Showing papers on "Fatigue limit published in 2013"

General relation between tensile strength and fatigue strength of metallic materials

Abstract: With the development of high-strength materials, the existing fatigue strength formulae cannot satisfactorily describe the relation between fatigue strength sigma(w) and tensile strength sigma(b) of metallic materials with a wide range of strength. For a simple but more precise prediction, the tensile and fatigue properties of SAE 4340 steel with the tensile strengths ranging from 1290 MPa to 2130 MPa obtained in virtue of different tempering temperatures were studied in this paper. Based on the experimental results of SAE 4340 steel and numerous other data available (conventional and newly developed materials), through introducing a sensitive factor of defects P, a new universal fatigue strength formula sigma(w)=sigma(b)(C-P.sigma(b)) was established for the first time. Combining the variation tendency of fatigue crack initiation sites and the competition of defects, the fatigue damage mechanisms associated with different tensile strengths and cracking sites are explained well. The decrease in the fatigue strength at high-strength level can be explained by fracture mechanics and attributed to the transition of fatigue cracking sites from surface to the inner inclusions, resulting in the maximum fatigue strength sigma(max)(w) at an appropriate tensile strength level. Therefore, the universal fatigue strength formula cannot only explain why many metallic materials with excessively high strength do not display high fatigue strength, but also provide a new clue for designing the materials or eliminating the processing defects of the materials. (c) 2012 Elsevier B.V. All rights reserved.

High Cycle Fatigue (HCF) Performance of Ti-6Al-4V Alloy Processed by Selective Laser Melting

Abstract: Selective laser melting (SLM) is a relatively new additive manufacturing (AM) technology which uses laser energy for manufacturing in a layered pattern. The unique manufacturing process of SLM offers a competitive advantage in case of very complex and highly customized parts having quasi-static mechanical properties comparable to those of wrought materials. However, it is not currently being harnessed in dynamic applications due to the lack of reliable fatigue data. The manufacturing process shows competitive advantages particularly in the aerospace and medical industry in which Ti-6Al-4V is commonly used, especially for high performance and dynamic applications. Therefore, in this exploratory research, high cycle fatigue (HCF) tests were performed for as-built, polished and shot-peened samples to investigate the capability of SLM for these applications. As-built samples showed a drastic decrement of fatigue limit due to poor surface quality (Ra ≈ 13 µm) obtained from the SLM process. Polishing improved the fatigue limit to more than 500 MPa, the typical value for base material. The effect of shot-peening proved to be antithetical to the expected results. In this context, fractographic analysis showed that very small remnant porosity (less than 0.4%) played a critical role in fatigue performance.

Properties of open-cell porous metals and alloys for orthopaedic applications.

Abstract: One shortcoming of metals and alloys used to fabricate various components of orthopaedic systems, such as the femoral stem of a total hip joint replacement and the tibial plate of a total knee joint replacement, is well-recognized. This is that the material modulus of elasticity (E′) is substantially larger than that of the contiguous cancellous bone, a consequence of which is stress shielding which, in turn, has been postulated to be implicated in a cascade of events that culminates in the principal life-limiting phenomenon of these systems, namely, aseptic loosening. Thus, over the years, a host of research programs have focused on the synthesis of metallic biomaterials whose E′ can be tailored to match that of cancellous bone. The present work is a review of the extant large volume of literature on these materials, which are called open-cell porous metals/alloys (or, sometimes, metal foams or cellular materials). As such, its range is wide, covering myriad aspects such as production methods, characterization studies, in vitro evaluations, and in vivo performance. The review also includes discussion of seven areas for future research, such as parametric studies of the influence of an assortment of process variables (such as the space holder material and the laser power in the space holder method and the laser-engineered net-shaping process, respectively) on various properties (notably, permeability, fatigue strength, and corrosion resistance) of a given porous metal/alloy, innovative methods of determining fatigue strength, and modeling of corrosion behavior.

