Showing papers on "Fatigue limit published in 2020"

A review on heat treatment efficiency in metastable β titanium alloys: the role of treatment process and parameters

Abstract: Metastable β titanium alloys are widely used in the biomedical, automotive, and aerospace industry, due to their excellent corrosion resistance, fatigue strength, biocompatibility, and easy formability. Besides all these use areas, the suitable microstructure of the alloys for heat treatment increases the efficient usability day by day. In literature research, it has been found that heat treatment types such as cryogenic treatment and precipitation hardening can be applied efficiently to the alloys. Optimum strength/ductility, wear resistance, creep strength, and fatigue strength can be obtained with these heat treatments. For this reason, it has become important to understand the effect of heat treatments on the microstructural and mechanical properties of the alloys and parameters affecting this heat treatment efficiency. Precipitation hardening includes solution and aging treatment steps. The solution treatment can be applied at temperatures below and above the β transition temperature. While the aging treatment can be applied in four different ways, in the review article, the effects of single step and duplex aging treatment, which are applied with high efficiency, are emphasized. Precipitation hardening efficiency affects the chemical composition of the alloys, heat treatment steps, treatment temperature and times, and heating/cooling rate. The cryogenic treatment provides the formation of martensite α phases in metastable β titanium alloys cooled below the martensitic transformation temperature. Higher-strength and hardness have been obtained in the studies where aging treatment was applied after cryogenic treatment. Cryogenic treatment efficiency determines chemical composition of the alloys, treatment temperature and time, the heating/cooling rates, and heat treatments applied after cryogenic treatment.

Killer notches: The effect of as-built surface roughness on fatigue failure in AlSi10Mg produced by laser powder bed fusion

Abstract: Additive manufacturing allows the production of complex components for critical applications such as in the aerospace industry Laser powder bed fusion is the most widely used form of additive manufacturing and good progress has been made in improved material quality in recent years Despite the progress, fatigue properties are sometimes still problematic and this requires further investigation The fatigue properties of additively manufactured metals depend on a variety of factors including surface roughness, microstructure, porosity and residual stress, amongst others In this work the role of surface roughness in particular is evaluated using micro computed tomography (microCT) scans before and after fatigue tests The crack locations are identified in scans after fatigue testing and correlated with surface features prior to fatigue tests by careful alignment of CT images In this way notches on the surface which act as “killer notches” are measured and compared with other defects (both roughness notches and pores) in the vicinity Direct evidence is thereby provided for specific features acting as killer defects, studied with varying surface topographies depending on build orientation A statistical analysis using stress intensity factor and fatigue test results of the same samples directly validate the effect of the notches, in comparison to other similar notches across the sample This is the first notch-based surface roughness evaluation method reported using X-ray tomography, showing promise as analytical methodology In addition, the experimental campaign shows for the first time a direct correlation of fatigue strength with surface roughness using different typical as-built surfaces This work lays the foundation for improved non-destructive testing, predictive modelling and overall improvement and management of the performance of additively manufactured parts based on surface features and surface characterization

Training high-strength aluminum alloys to withstand fatigue.

Abstract: The fatigue performance of high strength aluminum alloys used in planes, trains, trucks and automobiles is notoriously poor. Engineers must design around this important limitation to use Al alloys for light-weighting of transportation structures. An alternative concept for microstructure design for improved fatigue strength is demonstrated in this work. Microstructures are designed to exploit the mechanical energy imparted during the initial cycles of fatigue to dynamically heal the inherent weak points in the microstructure. The fatigue life of the highest strength Aluminum alloys is improved by 25x, and the fatigue strength is raised to ~1/2 the tensile strength. The approach embraces the difference between static and dynamic loading and represents a conceptual change in microstructural design for fatigue.

