bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of the fatigue behavior of notched reinforced thermoplastics
Authors: Raphael Ilan , Saintier Nicolas , Laiarinandrasana Lucien , Robert Gilles
Published: May 2018

Understanding and modeling of fatigue damage and behavior in short glass fiber reinforced thermoplastics is an important issue in order to improve material processing and the design of composite parts for the industry. The present work aims at a more precise description of the fatigue damage mechanisms and the establishment of a simple, yet efficient, durability criterion. Observations bring new insights on the fatigue damage mechanisms: fiber failure triggered by fretting fatigue and crack propagation by intra-spherulitic failure in their equatorial plane. Finally, a cyclic strain rate criterion is introduced for notched specimens and the identification of a behavior model is performed for later implementation of the durability criterion.
Fast screening of the fatigue properties of thermoplastics reinforced with short carbon fibers based on a heat build-up protocol
Authors: Leveuf Louis , Marco Yann , Le Saux Vincent , Navratil Libor , Leclercq Sylvain , Olhagaray Jérôme
Published: May 2018

This study deals with the characterization of the fatigue lifetime of a short carbon fiber reinforced PEEK matrix thermoplastic composite using a heat build-up protocol. Several commercial grades and fillers ratios are considered in order to challenge the ability of the technique to capture the influence of these variations. First, the materials investigated and the experimental protocols are described. The way to build the heat build-up curves is outlined. Then the results obtained from the heat build-up and the fatigue experiments are presented. Finally these results are used to discuss two main points. The first one is the validity of the hypotheses needed to apply the approach, ranging from the evaluation of the dissipation to the response during the fatigue tests. The second one is the capability to predict the fatigue curves accurately throughout an energetic criterion and to catch the influence of the variation of material on the fatigue properties. The results presented here are published with more details in [47].
Fatigue life assessment of a railway wheel material under HCF and VHCF conditions
Authors: Soares H. , Costa P. , Freitas M. , Reis L.
Published: May 2018

Fatigue damage and life assessment is still an issue and a challenge nowadays. Many different tests can be performed for the assessment of fatigue properties of any given material. In the present study a worn out railway wheel goes under uniaxial fatigue analyses for the high cycle and very high cycle fatigue regimes through the use of a conventional hydraulic machine and an ultrasonic fatigue test, both with a stress ratio of R=-1. For every used specimen, a life cycle is obtained for the corresponding induced specimen and the data is afterwards plotted. All the fracture surfaces of all failed specimens were analysed. The study and initial application of multiaxial ultrasonic fatigue condition and the design of the corresponding specimen for evaluating the wheel material is also presented.
Simulation of Structure Dynamic Stress under Multi-load Condition in Resonance State
Authors: Li Fansong , Wu Pingbo , Liu Chaotao
Published: May 2018

Some metro vehicle lifeguard structure cracking emerged during operation, and the reason is due to resonance. In order to simulate the dynamic stress accurately in resonance state, the formula for calculating the dynamic stress under multi-load condition was deduced based on structural dynamics mechanics. During the dynamic stress simulation, firstly, the frequency response function (FRF) of each external load was calculated, which including the phase information. Then, the method was based on Fast Fourier Transform (FFT) to get the frequency domain with real and imaginary part of each external load rather than power spectrum density (PSD) calculation, avoiding loss of the phase information due to PSD method. Finally, the dynamic stress frequency domain data could be calculated by FFT results and FRFs. Based on above, the dynamic stress time history could be acquired by Inverse FFT (IFFT) of stress frequency domain data, and the stress simulation method was verified by the line test result. Compared with the conventional stress PSD calculation method, the method in this paper can retain the phase information of the each external load. Meanwhile, the conventional stress simulation method based on PSD could not get satisfactory result.
Reliability-based fatigue life estimation of shear riveted connections considering dependency of rivet hole failures
Authors: Leonetti Davide , Maljaars Johan , Snijder H.H. (Bert)
Published: May 2018

Standards and guidelines for the fatigue design of riveted connections make use of a stress range-endurance (S-N) curve based on the net section stress range regardless of the number and the position of the rivets. Almost all tests on which S-N curves are based, are performed with a minimum number of rivets. However, the number of rivets in a row is expected to increase the fail-safe behaviour of the connection, whereas the number of rows is supposed to decrease the theoretical stress concentration at the critical locations, and hence these aspects are not considered in the S-N curves. This paper presents a numerical model predicting the fatigue life of riveted connections by performing a system reliability analysis on a double cover plated riveted butt joint. The connection is considered in three geometries, with different number of rivets in a row and different number of rows. The stress state in the connection is evaluated using a finite element model in which the friction coefficient and the clamping force in the rivets are considered in a deterministic manner. The probability of failure is evaluated for the main plate, and fatigue failure is assumed to be originating at the sides of the rivet holes, the critical locations, or hot-spots. The notch stress approach is applied to assess the fatigue life, considered to be a stochastic quantity. Unlike other system reliability models available in the literature, the evaluation of the probability of failure takes into account the stochastic dependence between the failures at each critical location modelled as a parallel system, which means considering the change of the state of stress in the connection when a ligament between two rivets fails. A sensitivity study is performed to evaluate the effect of the pretension in the rivet and the friction coefficient on the fatigue life.
Fatigue of automotive engine cylinder heads – A new model based on crack propagation
Authors: Béranger Mathieu , Morin Guillaume , Saanouni Sarah , Koster Alain , Maurel Vincent
Published: May 2018

