2,449 research outputs found

    Cyclic Nano-indentation Investigation of Plasticity in Case Hardened Bearing Steels

    Get PDF
    Demands for performance and reliability of bearings present a challenge for materials and process development in that a knowledge gap exists between the understanding ratcheting mechanisms that lead to crack initiation and mechanical predictions of bearing endurance. Current bearing research suggests the very high cycle fatigue endurance of low-inclusion content bearing steels is dominated by sub-micrometer plasticity and features uncharacterized by standard specifications. This effort investigates whether mechanical measurements on the scale of the sub-micron structures with cyclic nano-indentation relate to bearing steel fatigue performance. Cyclic indentation within the graded microstructures of case hardened steel samples is performed near the typical active fatigue depths in rolling element bearings. With a dynamic mechanical analysis (DMA) technique, differences in elastic losses in the matrix can be discerned between structures. Additionally, these differences in elastic losses resemble the bending fatigue performance and are isolated from residual stress and other variables that cloud fatigue analysis. This method may be utilized in development and optimization of alloys and processing techniques for high cycle and contact fatigue applications

    Life prediction of materials exposed to monotonic and cyclic loading: A new technology survey

    Get PDF
    Reviewed and evaluated technical abstracts for about 100 significant documents are reported relating primarily to life prediction for structural materials exposed to monotonic and cyclic loading, particularly in elevated temperature environments. The abstracts in the report are mostly for publications in the period April 1962 through April 1974. The purpose of this report is to provide, in quick reference form, a dependable source for current informatio

    Microstructural features controlling very high cycle fatigue of nitrided maraging steel

    Get PDF
    Maraging steels belong to the group of ultra-high strength materials and are often used in critical aerospace, automotive and tooling components. By applying a surface treatment such as nitriding, the fatigue and wear resistance can be improved. The microstructural features that influence the (very) high cycle fatigue response of nitrided maraging steels are studied in this work. Although the used steel has practically no inclusions, it was found that small surface imperfections, introduced during processing, may form potential fatigue initiation points. The samples are nitrided during aging in order to form nitrided layers with various thicknesses, microstructures and hardness profiles without formation of a continuous (compound) iron nitride layer. Data from microhardness tests, scanning electron microscopy, electron backscatter diffraction, x-ray diffraction and transmission electron microscopy were used to characterize the microstructure of the layers. Bending fatigue tests were employed to evaluate the fatigue response of the steel. It was found that the best fatigue behavior is obtained in samples with a thin diffusion zone with a narrow constant hardness region. In this zone, coherent disc-shaped nitride precipitates are detected with TEM

    Doctor of Philosophy

    Get PDF
    dissertationPowder metallurgy (PM) offers a cost-effective approach to produce titanium alloys in near net shape. Conventionally, two general processes have been followed to produce PM titanium alloys: Blended Elements (BE) and the Pre-Alloyed (PA) methods. Cost con

    Plasma Nitriding of Titanium Alloys

    Get PDF
    Titanium alloys are found in many applications where weight saving, strength, corrosion resistance, and biocompatibility are important design priorities. However, their poor tribological behavior is a major drawback, and many surface engineering processes have been developed to enhance wear in titanium alloys such as nitriding. Plasma (ion) nitriding, originally developed for ferrous alloys, has been adopted to address wear concerns in titanium alloys. Plasma nitriding improves the wear resistance of titanium alloys by the formation of a thin surface layer composed of TiN and Ti2N titanium nitrides (e.g., compound layer). Nonetheless, plasma nitriding treatments of titanium alloys typically involve high temperatures (700–1100°C) that promote detrimental microstructural changes in titanium substrates, formation of brittle surface layers, and deterioration of mechanical properties especially fatigue strength. This chapter summarizes the previous and ongoing investigations in the field of plasma nitriding of titanium alloys, with particular emphasis on the authors’ recent efforts in optimization of the process to achieve tribological improvements while maintaining mechanical properties. The development of low-temperature plasma nitriding treatments for α + β and near-β titanium alloys and further wear improvements by alteration of near-surface microstructure prior to nitriding are also briefly reviewed

    Microstructural Behavior And Multiscale Structure-Property Relations For Cyclic Loading Of Metallic Alloys Procured From Additive Manufacturing (Laser Engineered Net Shaping -- LENS)

