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At present, due to advanced fatigue calculation models, it is becoming more crucial to find a reliable source for design S–N curves, especially in the case of new 3D-printed materials. Such obtained steel components are becoming very popular and are often used for important parts of dynamically loaded structures. One of the commonly used printing steels is EN 1.2709 tool steel, which has good strength properties and high abrasion resistance, and can be hardened. The research shows, however, that its fatigue strength may differ depending on the printing method, and may be characterized by a wide scatter of the fatigue life. This paper presents selected S–N curves for EN 1.2709 steel after printing with the selective laser melting method. The characteristics are compared, and conclusions are presented regarding the resistance of this material to fatigue loading, especially in the tension–compression state. A combined general mean reference and design fatigue curve is presented, which incorporates our own experimental results as well as those from the literature for the tension–compression loading state. The design curve may be implemented in the finite element method by engineers and scientists in order to calculate the fatigue life.

Implantoplasty (IP) is used in dental implants with peri-implantitis and aims to remove threads and polish rough surfaces in order to prevent bacterial colonization. As a result of this procedure, implant strength might be compromised. We tested 20 tapered screw-shaped Ti6Al4V dental implants with a simulated bone loss of 50%. Ten implants underwent IP and 10 served as controls. Surface topography (Sa, Sz, Ssk, and Sdr) was analyzed with a confocal optical microscope. Subsequently, a minimum of four series of cyclic loads were applied with a servo-hydraulic mechanical testing machine (5 × 106 cycles at 15 Hz, between a maximal nominal value–starting at 529 N in the IP group and 735 N in the control group–and 10% of that force). We recorded the number of cycles until failure and the type of failure. Implant failure was analyzed by visual inspection and scanning electron microscopy. Open circuit potential and potenctiodynamic tests were carried out with high precision potentiostat using Hank’s solution at 37 °C to evaluate the effect of the implantoplasty on the corrosion resistance. Implantoplasty significantly reduced the surface topography values (median) and interquartile range (IQR); Sa from 1.76 (IQR = 0.11) to 0.49 (IQR = 0.16), Sz from 20.98 (IQR = 8.14) to 8.19 (IQR = 4.16), Ssk from 0.01 (IQR = 0.34) to −0.74 (IQR = 0.53) and Sdr from 18.20 (IQR = 2.26) to 2.67 (IQR = 0.87). The fatigue limits of the control and implantoplasty groups were 551 N and 529 N, respectively. The scanning electron micrographs showed fatigue striations indicating fatigue failure. Besides, the fractographic analysis revealed a typical brittle intergranular fracture mechanism. The infinite life range of the dental implants evaluated was largely above the threshold of usual chewing forces. Implantoplasty seems to render a fairly smooth surface and has a limited impact on fatigue resistance. In addition, implantoplasty produces a decrease in the corrosion resistance of the implant. Corrosion current density from 0.019 μA/cm2 for as-received to 0.069 μA/cm2 in the interface smooth-roughened dental implant. These places between the machining and the rough area of the implant are the most susceptible, with the appearance of pitting.

Wire arc additive manufacturing (WAAM), also known as directed energy deposition (DED) process, is an efficient additive manufacturing technology, offers high potential to rapidly fabricate large-scale parts with complex geometries layer-by-layer. However, the fundamental understanding of the fatigue behaviour of such parts and the material requirements need to be significantly improved at all levels before this unique technology can be implemented for critical applications. This work aims to investigate the fatigue behaviour of WAAM built ER70S-6 steel under uniaxial, torsion and multiaxial loading conditions. Specimens were extracted in two different orientations: vertical and horizontal, to explore if the orientation direction has any effect on the fatigue results. Scanning Electron Microscopy (SEM) was conducted to examine the fracture surface of broken specimens and identify crack initiation regions and fracture mechanisms. The obtained results were compared with the fatigue data available in the literature on common structural steels fabricated using conventional welding and WAAM technique, showing similar fatigue behaviour with wrought S355 specimens. Moreover, the uniaxial data set on ER70S-6 WAAM specimens was evaluated according to the DNV RP-C203 standard for continuous welds, demonstrating advantageous fatigue resistance in the examined material.

This study reports the variability of the fatigue strength of specimens manufactured by the laser powder bed fusion process with respect to their location on the build plate. Specimens from the right-hand and left-hand halves of the build plate were tested under high cycle fatigue. Comparison of the fatigue data suggests that the specimens manufactured on the right-hand half of the build plate have a higher fatigue strength than those manufactured on the left-hand half. One reason for the observed discrepancy in fatigue strength was the higher accumulation of spattered powder particles on the left-hand side as compared to the right-hand side of the build plate. These spattered particles are oxidised, and form defects such as inclusions within the specimen. © 2021 The Author(s)