The main reliability issue encountered during cylinder head development is the appearance of fatigue cracks during severe endurance tests. Some of these cracks can initiate in the coolant water jacket and lead to the complete part failure. They are mainly associated with the combination of combustion pressure alternate stresses and high cycle fatigue load in infinite life domain. However, Haigh or Dang Van analysis issued from 3D Finite Element Analysis reveals several issues: crack initiations are not correctly located and scatter of crack sizes are not predicted. To overcome these difficulties, a new fatigue model dedicated to cylinder heads in Aluminum - Silicon alloy has been developed in Renault powertrain division. The model is based on fatigue crack growth modelling in order to take into account various and combined load ratio in one hand, and typical characteristics of the microstructure of casting alloy in the other hand. This paper presents the theoretical basis of this model and the associated identification methodology for two different aluminum alloys.
Mean stress effect in stress-life fatigue prediction re-evaluated
Authors: Papuga Jan , Vízková Ivona , Lutovinov Maxim , Nesládek Martin
Published: May 2018

The paper compares various methods for computing the equivalent stress amplitude for stress cycles of non-zero mean value in stress-life fatigue prediction. A set of 11 calculation methods is evaluated. In addition to formulations based on common static or fatigue properties, the Walker formula and the generalized Linear formula are included in the investigation. These two methods use an optimization routine to find the material parameters. The final response of the methods is compared and discussed. The Walker method provides a better solution. The generalized Linear method produces inferior results, i.e. the linear fit of the segment of the Haigh diagram is not an optimal solution.
The effect of mean stress on the fatigue behaviour of overhead conductor function of theH/wparameter
Authors: Badibanga Remy , Miranda Thiago , Rocha Pedro , Ferreira Jorge , da Silva Cosme , Araújo José
Published: May 2018

The objective of this work is to evaluate the effects of mean stress on the fatigue behaviour of an All Aluminium Conductor (AAC Orchid), Aluminium Conductor Steel Reinforced (ACSR Tern), and an Aluminium Conductor Alloy Reinforced (ACAR 750 MCM). In this sense, 72 fatigue tests on overhead conductors were performed using different values ofH/wparameter. Based on the experimental results, the parameters which describe the fatigue behaviour of the conductors were determined after generating theirs S-N curves. In the assessment of the mean stress effects on the fatigue life, Goodman and Gerber’s relations were fitted to evaluate the use of such models for the conductors. It was observed that the evaluation of the mean stress effect on the overhead conductor could be made by using the fatigue relations.
Phase transformation in white etching area in rolling contact fatigue
Authors: Su Yun-Shuai , Li Shu-Xin , Lu Si-Yuan , Wan Li-Biao
Published: May 2018

Rolling contact fatigue (RCF) involves microstructural change in the subsurface of contact. The changed microstructure is generally termed as white etching area (WEA) as it appears white under optical microscope when etching in nital solution. WEA has been acknowledged as one of the primary failure modes in RCF since it causes severe local inhomogeneity of microstructure. It was reported that WEA consists of nano ferrites as martensite grains and carbides are significantly refined in the WEA. Some carbides are dissolved. In some cases, an amorphous-like structure was occasionally observed in the WEA, indicating that phase transformation may possibly occur. The WEAs were studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Electron back scattered diffraction (EBSD). The result showed that WEA is dominated with an amorphous phase with martensite, austenite and carbides embedded interior. A distinct interface between the matrix and the WEA was present. In addition to grain refinement down to nanometers, phase transformation including amorphization and austenitization happened in WEAs. The content of austenite was increased from 2% in the matrix to 20% in the WEA. The analysis showed that phase transformation is controlled by plastic deformation mechanism.
Probabilistic fatigue analysis of ultra-high-performance fibre-reinforced concrete under thermal effects
Authors: Ríos José D. , Cifuentes Héctor
Published: May 2018

