Excellent Fatigue Life Research Articles

Tailoring microstructure with special bimodal structure to improve the ductility and fatigue life of laser powder bed fused Ti6Al4V alloy

Achieving the high strength and ductility, as well as the excellent fatigue life, was the primary consideration for aerospace application of the additively manufactured (AM) titanium alloys, but it was limited to the high residual stress, huge columnar β grains arising from the severe directional heat flow and temperature gradient. To address these challenges and then improve the comprehensive mechanical properties, this work newly designed heat treatment schedules to tailor the microstructure to improve tensile/fatigue properties in the laser-powder bed fusion (L-PBF) of Ti6Al4V. The optimized schedule (920 °C/1 h/water cooling + 800 °C/2 h/furnace cooling) obtained the special bimodal structure, including the thin lamellar α+β phases (thickness of 1.8 μm) and the discontinuous equiaxed grain boundary α phases (GB-α) (diameter in ∼6 μm), achieving the good combination of tensile strength (1025 MPa) and ductility (17.2%). The heated-treated Ti6Al4V parts with thickness of 2∼10 mm exhibited the good stability of strength and eliminated the strength anisotropy. The dimensional effect of specimen resulted in the significant discrepancy in ductility. The tailored Ti6Al4V exhibited the superior low-cycle fatigue life equivalent to the wrought counterpart due to the refined α phases, whereas presenting the relatively low stability in the high cycle fatigue life. The fracture morphology showed the crack initiation and propagation resulting from the aligned pores and uncontrollable defects.

Investigation on microhardness and fatigue life in spot welding of quenching and partitioning 1180 steel

Quenching and partitioning (Q&P) steels are among the most critical third-generation steels for realizing vehicle weight reduction and improving safety performance. It is critical for the structure durability to have an excellent fatigue life of the welds. In this study, Q&P1180 steel was laser spot welded (LSW) in order to evaluate the microhardness, mechanical properties, and fatigue behavior of its joints, which were compared with resistance spot welding (RSW). In both types of joining methods, the welded zone is composed of lath martensite with high microhardness value. Due to the martensite tempering, the sub-critical heat affected zone of Q&P RSW welds was soften, resulting in a decrease of 52 HV of microhardness than the LSW joint. The presence of stress concentration in the boundary influenced tensile strength and all-welded samples failed in the boundary. Furthermore, the fatigue limit of the RSW joint (1.139 kN) was about 91.5%t of the LSW joint (1.245 kN). Fatigue specimens of the LSW joint failed between the fusion zone and heat-affected zone because of their higher sensitivity to stress concentration than the RSW joint, resulting in double eyebrow failure and pull-out failure. Fatigue crack originated from the specimen surface and propagated through fatigue striations together with secondary cracks. In addition, it was found that the strength of the base metal has inconsequentially affected the fatigue life of the welds when these welds were produced by the same welding method and have similar geometries.

Influence of Complete Tool-Offset Position on Intrinsic Microstructure and Its Effect on Fatigue Life of AA2024 Alloy-Copper Joint Made by Friction Stir Welding

Abstract Tool offset position is the key parameter to consider during the welding of dissimilar metals by friction stir welding technique. It helps to control the formation of brittle intermetallics and volume defects. Even tool offset position can leave defects at the interface when it exceeds a certain limits, and it has been reported as two-thirds of the bottom radius of the tool. In this work, the effect of the maximum tool deviation (complete offset) on intrinsic microstructure and fatigue life of AA 2024-copper weld was studied. For this, an experiment was carried out with complete tool offset position. The results highlighted that the stir zone was formed mainly by aluminum and the total volume of copper transferred into the stir zone was measured as 5.6 % of the total volume of the stir zone. This kind of structure gives excellent fatigue life to the weld compared to the regular microstructure formed during friction stir welding.

Effects of aging treatments on low-cycle fatigue behavior of extruded AZ80 for automobile wheel disks

The effect of precipitates on the strain-controlled fatigue of automobile wheel disks made of extruded AZ80 magnesium alloy was investigated via T5 and T6 peak aging. The age-hardening behavior, stress–strain response, fatigue fracture morphology, and fatigue crack path of the AZ80 wheel disks were characterized to determine their monotonic and cyclic deformation behaviors. The T5 and T6 peak-aging treatments significantly enhanced their hardness and monotonic strengths via precipitation hardening; as the precipitate amount increased, the twinning–detwinning behavior in the initial stage of cyclic deformation weakened accompany the strain hardening in the half-life cycle. According to the crack growth analysis, the propagation pattern was related to the coalescence of voids in the cyclic plastic zone. Compared with the extruded and T6-peak-aged counterparts, the highly dense lamellar phases in the T5 peak-aged sample suppressed the cyclic plastic deformation at the crack tip, deflecting the cracks and resulting in longer fatigue life at certain strain amplitudes. Thus, the T5 peak-aged sample exhibited excellent fatigue life at intermediate strain amplitudes (0.3%–0.8%).