Wind turbine rotor blades are subject to highly dynamic loads and designed for life cycles of at least 20 years, which means that materials are subjected to high-cycle fatigue. Fatigue is a design-driving loading for current and future blades. Bond lines of blades are exposed to a multi-axial stress-state due to the anisotropic thin-walled blade structure and curved, tapered, twisted, and airfoil-shaped blade geometry. To eliminate undesirable failure modes and thus increase the reliability of wind turbine rotor blades, standards and guidelines recommend that the multi-axial stress-states be taken into consideration for the limit state analysis. In addition, thermal residual stresses that develop during manufacture can have a significant impact on the fatigue life of the bond line. By means of a cyclic full-scale blade test of a commercial 81.6m long offshore blade, we validate a crack initiation model, which takes into account multi-axial thermal and mechanical stress-states, as well as the probabilistic stress-life, to predict the edge of crack initiation in the adhesive as well as the span-wise position. Both observations agreed well with the simulations. All residual normal stress components and cross-sectional plane shear stress made up the major part of the mean equivalent stress, while the mechanical stress amplitude components - longitudinal, peel, and cross-sectional plane shear stress - made up the major part of the equivalent stress amplitude. Paper: https://doi.org/10.1088/1742-6596/2265/3/032045

Fatigue causes cracking or breakage in a material due to repeated loads; it causes the material to become unusable. Therefore, knowing the fatigue life of materials is crucial for the implementation of designs, economy and human life. Soft computing methodologies, a subset of artificial intelligence emerging to simulate human intelligence, deal with approximate models and seek solutions to complex real-life problems relying on both computational power of machines and the high accuracy of the algorithms. In this study, soft computing methods adapted for estimating/predicting the fatigue life of engineering structures and materials are investigated. For this purpose, 95 articles published between 1995 and 2020 have been examined in detail. With this review, it is aimed to reveal the efficiency of soft computing methods and contribute to their development. Recommendations have been made to draw attention to these methodologies which are expected to be used in many areas in the future. © 2020 Wiley Publishing Ltd.

The fatigue behaviour of notched and unnotched specimens produced by additively manufactured Inconel 718 were analysed in the as‐built and heat‐treated conditions. The surfaces display high roughness and defects acting as fatigue initiation sites. In the as‐built condition, fine subgrains were found, while in in the heat‐treated state, the subgrains were removed and the dislocation density recovered. SN‐curves are predicted based on tensile properties, hardness and defects obtained by fractography, using the urn:x-wiley:ffe:media:ffe13300:ffe13300-math-0001‐method. © 2020 The Authors. Fatigue & Fracture of Engineering Materials & Structures published by John Wiley & Sons Ltd This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

In this study, the fatigue strength of 316 L stainless steel manufactured by selective laser melting (SLM) is evaluated. The effect of powder layer thickness and postmachining is investigated. Specimens were produced with 30 and 50 µm layer thickness and tested under high cycle fatigue in as-printed and postmachined conditions. Examination of the specimens reveals that in the as-printed condition, fatigue strength suffers from high roughness and surface tensile residual stresses as well as defects such as pores and lack of fusion voids. After machining, the fatigue strength was improved due to lower surface roughness, presence of compressive residual stresses, and removal of surface porosity. The results show that increasing the layer thickness (within the range tested) has a minor negative impact on fatigue strength; however, it has a major positive impact on the productivity of the SLM process. In addition, it is clear that the impact of postmachining on fatigue is far greater than that of the layer thickness. © 2020, The Author(s). Funding details: 2015-03457; Funding text 1: Open access funding provided by RISE Research Institutes of Sweden. This work was supported by Sweden’s Innovation Agency [Grant Number: 2015-03457]. Ms. Anna Larsson and Mr. Heike Henrich from Höganäs AB are acknowledged for performing porosity and metallography analyses.

The present investigation is concerned with the high-cycle axial fatigue behaviour of a third generation Al-steel butt weld made by Hybrid Metal Extrusion & Bonding (HYB). In this particular weld, metallurgical bonding is achieved by a combination of microscale mechanical interlocking and intermetallic compound (IMC) formation, where the IMC layer is in the sub-micrometre range (

Nominal stresses have been used for a long time for the assessment of fatigue resistance of welded joints, however, this approach has strong limitations since the definition of the nominal stress may be subjective for complex welded details and/or complex loading. On the other hand, the hot-spot stress approach has been proposed to overcome these limitations considering the structural geometrical discontinuities. However, the hot-spot stress methods also present certain limitations, and the present study aims at evaluating the available numerical and analytical hot-spot stress methods proposed by DNVGL (2016) and IIW (2014). The particular case of an offshore tubular KT joint has been considered herein and discretized in two planes. It has been studied numerically using the ABAQUS software coupled with the hot-spot stress extrapolation methods described in IIW (2014) and DNVGL (2016). The influence of the weld geometry has been considered and evaluated. In addition to the numerical method, the present study has also considered the analytical approach proposed in DNVGL (2016) derived from the combination of Efthymiou solutions for the stress concentration factor with the method of superposition of stresses. The numerical models according to IIW (2014) have been found to be more conservative when compared with the mesh-size methods proposed by DNVGL (2016), both in numerical modelling without the weld or with weld. For the numerical models with weld cord, the mean values of normalized difference index obtained for all braces together, as a result of comparing numerical results with analytical solutions, are lower, when compared with results obtained from the numerical models without weld cord.

This paper proposes a thermal method for estimating fatigue life of metallic ropes. Rotating bending fatigue tests are performed on rope specimens and their temperature evolution is monitored till specimen failure. The proposed method is justified through a theoretical framework based on thermally dissipated energy and energy spent for mechanical damage. Experimental fatigue tests are carried out in different conditions by primarily varying the bending load and the rotational speed. Experimental results are in agreement with the theoretical model allowing the estimation of damage evolution and rope life via thermal measurements.