This paper describes the influence of the temperature and the fibre reinforcement on the flexural fatigue behaviour of an ultra-high-performance fibre-reinforced concrete. Three-point bending fatigue tests were carried out for an ultra-high-performance concrete subjected to different temperatures ranging from room temperature up to 300 ºC and considering three different types of reinforcement: a) steel fibres, b) hybrid steel and polypropylene fibres and c) non-reinforced (reference matrix). The fatigue behaviour was assessed from the S-N fields obtained through a probabilistic fatigue model developed by Castillo and Fernández-Canteli. The influence of the type of reinforcement on the fatigue behaviour was analysed by SN curves. An analysis of the thermal effects in the fatigue life of the ultra-high-performance concrete has also been carried out. The results showed the most suitable fibre reinforcement among the analysed options to get the best fatigue behaviour in accordance to the exposure temperature.
Predicting creep-fatigue crack growth rates in Alloy 709 using finite element simulations of plasticity and creep-induced crack closure
Authors: Ramirez Jose , Potirniche Gabriel P. , Pugesek Henry , Shaber Nicholas , Taylor Martin , Stephens Robert , Charit Indrajit
Published: May 2018

This paper reports on a computational study and experimental validation of creep-fatigue crack growth rates at high temperature in two structural materials. The objectives are to develop a methodology to predict creep-fatigue crack growth rates using plasticity-induced crack closure under creep-fatigue loading conditions by characterizing the effect of hold time on crack growth rates during cyclic loading. In this study, the computation of fatigue crack growth rates is based on the crack closure phenomenon. The total crack growth rate during creep-fatigue loading is based on the addition of fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. The study identifies the effects of frequency and shape of loading cycle on crack-tip opening stresses induced by the combined action of the plasticity-induced crack closure and creep relaxation at the crack tip. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions. Elastic-plastic-creep material behavior is considered in these simulations. Closure levels are computed for high temperature structural materials such as 9Cr-1Mo steel and Alloy 709. The numerical predictions provide satisfactory agreement with experimental data of creep-fatigue crack growth rates in modified 9Cr-1Mo and Alloy 709 steels at high temperatures.
Energy Dispersive X-ray technique applied to fatigue fracture surfaces oxidation to provide information on growth rate
Authors: Hernandez Iñigo , Saunders Edward A. , Madariaga Iñaki
Published: May 2018

Gas turbine components are strongly affected by cyclic loading during operation, which makes fatigue development the most significant cause of failure or cracking within them. The nature and growth rate of that fatigue is dependent on the stress experienced at a particular area of a part, the temperature, the material properties and the applied loading frequency. Various techniques, such as striation and/or band counting can, in some cases, provide information on fatigue life. However, often such techniques are rendered useless by lack of formation or obliteration by damage and/or oxidation of such features. Once a crack has initiated and is exposed to the engine atmosphere, the level of oxidation and other contaminant elements observed on the crack surface will evolve with time, and, correspondingly, crack depth. Element distribution analysis by an Energy Dispersive x-ray Spectroscopy (EDS) technique applied along a fatigue fracture surface could help to understand the evolution of the crack with time.

Using EDS, an experimental procedure was carried out to ascertain a perceived measured percentage of oxygen plus other additional chemical species at a large number of stations along the length of a fatigue crack. The analysis was performed from the initiation site to the end of the propagation area, and also within the final rupture zone which had less time exposed to air and gasses compared to the fatigue system. An exercise of comparison between that progression in oxidation across cracked in-service components versus perceived measured oxidation level analyses on trial fracture surfaces exposed in atmospheric oven conditions at high temperatures for a number of different durations was performed. The obtained results provided information on specific questions concerning the fatigue propagation life of the in-service components; thereby amplifying the use of the EDS technique in this aspect of the materials forensic field.
Understanding the near-threshold crack growth behavior in an aluminum alloy by x-ray tomography
Authors: Wicke Marcel , Brueckner-Foit Angelika , Kirsten Tina , Zimmermann Martina , Buelbuel Fatih , Christ Hans-Juergen
Published: May 2018

The near-threshold behavior of long cracks is evaluated using pre-cracked flat dogbone specimens of a commercial aluminum alloy in two heat treatment states. Once the threshold was known, the crack initially introduced by cyclic compression was propagated under load control with approximately constant range of the stress intensity factor at values close to the threshold values. The 3D morphology of the crack and the spatial distribution of the primary precipitates obtained by μ-CT were reconstructed with the aim of analyzing the role of the microstructure and primary precipitates on crack growth. Results pointed out that there were two major mechanism keeping the crack from continuous extension. First, the crack front was pinned by primary precipitates leading to kinking of the crack front. The second mechanism was pronounced shear-controlled crack growth of long cracks seen in terms of hilly structures on the crack faces. The first mechanism was found to be dominant at R=-1, whereas tests performed with R=0.1 showed an increased propensity towards shear controlled crack growth.
Influence of static strength on the fatigue resistance of welds
Authors: Steimbreger Ceferino , Gubeljak Nenad , Enzinger Norbert , Ernst Wolfgang , Chapetti Mirco
Published: May 2018

