Increase In Fatigue Limit Research Articles

Quasi-static and fatigue performance of Ti-6Al-4V triply periodic minimal surface scaffolds manufactured via laser powder bed fusion for hard-tissue engineering

Additive manufacturing of Triply Periodic Minimum Surface scaffolds offers a promising solution for bone defect restoration and an alternative to traditional implants in hard tissue engineering. This study investigates the quasi-static and tension-compression cyclic properties of laser powder bed fusion-produced Ti6Al4V lattice-based architectures, specifically gyroid and diamond scaffold geometries. The scaffolds were designed to emulate the mechanical properties of bones, with pore sizes (650–1000 μm) suitable for tissue engineering, and porosities ranging from to 30–60 %. The quasi-static tensile properties of the stiffness and yield strength were characterized experimentally and validated numerically using the finite-element method. The results showed that the stiffness and tensile strength of the scaffolds were comparable to those of cortical bone, reducing the risk of scaffold failure. The influence of post-processing by heat treatment and hot isostatic pressing resulted in an increased fatigue limit and ductility compared with non-treated materials reported in the literature. Fractography analysis revealed that fatigue fracture initiation occurred at the outer surface of the skeleton structures, where the maximum tensile stress was concentrated. These findings support the assumption that the designed scaffolds accurately replicate the mechanical and fatigue characteristics of cortical bone, offering suitable alternatives for bone replacement and orthopedic implants.

Study on Axial Fatigue Performance and Life Prediction of High-Strength Bolts at Low Temperatures

High-strength bolts are widely used in outdoor steel structures such as transmission towers and bridges, where they not only endure cyclic wind loads and vehicle loads but also frequently operate in low-temperature environments. However, there is limited research on the axial fatigue performance of high-strength bolts, particularly regarding their mechanical behavior at low temperatures. Therefore, this study conducted a series of fatigue tests on high-strength bolts at 20 °C and 0 °C, both with and without pretension. We established S-N curves and fatigue limits for the three scenarios, revealing that pretension significantly enhances the fatigue life of the bolts, with a 10% increase in fatigue limit at 0 °C compared to 20 °C. However, due to the influence of pretension, the external load has a minimal effect on the actual stress experienced by the bolts, resulting in S-N curves for bolts with pretension being very similar to those for bolts without pretension during cyclic loading. Additionally, we obtained the load–displacement curves and corresponding stiffness degradation patterns of the bolts at both temperatures, finding that all bolts exhibited significant stiffness degradation after reaching 0.8 times their fatigue life. The high-strength bolts at 0 °C demonstrated greater stiffness and faster crack propagation rates, with increases of approximately 6% and 8%, respectively. Furthermore, electron microscope scans were used to clarify the fatigue crack initiation and the evolution of fatigue striations at both temperatures. Finally, by combining refined numerical simulations with the local stress–strain method, the effectiveness of the local stress–strain method for evaluating the fatigue life of bolts without pretension was validated. Building on this, we extended the method to bolts at 0 °C and those subjected to pretension, recommending notch sizes of 0.4 mm and 1.1 mm for fatigue life assessment of bolts with pretension at 0 °C and 20 °C, respectively.

Correlation between microstructure and fatigue properties of hot-rolled thick-plate complex-phase steel

Complex-phase (CP) steels, with a multiphase microstructure, offer an excellent combination of high strength, ductility, and formability, making them an attractive alternative to conventional high-strength low-alloy (HSLA) steels in the automotive industry. However, the microstructure and fatigue property relation in CP steels is complex. This limits the full exploitation of CP steels in applications, such as heavy-vehicles, where excellent fatigue performance of thick-plates after punching holes is the critical parameter. In this work, we initiate the study of the relation between microstructure and fatigue properties of a commercial CP steel (800CP) and compare it with a conventional HSLA (500MC) steel. Fatigue property, tensile property, and fatigue crack growth rate (FCGR) testing are conducted and the performance of the two steels is rationalized using detailed microstructure characterization, before and after fatigue testing. FCGR testing shows that, despite a higher yield strength of the 800CP, both steels have a similar propagation rate due to a more tortuous crack propagation path and a higher quantity of secondary crack formation in the 800CP microstructure. The high cycle fatigue (HCF) testing shows that the fatigue limit in the 800CP is 25% higher. This increase in fatigue limit is attributed to the improved resistance to fatigue crack initiation in the 800CP due to its larger fraction of bainite.