An estimation of the elastic-plastic stress state using elasticity-theoretical input data is an essential part of the service life estimation with the local strain approach in general and a German guideline based on it, in particular. This guideline uses two different notch root approximations (an extended version of Neuber’s rule and an approach according to Seeger and Beste) for this estimation. Both require the implementation of Newton’s method to be iteratively solved. However, many options are left open to the user concerning implementation in program code. This paper discusses ways in which notch root approximation methods can be implemented efficiently for use in software systems and elaborates an application recommendation. The following aspects and their influence on the computational accuracy and performance of Newton’s method are considered in detail: influence of the formulation of the root finding problem, determination of the derivative required for Newton’s method and influence of the termination criterion. The investigation shows that the advice given in the abovementioned guideline indeed leads to a conservative implementation. By carefully considering the investigated aspects, however, the computational performance can be increased by approximately a factor of 2–3 without influencing the accuracy of the service life estimation.

Accurate knowledge of the plastic zone of fatigue cracks is a very direct and effective way to quantify the damage of components subjected to cyclic loads. In this work, we propose an ultra-fine experimental characterisation of the plastic zone based on Vickers micro-indentations. The methodology is applied to different compact tension (CT) specimens made of aluminium alloy 2024-T351 subjected to increasing stress intensity factors. The experimental work and sensitivity analysis showed that polishing the surface to #3 μm surface finish and applying a 25 g-force load for 15 s produced the best results in terms of resolution and quality of the data. The methodology allowed the size and shape of both the cyclic and the monotonic plastic zones to be visualised through 2D contour maps. Comparison with Westergaard’s analytical model indicates that the methodology, in general, overestimates the plastic zone. Comparison with S355 low carbon steel suggests that the methodology works best for alloys exhibiting a high strain hardening ratio.

cited By 0; International audience; This work presents a new ultrasonic fatigue testing device for studying the initiation and propagation mechanisms of internal microstructurally short fatigue cracks using in situ synchrotron tomography. Its principle is described as well as the method used for automatically detecting crack initiation and its subsequent growth. To promote internal crack initiation, specimens containing internal casting defects were tested between the high cycle and very high cycle fatigue regimes (107-109 cycles). Preliminary results show the ability of this new device to initiate an internal microstructurally short crack in a reasonable testing time and monitor its growth. © 2019 Wiley Publishing Ltd.

cited By 0; Non-proportional axial-torsional loading fatigue tests on nodular cast iron revealed a non-symmetrical behavior of the fatigue life relatively to the phase shift angle δ = 90° between axial and torsion loads. This behavior was in agreement with fatigue crack orientation investigated using optical and electron microscopy. Elasto-plastic finite element model (FEM) were used to assess the crack orientation behavior and it could explain this non-symmetrical behavior by the non-symmetrical values of both σ I and σ I / σ II ratio relatively to δ = 90°. FEM revealed that for the δ < 90° loads, principal stress was applied essentially at critical planes of low angle ϕ n , thus occurrence of tensile cracks mode was higher for δ < 90° than for δ > 90° load, which reduces the fatigue life. σ I / σ II ratio strongly influenced the dominant crack mode. When the applied loads were torsion σ I /σ II = 1, cracks were observed to occur, equally, in modes I and II. Presence of the non-propagating failure (mode II) significantly increased the fatigue life. Conversely, for high phase shift, where σ I / σ II ≫ 1, crack mode I dominated and crack driving force remained high during the whole fatigue cycle, inducing a lower fatigue limit. © 2019

cited By 0; International audience; This study investigates the influence of a rapid heat treatment followed by water-quenching on the mechanical properties of Ti[sbnd]6Al[sbnd]4V ELI alloy to improve its strength for use in implants. Prior to the experiment, a dilatometry test was performed to understand the progressive α-to β-phase transformation taking place during heating. The results were then used to carry out heat treatments. Microstructure was analysed using SEM, EBSD, EDX and XRD techniques. Vickers micro-hardness, tensile and high cycle rotating bending tests were used to analyse the influence of the α'-phase fraction on the strength of the studied alloy. Results show that this process can provide a Ti[sbnd]6Al[sbnd]4V ELI alloy with a better Yield Strength (YS)/uniform deformation (ε u ) ratio and improved high cycle fatigue strength than those observed in the current microstructure used in medical implants. Lastly, cytotoxicity tests were performed on two types of human cells, namely MG63 osteoblast-like cells and fibroblasts. The results reveal the non-toxicity of the heat-treated Ti[sbnd]6Al[sbnd]4V ELI alloy. © 2019 Elsevier Ltd

The fatigue behaviour of additively manufactured (AM) 316L stainless steel is investigated with the main emphasis on internal porosity and surface roughness. A transition between two cases of failure are found: failure from defects in the surface region and failure from the internal defects. At low applied load level (and consequently a high number of cycles to failure), fatigue is initiating from defects in the surface region, while for high load levels, fatigue is initiating from internal defects. Porosities captured by X‐ray computed tomography (XCT) are compared with the defects initiating fatigue cracks, obtained from fractography. The fatigue data are synthesised using stress intensity factor (SIF) of the internal and surface defects on the fracture surface. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. ©2019 The Authors Fatigue & Fracture of Engineering Materials & Structures Published by John Wiley & Sons Ltd.