    Get PDF
    The goal of this study is to investigate the microstructure and microstructure-based fatigue (MSF) model of additively-manufactured (AM) metallic materials. Several challenges associated with different metals produced through additive manufacturing (Laser Enhanced Net Shaping – LENS®) have been addressed experimentally and numerically. Significant research efforts are focused on optimizing the process parameters for AM manufacturing; however, achieving a homogenous, defectree AM product immediately after its fabrication without postabrication processing has not been fully established yet. Thus, in order to adopt AM materials for applications, a thorough understanding of the impact of AM process parameters on the mechanical behavior of AM parts based on their resultant microstructure is required. Therefore, experiments in this study elucidate the effects of process parameters – i.e. laser power, traverse speed and powder feed rate – on the microstructural characteristics and mechanical properties of AM specimens. A majority of fatigue data in the literature are on rotation/bending test of wrought specimens; however, few studies examined the fatigue behavior of AM specimens. So, investigating the fatigue resistance and failure mechanism of AM specimens fabricated via LENS® is crucial. Finally, a microstructure-based MultiStage Fatigue (MSF) model for AM specimens is proposed. For calibration of the model, fatigue experiments were exploited to determine structure-property relations for an AM alloy. Additional modifications to the microstructurally-based MSF Model were implemented based on microstructural analysis of the fracture surfaces – e.g. grain misorientation and grain orientation angles were added to the MSF code

    Predictive models for fatigue property of laser powder bed fusion stainless steel 316L

    Get PDF
    The selection of appropriate processing parameters is crucial for producing parts with target properties via the laser powder bed fusion (L-PBF) process. In this work, the fatigue properties of L-PBF stainless steel 316L under controlled changes in laser power and scan speed were studied by employing the statistical response surface method. Processing regions corresponding to different fatigue failure mechanisms were identified. The optimum fatigue properties are associated with crack initiation from microstructure defect, which, by acting as the weakest link, creates enhanced porosity-tolerance at applied stress approaching the fatigue limit. Deviations from the optimum processing condition lead to strength degradation and porosity-driven cracking. Based on the observed relations between microstructural features and failure behaviour, a processing-independent fatigue prediction model was proposed. The microstructure-driven failure was modelled by a reference S-N curve where the intrinsic effect of microstructure inhomogeneity was accounted for by applying a reduction factor on fatigue life. For the porosity-driven failure, high cycle fatigue life follows an inverse-square-root relation with porosity fraction. This relation was incorporated into the Basquin equation for predicting the fatigue strength parameters.Accepted versio

    Fatigue fracture and microstructural analysis of Friction Stir Welded butt joints of aerospace aluminum alloys

    Get PDF
    Friction-Stir-Welding (FSW) has been adopted as a major process for welding Aluminum aerospace structures. Al-2195, which is one of the new-generation Aluminum alloys that has been used on the external tank of the new super lightweight external tank of the space shuttle. The Lockheed Martin Space Systems (LMSS), Michoud Operations in New Orleans is continuously pursuing Friction-Stir-Welding technologies in its efforts to advance fabrication of the external tanks of the space shuttle. The future launch vehicles which will have to be reusable, m, an dates the structure to have good fatigue properties, which prompts an investigation into the fatigue behavior of the friction-stir-welded aerospace structures. The butt joint specimens of Al-2195 and Al-2219 are fatigue tested according to ASTM-E647. The effects of: (i) Stress ratios, (ii) Corrosion Preventive Compound (CPC), and (iii) Periodic Overloading on fatigue life are investigated. Scanning electron microscopy (SEM) is used to examine the failure surface, and examine the different modes of crack propagation i.e. tensile, shear, and brittle modes. It is found that fatigue life increases with increase in stress ratio; the fatigue life increases from 30-38% with the use of CPC, the fatigue life increases 8-12 times with periodic overloading, , and crack closure phenomenon predominates the fatigue facture. Numerical Analysis in FEA has been used to model a fatigue life prediction scheme for these structures, the interface element technique with critical bonding strength criterion for formation of new surface has been used to model crack propagation. The Linear Elastic Fracture Mechanics (LEFM) stress intensity factor is calculated using FEA, and the fatigue life predictions made using this method are within acceptable 10-20% of the experimental fatigue life obtained. This method overcomes the limitation of the traditional node release scheme, and closely matches the physics of crack propagation

    Study of the effect of microstructural constituents on fatigue crack propagation of high-performance PM steels

