Characterising the impact of surface integrity on the fatigue behaviour of forged components

The present study focusses on analysing and modelling the influence on fatigue behaviour of the surface of a hot-forged C70 connecting rod which undergoes a shot-blasting treatment. The shot-blasting heavily affects the surface and thus the fatigue properties. In addition, the forging process introduces large defects which also have an effect on the fatigue strength. So as to be able to determine which aspects of the surface integrity are the most influential in fatigue, various surface states were thoroughly characterised and then tested in high cycle fatigue in bending. The various aspects studied are the surface roughness and large defects, residual stresses, microstructure and hardness

Characterising the impact of surface integrity on the fatigue behaviour of a shot-peened connecting rod

The present study focuses on analysing and modelling the influence on fatigue behaviour of the surface of a hot-forged C70 connecting rod which undergoes a shot-blasting treatment. The shot-blasting heavily affects the surface and thus the fatigue properties. In addition, the forging process introduces large defects which also have an effect on the fatigue strength. So as to be able to determine which aspects of the surface integrity are the most influential in fatigue, additional surface states were generated by shot-peening the as-forged surface. The various surface states were thoroughly characterised and then tested in high cycle fatigue in bending. The various aspects studied are the surface roughness and large defects, residual stresses, and microstructure

Influence of forging conditions on the fatigue mechanisms of low alloy steels: a 3D study

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 ?m wide x 50 ?m deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface

Competition between surface defects and residual stresses on fatigue behaviour of shot peened forged components

The present study focuses on analysing and modelling the influence on fatigue behaviour of the surface of a hot-forged C70 connecting rod which undergoes a shot-blasting treatment. The shot-blasting heavily affects the surface and thus the fatigue properties. In addition, the forging process induces large defects which also have an effect on the fatigue strength. So as to be able to determine which aspects of the surface integrity are the most influential in fatigue, various surface states were thoroughly characterised and then tested in high cycle fatigue in bending. The various aspects studied are the surface roughness and large defects, residual stresses, microstructure and hardness. The aim of this work is to develop a fatigue design approach that can take into account both the effect of the surface defects and that of the residual stresses on fatigue

Beneficial effect of prestrain due to cold extrusion on the multiaxial fatigue strength of a 27MnCr5 steel

Cold extrusion is a process commonly used to manufacture drive train components in the automotive industry. Large plastic strains can be applied during this operation (up to 1.5) and greatly changes the mechanical properties of the resulting material. This study focuses on the impact of cold extrusion process parameters on the multiaxial fatigue behaviour of steel components. A specific set of forward rod extrusion tools was developed to get original fatigue specimen able to characterise the effect of the manufacturing process on the fatigue behaviour. The specimens were extruded from two different initial diameters, giving two different reductions in cross-section of 18% and 75% respectively. To understand the influence of cold extrusion, the following analyses have been undertaken for each condition and on the initial material: monotonic tensile properties, microstructure, EBSD, residual stresses and hardness. Simulation of the forming process and microstructural observations show that the plastic strain is homogeneous in the specimen section. For both reduction factors, the forming process has a positive effect on the components properties: induced residual stresses in compression and improved hardness and roughness (Ra decreasing). Tension, plane bending and torsion fatigue tests show that the fatigue strength is about 30% higher for the batch with 75% reduced cross-section. All investigations show that strain hardening is the principal material parameter responsible for the increase in fatigue strength. A multiaxial fatigue criterion taking into account the effects of the forward rod extrusion process was also developed.This work has been performed within the ANR (National Research Agency) DEFISURF project, in a partnership including several industrial (Ascometal, Cetim, PSA, Transvalor, Atelier des Janves, Gévelot) and academic (INSA Lyon MATEIS, ENSMP-CEMEF, Arts et Métiers ParisTech LAMPA) institutions

Effet bénéfique de l'extrusion à froid sur la tenue en fatigue de pièces automobiles

La déformation introduite dans le matériau par les procédés de forgeage à froid peut être considérée comme un type de pré-déformation, appliquée avant l’utilisation en fonctionnement du composant. Cette déformation modifie le matériau, en introduisant écrouissage et contraintes résiduelles. Les caractéristiques mécaniques du matériau en sont donc modifiées, ainsi que la dureté et la microstructure. De plus, la surface du matériau est affectée, l’extrusion permet d’obtenir par exemple un très bon état de surface (Ra = 0,1 à 0,5μm) [Caminaga et al., 2007]. Les modifications que subit le matériau dépendent des conditions de forgeage : taux de corroyage, forme de la matrice, type de lubrification… Le pivot de direction présenté en figure 1 est un exemple de pièce extrudée à froid. Seules les zones fonctionnelles du composant ont été usinées (gorges, rotule...), le reste de sa surface a été laissée brut de forge. Le composant présente plusieurs réductions de sections consécutives. Ces réductions de section témoignent des différents niveaux de prédéformation introduits lors de la mise en forme. Les caractéristiques mécaniques et la tenue en fatigue dépendent de l’intensité de cette prédéformation, et donc de la réduction de section (aussi appelé taux de corroyage)