Effect of multifunction cavitation on rotating bending fatigue properties of steel rods and its fatigue limit estimation

The effect of multifunction cavitation (MFC) on the rotating bending fatigue properties of low-alloy steel rods was examined. MFC generates compressive residual stress on the surface of steel rods, in contrast to conventional water jet peening, which improves the fatigue limit of steel rods. The fatigue limit of MFC-treated steel rods can be estimated using a modified Goodman diagram. The proposed equation gives the maximum value of the fatigue limit of MFC-treated steel rods (227.7 MPa) and the maximum value of the compressive residual stress contributing to the increased fatigue limit (-117.9 MPa).

Impact of Cooling Rate during High-Pressure Gas Quenching on Fatigue Performance of Low Pressure Carburized Gears

The impact of cooling rate during high-pressure gas quenching on the fatigue performance of low-pressure carburized spur gears was studied for steel grades 20MnCr5 and 17NiCrMo6-4. The results show an increased fatigue limit by 10 to 11% when applying a slower cooling rate for both steel grades. Moreover, for 20MnCr5 the slower cooled gears show an increase in compressive residual stresses by 130 MPa compared to the faster cooling, although no significant difference was observed for 17NiCrMo6-4. It is also seen that the cooling rate affects the core hardness for both steel grades, while other properties like surface hardness, case-hardness depth and martensite variant pairing were unaffected. The results for the retained austenite content and average martensite unit size show no clear effect of the cooling rate. The possible influence of different carbon distributions after quenching for the two used cooling rates on the carbide precipitation and fatigue limit is discussed.

Effect of Diamond Burnishing on Fatigue Behaviour of AISI 304 Chromium-Nickel Austenitic Stainless Steel.

The disadvantages of widely used austenitic stainless steels are their low hardness and relatively low fatigue strength. Conventional chemical-thermal surface treatments are unsuitable for these steels since they create conditions for inter-granular corrosion. An effective alternative is a low-temperature surface treatment, creating an S-phase within the surface layer, but it has a high cost/quality ratio. Austenitic steels can increase their surface micro-hardness and fatigue strength via surface cold working. When the goal is to increase the rotating bending fatigue strength of austenitic chromium-nickel steels, and the requirements for significant wear resistance are not paramount, diamond burnishing (DB) has significant potential to increase the fatigue strength and, based on the cost/quality ratio, can successfully compete with low-temperature chemical-thermal treatments. The main objective of this study is to establish the effect of DB on the rotating fatigue strength of AISI 304 L chromium-nickel austenitic steel. The influence of DB parameters on the surface integrity (SI) characteristics was studied. Optimal DB parameters under minimum roughness and maximum micro-hardness criteria were obtained. Rotating bending fatigue tests of the diamond burnished (in a different manner) and untreated specimens were performed. DB implemented via parameters providing maximum micro-hardness increased fatigue limit by 38% compared to untreated specimens.