The present paper deals with a fracture mechanic approach that employs the Resistance-Curve concept, in order to predict fatigue endurances of welded components, with different tensile strengths of the base metal. The Resistance-Curve method compares the total driving force applied to a crack with its threshold for propagation, both defined as a function of crack length. The former depends on load scheme and weld geometry and it can be obtained from finite element analyses, while the second is inherently related to weld resistance. Results obtained herein showed that threshold curve shape is changed when static strength of the base material is modified. Consequently, its interaction with the driving force differed, giving raise to different fatigue endurances for various values of the tensile strength. However, this effect is only likely to be leveraged, provided that the initial crack length is small enough. In real welded structures, the presence of defects demands longer initial crack lengths to be used in calculations, at which the benefit of enhanced strength is minimised or even inverted. Moreover, at these lengths, the growing process is mainly controlled by weld geometry and long crack propagation threshold, whereas local properties become less important in fatigue limit prediction.
Fatigue strength characterization of Al-Si cast material incorporating statistical size effect
Authors: Aigner Roman , Leitner Martin , Stoschka Michael
Published: May 2018

Cast aluminium components may exhibit material imperfections such as shrinkage and gas pores, or oxide inclusions. Therefore, the fatigue resistance is significantly influenced by the size and location of these inhomogenities. In this work, two different specimen geometries are manufactured from varying positions of an Al-Si-Cu alloy casting. The specimen geometries are designed by means of shape optimization based on a finite element analysis and exhibit different highly-stressed volumes. The numerically optimized specimen curvature enforces a notch factor of only two percent. To enable the evaluation of a statistical size effect, the length of the constant testing region and hence, the size of the highly-stressed volume varies by a ratio of one to ten between the two specimen geometries. Furthermore, the location of the crack initiation is dominated by the comparably greatest defects in this highly-stressed volume, which is also known as Weibull’s weakest link model. The crack initiating defect sizes are evaluated by means of light microscopy and modern scanning electron microscope methods. Finally, the statistical size effect is analysed based on the extreme value distribution of the occurring defects, whereby the size and location of the pores is non-destructively obtained by computed tomography (CT) scanning. This elaborated procedure facilitates a size-effect based methodology to study the defect distribution and the associated local fatigue life of CPS casted Al-Si lightweight components.
Influence of inclusion type and size on the fatigue strength of high strength steels
Authors: Schumacher Jens , Clausen Brigitte , Zoch Hans-Werner
Published: May 2018

In this investigation the influence of the inclusion type and size on the fatigue strength of different steels is analysed. As database case hardened, quenched and tempered as well as bearing steels in different heat treatment conditions, which have been investigated in several research projects over the last two decades, are used. Based on the approach of Murakami the local stress intensity at inclusions was determined to evaluate the influence of fracture causing inclusions on the fatigue strength. Different fatigue criteria have been used to calculate the local equivalent stress amplitudes considering residual stresses in the specimens, mean stresses during loading and multiaxial stresses in notched specimens. Since many run out specimens were subsequently fatigued at a higher stress amplitude, the critical inclusion type and size which have survived the initial stress amplitude could be determined. As a result the local stress intensity at inclusions which leads to no fatigue failure could be calculated as well. With the knowledge of the stress intensity factor range of the fractured and run out specimens a threshold stress intensity factor range could be derived for the different steels and inclusion sizes.
Interactions between geometrical defects and microstructure during high cycle fatigue of polycrystalline aluminium with different grain sizes
Authors: Bracquart Benoît , Mareau Charles , Saintier Nicolas , Morel Franck
Published: May 2018

In this work, the influence of the geometrical defect size on the high cycle fatigue behavior of polycrystalline aluminium with different grain sizes is investigated, to better understand the role of internal length scales. Two sizes of grains and defect are used: 100 μm and 1000 μm, the grain size being controlled with thermomechanical treatments. Fully reversed stress-controlled fatigue tests are then carried out. According to fatigue test results, surface crack initiation is delayed when the grain size is reduced, while an approximation of the fatigue limit shows that it is not much influenced by the average grain size. The relative defect diameter (compared to the grain size) seems to be the leading parameter influencing fatigue crack initiation from a defect. Finally, Electron BackScattered Diffraction (EBSD) maps are collected for specimens with large grains and small defects. Fatigue crack initiation from a defect is found to be strongly impacted by the crystallographic orientation of the surrounding grain, crack initiation preferably occurring in crystals being favorably oriented for plastic slip.
Development of a generalized Kitagawa diagram for cast G21Mn5 steel
Authors: Schuscha Manuel , Leitner Martin , Stoschka Michael
Published: May 2018