Low cycle fatigue behaviour of a ferritic steel strengthened with nano-meter sized precipitates

Low-cycle-fatigue behaviour (Δεt/2 = ± 0.002 to ± 0.01) of a ferritic-steel (tensile strength ∼ 800 MPa) strengthened with nano-meter-sized-precipitates has been examined. The selected steel is a hot-rolled Ti-Mo-bearing low-carbon steel (grain size ∼ 3 μm) strengthened predominantly with Ti-Mo-C and Ti-C type precipitates of size ≤ 30 nm distributed randomly on the grain-boundaries and the matrix.The spread of cyclic-plasticity is negligible in the steel till Δεt/2 = 0.0025 leading to cyclically-stable behaviour and excellent fatigue-life (no failure till 105 loading cycles). At Δεt/2 = 0.003, significant initial-cyclic-hardening is noticed followed by minor amount of cyclic-softening. The latter is attributed to formation of low-energy-dislocation-structures e.g. dislocation-walls and channels. At Δεt/2 = 0.004–0.01, predominantly cyclic-hardening is noticed till failure; the intensity of hardening increases with increase in Δεt/2. The cyclic-hardening is attributed to increase in dislocation-density and dislocation-precipitate-interaction, formation of subgrains, microbands and dislocation-clusters within both ferrite-grains and microbands. All the above types of dislocation-substructures hinder movement of dislocations and reduce opportunity for cyclic-strain-hardening at Δεt/2 ≥ 0.004 compared to that at Δεt/2 = 0.002–0.003. This phenomenon leads to two stages of cyclic-hardening with two distinct values of cyclic-strain-hardening-exponents (n1′ = 0.3 at low Δεt/2 and n2′ = 0.04 at Δεt/2 ≥ 0.004). Two-stage cyclic-hardening is also reflected in the bilinear Coffin-Manson-relationship and fatigue-fracture. In sharp contrast to fatigue-fracture-surface at Δεt/2 = 0.003, that at Δεt/2 ≥ 0.004 are characterised by multiple-origins of fatigue-crack, absence of fatigue-striations and occurrence of microcleavage/river-pattern indicating faster brittle-fracture at higher Δεt/2.

Interaction of shear and normal stresses in multiaxial fatigue damage analysis

Due to the abundance of engineering components subjected to complex multiaxial loading histories, being able to accurately estimate fatigue damage under multiaxial stress states is a fundamental step in many fatigue life analyses. In this respect, the Fatemi-Socie (FS) critical plane damage parameter has been shown to provide excellent fatigue life correlations for a variety of materials and loading conditions. In this parameter shear strain amplitude has a primary influence on fatigue damage and the maximum normal stress on the maximum shear plane has a secondary, but important, influence. In this parameter, the maximum normal stress is normalized by the material yield strength in order to preserve the unitless feature of strain. However, in examining some literature data it was found that in certain situations the FS parameter can result in better fatigue life predictions if the maximum normal stress is normalized by shear stress range instead. These data include uniaxial loadings with large tensile mean stress, and some non-proportional axial-torsion load paths with different normal-shear stress interactions. This modification to the FS parameter was investigated by using fatigue data from literature for 7075-T651 aluminum alloy, as well as additional data from 2024-T3 aluminum alloy fatigue tests performed in this study.

Predictive Crack Growth Technique for Laser Peening Process Development

Laser peening (LP) has shown excellent fatigue life extension in numerous tests with typical treatments garnering 2-4 times the fatigue performance of an untreated component. Initially, large test programs were implemented to determine the best LP parameters for a given scenario, eventually being augmented by physics-based modeling due to the large design space available to the LP process. Approval for these processes continues to be on a case-by-case basis, contingent on multiple factors: cost, applicability, time, % fatigue life extension, and ability to track crack growth. Because LP induces compressive residual stresses in the near surface region, the compensatory tensile residual stresses are shifted sub-surface. While an axial tensile load would be mitigated by surface compressive stresses, sub-surface a crack can propagate rapidly via tensile stresses. Current predictive methods lack the ability to track this sub-surface behavior, limiting the accuracy of fatigue crack growth predictions throughout the various design stages of an LP treatment. This work demonstrates a framework that incorporates user-defined geometry, material data, crack growth data, mechanical loading, and residual stresses to predict the crack front shape evolution in 3D solids. A baseline case with no residual stresses is simulated and compared with a closed form solution.