Mechanical surface treatment of EBM Ti6Al4V components: Effects of the resulting surface layer state on fatigue mechanisms and service life

Although additive manufacturing of the titanium alloy Ti6Al4V has been an established process for years, problems still exist with regard to the design of components subjected to cyclic loads. In this context, process-related defects play an important role, such as surface roughness and pores. Typically, AM Ti6Al4V components are machined and subjected to a hot isostatic pressure (HIP) treatment, which, however, strongly limits the cost advantage. Mechanical surface treatment processes can be used locally where high loads are expected and thus make a cost-effective contribution to the service life of additively manufactured components. In the present work, the influence of the mechanical surface treatment process of deep rolling (DR) on the surface state of electron beam melted (EBM) and EBM+HIP components is investigated. In addition, rotating bending tests are presented in SN curves and these are evaluated in the context of the surface layer condition and the failure mechanisms. The fatigue strength could be improved from RAB=157MPa in the as built state (AB) to RAB+DR=251MPa in the deep rolled state (AB+DR). Through the deep rolling treatment, the surface could be strengthened to the extent that the location of the failure shifts into the component volume. The comparison of the fracture surfaces analysis of deep rolled samples shows that pores below the surface are the cause of failure, in a depth that is not influenced by the mechanical surface treatment. In an additional series of tests, the fatigue strength of the HIP and deep-rolled (HIP+DR) state was determined. Since HIP EBM specimens are nearly pore-free, the failure mechanism switches back to the strengthened surface, resulting in an increased fatigue limit of RHIP+DR=413MPa.

VHCF, Tribology Characteristics and UNSM Effects of Bainite and Martensite Spring Steels

It has been reported that the duplex bainite microstructure obtained by austempering (AT) shows higher strength, ductility and impact toughness than quench and tempered (QT) martensite structure in SAE9254 spring steel. However, there seems to be no research on the very high cycle fatigue (VHCF) and tribology characteristics of bainite structure for durability design of next generation spring steel from the perspective of engineering and industrial applications. This is a follow-up study that quantitatively analyzed the mechanical properties, microstructural deformation characteristics, and impact toughness of bainite and martensite using EBSD (Electron Backscatter Diffraction) and SEM (Scanning Electron Microscope) analyses. In this study, VHCF, HCF, tribology characteristics and UNSM (ultrasonic nanocrystal surface modification) effects under duplex bainite and single martensite microstructures were quantitatively studied and analyzed by fracture mechanics from the engineering and industrial point of view to improve durability and weight reduction in spring steels. The bainite AT and martensite QT specimens showed a 56% and 33% increase in fatigue limit for as received AR specimens. Fisheye cracks in duplex bainite AT specimens are similar to ‘facet internal cracks’ that initiated in the absence of inclusions. Generally fisheye crack fracture mode is preferred in VHCF, but fisheye crack was not found in the QT and the AR specimens at all. The UNSM-treated specimens showed fatigue limits that were about 33~50% higher than the untreated specimens.

Fatigue limit estimation for carburized steels with surface compressive residual stress considering residual stress relaxation

The mechanism of residual stress relaxation for carburized steels was elucidated and their fatigue limit was estimated. The compressive residual stress in the carburized steel after rotating bending fatigue tests is mainly influenced by the relaxation during the compressive loading phase, although it increases at the early stage. The fatigue limit of carburized steels can be estimated according to the modified Goodman diagram considering the residual stress relaxation. The proposed equation shows the maximum value of the fatigue limit (1013 MPa) of carburized steel and the maximum value of the compressive residual stress contributing to the increased fatigue limit.

Wear and Fatigue Behaviour of Deep Cryogenically Treated H21 Tool Steel

The influence of deep cryotreatment before and after double tempering on hardness, wear rate, surface finish and fatigue limit of AISI H21 tool steel has been examined. In the present work, H21 tool steel has been subjected to heat treatment at 1195 °C, double tempering at 540 °C, deep cryotreatment at − 185 °C and soft tempering at 100 °C. The microstructure of samples has been characterized for the number density of carbides, hardness, wear rate, surface roughness and fatigue limit. The fatigue test has been carried out using a rotating bending fatigue machine to study the fatigue strength of the material. The obtained results show that the HTTC24 specimen reduces the wear rate by ≈ 24% and surface roughness by ≈ 21% with an increase in fatigue limit by ≈ 13% compared to HTT specimens. This has been attributed to the increased number density of carbides and hardness; and approximately the complete conversion of retained austenite content into martensite.

