Including Fatigue Strength Research Articles

THE METHOD OF MECHANICAL TESTING AND INVESTIGATION OF THE POSSIBILITY OF FATIGUE STRENGTH CONTROL BY THE MAGNETO-NOISE METHOD OF STEEL SAMPLES MANUFACTURED BY ADDITIVE TECHNOLOGY

Steel products manufactured using additive technologies are an alternative to traditional methods of their production. The specific method of their forming actualizes the issue of strict control of their quality, including fatigue strength. To investigate the possibility of control on the samples of low-alloyed steel 09G2S produced by selective laser alloying and, for comparison, by casting method, it is proposed to use the magnetic noise method. For mechanical tests of specimens in bending a simple original portable stand was created, a methodology of fatigue strength research and a program of specimen testing were developed. The tests have shown the efficiency of using the methodology and the bench in the study of fatigue strength. The results of evaluation of local heterogeneity of specimen properties before cycling and the effect on the magnetic noise level of the number of cycles at the initial period of loading at low cycle amplitude are presented.

The Role of the Distance between Fine Non-Metallic Oxide Inclusions on the Fatigue Strength of Low-Carbon Steel

The fatigue strength of steel is an important parameter determining the use of the alloy. Conducting material durability tests depending on the working conditions of the material requires a lot of work. Therefore, the industry knows methods to estimate the fatigue life of steel on the basis of other parameters or measurements of other mechanical properties. One of such parameters is the fatigue strength coefficient, which allows one to link the fatigue strength with the hardness results of a specific steel grade. Alloys produced in industrial conditions contain impurities that can affect the properties of steel, including fatigue strength. Impurities in steel depend mainly on the technology of its production. One of the technologies that allows one to obtain high-purity steel is by subjecting it to secondary metallurgy treatment consisting of desulfurization and refining with argon. The fatigue strength of steel depends, among other things, on the morphology of impurities. In the work, the influence of the distance between small non-metallic inclusions with a diameter of less than 2 µm on the fatigue strength of steel, expressed by the fatigue resistance factor, was assessed. The research was carried out in industrial conditions on seven independent melts of low-carbon steel capable of forming a martensite microstructure. Several dozen fatigue strength tests were carried out for each of the casts. The volume fraction, size, and distribution of pollutants were examined. It was found that the main impurity is Al2O3, with a diameter of about 1.8 µm occurring at a distance of about 12 µm. The distance between small non-metallic inclusions affects the fatigue resistance factor, and small non-metallic inclusions with a diameter of less than 2 µm hinder the destruction of high-ductility steel. The paper presents an example of the structure of non-metallic inclusions for heat, the relative volume of inclusions, the average impurity diameter and impurity spacing for impurity dimensional ranges, the impurity spacing λ for the total volume of impurities, and the bending fatigue strength coefficient tested in steel after hardening and tempering at different tempering temperatures.

Under ballast mat – A review of recent developments, limitations, and future prospects

A high frequency of loadings and an excessive amount of axle loads are rising as trains are becoming faster and heavier. In this regard, improvements in track design, such as using an under ballast mat, are vital to performance improvement. In addition to reducing noise, vibration, and ballast degradation, UBMs are designed to improve the dynamic behavior of tracks, thus extending their service life and increasing safety. This paper provides an overview of the recent developments on UBMs, identifies the crucial characteristics of different kinds of mats, including fatigue strength, environmental conditions, stiffness, and ballast deformation, and describes challenges for more widespread adoption. Limitations of using a UBM include their high cost and stiffness, limited knowledge of long-term efficiency, and interlocking reliability. This paper also provides future perspectives and prospects for overcoming the challenges in the utilization and acceptance of UBMs.

Microstructure evolution for isothermal sintering of binder jet 3D printed alloy 625 above and below the solidus temperature

When compared to beam-based melting methods, binder jet 3D printing is less time consuming, and the printed material is less prone to residual stresses. However, binder jet printing often contains remnant porosity that survives pressureless sintering and limits the mechanical properties of metals including fatigue strength and ductility. In this work, gas atomized nickel-based superalloy 625 powders with three different particle size distributions are binder jet printed and subsequently isothermally sintered at subsolidus and supersolidus temperatures. Individual feature measurements were conducted on two-dimensional sections and pore size distribution frequencies were plotted to reveal the difference in microstructural evolution between subsolidus and supersolidus sintering. Supersolidus liquid phase sintering is thought to have facilitated particle rearrangement under the surface tension of the liquid phase, resulting in a homogeneous microstructure that favored subsequent densification and higher final density. Additionally, printed powder with a wide particle size distribution not only reached high green density of ~52%, but also achieved final densities above 99%.