Fatigue life prediction: A comparative study for a three layer EN45A parabolic leaf spring

Article history: Received 6 January, 2015 Accepted 10 May 2015 Available online 12 May 2015 There are literally several studies accomplished to predict the fatigue life of leaf springs but estimation of fatigue life of a parabolic leaf spring by using CAE tools has not yet been executed in the past. Parabolic spring is an important component in a vehicle suspension system. It needs to have excellent fatigue life and in today’s scenario manufacturers rely on constant loading fatigue analysis. The objective of this work is to perform the fatigue analysis of parabolic leaf spring by three different methods where CAE analysis is performed to observe the distribution of stress fatigue life and damage using Goodman approach. In this work, fatigue life of the parabolic leaf spring is determined as per SAE spring design manual and experimentally by testing on full scale fatigue testing machine. ANSYS is used for CAE solution for the prediction of leaf springs fatigue life considering stress theory. The fatigue life estimated by all three modes is then compared for the purpose of validation. The methodology used in this paper brings a practical approach to the professionals in the industries who are engaged for design of mechanical components. © 2015 Growing Science Ltd. All rights reserved.

A Highly Fatigue-Resistant Zr-Based Bulk Metallic Glass

The strength-normalized fatigue endurance strength of the bulk metallic glass (BMG) Zr52.5Cu17.9Ni14.6Al10Ti5 (Vitreloy 105) has been reported to be the highest for any BMG; however, to date, there has been no explanation of why this material is so much better than other Zr-based compositions. In this study, the fatigue-crack growth behavior of Zr52.5Cu17.9Ni14.6Al10Ti5 was compared in ambient air vs dry nitrogen environment. The excellent fatigue life behavior is attributed to a relatively high fatigue threshold (ΔKTH ≈ 2 MPa√m) and a lack of sensitivity to environmental effects on fatigue-crack growth in ambient air, as compared to other Zr-based BMGs. Fatigue life experiments conducted in ambient air confirmed the excellent fatigue life properties with a 107-cycle endurance strength of ~0.24 of the ultimate tensile strength; however, it was also found that casting porosity, even in limited amounts, could reduce this endurance strength by as much as ~60 pct. Overall, the BMG Zr52.5Cu17.9Ni14.6Al10Ti5 appears to have excellent strength and fatigue properties and should be considered as a prime candidate material for future applications where good mechanical fatigue resistance is required.

Life Assessment of A Parabolic Spring Under Cyclic Stress & Cyclic Strain Loading Using Finite Element Method

This study presents a fatigue life prediction based on finite element analysis under non constant amplitude proportional loading. Parabolic spring is the vital component in a vehicle suspension system, commonly used in trucks. It needs to have excellent fatigue life and recently, manufacturers rely on constant loading fatigue data. The objective of this study is to simulate the non constant amplitude proportional loading for the fatigue life analysis. The finite element method (FEM) was performed on the spring model to observe the distribution of stress and damage. The fatigue life simulation was performed and analyzed for materials AISI6150, SAE1045-595-QT. when using the loading sequences is predominantly tensile in the nature; the life of mounting in Goodman approach is more conservative. When the loading is predominantly tensile in nature, the life of the component in Morrow approach is more sensitive and is therefore recommended. It can be concluded that material SAE 1045-595-QT gives constantly higher life than material AISI6150for all loading conditions under both methods.

Effect of cooling rate during hot stamping on low cyclic fatigue of boron steel sheet

Boron steel is widely used throughout the automobile industry due to its high tensile strength and hardenability. When boron steel is used for body parts, only high strength is required for crashworthiness. However, when boron steel is used for chassis parts, a high fatigue life is needed. The microstructure of boron steel is mainly affected by the cooling rate during hot stamping. Therefore, this study investigated the low cyclic fatigue life according to the cooling rate. The fatigue life increased at a low strain amplitude when the cooling rate was fast. However, at a high strain amplitude, the fatigue life decreased, due to the low ductility and fracture toughness of the martensite formed by rapid cooling. Martensite formed by a fast cooling rate shows excellent fatigue life at a low total strain amplitude; however, a multiphase microstructure formed by a slow cooling rate is recommended if the parts experience high and low total strain amplitudes alternately. In addition, the cooling rate has little effect on the distribution of solute boron and boron precipitations, so it is expected that boron rarely affects low cyclic fatigue.