Behavior of UDM processed epoxy based TiO2 nano filler composite adhesive joints under fatigue loading

The present research aims at comparing the behaviour of neat epoxy adhesive joints versus UDM (ultrasonic dual mixing) processed epoxy based TiO2 nano composite adhesive joints under fatigue loading. Single lap shear adhesive joints of both adhesives were tested under static and variable loading. 15% increase in toughness and 33.2% increase in fatigue limit of TiO2 nano composite adhesive joints over neat epoxy adhesive joints were observed. Finite element analysis showed that local stresses were maximum near the edges of the adhesive layer, making them prone to crack initiation.

Methods of Improving Mechanical Integrity of Center-Link Chains for a Trolley Conveyor System

The purpose of this study is to identify the causes of the continuous failure of center-link chains for a trolley conveyor system by analyzing their failure characteristics under static and dynamic loading conditions and to propose methods for securing their mechanical integrity. For this purpose, center-link chains were minimally processed to prepare testing specimens with their original surface property and structural dimension maintained. These specimens were used to measure the tensile strength (1172.1 MPa), elongation (8.7%), yield strength (1073.4 MPa), hardness (44.1 HRC), and surface roughness (R_{a} = 10.5 μm). A three-point bending fatigue test (R = 0.1) resulted in a fatigue limit of 746.7 MPa. The results confirmed that the rough surface of the center-link chain caused a reduction in the elongation and fatigue limit and an increase in the scattering of the surface hardness and fatigue life, which were the direct causes of frequent chain failure in the field. To improve the fatigue life of the center-link chain, two methods were reviewed in this study. The first method was to reduce its surface roughness through surface machining of the center-link chain, and the second was to modify its shape design to allow improved structural integrity, even if the current surface roughness is retained. In the case of the first method, the results of the fatigue test using the specimen with reduced surface roughness (from R_{a} = 10.5 to 0.9 μm) by surface machining demonstrated reduced scattering and increased fatigue limit (from 746.7 to 920.3 MPa). As for the second method, a stiffener was added and two design variables (slope angle and fillet radius) of the stiffener were selected to propose a new design that can reduce the maximum stress ~ 1.6 times compared to the conventional design. While both methods for improving the structural integrity of the center-link chain were effective enough to improve the fatigue life of the chain, the second method, which requires only the initial mold manufacturing cost for modifying the shape design of the chain, was finally selected, because it exhibited better economic feasibility than the first method that increases the product cost and overall process time due to addition of the surface machining process.

Fatigue performances of selective laser melted Ti-6Al-4V alloy: Influence of surface finishing, hot isostatic pressing and heat treatments

In this study, a detailed evaluation on the effects of different surface finishing processes, hot isostatic pressing (HIP) and heat treatments on the fatigue performance of SLMed Ti-6Al-4V is conducted. It is found that surface finishing processes of turning, grinding, grinding followed by sandblasting and polishing can effectively reduce the surface roughness of as-SLMed surfaces (Rz = 68.66 μm) and significantly improve the fatigue performances. The HIP, HT-920 and HT-850-550 treatments can significantly improve the ductilities of SLMed Ti-6Al-4V due to the coarsening of α laths and formation of β grains. The ultimate tensile strengths (UTSs) and yield strengths (YSs) of HIPed and HT-920 specimens reduce more than 140 MPa, while those of the HT-850-550 specimens show no obvious reduction. The SLMed Ti-6Al-4V specimen has a fatigue limit lower than 300 MPa, while the HT-920 and HT-850-550 treatments enhance the threshold of crack growth and increase the fatigue limit to 350–400 MPa. The reduction or elimination of pores and the modification of microstructure in HIPed specimens both significantly extend the period of fatigue crack initiation, leading to an increased fatigue limit of 450–500 MPa.

