

Simultaneous creep–fatigue damage accumulation of forged turbine disc of IN 718 superalloy

In the present study, simultaneous creep–fatigue interaction of rim, hub and bore section of forged turbine disc of IN 718 superalloy as per the Kartik–Vikas test method has been presented. Metallographic assessment of interrupted test specimens confirmed the ‘true interaction’ mode of simultaneous creep–fatigue interaction where transgranular initiated crack gets perturbed by accumulating creep damage and becomes intergranular. The cause of intergranular cracking can be attributed to the combined effect of planar slip and oxidation. The superalloy showed significant reduction in fatigue life under simultaneous creep–fatigue interaction as compared to pure fatigue loading conditions. This was attributed to high activity of dislocations and grain boundary damage in creep–fatigue interaction as compared to pure fatigue loading conditions.

Prediction of deformation and failure of modified 9Cr–1Mo steel under creep-fatigue interaction

Plant components often undergo loadings which have aspects of both fatigue and creep by experiencing repeated start-up/shut-downs between steady-state operation at high temperature. In such a case, structural materials show inelastic deformation different from that observed under pure fatigue or pure creep conditions. This finding prompted the development of “unified” constitutive models in which cyclic and creep deformations are treated in a unified way. The author’s group has been developing such a unified model for modified 9Cr–1Mo steel which is widely used in ultrasupercritical fossil power plants. Although the latest model has been demonstrated to be capable of describing deformation behaviour under various loading conditions, including cyclic and steady-state loadings, the model still has room for improvement, such as the need to represent accelerated deformation in the tertiary creep regime. In this study, an attempt is made to improve the capability of the model from various perspectives. A failure prediction model which can deal with various failure modes is also formulated. The deformation and failure lives predicted using these models show reasonable agreement with the results of various load sequence tests obtained on modified 9Cr–1Mo steel.

Fatigue Resistance and Crack Initiation Mechanisms in Nitrided Steel Treated with Selected Types of Surface Coating

Surface treatment processes are usually applied to improve surface characteristics and resistance. Particularly duplex processes like nitriding combined with nanostructured based coatings are considered as the most advanced ones, with a large application potential in automotive industry. An example is an application in gear wheels, where fatigue loading in gear teeth roots exists besides contact fatigue on teeth sides. Contact fatigue, has to be therefore well balanced with pure fatigue resistance. Some surface treatments leading to excellent contact properties, wear resistance and extremely low friction coefficient may reduce resistance to pure fatigue crack initiation and subsequent growth to premature failure. In the paper, results of an experimental programme aimed at evaluation of fatigue resistance at repeated loading of nitrided steel after application of two types of coating, (i) nanostructured C-layer deposited by a modified cathodic arc technique according to Microcoat patent and (ii) Cr-WC:H-DLC layer produced according to Hauzer patent [1], are presented and discussed. Fatigue tests were carried out using small samples with 2.1 mm diameter, with nitrided layer of 0.35 mm thickness. Tests were completed with fractographical analyses of initiation areas. Results and analyses showed that coating effects depended on the specific technology and parameters. The Cr-WC:H-DLC layer affected fatigue limit favourably unlike the Microcoat C-layer, which resulted in reduction of fatigue strength. A detailed analysis of recorded data during static tensile tests indicated an occurrence of significant surface and subsurface residual stresses with a strong possible effect on fatigue crack initiation mechanisms. A significant, unfavourable effect of inclusions in the subsurface layer was demonstrated.

The development of microstructural damage during high temperature creep–fatigue of a nickel alloy

Alloy 617 is the leading candidate material for an Intermediate Heat Exchanger (IHX) of the Very High Temperature Reactor (VHTR). To evaluate the behavior of this material in the expected service conditions, strain-controlled cyclic tests that include hold times up to 9000s at maximum tensile strain were conducted at 950°C. The fatigue resistance decreased when a hold time was added at peak tensile strain, owing to the mechanisms resulting in a change in fracture mode from transgranular in pure fatigue to intergranular in creep–fatigue. Increases in the tensile hold duration beyond an initial value were not detrimental to the creep–fatigue resistance. An analysis of the evolving failure modes was facilitated by interrupting tests during cycling for ex situ microstructural investigation.

Mechanism‐based life model for out‐of‐phase thermomechanical fatigue in single crystal Ni‐base superalloys

ABSTRACTThis paper describes an enhanced physics‐inspired model to predict the life of the second‐generation single crystal superalloy PWA 1484 experiencing out‐of‐phase (OP) thermomechanical fatigue (TMF). Degradation due to either pure fatigue or a coupling between fatigue and environmental attack are the primary concerns under this loading. The life model incorporates the effects of material anisotropy by utilizing the inelastic shear strain on the slip system having the highest Schmid factor while accounting for the effects of temperature‐dependent slip spacing and stress‐assisted γ′ depletion. Both conventional TMF and special bithermal fatigue (BiF) experiments were conducted to isolate and therefore better understand the interactions between these degradation mechanisms. The influences of crystallographic orientation, applied mechanical strain range, cycle maximum temperature and high temperature hold times were assessed. The resulting physics‐inspired life estimation model for OP TMF and OP BiF predicts the number of cycles to crack initiation as a function of crystal orientation, applied strain amplitude and stresses, temperature, cycle time (including dwells), and surface roughness within a factor of 2.