An accurate assessment of cast steel components including cast imperfections minimizes production rejects and moreover facilitates lightweight design. In order to improve the fatigue assessment of cast steel structures exhibiting macroscopic imperfections, a generalized Kitagawa diagram based on the linear elastic fracture mechanical theory is presented. Therefore, single-edge-notch-bending crack propagation tests as well as extensive fatigue tests utilizing un-notched base material specimens are conducted. Both the stress intensity factor threshold range and the plain material fatigue limit act as basis to set-up the generalized Kitagawa diagram. In addition, fatigue tests under rotating bending as well as axial loading using round V-notched specimens with varying opening angles are additionally conducted for validation. It was observed that the notch-stress-intensity-factor (NSIF) calculation based on El-Haddad’s material length leads to slightly non-conservative designs for this cast material. But if the intrinsic material length is calculated by Neuber’s stress averaging approach or Peterson’s microstructural length instead, the presented NSIF-based fatigue assessment reveals a sound comparability to the experimentally determined fatigue reference strength. Summing up, the presented generalized Kitagawa diagram provides a feasible engineering-applicable fatigue assessment tool incorporating varying imperfection opening angles and load conditions for G21Mn5 cast steel.
The use of areal surface topography characterisation in relation
to fatigue performance
Authors: Zabala Alaitz , Blunt Liam , Tato Wilson , Aginagalde Andrea , Gomez Xabier , Llavori Iñigo
Published: May 2018

Although the effect of surface topography on fatigue life is widely accepted, the underlying role of surface roughness from first principles is still poorly understood. Currently approaches which consider the influence of surface roughness on fatigue life prediction can be broadly classified into surface corrector factors (Cs) and stress concentration factors (Kt). Those approaches describe the surface according to the manufacturing process (machined, grounded…), or using 2D height descriptor parameters (Ra, Rz…). However, these approaches are not able to correctly describe the effect of roughness on the fatigue performance where it is anticipated a richer set of surface descriptors would show correlation. The present work aims to highlight the inherent limitations of the most commonly employed 2D surface measurement and characterization techniques, and provides an insight into the application of 3D areal surface characterization processes including the use of the latest areal surface topography parameters quantifying amplitude, spatial, and hybrid topographical information which is considered to be important for fatigue performance correlation.
Study of load history effects on the high cycle fatigue properties of high-strength low-alloy steel from self-heating measurements
Authors: Louge Julien , Doudard Cédric , Calloch Sylvain , Weber Bastien
Published: May 2018

In the context of high cycle fatigue (HCF), the experimental characterization of the fatigue properties is often performed by using specimens in a virgin state (i.e., without preliminary loading), and with a constant stress amplitude for each specimen. However, the load history applied to a real structure is more complex and the fatigue life prediction remains a difficult task because of the time dedicated to the classical fatigue tests (i.e., the specimen is loaded until failure) and the dispersion of fatigue lives. The load history effects on the HCF properties is characterized using an alternative method: self-heating measurements under cyclic loadings. This method is based on the observation of the mean steady state temperature evolution of a specimen under a successive series of cyclic loadings with increasing stress amplitude for each loading series. A probabilistic two-scale model was developed from the self-heating method able to predict HCF properties. Some self-heating tests are performed to study the influence of a load history effects. It seems that the plasticity is the most influential factor. So, the evolution of the plasticity is observed at the surface of the material under cyclic loading. There is a significant evolution in function of the plastic pre-strain.
Effect of size of alpha phases on cyclic deformation and fatigue crack initiation during fatigue of an alpha-beta titanium alloy
Authors: Sun Qiaoyan , Tan Changsheng , Xiao Lin , Sun Jun
Published: May 2018

Alpha phase exhibits equiaxed or lamellar morphologies with size from submicron to microns in an alpha-beta titanium alloy. Cyclic deformation, slip characteristics and crack nucleation during fatigue in different microstructures of TC21 alloy (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si) were systematically investigated and analyzed. During low-cycle fatigue, equiaxed microstructure (EM) in TC21 alloy exhibits higher strength, ductility and longer low-cycle fatigue life than those of the lamellar microstructure (LM). There are more voids in the single lamellar alpha than the equiaxed alpha grains. As a result, voids more easily link up to form crack in the lamellar alpha phase than the equiaxed alpha phase. However, during high-cycle fatigue, the fine lamellar microstructure (FLM) shows higher fatigue limit than bimodal microstructure (BM). The localized plastic deformation can be induced during high-cycle fatigue. The slip bands or twins are observed in the equiaxed and lamellar alpha phases(>1micron), which tends to form strain concentration and initiate fatigue crack. The localized slip within nanoscale alpha plates is seldom observed and extrusion/intrusion dispersedly distributed on the sample surface in FLM. This indicates that FLM show super resistance to fatigue crack which bring about higher fatigue limit than BM.
Multiaxial fatigue life estimation in low-cycle fatigue regime including the mean stress effect
Authors: Scorza Daniela , Carpinteri Andrea , Fortese Giovanni , Ronchei Camilla , Vantadori Sabrina , Zanichelli Andrea
Published: May 2018