Fatigue behavior of 316L austenitic stainless steel in air and LWR environment with and without mean stress

The fatigue life design curves in nuclear codes are generally derived from uniaxial straincontrolled fatigue test results. Evidently, the test conditions are very different from the actual components loading context, which involves much more complex thermo-mechanical loading including mean stress, static load holding time and variation in water chemistry, etc. In this work, the mean stress and environmental effects on fatigue life of 316L austenitic stainless steel in air and light water reactor (LWR) environment were studied using hollow fatigue specimens and testing under load-controlled condition. Both positive (+50 MPa) and negative (-20 MPa) mean stresses showed beneficial effect on fatigue life in LWR environment and in air. This is tentatively attributed to mean stress enhanced cyclic hardening, which leads to smaller strain response at the same loading force. -20 MPa mean stress was found to increase fatigue limit, whereas the effect of +50 MPa mean stress on fatigue limit is still unclear. The preliminary results illustrate that the environmental reduction of fatigue life is amplified in load-controlled fatigue tests with tensile mean stress.

Very high cycle fatigue behaviour of 42CrMo4 steel with plate-like alumina inclusions

Recent progress in steel refining shows significant reduction of non-metallic inclusions (NMIs) of which alumina (Al2O3) is one of the most problematic. Among other refining methods, metal melt filtration by ceramic foam filters shows promising results in steel cleaning. In the present work the influence of alumina inclusions on the fatigue behavior is investigated after the reaction of steel melt with filters. Different batches are compared where carbon-bonded alumina foam filters with different coatings were introduced into the steel melt of 42CrMo4 for 10 s (so called “finger test”). Fatigue tests were performed using ultrasonic fatigue testing (USFT) up to 109 cycles. Specimens were nitrided in order to prevent crack initiation from the surface and to study internal failure on NMIs. Surface hardening of quenched steel increased fatigue limit significantly. Metallographic sections were analyzed using optical and scanning electron microscopy (SEM) for the estimation of NMIs distribution properties. NMI size distribution analysis based on maximum Feret diameter (instead of area) is found to be an effective method for detecting plate-shaped inclusions. Fracture surfaces after fatigue tests were investigated by methods of SEM and confocal laser scanning microscopy (CLSM), revealing that plate-like NMIs initiate crack with all their area even being inclined to the crack plane. Properties of crack initiating NMIs – alumina plates and MnS dendrites – are compared and analyzed. Formation of alumina NMI as plate lead to significant enlargement of its stress-concentrating area in comparison to the spherical shape of the same volume. Thus, total NMI content reduction in steel could give no fatigue limit improvement if NMI morphology changes from spherical to plate-like.

Effects of thermomechanical history and environment on the fatigue behavior of (β)-Ti-Nb implant alloys

This study examined the fatigue properties of a newly developed cast and thermomechanical processed (β)-Ti-40Nb alloy for a possible application as biomedical alloy due to exceptional low Young’s modulus (64-73 GPa), high corrosion resistance and ductility (20-26%). Focusing on the influence of two microstructural states with fully recrystallized β-grain structure as well as an aged condition with nanometer-sized ω-precipitates, tension-compression fatigue tests (R=-1) were carried out under lab-air and showed significant differences depending on the β-phase stability under cyclic loading. Present ω- precipitates stabilized the β-phase against martensitic α’’ phase transformations leading to an increased fatigue limit of 288 MPa compared to the recrystallized state (225 MPa), where mechanical polishing and subsequent cyclic loading led to formation of α’’-phase due to the metastability of the β-phase. Additional studied commercially available (β)-Ti-45Nb alloy revealed slightly higher fatigue strength (300 MPa) and suggest a change in the dominating cyclic deformation mechanisms according to the sensitive dependence on the Nb-content. Further tests in simulated body fluid (SBF) at 37°C showed no decrease in fatigue strength due to the effect of corrosion and prove the excellent corrosion fatigue resistance of this alloy type under given test conditions.