Effet bénéfique de l'extrusion à froid sur la tenue en fatigue de pièces automobiles

La déformation introduite dans le matériau par les procédés de forgeage à froid peut être considérée comme un type de pré-déformation, appliquée avant l’utilisation en fonctionnement du composant. Cette déformation modifie le matériau, en introduisant écrouissage et contraintes résiduelles. Les caractéristiques mécaniques du matériau en sont donc modifiées, ainsi que la dureté et la microstructure. De plus, la surface du matériau est affectée, l’extrusion permet d’obtenir par exemple un très bon état de surface (Ra = 0,1 à 0,5μm) [Caminaga et al., 2007]. Les modifications que subit le matériau dépendent des conditions de forgeage : taux de corroyage, forme de la matrice, type de lubrification… Le pivot de direction présenté en figure 1 est un exemple de pièce extrudée à froid. Seules les zones fonctionnelles du composant ont été usinées (gorges, rotule...), le reste de sa surface a été laissée brut de forge. Le composant présente plusieurs réductions de sections consécutives. Ces réductions de section témoignent des différents niveaux de prédéformation introduits lors de la mise en forme. Les caractéristiques mécaniques et la tenue en fatigue dépendent de l’intensité de cette prédéformation, et donc de la réduction de section (aussi appelé taux de corroyage)

Resolving the ancestry of Austronesian-speaking populations

There are two very different interpretations of the prehistory of Island Southeast Asia (ISEA), with genetic evidence invoked in support of both. The “out-of-Taiwan” model proposes a major Late Holocene expansion of Neolithic Austronesian speakers from Taiwan. An alternative, proposing that Late Glacial/postglacial sea-level rises triggered largely autochthonous dispersals, accounts for some otherwise enigmatic genetic patterns, but fails to explain the Austronesian language dispersal. Combining mitochondrial DNA (mtDNA), Y-chromosome and genome-wide data, we performed the most comprehensive analysis of the region to date, obtaining highly consistent results across all three systems and allowing us to reconcile the models. We infer a primarily common ancestry for Taiwan/ISEA populations established before the Neolithic, but also detected clear signals of two minor Late Holocene migrations, probably representing Neolithic input from both Mainland Southeast Asia and South China, via Taiwan. This latter may therefore have mediated the Austronesian language dispersal, implying small-scale migration and language shift rather than large-scale expansion

Predicting haplogroups using a versatile machine learning program (PredYMaLe) on a new mutationally balanced 32 Y-STR multiplex (CombYplex): Unlocking the full potential of the human STR mutation rate spectrum to estimate forensic parameters

We developed a new mutationally well-balanced 32 Y-STR multiplex (CombYplex) together with a machine learning (ML) program PredYMaLe to assess the impact of STR mutability on haplogourp prediction, while respecting forensic community criteria (high DC/HD). We designed CombYplex around two sub-panels M1 and M2 characterized by average and high-mutation STR panels. Using these two sub-panels, we tested how our program PredYmale reacts to mutability when considering basal branches and, moving down, terminal branches. We tested first the discrimination capacity of CombYplex on 996 human samples using various forensic and statistical parameters and showed that its resolution is sufficient to separate haplogroup classes. In parallel, PredYMaLe was designed and used to test whether a ML approach can predict haplogroup classes from Y-STR profiles. Applied to our kit, SVM and Random Forest classifiers perform very well (average 97 %), better than Neural Network (average 91 %) and Bayesian methods (< 90 %)

Microstructural heterogeneities and fatigue anisotropy

In this study, various experimental methods are employed to determine the anisotropic fatigue behavior of a 25MnCrSiVB6 forged steel (Metasco MC). This material has a bainitic microstructure and contains many elongated non-metallic inclusions in the rolled direction, which are grouped into clusters. Specimens with different orientations relative to the rolling direction have been extracted from a hot rolled bar and the ability of certain experimental techniques to capture the fatigue anisotropy has been tested. Results obtained from monotonic tensile tests and Charpy impact tests show that the material has isotropic fracture strength and anisotropic ductility. The influence of the inclusion clusters is clearly demonstrated via observation of the fracture surfaces. Concerning the fatigue behavior, results from a classical staircase experimental procedure are compared to results from self-heating fatigue tests. For specimens orientated at 0° relative to the rolled direction, microcrack initiation is controlled by the material matrix and the prediction of the fatigue strength with the self-heating method has been observed to be correct. For specimens orientated at 45° and 90°, the elongated manganese sulfide inclusion clusters are the origin of crack initiation and the fatigue strength drops significantly. For this case, it appears that the self-heating method has difficulty predicting the fatigue behavior