Modifications on porous absorbable Fe-based scaffolds for bone applications: A review from corrosion and biocompatibility viewpoints.

Iron (Fe) and Fe-based scaffolds have become a research frontier in absorbable materials which is inherent to their promising mechanical properties including fatigue strength and ductility. Nevertheless, their slow corrosion rate and low biocompatibility have been their major obstacles to be applied in clinical applications. Over the last decade, various modifications on porous Fe-based scaffolds have been performed to ameliorate both properties encompassing surface coating, microstructural alteration via alloying, and advanced topologically order structural design produced by additive manufacturing (AM) techniques. The recent advent of AM produces topologically ordered porous Fe-based structures with an optimized architecture having controllable pore size and strut thickness, intricate internal design, and larger exposed surface area. This undoubtedly opens up new options for controlling Fe corrosion and its structural strengths. However, the in vitro biocompatibility of the AM porous Fe still needs to be addressed considering its higher corrosion rate due to the larger exposed surface area. This review summarizes the latest progress of the modifications on porous Fe-based scaffolds with a specific focus on their responses on the corrosion behavior and biocompatibility.

Influences of particle collision treatments on surface characteristics and fatigue strength of Ti-6Al-4V alloy

In this study, the particle collision treatments (fine particle bombarding (FPB) and conventional shot peening (SP)) were performed for Ti–6Al–4V alloy in order to comprehensively investigate the influences of their treatment conditions on the surface conditions, microstructures, hardness distributions and residual stress. Moreover, we examined the relationship between the surface characteristics of the treated materials and their mechanical properties including fatigue strength in detail. According to the obtained experimental results, the values of the maximum height waviness increased with increasing in the size of collision particles and injection pressure. Among all the treated materials, the differences in hardness distribution were relatively small because the work hardening exponent of Ti–6Al–4V alloy is low and the degree of work hardening is insensitive to the amount of plastic deformation. The absolute values of compressive residual stress generated at the surfaces became higher when finer particles were used as collision particles. The increase in injection pressure increased the surface temperature and released the introduced residual stress. As the particle collision treatments affected only the surface regions, the mechanical properties of the treated materials were almost the same as those of the annealed one. In the range of the conditions examined in this study, the most effective treatment to improve the fatigue strength of Ti–6Al–4V alloy was FPB under a relatively low injection pressure. This was because the value of the maximum height waviness was the smallest and the highest compressive residual stress was introduced at the same time.

Ultrafine‐Grained Titanium‐Based Alloys: Structure and Service Properties for Engineering Applications

Herein, an overview of the recent research on the relationship between the ultrafine‐grained (UFG) structure of titanium and its alloys and the physical and mechanical properties of the materials, as well as the formation of advanced functional and service properties, including fatigue strength, creep behavior, and impact and fracture toughness is presented. It is shown that due to the record properties achieved in UFG titanium and its alloys, these materials have an innovative potential for successful application in medicine and engineering industries. Some of these works are carried out in cooperation with the team of Prof. Terence G. Langdon. Professor Langdon made a significant contribution to the development of modern materials science in the field of nanostructured metallic materials.

Analytical modeling of residual stress formation in workpiece material due to cutting

Residual stress on surface layer of workpiece has significant effects on service life of parts, including fatigue strength and corrosion resistance. Metal cutting process is widely used on manufacturing parts, which will cause severe mechanical and thermal stress on the material and then result in the formation of residual stress on workpiece surface layer. A thorough analysis of residual stress formation due to cutting is essential to create parts with desired residual stress distribution. After analyzing stress field in workpiece based on previous work, the authors in this paper observe that stress of a point during cutting is time dependent, and then proposed that initial condition is mandatory in order to determine initial stress state in stress relaxation procedure. Then the authors propose a criterion to determine initial condition based on the calibration of surface residual stress. Using this criterion and calibration method, the residual stress in different depth of machined surface can be compute using the proposed analytical residual stress model, and the computed results are consistent with experiment results. Moreover, the computed results show that the residual stress component in cutting speed direction is higher than that in axial direction, which is consistent with the general conclusion in the existing literatures. The proposed concept of initial condition and its calibration method in stress relaxation promote the understanding of physical mechanism of residual stress formation in cutting, and can be extended to the situations where the evolution of flow stress in workpiece during cutting is similar with that of in orthogonal cutting.