Effect of Stress Frequency and Chemical Medium on the Corrosion-Fatigue Behaviour of 316L Stainless Steel

Among most used materials in implantology and the manufacture of surgical instruments, one finds the austenitic stainless steel AISI 316L, considered for its well adapted mechanical characteristics, its biocompatibility and in particular its resistance to uniform corrosion. The implants are often subjected to cyclic mechanical loads during normal activity of the human body, but they can also be attacked chemically by the physiological medium, under certain conditions. Between several mechanical and chemical parameters that can influence the corrosion-fatigue behavior of such material, the load frequency parameter is highlighted in this work. The aim is to determinate the effect of load frequency changes on the crack growth rate in corrosion-fatigue and to compare this effect in pure fatigue. To make experimental evidences, notched austenitic steel specimens have been submitted to cyclic bending tests inside a chlorine solution simulating the physiological medium (NaCl 0.9%). The bending stress value was taken equal to 200 MPa with a stress ratio R of 0, at different stress frequency values, respectively of 0.5, 1 and 2 Hz. When immersed in the chemical medium, the mechanical behavior of the steel sheets appears to be worst as the frequency decreases, as long as the crack size remains less than the critical one. In pure fatigue, the mechanical behavior changes with the increase of frequency and becomes detrimental. This corroborates different author works for such material.

Effect of Stress Frequency and Chemical Medium on the Corrosion-Fatigue Behaviour of 316L Stainless Steel

Among most used materials in implantology and the manufacture of surgical instruments, one finds the austenitic stainless steel AISI 316L, considered for its well adapted mechanical characteristics, its biocompatibility and in particular its resistance to uniform corrosion. The implants are often subjected to cyclic mechanical loads during normal activity of the human body, but they can also be attacked chemically by the physiological medium, under certain conditions. Between several mechanical and chemical parameters that can influence the corrosion-fatigue behavior of such material, the load frequency parameter is highlighted in this work. The aim is to determinate the effect of load frequency changes on the crack growth rate in corrosion-fatigue and to compare this effect in pure fatigue. To make experimental evidences, notched austenitic steel specimens have been submitted to cyclic bending tests inside a chlorine solution simulating the physiological medium (NaCl 0.9%). The bending stress value was taken equal to 200 MPa with a stress ratio R of 0, at different stress frequency values, respectively of 0.5, 1 and 2 Hz. When immersed in the chemical medium, the mechanical behavior of the steel sheets appears to be worst as the frequency decreases, as long as the crack size remains less than the critical one. In pure fatigue, the mechanical behavior changes with the increase of frequency and becomes detrimental. This corroborates different author works for such material.

Prediction of deformation and failure of modified 9Cr–1Mo steel under creep-fatigue interaction

Plant components often undergo loadings which have aspects of both fatigue and creep by experiencing repeated start-up/shut-downs between steady-state operation at high temperature. In such a case, structural materials show inelastic deformation different from that observed under pure fatigue or pure creep conditions. This finding prompted the development of “unified” constitutive models in which cyclic and creep deformations are treated in a unified way. The author’s group has been developing such a unified model for modified 9Cr–1Mo steel which is widely used in ultrasupercritical fossil power plants. Although the latest model has been demonstrated to be capable of describing deformation behaviour under various loading conditions, including cyclic and steady-state loadings, the model still has room for improvement, such as the need to represent accelerated deformation in the tertiary creep regime. In this study, an attempt is made to improve the capability of the model from various perspectives. A failure prediction model which can deal with various failure modes is also formulated. The deformation and failure lives predicted using these models show reasonable agreement with the results of various load sequence tests obtained on modified 9Cr–1Mo steel.

Constitutive Laws for the Deformation Estimation of Extrusion Die in the Creep-Fatigue Regime

Tools are exposed to severe working conditions during the hot extrusion process. In particular, dies and mandrels can be subjected to an excessive amount of deformation as a result of the developed high cyclic loads and temperatures. In this scenario, a physical experiment reproducing the thermo-mechanical conditions of a mandrel in a porthole die was performed with the Gleeble machine on the AISI H11 tool steel with the aim to investigate the mechanisms that influence the die deformation. The design of experiment consisted of 4 levels of temperature, 3 levels of stress and 3 types of load, i.e. pure creep, pure fatigue and creep-fatigue. In all the testing conditions, a comparable pattern of the mandrel displacement-time curve was found reproducing the 3 stages of softening typical of the strain evolution in a standard creep test but with a marked primary phase. Thus, with the aim to identify an easy-applicable equation to estimate the die deformation, the time hardening creep law was chosen. Coefficients of the time-hardening law were optimized, for each testing condition, on the basis of experimental data starting from values for similar alloys taken from the literature. Results in terms of mandrel displacement were then compared to experimental data for the creep-fatigue condition at different stress and temperature levels. The values found were validated against additional experimental data performed with different specimen geometries. A good average agreement was found between experimental and numerical results. The developed procedure was then applied to an industrial die.

P.1.g.026 Relationships between affective symptoms, psychomotor performance, vigilance impairment and sleep quality in pure chronic fatigue

P.1.g.026 Relationships between affective symptoms, psychomotor performance, vigilance impairment and sleep quality in pure chronic fatigue