An Examination of Fatigue Initiation Mechanisms in Thin 35Co-35Ni-20Cr-10Mo Medical Grade Wires

Abstract Knowledge of the intrinsic defect size distribution, surface grain size distribution, and prior deformation history are important factors in determining fatigue crack initiation mechanisms and total life variability in thin, metallic, medical grade wires. The ASTM F562 alloy system is used extensively as a fine wire coil or cable in the production of cardiac rhythm management leads which require excellent fatigue life, and a good understanding of life variability. In the present investigation, samples of 0.0070 in. diameter ASTM F562, 35Co-35Ni-20Cr-10Mo wires were produced with a variety of grain sizes and strain hardening conditions. Samples were then cyclically loaded to failure in rotary beam testing and preserved for post mortem fractography using high resolution scanning electron microscopy (HR-SEM). Fatigue cracks were found to initiate from three sources: intrinsic microstructural inhomogeneities, persistent slip bands (PSBs), and extrinsic surface defects. The dominance of the various initiation mechanisms was shown to be a function of the constituent particle and grain size as well as the fatigue loading conditions and prior deformation history. In samples exhibiting a surface grain size significantly larger than the constitutive particle distribution, cracks were observed to preferentially nucleate from surface intersecting PSBs rather than near-surface-particles. Understanding of these phenomena is important in the design of robust cardiac lead systems that will outlive the patient.

Fatigue testing of Inconel‐625 coatings on carbon and stainless steel after one and three weeks of exposure to aqueous electrolytic solution

PurposeThis paper seeks to examine the fatigue properties of HVOF sprayed Inconel‐625 coating of steel substrate before and after the aqueous corrosion.Design/methodology/approachWorkpieces were cut from steel sheets. After chemical and ultrasonic cleaning, workpiece surfaces were sand‐blasted and HVOF sprayed Inconel‐625 coated. The coated and un‐coated surfaces were subjected to the aqueous corrosion tests for one and three weeks. After the completion of the corrosion tests, fatigue properties of the workpieces were examined.FindingsStainless steel coated workpieces demonstrated excellent fatigue life resistance versus coated carbon steel workpieces. Stainless steel workpieces apparently have a high‐cycle fatigue represented by in excess of 1.50 million cycles without cracking, thereby assuring a high‐fatigue life. The carbon steel specimens have low‐cycle fatigue and consequently a short fatigue life. In addition, high velocity impacting of splats on to the workpiece enhances the hardness of the surface. This, in turn, improves fatigue properties at the interface, particularly for stainless steel workpieces.Research limitations/implicationsThe tests can be extended to include the duplex treated workpieces such as the laser treatment of surface after HVOF sprayed coating. This enhances the bonding of the coating through thermal integration of the coating and the base substrate material.Practical implicationsThe results can be used to assess the HVOF sprayed coatings.Originality/valueThis paper provides information on the fatigue behavior of HVOF sprayed coatings when subjected to the cyclic load and offers practical help for the researchers and scientists working in the coatings area.

Mechanical Properties of Date Palm Fiber Reinforced Composites

Mechanical properties of several laminates of date palm leave (DPL) reinforced composites are investigated. Three different processes to construct these composite laminates are assessed. These are wet lay-up with simple vacuum bagging, autoclaving with vacuum bagging and Vulcan press moulding. Several fiber orientation and dimensions are tested. Two types of resins are selected. The first is a high temperature curing Phenolic (phenol formaldehyde) resin. The second is a two-component Bisphenol resin with amine-based slow curing agent. The average tensile strength ranged between 127.4 and 152.3 MPa for long unidirectional fibers and 8.4 to 62.6 MPa for short and medium length fibers respectively (depending on process and resin type). The Bisphenol laminates have better mechanical properties. These laminates have consistently 25 to 50% higher tensile strength than Phenolic laminates. The bending strength ranged between 120.6 and 342.3 MPa (at 500 mm/min loading speed) depending on fiber length. Short fiber Bisphenol laminates have excellent fatigue life characteristics (over 466,000 and 653,000 cycles at 70% and 60% of maximum stress loads respectively). The process parameters and resin types have significant effects on the laminate properties. The water absorption of the laminates ranged between 2.3 to 5.8% for the Phenolic laminates (depending on the test method and manufacturing process used). The water absorption of Bisphenol laminates ranged between 10 and 40%. The produced laminates were very stable to handle all required machining processes as construction panels. They were successfully subjected to several machining processes such as milling, end milling, hack sawing and drilling.