Effects of micro- and nano-scale wave-like structures on fatigue strength of a beta-type titanium alloy developed as a biomaterial

Some newly developed β-type titanium alloys for biomedical applications exhibit distinctive heterogeneous structures. The formation mechanisms for these structures have not been completely revealed; however, understanding these mechanisms could lead to improving their properties. In this study, the heterogeneous structures of a Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), which is a candidate for next-generation metallic biomaterials, were analyzed. Furthermore, the effects of such heterogeneous structures on the mechanical strength of this alloy, including fatigue strength, were revealed by comparing its strength to that of homogenous TNTZ. The heterogeneous structures were characterized micro-, submicro- and nano-scale wave-like structures. The formation mechanisms of these wave-like structures are found to be different from each other even though their morphologies are similar. It is revealed that the micro-, submicro- and nano-scale wave-like structures are caused by elemental segregation, crystal distortion related to kink band and phase separation into β and β′, respectively. However, these structures have no significant effect on both tensile properties and fatigue strength comparison with homogeneous structure in this study.

Life-cycle prediction of steel bridges using reliability-based fatigue deterioration profile: Case study of neka bridge

Fatigue deterioration profiles of critical elements in a railway truss bridge are determined using a probabilistic approach. A performance function including fatigue strength, R, and fatigue action, D, is used, where R is a material property with lognormal distribution. The random variable D depends on AASHTO category, stress spectrum and traffic features. By applying Monte-Carlo simulations on the performance function of each year, reliability index profile is calculated. The reliability profile decreases with life, and whenever reaches to a critical value, an MR&R action should be implemented. A numerical approach is presented to specify the effects of MR&R actions on the deterioration profile. These actions include re-welding, attaching CFRP and combination of them. The structural model is calibrated by results of a loading test. In addition, the uncertainties arisen from future traffic volume and the relevant S-N curve are considered by a statistical analysis. For a target reliability of 2.0, the retrofit time for deck and truss are estimated. Finally, the outputs of the research are used to recommend a plan for maintenance of Neka Bridge.

Changes in mechanical properties of vehicle components after strengthening under low‐amplitude loads below the fatigue limit

ABSTRACTChanges in the mechanical properties of vehicle components were investigated after strengthening under low‐amplitude loads below the fatigue limit (SLAL). The strengths of vehicle components including fatigue strength, static strength and yield strength were significantly increased by SLAL. The surface hardness of the front axle, composed of a kind of low strength steel without heat treatment, was enhanced with increased strengthening and damaging. The change of surface hardness was reversed completely for the transmission gear, composed of a kind of high strength steel with surface heat treatment. The natural frequency of the transmission gear was appreciably increased by SLAL.

R5 procedures for assessing structural integrity of components under creep and creep–fatigue conditions

The R5 high temperature assessment procedures have been developed in the UK and address both creep–fatigue crack initiation in initially defect free components and the growth of flaws by creep and creep–fatigue mechanisms. The procedures were developed some time ago and included a number of novel features such as: the shakedown reference stress approach for structural assessment; the ductility exhaustion method for estimating creep damage; the inclusion of size effects in fatigue damage calculations to enable assessments of thin section components; and the treatment of the effects of crack closure on creep–fatigue crack growth rates. Recently, the R5 procedures have undergone a major revision leading to Issue 3 of the document. In the revision, the above novel features have been retained but other parts of the procedure have been modified. These include: a restructuring of the document with associated new step-by-step procedures to enable easier application; modifications to the ductility exhaustion model to address multiaxial stresses including the triaxial stresses which may be present in regions of high welding residual stress; additional advice for assessment of weldments including fatigue strength reduction factors based on experimental data on austenitic weldments; generalised hysteresis loop construction methods for complex non-isothermal cycles, supported by laboratory data collected under non-isothermal conditions; advice on inelastic analysis when simplified shakedown methods are inapplicable; and rules for determination of interactions between creep and fatigue crack growth. The present review first describes in outline the overall structure and content of R5 Issue 3. Then the new R5 creep–fatigue crack initiation assessment procedures are set out in more detail. This includes the recent modifications and some associated background experimental and analytical work on which these modifications are based. Finally, the paper sets out the new creep–fatigue crack growth procedures in R5, again with some background information on the recent developments.

