The Laser Powder Bed Fusion (L-PBF) process allows to manufacture parts with both a complex geometry and a high mechanical performance. The as-built and net-shape L-PBF 316L stainless steel parts-i.e. without any heat nor surface treatment, have tensile residual stresses, subsurface pores, a rough surface and a contour-core microstructure that synergistically lead to a low fatigue strength. Residual stresses can be relieved with a heat-treatment to enhance the fatigue properties, but the best performance is obtained after machining and polishing. The differences between the polished and the heat-treated net-shape conditions lie in the subsurface microstructure, the surface topography, and the population of subsurface pores. This study aims to quantify the impact of each of these subsurface parameters on the fatigue behaviour. To do so, the subsurface microstructure of the net-shape condition is characterised. Then, uni-axial fatigue tests are carried out on stress-relieved specimens with the following surface conditions: net-shape, partially polished, and "machined and polished". The Kitagawa-Takahashi representation shows that for defects smaller than 200 mu m, the subsurface microstructure is the most influential parameter on the fatigue strength. Conversely, the surface topography has a limited influence. After examining the microstructure surrounding killer defects of both net-shape and polished specimens, the grain size under the surface of all surface conditions is considered in the Kitagawa-Takahashi diagram relatively to the killer defect size: this allows to align the various batches' results.

Aeronautical components are frequently subjected to multiaxial stresses. Producing representative fatigue data is compromised by the high costs and extended durations of traditional tests. This work proposes a novel high cycle vibration-based fatigue method for pure shear. A design protocol was followed to develop a specimen capable of inducing this specific stress state during resonance conditions. The stress state was validated experimentally using a strain rosette. The proposed methodology was subsequently applied to characterize the fatigue behavior of Ti-6Al-4V fabricated via L-PBF with as-built surfaces. The fatigue strengths obtained at around 700 Hz proved to be in accordance with available results from standard torsion tests at 5 Hz. All fatigue initiations occurred within the high stress zone and far from the edges. Fractography analysis revealed that all failures were linked to typical additive manufacturing discontinuities, such as lack of fusion and surface roughness features. The use of flat plate specimens proved advantageous for analyzing the influence of surface characteristics on fatigue behavior, as they offer superior visibility of crack initiation and propagation compared to cylindrical geometries. Furthermore, the significant reduction in test duration underscores the efficiency of the proposed approach for investigating the effects of cyclic shear stress.

Many modern wheel bearing hubs are assembled with a specified preload using the orbital forming process. This operation introduces a zone of high stress concentration and a high preload on a plastically deformed zone that can be critical to the structural fatigue performance of the component. In this study, fatigue tests were performed on notched and unnotched specimens, representative of the component, made of the untreated C56 bearing steel under axial loading, at load ratios ranging from R = -1 to R = 0.8. The results revealed that strain hardening at the notch root during the initial loading cycles modified the local stress ratio, consequently affecting the fatigue strength. A combined approach involving numerical simulations with a cyclic hardening material model and experimental X-ray diffraction measurements were employed to investigate the stress state at the notch root and the results showed a good correlation. A modified mean stress correction model was proposed to take into account the effect of high load ratio on the notch fatigue behavior of the material. The clear link between the specimens and the component in terms of fatigue failure mechanisms was established, reinforcing the applicability of the methodology to fatigue life assessment and design optimization of bearing hubs. Data from the literature were consulted and compared with the findings of this study.

The gears used in aircraft engines are typically made from high-strength steels reinforced by thermochemical treatments (TCT). These treatments increase surface fatigue strength through microstructural modifications, enhancing hardness and adding compressive residual stresses. In some cases, the combination of material, TCT, and applied stress can lead to a bi-modal fatigue behavior, notably in failures at the gear tooth root. This work investigates the bi-modal fatigue response of M50NiL case-hardened steel by characterizing and analyzing crack initiation mechanisms to propose a relevant fatigue modeling approach. A comprehensive experimental fatigue test campaign was carried out on notched specimens under plane bending and on gear specimens using a Single Tooth Bending Fatigue (STBF) method. The resulting W & ouml;hler diagram shows significant scatter in fatigue life for several stress levels, suggesting a bi-modal behavior with two distinct populations. Fractographic analyses confirmed the competition between two different crack initiation mechanisms depending on stress level and number of cycles to failure. A statistical analysis using a mixture model also indicates that a bimodal distribution best represents the results. Accordingly, a probabilistic model is proposed to describe the bi-modal fatigue behavior from a global perspective, based on the maximum applied hot-spot surface stress for a fixed stress ratio. Finally, a complementary local stress analysis shows that the combined effect of stress and material property distributions significantly influences local maximum stress variation. Correcting for these factors reduces scatter in the bi-modal stress levels.