The goal of the present paper is to discuss the reliability of a strain-based multiaxial Low-Cycle Fatigue (LCF) criterion in estimating the fatigue lifetime of metallic structural components subjected to multiaxial sinusoidal loading with zero and non-zero mean value. Since it is well-known that a tensile mean normal stress reduces the fatigue life of structural components, three different models available in the literature are implemented in the present criterion in order to take into account the above mean stress effect. In particular, such a criterion is formulated in terms of strains by employing the displacement components acting on the critical plane and, then, by defining an equivalent strain related to such a plane. The Morrow model, the Smith-Watson-Topper model and the Manson-Halford model are applied to define such an equivalent strain. The effectiveness of the new formulations is evaluated through comparison with some experimental data reported in the literature, related to biaxial fatigue tests performed on metallic specimens under in-and out-of-phase loadings characterised by non-zero mean stress values.
PWR Effect On Crack Initiation Under Equi-biaxial Loading
Authors: Gourdin Cédric , Dhahri Hager , Perez Grégory , Courtin Stéphan , Le Roux Jean-Christophe , Matirouman Habibou
Published: May 2018

The lifetime extension of the nuclear power stations is considered as an energy challenge worldwide. That is why, the risk analysis and the study of various effects of different factors that could potentially prevent safe long term operation are necessary. These structures, often of great dimensions, are subjected during their life to complex loading combining varying mechanical loads, multiaxial, with nonzero mean values associated with temperature fluctuations and also PWR environment. Based on more recent fatigue data (including tests at 300°C in air and PWR environment, etc…), some international codes (RCC-M, ASME and others) have proposed and suggested a modification of the austenitic stainless steels fatigue curve combined with a calculation of an environmental penalty factor, namely Fen, which has to be multiplied by the usual fatigue usage factor. The aim of this paper is to present a new device "FABIME2E" developed in the LISN in collaboration with EDF and AREVA. These new tests allow quantifying the effect of PWR environment on disk specimen. This new device combines the structural effect like equi-biaxiality and mean strain and the environmental penalty effect with the use of PWR environment during the fatigue tests.
An innovative system for uncoupled bending/torsion tests by tri-axis shaker: numerical simulations and experimental results
Authors: Zanellati Davide , Benasciutti Denis , Tovo Roberto
Published: May 2018

This work describes a new testing system for applying a coupled/uncoupled bendingtorsion loading in vibratory tests by a tri-axis shaker. The system is composed of a cylindrical specimen with eccentric tip masses, excited by horizontal and/or vertical base accelerations. The specimen tip is constrained by a lateral thin and flexible plate which impedes any bending when the specimen is excited horizontally, but which permits the specimen torsional rotation. This layout then allows torsional and bending deformations to be produced and controlled independently, when vertical and horizontal base accelerations are applied simultaneously. A finite element model is first used to estimate the system dynamic response and the stresses in the notched specimen section. The model is then validated through experimental tests under harmonic base accelerations. The strains at clamping system are also monitored to indirectly estimate the bending and torsion moment in the specimen. Comparison of numerical and experimental results showed a close correlation and proved that bending-torsion loading are truly uncoupled. Preliminary fatigue tests with harmonic bending loading (vertical base excitation) are finally compared to the constant amplitude S-N curve, showing a quite satisfactory agreement.
On the applicability of multiaxial high cycle fatigue criteria to metallic materials
Authors: Gonçalves Roberta A. , Pereira Marcos V. , Darwish Fathi A.
Published: May 2018

A comparative study is made of the applicability of critical plane based multiaxial high cycle fatigue models to predicting the fatigue behavior of metallic materials. A number of models, namely Matake, McDiarmid, Carpinteri and Spagnoli, Liu and Mahadevan and Papadopoulos, were applied to fatigue limit states, involving synchronous fully reversed in-phase sinusoidal bend and torsion loading. The results obtained indicated a good predictive capability of the models with an average error index situated approximately between -5,5% and 4,5%. However, this average was limited to less than 3% for the latter three models. Finally, the critical plane orientation, which, for a given material, is characteristic of the proper model, is compared with that of the fracture plane, exclusively determined by the ratio between the shear stress and normal stress amplitudes.
Cyclical fatigue of annealed and of thermally tempered soda-lime-silica glass
Authors: Schneider Jens , Hilcken Jonas
Published: May 2018

We present experimental and theoretical investigations on the cyclic fatigue of annealed and of thermally tempered soda-lime-silica glass. Static fatigue due to subcritical crack growth at micro cracks significantly decreases the macroscopic strength of soda-lime-silica glass and causes a time-dependent strength reduction. A subsequent thermal tempering process is typically used to induce residual surface compression stresses, which inhibit the crack growth of surface cracks, and corresponding bulk tension stresses. From the experimental results we show that the existing models for static fatigue used in linear elastic fracture mechanics can be used for the lifetime prediction of cyclically loaded annealed glass and thermally tempered glass, although the (static) crack growth exponent slightly decreases in cyclic loading. The equivalent duration of tensile stress at the crack tip of a micro crack governs the crack growths and not the number of cycles. The threshold for subcritical crack growth determined from the cyclic experiments was found to be in good agreement with data from literature. But unlike in strength tests with singular and quasi-static re-loading, it could be found that periodic loading with load free intervals does not lead to a strength increase by crack healing effects. Based on the results, an engineering design concept for cyclically loaded glass is presented.
Experimental Life Improvement Quantification of Shot Peening and Fastener Modifications
Authors: Renaud Guillaume , Martin Pascal , Liao Min
Published: May 2018