Fatigue strength improvement of steel structures by high-frequency mechanical impact: proposed fatigue assessment guidelines

Abstract: In the past decade, high-frequency mechanical impact (HFMI) has significantly developed as a reliable, effective, and user-friendly method for post-weld fatigue strength improvement technique for welded structures. During this time, period 46 documents on HFMI technology or fatigue improvements have been presented within Commission XIII of the International Institute of Welding. This paper presents one possible approach to fatigue assessment for HFMI-improved joints. Stress analysis methods based on nominal stress, structural hot spot stress, and effective notch stress are all discussed. The document considered the observed extra benefit that has been experimentally observed for HFMI-treated high-strength steels. Some observations and proposals on the effect of loading conditions like high mean stress fatigue cycles, variable amplitude loading, and large amplitude/low cycle fatigue cycles are given. Special considerations for low stress concentration details are also given. Several fatigue assessment examples are provided in an appendix. A companion paper has also been prepared concerning HFMI equipment, proper procedures, safety, training, quality control measures, and documentation has also been prepared. It is hoped that these guidelines will provide stimulus to researchers working in the field to test and constructively criticize the proposals made with the goal of developing international guidelines relevant to a variety of HFMI technologies and applicable to many industrial sectors. The proposal can also be used as a means of verifying the effectiveness of new equipment as it comes to the market.

IIW guideline for the assessment of weld root fatigue

Abstract: At welded joints, fatigue cracks usually initiate either at the weld toe or the weld root. The latter may be influenced by small defects or other irregularities or even by non-fused root faces forming a slit which acts like a crack. Weld root failure has therefore to be checked for several weld types. Different approaches exist for fatigue assessment, including the rather simple nominal stress approach, local stress approaches and the crack propagation approach which are partly well suited for the assessment of weld root fatigue. However, it is not easy to keep the overview and to decide which approach should be applied in the case in question. For this purpose, the guideline has been established giving an overview of the different approaches with special emphasis on weld roots and discussing their suitability and the limitations. Six typical examples are described where different approaches are applied and in some cases compared with fatigue tests, thus giving insight into the practical application and allowing own judgement.

Investigating the effect of machining processes on the mechanical behavior of composite plates with circular holes

Abstract: The influence of the machining quality on the mechanical behavior of CFRP composites is yet not fully understood. There are only few works in the literature that have investigated the effect of the machining quality on CFRP. In fact, most of these works focus only on conventional machining such as axial or orbital drilling. The aim of this paper is to examine the influence of two machining processes namely conventional machining (CM) and abrasive water jet machining (AWJM) on the mechanical behavior of composite plates under cyclic loading. For this purpose, an experimental study using several composite plates drilled with a cutting tool and an abrasive water jet machining was carried out. In order to study the impact of the process of machining on the mechanical behavior, thermographic infrared testing and fatigue cyclic tests were performed to assess temperature evolutions, stiffness degradation, and the damage evolution in these plates. Fatigue testing results have shown that the damage accumulation in specimens drilled with CM process was higher than the AWJM specimens. Furthermore, the endurance limit for a composite plate drilled with CM was approximately 10% inferior compared to specimens drilled with AWJM. This difference can be related to the initial surface integrity after machining induced by the difference in the mechanism of material’s removal between the two processes used.

Uniaxial ratcheting and fatigue failure behaviors of hot-rolled AZ31B magnesium alloy under asymmetrical cyclic stress-controlled loadings