Hot-forged wheel bearing hubs are manufactured using a complicated fabrication process chain to obtain the desired microstructure and mechanical properties in different zones that experience varying local stress states. The different process steps modify the surface roughness and the residual stresses distribution and therefore affect the fatigue behavior of the component. This study focuses on the effects of the shot-blasting and the induction hardening stages on the fatigue failure mechanisms and aims to propose an appropriate fatigue design method. Fatigue tests are performed on industrial components and on conventional material specimens to evaluate the influence of the surface roughness and the residual stresses on the fatigue strength. The relaxation of residual stresses, caused by the heat treatment and the cyclic loading during fatigue testing, is shown to annihilate the positive effect of shot-blasting, and the fatigue strength decreases because of the detrimental surface roughness, created by shot-blasting. Different multiaxial fatigue criteria are tested to take into account the manufacturing process operations.

Laser Powder Bed Fusion (L-PBF) additive manufacturing enables the production of complex-shaped parts with high mechanical resistance. Such components exhibit a multi-scale microstructure, internal, sub-surface, and surface defects, a rough surface finish and a residual stress gradient in the as-built net-shape condition. All of these factors can alter the fatigue behaviour. This study aims to improve the understanding of the combined effect of various surface parameters on the fatigue behaviour of L-PBF 316L stainless steel by conducting an extensive experimental campaign. Uni-axial fatigue tests were carried out on six batches having an as-built or heat-treated microstructure and a net-shape, pre-corroded net-shape, single-defect net-shape or polished surface condition. Results were compared with data from the literature: as-built polished, pre-corroded polished, or single-defect polished specimens. Studied defects were process-induced (e.g. lack of fusion, spatter, gas pore) and artificial (i.e. corrosion pit, electric discharge machined defect). Their sizes ranged from 10 to 700 mu m. The residual stresses gradients were characterized by X-ray diffraction. A Kitagawa-Takahashi diagram was used to illustrate the effects of the various parameters on the fatigue behaviour. Residual stresses and defects were the most influential factors on the fatigue strength of net-shape specimens over surface condition and sub-surface microstructure.

This study focuses on the evolution of the fatigue strength of Laser Powder Bed Fusion (L-PBF) produced Ti-6Al-4V as a function of the chemical etching finishing process. The aim is to identify the critical fatigue crack initiation mechanisms and the transitions between them in terms of the evolution of the surface micro-geometry. This is done using three different geometries and six different surface states. The evolution of the crack initiation mechanisms is then used to explain the evolutions of the fatigue strength and the fatigue scatter. Chemical etching affects the fatigue life via a polishing effect, which directly influences both the finite and the high cycle fatigue domains. It is shown that chemical etching makes it possible to obtain fatigue strengths that are almost similar to those of the machined surface. However, it is also observed that etching cannot fully counteract the effects of large surface cavities caused by surface connected porosities.

This paper deals with the effect of defect size and shape under high cycle fatigue for metallic alloys. A large simulation campaign based on a multiaxial fatigue criterion and a non -local approach is presented. A relative defect size based on a ratio between the defect size and a characteristic length introduced by the non -local approach is defined. A normalized Kitagawa-Takahashi diagram is then obtained. A competition between the highly stressed volume size and the local maxima due to the defect is observed and seem dependent on the relative defect size. The effect of the loading mode (uniaxial and pure shear) and of the plasticity are discussed. Finally, a comparison of the simulation results with experimental data on a 316L L-PBF demonstrates the robustness of the proposed approach and explains the negligible effect of the defect morphology compared to its size.