The purpose of this paper is to investigate the effect of variable amplitude service loading conditions and wear phenomenon on reliability analysis for rail steel. For this purpose, the rail with the exact geometry of the profile in the railway systems and the boundary element method has been used. Also, the geometry correction functions of the five cracks, as a function of a crack size are determined by using nonlinear regression analysis estimating the statistical descriptors of the five geometry correction functions. Fatigue reliability is evaluated for each failure mode due to multiple site fatigue cracks propagation in rail steel.

Increasing the corrosion resistance of low-alloy steels containing less than 2.5% of doping components (except carbon), which are actively used in the construction of various buildings and structures, is one of the priorities in the development of the modern construction industry. This problem becomes especially urgent during operation of stressed elements of various structures, which are used in the conditions of possible surface exposure to moisture. As part of the research, the results of which are presented in this article, we have examined the possible reduction in the rate of electrochemical corrosion of low-alloy steels in a natural corrosive environment. The purpose of the study was to establish the effect of the composition of steels on their corrosive properties. The main method chosen was a method of thermodynamic potentials, used in conjunction with model ideas about ideal gas and free energy of diffusion of liquid, solid and gaseous solutions. The present study analyzes the process of formation of ideal binary alloy, based on which the relationship between the thermodynamic characteristics of its original components and the corrosive behavior of the alloy was established. In order to verify the detected corrosion mechanism, the process of amalgam anode dissolution was considered, which fully confirmed the results obtained earlier. Based on the analysis conducted, it can be concluded that the equilibrium potential of the ideal binary alloy in the electrolyte in no way depends on the process of its formation from the pure original components. It is determined solely by the ratio of energy characteristics of metal ions actually in the alloy, as well as the energy characteristics of the same ions in the solution of electrolyte, which comes into contact with the alloy.

In this work, the phase-field approach to fracture is extended to model fatigue failure in high- and low-cycle regime. The fracture energy degradation due to the repeated externally applied loads is introduced as a function of a local energy accumulation variable, which takes the structural loading history into account. To this end, a novel definition of the energy accumulation variable is proposed, allowing the fracture analysis at monotonic loading without the interference of the fatigue extension, thus making the framework generalised. Moreover, this definition includes the mean load influence of implicitly. The elastoplastic material model with the combined nonlinear isotropic and nonlinear kinematic hardening is introduced to account for cyclic plasticity. The ability of the proposed phenomenological approach to naturally recover main features of fatigue, including Paris law and Wöhler curve under different load ratios is presented through numerical examples and compared with experimental data from the author’s previous work. Physical interpretation of additional fatigue material parameter is explored through the parametric study.

Surfaces of metals produced by additive manufacturing (AM) are known to be rough and populated with defects, this is, in particular, true for downward-facing (down-skin) surfaces. When dealing with the fatigue of as-built surfaces produced by AM, fatigue is typically initiating from surface defects. In this work, the fatigue behaviour of maraging steel grade 300 (18Ni300) produced by laser beam powder bed fusion (PBF-LB) is investigated. Fatigue initiation from surfaces built both up- and down-skin are investigated. This is done by using specimens where all surfaces are machined, except the one at interest. Specimens were built in 10 orientation ranging from 0° (horizontal, up-skin) to 135° (down-skin). The surface roughness was measured for all orientations; high surface roughness was found for down-skin surfaces while wavy surfaces were found for up-skin surfaces. The fatigue behaviour was found to be correlated to the build orientation and the surface roughness.

Fatigue is the main phenomenon causing the failure of mechanical structures. For very long service lives the initiation of cracks leading to the failure of in service mechanical parts changes from surface to volume. Due to experimental difficulties the propagation of internal cracks has only been studied to a limited extent in situ and in 3D. However, it is crucial from a dimensional point of view to know the behaviour of such cracks. In this work, in situ monitoring of the initiation and propagation of internal cracks was carried out by X-ray tomography (synchrotron source). Firstly, a method for manufacturing specimens was developed to enable systematic, non-destructive, and 3D visualization of the propagation of internal cracks in a titanium alloy (Ti-6Al-4V) at 20 Hz. For this purpose, an artificial defect was drilled on the surface of a rolled sheet of this material. A second sheet, placed on top of the first one, was diffusion bonded by spark plasma sintering, making it possible to make the defect volumetric. In situ fatigue tests showed the crucial role of the environment (vacuum) and, to a lesser extent, that of the crystallographic texture on the internal crack propagation rates. Secondly, an ultrasonic fatigue machine (cycling frequency of 20 kHz) allowing in situ testing at the synchrotron was used to study the 3D propagation of internal cracks in A357-T6 aluminium alloy specimens containing an artificial internal defect. The 3D grain distribution obtained by Diffraction Contrast Tomography (DCT) was used to study the interaction mechanisms between the microstructure and the 3D crack path., La fatigue est le principal phénomène causant la rupture de structures mécaniques. Pour les très grandes durées de vie, l’amorçage des fissures menant à la rupture des pièces en service passe de la surface au volume. Du fait des difficultés expérimentales, la propagation des fissures internes n’a que très peu été étudiée in situ et en 3D. Il est cependant crucial du point de vue du dimensionnement de connaître le comportement de telles fissures. Dans ces travaux, le suivi in situ de l’amorçage et de la propagation de fissures internes a été réalisé par tomographie aux rayons X (source synchrotron). Premièrement, une méthode de fabrication d’éprouvettes a été élaborée pour permettre de visualiser de manière systématique, non destructive et en 3D la propagation à 20 Hz de fissures internes dans un alliage de titane (Ti-6Al-4V). Un défaut artificiel a pour cela été usiné à la surface d’une tôle laminée de ce matériau. Une seconde tôle, placée sur la première, a été soudée par diffusion au frittage flash, permettant ainsi de rendre le défaut volumique. Des essais de fatigue in situ ont permis de montrer le rôle crucial de l’environnement (vide) ainsi que, dans une moindre mesure, celui de la texture cristallographique sur les vitesses de propagation des fissures internes. Deuxièmement, une machine de fatigue ultrasonique (fréquence de cyclage de 20 kHz) permettant de réaliser des essais in situ au synchrotron a été utilisée pour étudier la propagation 3D de fissures internes dans des éprouvettes d’alliage d’aluminium A357-T6 contenant un défaut interne artificiel. L’obtention préalable de la distribution 3D des grains par tomographie en contraste de diffraction (DCT) a permis d’étudier les mécanismes d’interaction entre la microstructure et le chemin de fissuration en 3D.