Development of High Strength Valve Spring Steel with Excellent Fatigue Life Performance

In order to respond to the needs for high-stress valve spring designing, we developed a new valve spring steel which has superior fatigue property and sag resistance without sacrificing the cold-coiling performance of wire. This paper describes the following results:1) The tensile strength of newly developed steel was 200MPa higher than that of conventional SAE9254 after heat treatment which simulated the nitriding process.2) Austenite grain size number of the developed steel was 2 point higher than that of SAE9254 after oil tempering process.3) Fracture toughness of the developed steel was 5MPa√m higher than that of SAE9254 compared at the same hardness level. This is thought to be the effect of Ni addition and higher tempering temperature.4) It was confirmed that conventional cold coiling process being adopted to SAE9254 is applicable to this newly developed steel.5) The developed steel is less sensitive against inclusions during fatigue test. If the inclusion size is same level, 16% higher cyclic stress can be applied for the developed steel than for SAE9254.6) We have also developed the inclusion control technique for high Si steel, and high fatigue performance are being stably obtained.

Magnetic properties of Ni-Co-Cr-base Elgiloy.

The Ni-Co-Cr-base Elgiloy is one of the commonly used engineering materials. Most applications rely on its high strength, ductility, corrosion resistance and excellent fatigue life over a wide temperature range. However, in the medical application of cerebral aneurysm clips, the alloy is often subjected to strong magnetic fields associated with magnetic resonance imaging (MRI). Its paramagnetic behavior meets MRI safety requirements, but is the source of relatively large artifacts and thus less MRI-compatible for MRI procedure involving the brain. This article reports superconducting quantum interference device (SQUID) measurements on the magnetic properties of a series of Elgiloy wires in either as-drawn or heat-treated conditions. Furthermore, low-temperature calorimetry was employed to reveal the existence of submicroscopic clusters containing ferromagnetic elements such as Ni or Co in the macroscopically paramagnetic matrix.

ANALYSIS AND TESTING OF SURFACE COLD‐WORK PROCEDURES APPLIED TO NOTCHED, FLAT ELEMENTS

Abstract— Several surface cold work procedures are developed to induce compressive residual stresses adjacent to a notch in a flat element. It would appear that they should all lead to fatigue life enhancement. For sharp notched specimens excellent fatigue life enhancement is observed. For moderately notched specimens only one of the procedures produces this enhancement. In order to shed light on the shortcomings of two of the procedures, the residual stress field is determined by means of finite element analyses in two and three dimensions. It is observed that the two‐dimensional analyses predict a residual stress field which should lead to fatigue life enhancement. It is the three‐dimensional analyses which help to explain why two of the cold work procedures fail.

Thermal Hysteresis in Ni-Ti and Ni-Ti-X Alloys and Their Applications

AbstractThe first practical shape memory alloy, NITINOL, has a transformation thermal hysteresis(Hs) of about 30K and a monoclinic martensite structure. The first application of this alloy was a pipe coupling. Although various ideas were proposed on thermal actuators, they were not easily industrialized at least partly because the amount of Hs of the binary Ni-Ti alloy was too large. While the reduction of Hs is not so easy as transformation temperature control, the alloy was improved in terms of Hs through the discovery of new martensite phases. The first Hs reduction was realized in a Ni-Ti-Cu alloy. The copper addition changes the martensite structure to orthorhombic and reduces Hs to 10-15K. The alloy was first applied to a thermal actuator in automobile fog-lamps. Subsequently the R-phase transformation was identified in the binary alloy under the combination of cold working and a heat treatment at relatively low temperatures. Hs was further reduced to 2K in the R-phase transformation. The R-phase transformation is also featured by an excellent fatigue life which reaches one million cycles. The small Hs and long repetition life rapidly widened the application field of shape memory actuators. An expansion of Hs up to 100K was also achieved by a combination of fine dispersion of niobium and cold working. At present, the Hs of the Ni-Ti base alloys ranges from 2K to 100K. An appropriate alloy system can be selected according to applications. The binary alloy has found a wider application field in the super elastic application. In this paper, we summarize the progress in the modification of Hs and present recent applications of each transformation.