    Get PDF
    Cette recherche est principalement planifiée pour améliorer les propriétés en fatigue des aciers MP afin qu'ils remplacent largement leurs contreparties corroyées, principalement en raison de leurs avantages en lien avec leur coût de production. L’atteinte de cet objectif est fait en déterminant la microstructure la plus efficace des aciers MP au cours de chargements cycliques. La microstructure la plus efficace est celle qui permet de réduire ou d'arrêter la propagation de fissures de fatigue par la combinaison appropriée de ses phases constitutives. Il existe quelques travaux sur ce sujet, cependant, ils n'ont pas atteint de conclusions cohérentes en raison du manque de données suffisantes et / ou une comparaison inappropriée. Par conséquent, l'effet de différentes phases constitutives d'une microstructure hétérogène d'un acier MP est encore ambigu et inconnu. Afin d'élucider cette question, il convient d'étudier différentes phases microstructurales et le comportement de propagation des fissures de fatigue dans celles-ci. Notre étude commence par choisir deux aciers MP communs fabriqués à l'aide de deux techniques d'alliage soient : pré-mélangeage et pré-alliage, afin de produire respectivement des microstructures hétérogènes et homogènes. Deux types de traitements thermiques ont également été utilisés pour fournir différentes phases microstructurales qui sont nécessaires aux fins de cette étude. Les échantillons ont ensuite été testés en utilisant la charge cyclique et quatre ratios de contraintes pour étudier l'effet des conditions de fatigue. Une analyse quantitative des surfaces de rupture, qui comprend l'étude détaillée du cheminement des fissures en OM et en SEM a ensuite été effectuée sur les échantillons fracturés. Les données de vitesse de propagation de fissures de fatigue dans différents aciers MP ayant des microstructures différentes avec les données quantitatives acquises quant à leur parcours préférentiel nous ont amené à des résultats intéressants sur l'effet des constituants de microstructure sur le comportement de propagation de fissure de fatigue. Il s'est avéré que la fissure de fatigue se propage plus rapidement à travers la phase la moins résistante parmi celles présentes dans la microstructure. Ainsi, la perlite n'était pas favorable à la propagation des fissures en compagnie de ferrite riche en Ni, alors qu’en présence de martensite, on a trouvé que les grains perlitiques étaient le chemin de fissure préféré. De plus, l'austénite résiduelle, qui a été identifiée dans la littérature comme étant une phase bénéfique pour le retard de fissure de fatigue, s'est révélée inefficace. Bien que les fissures de fatigue contournent ces régions, la déformation causée par ce changement de chemin de fissure n'a pas montré d'effet positif sur le retard de fissure de fatigue.This research is mostly planned to enhance the fatigue properties of PM steels inasmuch as they are extensively replacing their equivalent wrought steels due mostly to production cost benefits. This goal is going to be achieved through determining the most effective microstructure of PM steels in cyclic loadings. The most effective microstructure is the one that can reduce or stop the fatigue crack propagation through the proper combination of its constituent phases. There exists some researches on this topic, however, they did not reach consistent conclusions due to the lack of sufficient data and/or improper comparison. Therefore, the effect of different constituent phases of a heterogeneous microstructure of a PM steel is still ambiguous and unknown. In order to study this issue, diverse microstructures and the fatigue crack propagation behaviour through them should be studied. Our research begins by choosing two common PM steels manufactured using two alloying techniques of admixed and pre-alloyed to produce heterogeneous and homogeneous microstructures respectively. Two types of heat-treatments namely sinter-hardening and oil-quenching were also utilized to provide more microstructural phases that is needed for the purpose of this study. The samples were then tested in cycling loading using different R-ratio in order to study the effect of fatigue conditions as well. Quantitative analysis of the fracture surfaces, which includes the detailed study of the crack path in OM and SEM, were then performed on the fractured samples. The fatigue crack growth rate data in different PM steels having different microstructures along with the quantitative data acquired from their crack path led us to interesting results on the effect of microstructural constituents on fatigue crack propagation behaviour. It was found that the fatigue crack will propagate more rapidly through the weakest i.e. lowest strength phase among the ones present in the microstructure. Thus, pearlite was not favourable for crack propagation in the company of Ni-rich ferrite while in the presence of martensite, pearlitic grains were found to be the preferred crack path. Moreover, the retained austenite, which was identified in literature to be a beneficial phase for fatigue crack retardation, was found to be ineffective on the matter. Although fatigue cracks circumvented these regions, the deflection caused by this change of crack path did not show any positive effect on fatigue crack retardation
    • …
    corecore