Two coupon test programs were recently carried out at the National Research Council of Canada to certify shot peening and fastener modifications that aim at extending the lives of military aircraft components that are short of their revised usage goals. The life improvement associated with each modification was experimentally quantified by comparing the lives resulting from variants of each modification with that of pre-modification baseline configurations. This approach allowed determining the effects of several parameters, such as the applied stress level, geometric configuration, and modification specifications, on the life extension.
Investigations On Crack Propagation Under Cyclical Isothermal And Thermo-mechanical Loadings For A Type 304-L Stainless Steel Used For Pressurized Water Reactor
Authors: Gourdin Cédric , Perez Gregory , Le Delliou Patrick , Leopold Gaëlle
Published: May 2018

The integrity of structures exhibiting flaws in Pressurized Water Reactor (PWR) has to be assessed to meet safety criteria. This paper deals with crack-propagation under cyclic thermo-mechanical loadings, as encountered in class I austenitic pipes of PWR’s. To have a conservative and reliable assessment of the crack propagation due to the in-service loading, various codes and standards use simplified method. For example, the RSE-M Code introduces a plastic correction depending on the proportion of the mechanical loading. An improvement of the current method requires additional investigations. Moreover, components loaded with transient or thermal fluctuations are not really in loadcontrolled conditions. To this end, a device called PROFATH was designed. The specimen is a pre-cracked thick-walled tube undergoing a set of thermal cycles and loaded with a static mechanical force. During the first part of the thermal cycle, a high frequency induction coil heats the external wall of the tube. Then, the heating system stops and the specimen is cooled down by running water inside the tube. Finite element calculations show that only a region half-way along the tube should be heated to ensure adequate structural effect. In the heated zone, the machining of a sharp circumferential groove ensures the propagation of a unique crack. An electro-mechanical jack controls the level of the mechanical static load. Tests have been carried out, and these tests allow having an evaluation of the pertinence of the correction proposed by the RSE-M Code for a significant plasticity.
Towards a temperature dependent and probabilistic lifetime concept for nodular ductile cast iron materials undergoing isothermal and thermo-mechanical fatigue
Authors: Garcia Trelles Elena , Schweizer Christoph , Eckmann Stefan
Published: May 2018

In this investigation, the fatigue behaviour of a ductile cast iron with high content of silicon and molybdenum, was experimentally characterized by performing isothermal low cycle fatigue (LCF) tests as well as out-of-phase thermomechanical fatigue (OPTMF) tests within the temperature range RT – 500 °C. The studied material shows an embrittlement at temperatures nearby 400 °C. A possible explanation for the observed lifetime reduction is intergranular embrittlement (IE). A mechanism based lifetime model is proposed for assessing the lifetime. The model is based on the assumption that the crack advance per cycle is correlated with the cyclic crack tip opening displacement (ΔCTOD) attributed to the crack tip blunting caused by accumulation of plastic and creep deformations ahead of the crack tip. Intergranular embrittlement is accounted for by introducing a temperature and strain rate dependent prefactor in the crack growth law, which only acts in a certain temperature range. The model is calibrated for a GJS material and successfully applied to predict the lifetime of this material when undergoing isothermal and non-isothermal mechanical loadings. A probabilistic interpretation of the scatter of the investigated material is presented in conjunction with the random nature of the initial defect size distribution.
Characterization and modelling of both high-cycle hightemperature fatigue behaviour of Stainless Steel Grades
Authors: Santacreu Pierre-Olivier , Prudhomme Cloé , Proult Benoit , Evenepoel Isabelle
Published: May 2018

In the same context of thermo-mechanical fatigue and high temperature applications of stainless steel, high-frequency vibration fatigue at high temperatures should be considered, in particular for automotive exhaust gas applications. In fact one of the most frequent incidents that can happen on exhaust components is an accumulation of low-cycle thermal fatigue and high-cycle fatigue. The prediction of the lifetime of a structure under such complex thermal and mechanical loading is therefore a constant challenge at high temperature due to the coupling of metallurgical, oxidation or creep effects. In order to better understand in a first approach, the high cycle fatigue of stainless steels at high temperatures, tractioncompression tests were performed on flat specimens at 25Hz, under air and in isothermal conditions from ambient temperature to 850°C. Two different stress ratios, R=-1 and 0.1, are characterized with the objective to assess a multiaxial model for high temperature. Different criteria are used to predict the ruin of a structure under high-cycle fatigue but in general for ambient-around temperatures. In particular, multiaxial and stress-based DangVan criterion is today widely used to evaluate the risk of fatigue cracks initiation and it has been implemented recently in our fatigue life processor Xhaust_Life®. Therefore the Dang Van criterion was identified from the isothermal high cycle fatigue using the 2 stress ratio and its validity is discussed especially for temperatures higher than 500°C where strain rate and creep effects have increasing influence. Results are presented for two ferritic stainless steel grades used in high temperature exhaust applications: K41X (AISI 441, EN 1.4509) and K44X (AISI 444Nb, EN 1.4521).
Fatigue crack initiation and propagation of 100Cr6 steel under torsional loading in very high cycle regime
Authors: Xue Hongqian , Gao Tao , Sun Zhidan , Zhang Xianjie
Published: May 2018