Fatigue behavior of a harmonic structure designed austenitic stainless steel under uniaxial stress loading

Harmonic structured materials present a good balance of high strength and high ductility due to their peculiar network structure topology. Since long-term durability is critical for their practical applications, so the present work investigated the fatigue properties of a harmonic structured austenitic stainless steel at room temperature under uniaxial stress loading. The harmonic structure designed SUS316L steels were prepared by mechanical milling and subsequent hot isostatic pressing. The enhanced tensile strength in the harmonic structured SUS316L steels was attributed to the ultrafine grains (shell region), which also resulted in the improved resistance to fatigue crack initiation during cyclic loading. Compared to the conventional SUS316L bulk, increased fatigue limit can be achieved in the harmonic structured SUS316L steels. However, the fatigue ratio tends to be a constant value in SUS316L steels having homogeneous grain structure or bimodal grain structure. Moreover, the change of grain size was not significant after fatigue, which demonstrated the harmonic structured SUS316L steels showed good cyclic stability. In addition, the fatigue cracks tended to initiate at core/shell surface due to strain localization. The harmonic structure designed stainless steel demonstrates great attraction for commercial applications due to its good combination of high yield strength, large uniform elongation, good fatigue resistance and cyclic stability.

Influence of Tertiary Carbides on Improving Fatigue Limit of H13 Die Steels

Forging dies are subjected to fatigue failures for which different surface treatments are used as most of the failures begin from the surface. In this work, the influence of cryogenic treatment on fatigue limit of die steels has been discussed with hardening of fatigue specimens of AISI H13 die steels at 1020 °C, oil quenching, double tempering at 500 °C, cryogenic treatment at minus 185 °C for 16 h followed by soft tempering at 100 °C. The fatigue limit analysis had been carried out with constant amplitude rotating bending fatigue testing at room temperature, at 3000 rpm with load variations which indicated improved fatigue limit for 16 h cryogenically treated specimens in the high cycle fatigue regime. This has been attributed to decreased surface roughness, increased hardness and precipitation of finer tertiary carbides as an influence of microstructure. The minimum value of fatigue strength exponent, in Basquin equation, has also depicted the increased fatigue limit for cryogenically treated H13 specimens.

Microstructural characterization and fatigue response of alloy Ti-46Al-5Nb-1W with varied surface quality and thermal exposure history

The stress controlled fatigue behaviour of a coarse-grained fully lamellar TiAl alloy Ti-46Al-5Nb-1W has been investigated under no exposure, block exposure and individual exposure-oxidation conditions (exposure for 10,000h at 700°C) to assess the effects of surface roughness, stress concentration, oxidation and inner microstructural changes. The alloy is observed to be sensitive to surface damage, but less sensitive to V-notch even after exposure because the surface area sampling the highest stress significantly reduces the EDM impact and exposure effect. Unlike grain-refined alloys, both shot peening and electropolishing offered a moderate and roughly the same increase in fatigue limit, indicating that for a coarse-grain fully lamellar alloy the control factor for fatigue is lamellar colony size. Fatigue performances are improved for all surfaces when subjected to block exposure, attributed to a long-term annealing which outweighs the harmful effects from microstructural embrittlement. After individual exposure-oxidation, fatigue performance deteriorates significantly for the shot peened and moderately for the electropolished but not for the EDM wired. The behaviour can be understood based on whether or not the beneficial changes outweigh the detrimental changes for a specific surface.

Local chill as a mean of increasing strength in grey cast iron

The influence of a chill on the mechanical properties and microstructural features in grey cast iron has been studied. Some of the main findings were that the chill refined the microstructure and modified the graphite distribution from A to D/E. Eutectic cell size was reduced by 60–70%. The Brinell hardness increased while the Vickers hardness, measured in dendrite arms, was unaffected. Fatigue testing in four point bending showed that the fatigue limit was increased by 20–30% in the chilled samples. An increase in tensile strength, proof strength and Young’s modulus was also observed in the chilled samples. The increase in fatigue limit was approximately twice as high as the increase in tensile strength. A possible explanation could be that the eutectic cell size had a more pronounced effect on the fatigue limit than on the tensile strength.