Additive manufacturing of industrially-relevant high-performance parts and products is today a reality, especially for metal additive manufacturing technologies. The design complexity that is now possible makes it particularly useful to improve product performance in a variety of applications. Metal additive manufacturing is especially well matured and is being used for production of end-use mission-critical parts. The next level of this development includes the use of intentionally designed porous metals - architected cellular or lattice structures. Cellular structures can be designed or tailored for specific mechanical or other performance characteristics and have numerous advantages due to their large surface area, low mass, regular repeated structure and open interconnected pore spaces. This is considered particularly useful for medical implants and for lightweight automotive and aerospace components, which are the main industry drivers at present. Architected cellular structures behave similar to open cell foams, which have found many other industrial applications to date, such as sandwich panels for impact absorption, radiators for thermal management, filters or catalyst materials, sound insulation, amongst others. The advantage of additively manufactured cellular structures is the precise control of the micro-architecture which becomes possible. The huge potential of these porous architected cellular materials manufactured by additive manufacturing is currently limited by concerns over their structural integrity. This is a valid concern, when considering the complexity of the manufacturing process, and the only recent maturation of metal additive manufacturing technologies. Many potential manufacturing errors can occur, which have so far resulted in a widely disparate set of results in the literature for these types of structures, with especially poor fatigue properties often found. These have improved over the years, matching the maturation and improvement of the metal additive manufacturing processes. As the causes of errors and effects of these on mechanical properties are now better understood, many of the underlying issues can be removed or mitigated. This makes additively manufactured cellular structures a highly valid option for disruptive new and improved industrial products. This review paper discusses the progress to date in the improvement of the fatigue performance of cellular structures manufactured by additive manufacturing, especially metal-based, providing insights and a glimpse to the future for fatigue-tolerant additively manufactured architected cellular materials.

Gadolinium zirconate (GZ)-based TBCs comprising GZ as the top layer and yttria stabilized zirconia (YSZ) as the base layer, are attractive double-layered thermal barrier coatings (TBCs) for high temperature gas turbine engine application. This work attempts to understand the influence of individual layer microstructure on the durability of GZ/YSZ double-layered TBCs processed by suspension plasma spray (SPS). Two different spray parameters were chosen to obtain a combination of three microstructurally distinct GZ/YSZ double-layered TBCs i.e. GZ porous (P)/YSZ porous (P), GZ dense (D)/YSZ porous (P) and GZ dense (D)/YSZ dense (D). Thermal diffusivity of the as-deposited coatings was measured using Laser Flash Analysis (LFA) technique and the thermal conductivity of the TBCs was calculated. The GZ/YSZ double-layered TBC specimens were subjected to two different durability tests, i.e. thermal cyclic fatigue (TCF) and burner rig test (BRT). Sintering behavior of the individual layer TBC microstructures was evaluated by comparing the porosity evolution in as-deposited and TCF tested TBCs. Fracture toughness measurements performed on each layer of the double-layered TBCs were correlated with the durability results. Thermal cycling results amply demonstrate that the individual layer microstructure of GZ/YSZ double-layered TBC influenced its durability. Detailed failure analysis of the TCF and BRT failed specimens revealed similar failure modes for GZ (P)/YSZ (P), GZ (D)/YSZ (P) and GZ (D/YSZ (D) TBCs under identical thermal cyclic test conditions. However, failure modes differed when subjected to different thermal cyclic test conditions (TCF and BRT) and the probable causes are discussed. Findings from this work provide key insights on designing durable GZ/YSZ double-layered TBCs. The authors extend their thanks to Mr. Andreas Ottosson from GKN Aerospace AB, Sweden, for performing the burner rig test.