Expert System for Fatigue Design of Spot-welded Joint.

In establishing a long life fatigue design criterion and estimating the reliability of spot-welded thin sheet structure such as the automobile and train, it is necessary to estimate fatigue strength of spot-welded joint. So far, many investigators have numerically and experimentally studied on the method of systematic fatigue strength estimation for various spot-welded joint. Also considerable amount of data on the fatigue strength of spot-welded joint has been accumulated. But, it is difficult to determine the fatigue design criterion based on the existing data when fatigue design is modified. Therefore, many tests were required to determine a new criterion for modified design. Then it takes a lot of money and time. Thus, an expert system was developed to predict the fatigue design criterion including fatigue strength and fatigue life of spot-welded joint. The expert system consists of the material destination database, the spot-welding condition database and the fatigue design module. And this expert system was developed based on the knowledge base, the numerical calculation module, interface engine and user interface. It is expected that the expert system can economically provide the designer and the manufacturer with useful information on the material properties and the fatigue design criterion for a modified design.

Fatigue Fracture Behavior of SiC Whisker/Aluminum Matrix Composite under In-Phase and Out-of-Phase Tension/Torsion Loading.

We describe an investigation on the fatigue fracture behavior of an SiC whisker reinforced A6061 aluminum alloy fabricated by a squeeze casting process under combined tension/torsion loading at room temperature. Attention was focused on in-phase and out-of-phase tension/torsion loading. The tests were conducted under a load-controlled condition keeping a constant value of the combined stress ratio, α=τmax/σmax. Irrespective of loading condition, the mechanical properties of the composite including fatigue strength were superior to those of an unreinforced A6061 alloy, not only under uniaxial loading, but also under combined in-phase and out-of-phase tension/torsion loading. In the case of the unreinforced matrix material, the fatigue strength under out-of-phase combined loading was smaller than that under in-phase loading. However, for the composite, the fatigue strength under out-of-phase combined loading was higher than that under in-phase combined loading. The crack initiation and propagation behavior was closely examined using a surface replication technique, and the fracture mechanisms under in-phase and out-of-phase combined loading were discussed.

Fatigue Fracture Behavior of SiC Whisker/Aluminum Matrix Composite under Out-of-Phase Tension/Torsion Combined Loading.

We describe an investigation on the fatigue fracture behavior of an SiC whisker reinforced A6061 aluminum alloy fabricated by a squeeze casting process under combined tension/torsion loading at room temperature. Attention was focused on in-phase and out-of-phase tension/torsion loading. The tests were conducted under a load-controlled condition keeping a constant value of the combined stress ratio, α=τ<max>/σ<max>. Irrespective of loading condition, the mechanical properties of the composite including fatigue strength were superior to those of an unreinforced A6061 alloy, not only under uniaxial loading, but also under combined in-phase and out-of-phase tension/torsion loading. In the case of the unreinforced matrix material, the fatigue strength under out-of-phase combined loading was smaller than that under in-phase loading. However, for the composite, the fatigue strength under out-of-phase combined loading was higher than that under in-phase combined loading. The crack initiation and propagation behavior was closely examined using a surface replication technique, and the fracture mechanisms under in-phase and out-of-phase combined loading were discussed.

Fracture Behavior of SiC Whisker Reinforced Aluminum Alloy under Combined Tension/Torsion Loading at Room and Elevated Temperatures.

This paper describes an investigation on the tensile and fatigue fracture behavior of an SiC whisker reinforced 6061 aluminum alloy fabricated by a squeeze casting process under combined tension/torsion loading at room and elevated temperatures. The tests were conducted under a load-controlled condition keeping a constant value of the combined stress ratio, α=τ/σ. Except for the elongation at failure, mechanical properties of the composite including fatigue strength were superior to those of an unreinforced 6061 alloy, not only under uniaxial loading, but also under combined tension/torsion loading. The static strength of the composite and the unreinforced matrix material showed good agreement with the Tsai-Hill failure criterion. Fatigue strength under αl1 was determined by maximum principal stress, whereas the strength under α>1 was determined by maximum equivalent stress. Fracture surfaces were closely examined with a scanning electron microscope, and the fracture mechanisms under combined tension/torsion loading were discussed.