Cyclic torsional fatigue properties of a high strength steel 100Cr6 are investigated using an ultrasonic torsional fatigue testing machine, and the results are compared with those obtained with fatigue tests under axial loading. Fatigue crack initiation and growth under torsion loading are observed in the very high cycle regime. Results show that fatigue cracks initiated from specimen surface as well as subsurface inclusions under torsion loading. However, subsurface MnS inclusions play a dominant role in crack initiation under torsion loading in the very high cycle regime. The initiation and early propagation of fatigue cracks are mostly controlled by the direction of the maximum shear stress. For surface crack initiation, cracks initiated in parallel to the longitudinal direction of the specimens. Once the shear crack propagated to a crack length of about 20-30 μm, crack branched to the angle close to the direction perpendicular to the remote maximum principal stresses. As for the subsurface fatigue crack initiation, the cracks parallel to the longitudinal direction of the specimens could not be observed, and crack propagation followed a spiral shape on a plane with an orientation of 45° with respect to the loading direction, which corresponds to the maximum principal stress plane.
Influence of surface morphology on the very high cycle fatigue behavior of metastable and stable austenitic Cr-Ni steels
Authors: Boemke Annika , Smaga Marek , Beck Tilmann
Published: May 2018

The present study investigates conventional and cryogenically turned specimens of metastable austenitic steel AISI 347 and stable austenitic steel AISI 904L in the VHCF regime. The cryogenic turning process includes cooling by CO2 snow and generates a surface layer on the specimens of metastable austenitic steel, which is characterized by a phase transformation from paramagnetic fcc - austenite to ferromagnetic bcc - martensite and grain refinement. The stable austenitic steel retains its purely austenitic structure after cryogenic turning, but also shows grain refinement in the surface layer. The specimens with different surface morphology were cyclically loaded at ambient temperature using an ultrasonic fatigue testing system developed and built at the authors’ institute. The testing machine operates at frequencies of approx. 20 kHz to achieve high numbers of load cycles within a reasonable time. To avoid self heating of the specimen, the tests were performed in pulse-pause mode. All specimens were tested with a load ratio of R = -1. During cyclic loading, the metastable austenitic steel partially transformed from paramagnetic fcc - austenite to ferromagnetic bcc - martensite, while no phase transformation could be detected in the stable austenitic steel.
Rotary bending as a mean for improving micro-cleanliness of stainless steels for high demanding applications
Authors: Sourisseau Thomas , Ferrière Dominique , Moser Frédéric , Libert Maximilien , Chemelle Pierre , Deville-Cavellin Christian
Published: May 2018

Stainless steels are used for automotive or medical applications which require a high fatigue resistance correlated to a high level of micro-cleanliness. A methodology based on rotary bending tests carried out on wires or bars has been defined to determine the material’s endurance limit (after 100 millions cycles) and identify the largest subcutaneous inclusions or precipitates where failures initiate. This methodology has been applied to EN 1.4568 spring wires. Failures were found to initiate both at oxide inclusions and AlN precipitates. For the same size, AlN precipitates were observed to be more critical towards crack initiation than oxide inclusions, due to their angular shape and lower thermal expansion at high temperatures. However, oxide inclusions larger than the AlN maximum size strongly impact the material’s fatigue limit, and their density and size should be reduced.
Fatigue behavior of 590MPa Tensile Strength galvanneal coated sheet steel laser welded blanks
Authors: Bhat Shrikant P. , Soldaat Ronald , Tandon Gagan
Published: May 2018

The use of Laser Welded Blanks (LWBs) with different grade/gauge combinations in automotive body structures is well established; however, the acceptance of LWBs in fatigue critical chassis and underbody components has been slower because of lack of reliable models for durability assessment of laser welded joints (LWJ). Most prior fatigue studies of LWBs are carried out in tension - tension loading mode, making it difficult to relate it to the cyclic deformation and fatigue behaviors of the substrate steel grade. In contrast, in this study, LWJ is conceptualized as a "notch" to estimate the local stresses from the strain - life data of the parent grade and the fatigue notch concentration factor (Kf) is estimated from the nominal stress values of LWJ. The method is illustrated with strain controlled fatigue data for 1.4 mm galvanneal coated 590 MPa steel and fully reversed, fatigue data for homogeneous and heterogeneous LWB combinations. The results indicate that for both homogeneous and heterogeneous LWJ configurations, Kf increases with fatigue life, but tends to saturate at life levels greater than about million cycles. Considering 105 cycles to failure as an example, Kf is estimated as 1.07 for the homogeneous and 1.25 for the heterogeneous combinations.