Additive manufacturing is nowadays already used on aeronautical serial parts but very few of them are highly loaded critical components. Titanium alloy Ti-6Al-4V and laser powder bed fusion (L-PBF) manufacturing process are both part of the most studied materials and processes. Many studies have been dedicated to L-PBF produced Ti-6Al-4V on a large range of subjects from defect generation and microstructure evolution to fatigue life. Finishing processes are also a significant part of the studied processes as the micro-geometry of additively manufactured components remains the principal drawback for the production of fatigue subjected critical parts. In this work, fatigue bending tests are undertaken on Ti-6Al-4V L-PBF made coupons. For each of them, the loaded surface has been scanned using an optical proflometer to identify the population of micro-geometric surface features. The effect of a chemical etching process on this population is described and the killer feature is highlighted in the population. Different indicators are discussed to understand if and how the killer feature can be predicted, knowing the loading conditions. It is concluded that Murakami's √area parameter may not be sufficient to determine the critical surface feature. A large scatter is observed in the fatigue strength, however, an approach based on the stress intensity range seems the most appropriate to identify the potentially critical surface micro-geometric features from the characterized population.

The present study investigates the effect of the high-speed milling (HSM), grinding and anodizing processes on the high cycle fatigue strength of the AA7050 aluminum alloy. These processes are systematically applied to certain aircraft components as specified by aeronautical standards, to attain a good surface finish. In order to understand the effect of each process on the fatigue strength, a vast experimental campaign consisting of fully reversed plane bending fatigue tests has been conducted on specimens with different surface states. A polished batch, three milled batches with different surface roughness, a grinded batch and two milled then anodized batches with different surface roughness have been tested. The results show that the roughness must be highly degraded (Sa > 3 $$\mu m$$ ) in order to impact the fatigue strength. It is also shown that anodizing has slight beneficial effect on the fatigue strength in the high cycle fatigue domain studied. The experimental results show a dependence of the fatigue strength on the surface roughness via the parameter $$\sqrt{Sa}$$ , previously proposed by the authors.

This paper focuses on the relationship between the fatigue performance and the presence of process or corrosion related defects for a 316L stainless steel manufactured by Laser Powder Bed Fusion. The fatigue responses of three batches (polished, pre-corroded and with an Electric Discharge Machined (EDM) defect) were investigated. A good correlation between Murakami’s area parameter and the fatigue strength was observed. Defect morphology does not seem to be an important factor for crack initiation as ellipsoidal corrosion pits, hemispherical EDM defects and flat lacks of fusion, although very different in terms of shape, are just as harmful at equivalent sizes.

The present work focuses on the modelling of the fatigue behaviour of a 316L produced by laser powder bed fusion containing various defect populations : Lacks of fusion, corrosion pits and one electric discharge machined hemispherical defect. As shown in previous experimental studies, the crack leading up to failure systematically initiated on a single surface defect. The nature and morphology of the critical defect did not show any influence on the fatigue strength, and only its size seemed to matter. To take into account the critical defect size, models based on linear elastic fracture mechanics were implemented and identified. A modified Paris propagation law was used to model the short crack regime. This approach was used to predict S–N curve domains based on critical defects size range.

This work is focused on the effect of natural defect on the fatigue resistance of a laser powder bed fusion additively manufactured Ti-6Al-4V titanium. To reveal the fatigue strength variability and its sensitivity to the defect size, push-pull fatigue tests have been undertaken on specimens with different sizes of highly loaded volume of material. In order to easily vary the size of the highly loaded volume, specimens containing different numbers of surface hemispherical shape holes of 600 mu m in diameter have been tested. This method also allowed to test small volume which triggered crack initiation from microstructural features. The fatigue damage mechanisms observed and the average natural defect size measured on the failure surfaces depend on the size of the highly stressed region. A higher fatigue strength is observed for smaller stressed volumes and defect free regions. To reduce the impact lack-of-fusion on fatigue and increase the probability of triggering crack initiation from a microstructural feature, the specimens were built in the horizontal direction. For specimens where fatigue cracks initiated at natural discontinuities, the results reported in a KitagawaTakahashi diagram revealed a critical defect size (root areav) in the range of 30 mu m. In addition, a probabilistic approach based on the weakest link theory is proposed. The model describes a probabilistic Kitagawa-Takahashi diagram accounting for the size of both the highly stressed volume and the natural defect.