Abstract: Uniaxial ratcheting and low-cycle fatigue failure behaviors of the hot-rolled AZ31B magnesium alloy are studied by uniaxial asymmetrical cyclic stress-controlled experiments at room temperature. The effects of mean stress, stress amplitude and stress rate on the uniaxial ratcheting response and ratcheting life of the studied magnesium alloy are analyzed. The microscopic observations near fracture surface are carried out. In addition, a fatigue parameter is introduced to describe the ratcheting damage and fatigue damage processes, and a new model is established to describe the relationship between the linear density of twins and the fatigue parameter. Results show that (1) the mean stress and stress amplitude has a great influence on the ratcheting strain and fatigue failure life. When the mean stress or stress amplitude is increased, the ratcheting strain and its rate rapidly increase. So, the fatigue failure life is reduced. (2) The increase of mean stress and stress amplitude can accelerate the generation of twins, which decreases the fatigue resistance and increases the ratcheting strain. However, the stress rate has little effect on the twinning. So, the effects of stress rate on ratcheting and low-cycle fatigue failure behaviors are not obvious under the tested conditions. This is because the studied magnesium alloy shows highly rate-independent property at room temperature within the tested strain rate range. (3) The established model can accurately describe the relationship between the linear density of twins and the fatigue parameter. Furthermore, the threshold fatigue parameter for twinning is found.

A critical review of the influence of hydrogen on the mechanical properties of medium-strength steels

Abstract: As medium-strength steels are promising candidates for the hydrogen economy, it is important to understand their interaction with hydrogen. However, there are only a limited number of investigations on the behavior of medium-strength steels in hydrogen. The existing literature indicates that the influences of hydrogen on the tensile properties of medium-strength steels are mainly the following: (i) the steel can be hardened by hydrogen, as demonstrated by an increase in the yield stress or ultimate tensile stress; (ii) some steels can be embrittled by hydrogen, as revealed by lower yield stress or ultimate tensile stress; (iii) in most cases, these steels may experience hydrogen embrittlement (HE), as indicated by a reduction in ductility. The degree of HE mainly depends on the test conditions and the steel. The embrittlement can lead to catastrophic brittle fracture in service. The influence of hydrogen on the fatigue properties of medium-strength steels is dependent on many factors such as the stress ratio, temperature, yield stress of the steel, and test frequency. Generally, the hydrogen influence on fatigue limit is small, whereas hydrogen can accelerate the fatigue crack growth rate, leading to a shorter fatigue life. Inclusions are an important factor influencing the properties of medium-strength steels in the presence of hydrogen. However, it is not possible to predict the influence of hydrogen for any particular steel that has not been experimentally evaluated or to predict service performance. It is not known why similar steels can have different behavior, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is needed.

Improvement of the fatigue strength of SUS304 austenite stainless steel using ultrasonic nanocrystal surface modification

Abstract: An ultrasonic nanocrystal surface modification (UNSM) treatment was applied to improve the fatigue strength of typical SUS304 austenite stainless steel. A gradient nanocrystalline plastic deformation layer with increased hardness was created at the specimen surface after UNSM treatment. The average grain size at the top surface was refined to a few nanometers, and the whole thickness of the plastic deformation layer was ~ 30 to 200 μm, depending on the various treatment conditions. The hardened surface layer was assumed to be the main reason for the improvement in the fatigue strength. The increase in the fatigue strength was related to the UNSM applied static loads. In this work, it was found that a static load of 90 N was optimum to obtain a large cycle fatigue strength limit. Plastic deformation (strain)-induced martensitic transformations were also observed in the surface layer. The mechanism for further grain refinement based on martensitic transformations is discussed.

Experimental Study on Fatigue Strength of Corroded Bridge Wires

Abstract: Fatigue tests were conducted for corroded galvanized steel wires on three corrosion levels, showing that fatigue strength of corroded wires lowers as corrosion progresses. Corrosion pits were measured on the corroded specimens, showing severer corrosion produced deeper pits in more condensed areas. Fatigue tests were then conducted for wire specimens with artificial pits whose sizes were decided by the measured corrosion pit data. Three different pit shapes were assumed: round, triangle, and triangle with a notch. The wire specimens with round pits did not break until 1 million cycles in the stress range of 400 MPa. The fatigue strength of wires with the triangular pit was lower than that with a round shape. Triangular pit specimens broke at fewer cycles for shorter pit length. The fatigue strength of wires with a notched triangle further decreased, and critical cycles did not depend on pit length. As the S-N relation of the wire specimens with triangular pits and notched triangular pits has a sim...