Fatigue behavior of PLA-wood composite manufactured by fused filament fabrication

Abstract: In this paper, the fatigue behavior of a polylactic acid-based composite reinforced with wood (trademarked as Timberfill) processed through fused filament fabrication is analyzed through rotating bending fatigue tests. Originally, this material was developed with the aim of enhancing the resistance of PLA by increasing the inner friction between polymer filaments. This work aims to confirm whether the inclusion of wood delivers the expected mechanical advantages from a functional point of view. The effect of wood on the mechanical behavior of the material is also assessed by comparing the results to similar experimental observed on non-reinforced PLA specimens. Firstly, the optimal set of parameters and levels resulting in the highest number of cycles to failure are determined, and compared to those found for PLA, by applying a Taguchi L27 experimental array. Layer height, infill level and nozzle diameter prove to be the most influential parameters on fatigue life. During a second phase, a set of specimens manufactured through the best parameter set were tested at different stress levels to represent the S-N curve. Infinite life was observed for specimens loaded with a maximum stress of 17.9 MPa. Therefore, this value can be considered as a lower threshold of the endurance limit for Timberfill cylindrical parts subjected to cyclic bending stress. SEM observations reveal that wood has a detrimental effect on the PLA matrix, as is reduces the cohesion of deposited filaments, and increases the ductile behavior of the base material.

Effects of laser shock peening on microstructure and fatigue behavior of Ti–6Al–4V alloy fabricated via electron beam melting

Abstract: Laser shock peening (LSP) is a post-treatment process that is widely used to modify the surface microstructure and mechanical properties of parts constructed by additive manufacturing (AM). In this study, the influence of LSP on the microstructure and fatigue behavior of Ti–6Al–4V alloy manufactured via electron beam melting (EBM), a popular method of AM, was investigated. The microstructure of the EBM sample consisted of the β phase (~6 vol%) and α lamellar phase. Grain refinement of the α phase occurred via both dislocation evolution and deformation twinning during LSP. A theoretical description of the microstructural evolution, particularly the distribution of deformation twins, was developed. The fatigue strength and micro-hardness of the EBM samples increased by approximately 17% and 11% after LSP treatment, respectively. The fatigue fracture morphologies at three defined damage stages (crack initiation, crack propagation, and instantaneous rupture) were examined for EBM samples before and after LSP. The dominant mechanism of fatigue strength enhancement by LSP was discussed. The effects of residual compressive stress assistant with adiabatic temperature increase and grain refinement of the α phase produced by LSP reduced the pre-existing crack size, suppressed crack initiation, and increased the required work for fatigue fracture.

Quantitative phase analysis of martensite-bainite steel using EBSD and its microstructure, tensile and high-cycle fatigue behaviors

Abstract: This study conducted a quantitative analysis of the martensite/bainite (M/B; bainitic structure region) fraction of martensite-bainite steel using electron back-scatter diffraction (EBSD) analysis. The M/B fraction analyzed using the EBSD analysis method was then compared with phase fraction measurement results with an optical microscope (OM), field emission scanning electron microscope (FE-SEM), and field-emission transmission electron microscopy (FE-TEM). In addition, microstructure, tensile and high-cycle fatigue behaviors according to M/B phase fraction were investigated. Initial microstructural observation measured a prior austenite grain size (PAGS) of 24 μm (alloy A) and 11 μm (alloy B). Both alloys were observed to have martensite and bainite structures. XRD phase analysis of the two alloys identified an α-Fe peak expected to be martensite or bainite. Quantitative phase fraction of M/B using EBSD analysis measured M: 40.37% and B: 59.63% for alloy A, and M: 53.03% and B: 46.97% for alloy B. Tensile tests of the above materials confirmed that alloy B, which had finer PAGS and a higher martensite fraction, had greater yield strength (1423 MPa) and tensile strength (1826 MPa) that were approximately 200 MPa higher than alloy A. The yield strength was calculated based on the M/B phase fraction using EBSD and the measured microstructure factors, with a consideration of the prediction model. The calculation value was similar to the actual tested strength one. In the high-cycle fatigue test, alloy B, with its greater strength, had an approximately 200 MPa higher fatigue limit (1075 MPa) than that of alloy A. EBSD analysis of the fatigue crack initiation area confirmed that the M/B interface can act as a fatigue crack initiation site. Based on the above findings, tensile and fracture surface analyses were performed, and attempts were made to identify the tensile and deformation mechanism according to the M/B phase fraction.