The results of tests under monotonically increasing load and cyclic compression load are often analysed by means of probabilistic methods. Although there is a considerable scattering in the results, especially in the number of cycles to failure, the cause of these cannot be completely explained. The imperfections of the specimens tested are among the causes of this scattering mentioned in the literature. Based on a round robin test the influence of HPC and UHPC production and specimen preparation techniques on the mean values of the compressive strengths, number of cycles to failure and data scattering have been evaluated. The main findings of the study are that the production techniques have an influence on the compressive strength, however, do not affect the mean number of cycles to failure. Moreover, the accurate preparation of the specimens has a positive influence on the compressive strength and the scattering of the results of both compression and cyclic load tests. The mean number of cycles to failure of HPC specimens is not influenced by the preparation techniques, whereas the polishing technique may have a positive influence on the mean number of cycles to failure of UHPC specimens.

International audience; This paper is focused on variable amplitude loading spectra applied in the automotive industry, more specifically for the chassis system parts with respect to high cycle fatigue design. A first analytical model referred to as Heuler's model is considered. It is of the simplest use, with one parameter to be identified. A whole identification process, based on road measurement realized on carmaker's proving ground with instrumented vehicles, is developed and discussed. Once the model is identified, the cycles influence on damage production is also investigated. In the light of the obtained results with this model, a second one, noted modified Heuler's model, is introduced and investigated. This model, similar to the first, requires the identification of two parameters. Once again, the whole model identification process is applied, and the lifetime accuracy is assessed. A new tool is presented, enabling the visualisation on the loading spectrum representation of the most and less damaging cycles. Hence, a methodology is set to legitimize a gate implementation when studying a variable amplitude loading spectrum.

The current study investigates the load controlled fatigue properties of the microplasma arc welded thin titanium Grade 5 (6Al-4V) sheets. In order to explore the effect of weld geometry on the fatigue, two different welded joints were used in the experimental studies. Load controlled fatigue test results were evaluated to present an outlook on the behaviour of microplasma welded titanium alloy Grade 5 sheets under cyclic loading. Even though the previously published monotonic tests showed successful use of microplasma arc welding to join thin titanium Grade 5 sheets with mechanical properties comparable to the base metal, fatigue life of the welded joints was lower than the lives of samples without welds. In particular, the fatigue performance of overlap joints was very poor. This was presumed to be due to the changed material properties of the heat affected zone which was formed by the excess heat of the welding process as fractures often occurred at such locations. Based on experimental findings and fractographic observations, a clear adverse effect of welding process in material behaviour was discovered. Despite the concentrated heat of microplasma arc welding, post-weld heat treatment of the weld area is recommended to improve the mechanical behaviour of the welded joints. © MDPI AG. All rights reserved.

This doctorate research project investigates the multiaxial deformation and fatigue behaviour of two Nickel-based superalloys used in the discs of aero-engines, named Waspaloy and RR1000, under proportional and non-proportional loading conditions. For that a systematic analysis has been performed to identify and quantify the biaxiality of loading effects on fatigue life, the effects of non-proportional multiaxial loading – including additional cyclic hardening and cross-hardening – on crack initiation and fatigue life, and lastly the role of material micro-structure and crystal orientation on fatigue crack nucleation. An extensive experimental programme with uniaxial and biaxial fatigue tests with Waspaloy and non-proportional tension-torsion tests with RR1000 was carried out. It was experimentally observed that the variation of biaxiality ratio, for the same maximum principal stress, was found to cause a significant change in fatigue life and failure location for Waspaloy. Furthermore, results show that RR1000 presents no load path dependency for stress levels near the yield strength of the material. On the other hand, fatigue lives and cyclic hardening are highly dependent on the degree of non-proportionality. Lives for non-proportional tests were around three times shorter than fatigue lives for proportional tests at comparable stress levels. The experimental programme was used for calibrating and benchmarking a wide range of stress-, strain- and energy-based multiaxial fatigue damage parameters. Most of them could provide a reasonable collapse of both Waspaloy and RR1000 test data. Particularly good fatigue life predictions were obtained with stress-based criteria, especially with Walker stress and Matake’s critical plane approach. Lastly, a GND-coupled crystal plasticity framework and stored energy criterion was implemented for predicting fatigue lives of RR1000. It was concluded that Wan et al.’s stored energy criterion, which combines stress, plastic slip and GNDs density represents an excellent tool for accurately predicting fatigue lives under complex multiaxial loading conditions.

The paper presents the experimental fatigue test results for cyclic constant amplitude loading conditions for the case of the torsion of the PA4 (AW-6082-T6), PA6 (AW-2017A-T4) and PA7 (AW-2024-T3) aluminum alloy for a drilled diabolo type test specimen. The tests have been performed for the stress asymmetry ratios R = −1, R = −0.7, R = −0.5 and R = −0.3. The experimental results have been used in the process of a fatigue life estimation performed for a random generated narrowband stress signal with a zero and a non-zero global mean stress value. The calculations have been performed within the time domain with the use of the rainflow cycle counting method and the Palmgren−Miner damage hypothesis. The mean stress compensation has been performed with the S-N curve mean stress model proposed by Niesłony and Böhm. The model has been modified in terms of torsional loading conditions. In order to obtain an appropriate R = 0 ratio S-N curve fatigue strength amplitude, the Smith−Watson−Topper model was used and compared with literature fatigue strength amplitudes. The presented solution extends the use of the correction model in terms of the torsional loading condition in order to obtain new S-N curves for other R values on the basis of the R = −1 results. The work includes the computational results for new fatigue curves with and without the mean stress effect correction. The results of the computations show that the mean stress effect plays a major role in the fatigue life assessment of the tested aluminum alloys and that the method can be used to assess the fatigue life under random conditions.