Optimized 316L steel samples were manufactured using laser powder bed fusion and tested in high cycle fatigue at R = 0.1. They showed microstructural crack initiation and outstanding fatigue properties. Additional fatigue testings were then carried out on samples containing deterministic defects of various sizes and positions. All results summarized in a Kitagawa-Takahashi diagram show that the critical defect size is around 20 mu m for surface defects and reach 380 mu m for internal pores. Fracture surface analysis revealed that the large size gap between surface and internal fatigue crack initiation could be linked to the local gaseous environment in the pores.

Selective laser melting SLM is investigated through a study of redesign and characterization of an aeronautic part made of titanium Ti6Al4V. The part must ensure an excellent static and fatigue behaviour. The methodology developed hereby follows 3 main steps: First, the influence of laser power, laser speed and hatch distance on the amount/rate of porosity is performed to define optimized process parameters. Then, the influence of building process strategy, i.e. building direction or as-built surface roughness on the static and fatigue behaviour are studied and understood by following a vast experimental campaign. Obtained properties are finally used in a topology optimization study to find the best compromise between part weight and fatigue behavior . 3 prototypes of simulated part are produced and then characterized. Fatigue tests are conducted on the component and confirm the fatigue design proposed. Obtained results are encouraging and illustrate the fatigue design optimization of a complex Additive Manufacturing component.

This work is focused on the influence of defects on scatter and statistical size effect of Ti-6Al-4V alloy fabricated by the SLM process. A vast fatigue test campaign has been undertaken, for two surface conditions (as-built and machined surfaces) and two specimen geometries with different highly loaded volume sizes. It was shown, for machined specimens, that a large variety of crack initiation mechanisms is the principal origin of the fatigue scatter. Regarding the size effect, the change of the mechanism is the first order factor that governs the size effect. For as-built specimens, these effects are much less pronounced.

Metal powder bed fusion techniques can be used to build parts with complex internal and external geometries. Process parameters are optimized in order to obtain parts with low surface roughness and porosity, while maintaining a high productivity rate. The goal of this work is to quantify the sensitivity to internal and surface defects on the fatigue endurance of additively manufactured metallic parts. 316L Stainless Steel samples were fabricated through powder bed fusion using identical contour parameters, but three different hatching strategies were applied by varying the scanning speeds in the internal portions of the parts. Samples were subsequently mirror-polished to smooth the rough as-built surface. X-ray computed tomography analysis revealed several defect populations in samples from all three parametric sets due to lack of fusion in the bulk, with a nearly fully dense external “shell”. High cycle fatigue tests at R = 0.1 were then performed on the specimens and combined with the X-ray computed tomography scans, helping to identify the largest and the critical defect size at which crack initiation occurred. Most fatigue failures initiated within the external contour zone for small (<100 μm) defects, even when larger (>200 μm) lack of fusion defects were widely present below the surface. It was determined that the high porosity (1% in volume or above 5% in area at some fabricated layers) observed in the bulk of parts manufactured with high scanning speeds had little impact on the fatigue limit of the material.

This work is focused on the influence of porosity when dealing with the fatigue behaviour of TA6V alloys fabricated by the selective laser melting (SLM) process. The presence of porosity is one of the major issues facing additive manufacturing (AM) of metallic components subjected to fatigue loading. In order to study the effect of porosity on the fatigue behaviour, a vast experimental campaign has been undertaken. Seven specimen batches, fabricated by the SLM process with different building directions (horizontal, vertical and diagonal) were tested and a large amount of data was obtained. The link between the applied stress, the fatigue life and the pore size is highlighted by using generalized Kitagawa-Takahashi maps. It is shown that the effect of porosity on the fatigue strength is much more pronounced compared to the effect of the microstructure. In the modelling section, two approaches based on fracture mechanics are considered. The first one is based on the Paris law which is used to model long fatigue crack growth. The second approach was proposed by Caton et al. (2001) for modelling small fatigue crack growth. Finally, a simulation of the generalized Kitagawa-Takahashi is presented and good agreement with the experimental data is shown.