Titanium-based alloys are susceptible to hydrogen embrittlement (HE), a phenomenon that deteriorates fatigue properties. Ti-6Al-4V is the most widely used titanium alloy and the effect of hydrogen embrittlement on fatigue crack growth (FCG) was investigated by carrying out crack propagation tests in air and high-pressure H2 environment. The FCG test in hydrogen environment resulted in a drastic increase in crack growth rate at a certain &Delta, K, with crack propagation rates up to 13 times higher than those observed in air. Possible reasons for such behavior were discussed in this paper. The relationship between FCG results in high-pressure H2 environment and microstructure was investigated by comparison with already published results of cast and forged Ti-6Al-4V. Coarser microstructure was found to be more sensitive to HE. Moreover, the electron beam melting (EBM) materials experienced a crack growth acceleration in-between that of cast and wrought Ti-6Al-4V.

Additive manufacturing is an emerging technique that is not only subjected to the interest of academic world because of its peculiar characteristics to obtain new material properties and optimized 3D geometries, but it also finds the interest of the industrial sector because of the possibility to build advanced components never realized until now. Among the additive manufacturing processes, Laser Powder Bed Fusion process is perhaps the most used in producing components out of metallic materials. In particular, thanks to its low density and its hypoeutectic favourable composition, AlSi10Mg alloy is particular suitable for the production of lightweight components by additive manufacturing. However, for safety reasons, their mechanical, static and cyclic, characteristics need to be well understood and predicted. Unfortunately, they are dramatically influenced by process parameters that in turn may promote killer defects dangerous for the fatigue strength of load bearing mechanical components. This contribution is aimed at highlighting the influence of defects on the fatigue resistance of AlSi10Mg samples produced by laser powder bed fusion. The combination of process parameters were obtained that maximizes the fatigue strength and reduces the scattering of the results.

A novel strain-energy–density (SED) based fatigue criterion is here proposed to account for the effect of mean stress and plasticity on the uniaxial fatigue strength of plain and notched components. It is based on the definition of four SED components: Δ W ¯ el , the elastic SED associated to the stress range, Δ W ¯ el , max , the maximum elastic SED in the stabilized cycle, Δ W ¯ pl , the plastic SED dissipated per stabilized cycle, W ¯ pl , max , the plastic SED dissipated over the cycles until stabilization. The mean stress effect is incorporated in a Walker-like expression, Δ W ¯ el α W ¯ el , max 1 - α , while W ¯ pl , max is added to the expression of the total SED to include the effect of mean stress relaxation. An energetic approach is proposed to identify the condition of cycle stabilization. The coefficients of the fatigue criterion are calibrated using experimental fatigue data. The criterion is validated by predictions of independent data.

Additively manufactured components are likely to contain defects deriving from the manufacturing process. They are also likely to be designed with high geometric complexity (including notches), due to very few design-limitations. In order to understand the influence of these two types of geometric features, we here investigate as-built Inconel 718 specimens produced by selective laser melting under quasi-static and cyclic loading. Notched and unnotched specimens are considered, and the main emphasis is set on the influence of the defects and on how they influence the failure locations at different load levels.

Selective Laser Melting (SLM) process is an Additive Manufacturing (AM) technique that allows producing metallic parts of any kind of geometry with densities greater than 99.5%. Complex shapes lead however to notches with different radii of curvature that may reduce load bearing capacities. This work is aimed to assess the fatigue strength of Ti-6Al-4V blunt V-notched samples produced by SLM. Results were compared with those of the corresponding smooth samples and Environmental Scanning Electron Microscopy (ESEM) have been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Finally, the strain energy density approach was used to evaluate the critical radius value. Despite the observed fatigue strength reduction induced by the notch, samples showed a sufficient low notch sensitivity that it was not possible to define a critical radius for the material analysed. This is a submitted manuscript of an article published by Elsevier Ltd in Theoretical and applied fracture mechanics , 29 June 2017.

In this study, epoxy-based composite materials reinforced with unidirectional glass fiber (GFRP) fabric were manufactured by adding nanosized multi-walled carbon nanotube (MWCNT) and nanoclay to epoxy resin during production process. The effects of those additives on fatigue strength of the composite materials were investigated experimentally. In this context, both nanoparticles by adding 0.5 wt% of epoxy resin were mixed with ultrasonic homogenization mixer. To examine the contribution of nanoparticles, composite plates were produced as MWCNT and nanoclay filled and unfilled. Mechanical properties and fatigue behavior (S-N diagram) were obtained and the results were compared with each other. From the results obtained, fatigue and tensile strength of composite specimens with MWCNT filled were found higher than unfilled specimens. © 2018 Polish Academy of Sciences Institute of Physics. All rights reserved.

Grouted connections are frequently used as structural detail of offshore wind turbines and platforms for the load transferring connection between piles and support structure. At latticed substructures this connection is commonly located at mudline. However, a potential influence of the surrounding water on the connection's fatigue behaviour was neglected in earlier tests and consequential design methods. Herein described experimental investigations at small and large-scale fatigue tests in submerged conditions showed a significant reduction of endurable load cycles. In addition, the water impact caused varied damage mechanism in the connection. © 2017 The Author(s).

cited By 0; International audience; The influence of lubricant additives on rolling contact fatigue crack propagation and the mechanisms responsible for the resulting micro-scale damage, was studied via experiments conducted on complete transmissions in a test cell. Bench-scale tribological tests and the exposure of steel surfaces to two different formulated lubricants were also carried out. Scanning and Transmission Electronic Microscopy observations, Electron Dispersive Energy and X-ray Photoelectron Spectroscopy analyses indicated that the sulphur present in the extreme pressure (EP) additives has a positive impact on limiting damage propagation. Thanks to TEM observations of cracks, it was demonstrated that a sulphur rich film is formed at the crack tip. This film can act as both a barrier film towards hydrogen permeation within the metal and / or as an inhibitor of oil decomposition. The latter is associated with the nascent surface's ability to limit hydrogen generation. Without such hydrogen embrittlement, crack propagation is slowed down. © 2017

Defects, particularly porosity and oxides, in high-pressure die casting can seriously compromise the in-service behavior and durability of products subjected to static or cyclic loadings. In this study, the influence of dimension, orientation, and position of casting defects on the mechanical properties of an AlSi12(b) (EN-AC 44100) aluminum alloy commercial component has been studied. A finite element model has been carried out in order to calculate the stress distribution induced by service loads and identify the crack initiation zones. Castings were qualitatively classified on the basis of porosities distribution detected by X-ray technique and oxides observed on fracture surfaces of specimens coming from fatigue and tensile tests. A damage criterion has been formulated which considers the influence of defects position and orientation on the mechanical strength of the components. Using the proposed damage criterion, it was possible to describe the mechanical behavior of the castings with good accuracy.

International audience; Natural Rubber (NR) exhibits a remarkable fatigue resistance, especially for non-relaxing loadings under which a strong lifetime reinforcement is observed (Cadwell et al. 1940). Such a resistance is classically attributed to strain-induced crystallization (SIC). At the microscopic scale, it has been shown that SIC induces striations on the fracture surface of NR samples tested under fatigue loadings (Le Cam and Toussaint 2010, Muñoz-Mejia 2011, Le Cam et al. 2013, Ruellan et al. 2018). In order to provide additional information on the role of SIC in the fatigue crack growth resistance of NR, striations are investigated through post-mortem analysis after fatigue experiments carried out under both relaxing and non-relaxing loadings. Results show that two striation regimes take place. Regime 1 corresponds to small striation patches with different orientations and Regime 2 induces zones with large and well-formed striations. As fatigue striations are observed for all the loading ratios applied, they are therefore not the signature of the reinforcement. Nevertheless, increasing the minimum value of the strain amplified the striation phenomenon and the occurrence of Regime 2. The analysis carried out unifies the results obtained in the literature for relaxing and fully relaxing loadings in the sense that increasing the loading, i.e. the tearing energy, leads to an increase in the crack growth rate Lindley (1973) and to a striation typology evolution, especially the striation size (Ruellan et al. 2018).

This paper assesses the accuracy and reliability of the Theory of Critical Distances (TCD) and the Strain Energy Density (SED) approach in estimating the lifetime of plain and notched specimens subjected to cyclic loading. To validate the two approaches for plain and notched components under uniaxial and multiaxial fatigue loading, a large bulk of experimental data taken from the literature was re-analyzed, with the state variables, i.e. the stress distributions and the strain energy density, being calculated via the Finite Element (FE) method. The results obtained demonstrate that both the TCD and the SED approach can provide highly accurate fatigue life estimation. In addition, these two approaches require little computational effort and little calibration information to be implemented and used for fatigue design in situations of practical interest.

International audience; In recent years, self-heating method, which is based on temperature measurements, has been proposed as a faster and cheaper alternative to classical fatigue tests. In the present study, one-dimensional thermomechanical constitutive equations of shape memory alloys (SMAs) were implemented into the self-heating method by using a probabilistic two-scale model in order to study fatigue of these alloys. This model was shown to be capable of predicting the S/N curve of these alloys as well as its scatter by using probability relations. To identify parameters of the two-scale model, self-heating experiments were conducted. The presented approach was developed to predict the S/N curve of both raw specimens and heterogeneous ones with surface treatments for any failure probability in a much shorter time compared to the classical methods. The heterogeneous samples were made by hydrogenating the surface of virgin specimens. To investigate validity of the numerical results, the theoretical predictions were compared with experimental data and a good agreement was observed.

The Strain Energy Density (SED) fatigue criterion is based on a material control radius. The value of this length is therefore required for an accurate assessment of the fatigue strength of any, especially severely, notched components. The singularity-based control radius is initially obtained by considering the hypothetical perfectly sharp V-notched specimen. The effect of the notch radius on the inverse search is then investigated with numerical simulations, and a new analytical procedure is introduced for the determination of the (actual) control radius. This procedure is applied to the experimental data of three metal alloys with different load ratios and manufacturing conditions.