Low Fatigue Strength Research Articles

Fatigue behavior of thin wires and foils of multi-directionally forged commercial purity grade 2 Ti

Thin wires and foils comprising of ultrafine-grained pure titanium (Ti) were made by combined processes of multi-directional forging (MDFing) and either drawing or rolling processes. The MDFed wires and foils exhibited higher tensile strength than the conventional pure Ti wires and plates due to the ultrafine-grained structure. The axial fatigue tests were conducted to investigate their fatigue behavior. The fatigue strengths of conventional pure Ti wires and plates were nearly the same, while the MDFed Ti wires and foils had higher strengths. When the fatigue strengths of MDFed Ti wires and foils were compared, the wires had higher fatigue strength than the foils. EBSD analyses revealed that strong textures were formed in both wires and foils. The basal planes were parallel to the specimen surfaces in wires, while they were inclined against the surface in foils. The texture with the inclined basal planes resulted in the lower crack initiation resistance, resulting in the lower fatigue strength of foils than wires of the MDFed Ti.

On the microstructure and high cycle fatigue of near-net shape PM-HIPed Inconel 625

This paper investigated the microstructure and fatigue behavior of PM-HIPed Inconel 625. The microstructure was composed of γ phase and (Mo, Nb) carbonitrides located mostly on prior particle boundaries. Despite the presence of these carbonitrides, the samples showed good tensile properties with high elongation. Two different surface conditions, pickled and machined, were used for high cycle fatigue testing under a four-point bending test. The results indicated that pickled samples had 6% lower fatigue strength (at 106 cycles) with three times higher standard deviation compared to the machined ones. Fatigue failure mechanisms were found to be dependent on surface conditions and showed different failure modes due to non-metallic oxide inclusions and surface defects in samples with machined and pickled surfaces, respectively. The effect of type, size, and location of defects, multiplicity of crack initiations, as well as surface roughness were analyzed and discussed.

Low-Cycle Fatigue Behavior of Wire and Arc Additively Manufactured Ti-6Al-4V Material.

Additive manufacturing (AM) techniques, such as wire arc additive manufacturing (WAAM), offer unique advantages in producing large, complex structures with reduced lead time and material waste. However, their application in fatigue-critical applications requires a thorough understanding of the material properties and behavior. Due to the layered nature of the manufacturing process, WAAM structures have different microstructures and mechanical properties compared to their substrate counterparts. This study investigated the mechanical behavior and fatigue performance of Ti-6Al-4V fabricated using WAAM compared to the substrate material. Tensile and low-cycle fatigue (LCF) tests were conducted on both materials, and the microstructure was analyzed using optical microscopy and scanning electron microscopy (SEM). The results showed that the WAAM material has a coarser and more heterogeneous grain structure, an increased amount of defects, and lower ultimate tensile strength and smaller elongation at fracture. Furthermore, strain-controlled LCF tests revealed a lower fatigue strength of the WAAM material compared to the substrate, with crack initiation occurring at pores in the specimen rather than microstructural features. Experimental data were used to fit the Ramberg-Osgood model for cyclic deformation behavior and the Manson-Coffin-Basquin model for strain-life curves. The fitted models were subsequently used to compare the two material conditions with other AM processes. In general, the quasi-static properties of WAAM material were found to be lower than those of powder-based processes like selective laser melting or electron beam melting due to smaller cooling rates within the WAAM process. Finally, two simplified estimation models for the strain-life relationship were compared to the experimentally fitted Manson-Coffin-Basquin parameters. The results showed that the simple "universal material law" is applicable and can be used for a quick and simple estimation of the material behavior in cyclic loading conditions. Overall, this study highlights the importance of understanding the mechanical behavior and fatigue performance of WAAM structures compared to their substrate counterparts, as well as the need for further research to improve the understanding of the effects of WAAM process parameters on the mechanical properties and fatigue performance of the fabricated structures.

Fatigue tests on connections of hollow sections made of high‐strength steel with shape‐optimized gusset plates

AbstractStructures made of hollow sections are often used in the construction of steel bridges, agricultural machinery and trucks. Due to secondary bending moments and local stress concentrations at the connection between brace and chord these joints have a low fatigue strength. In case of high‐strength steels, a decisive fatigue design leads to larger wall thicknesses and dimensions of the hollow sections and thus to uneconomical constructions. In the course of the research project FOSTA P1442 truss joints of hollow sections made of high‐strength steels were reinforced by attached and plugged through gussets plates to increase the fatigue strength by relocating the most critical detail at the weld toe to areas subjected to lower stresses. At the University of Applied Sciences Munich, 35 large‐scale fatigue tests on hollow section K‐joints with differently shaped gusset plates were performed to verify the results of the Finite Element Analysis. In order to verify the applicability of the results to real structures, two trusses with a span width of 6 meters were tested. Based on the data of the experimental tests, a fatigue design recommendation for connections of hollow sections with gusset plates could be developed. Within this paper the set‐up and results of the fatigue tests are summarized.

Numerical simulation and experimental study on hot rolling forming of spur face gears

Face gear transmission is the transmission mechanism composed of cylindrical gear and face gear. At present, the tooth making method of face gear is still tooth cutting, with low material utilization and low production efficiency. The metal streamlines are cut off, resulting in low fatigue strength of gear teeth. In this paper, the forming of hot rolling was numerically simulated, with which the characteristics of equivalent stress field, equivalent strain field, rolling force variation, and metal flow in the process of rolling were analyzed. Finally, a hot rolling experimental device was set up, by which the face gear test piece was rolled. Then its tooth surface precision was investigated. The research reveals that hot rolling spur face gears is technological feasible and has promising prospects in industrial applications.

Effect of temperature field on microstructural stability of Al–Cu–Mg alloy hemispherical component

Al–Cu–Mg alloy hemispherical components can be widely utilized in gyroscope navigation systems by virtue of low density, high deformability and fatigue strength. However, the nonuniformity in residual stress and microstructure of the component after deep drawing may lead to a complex microstructural evolution under the temperature environment, thereby decreasing the navigation precision. In this work, the evolution of grain sizes, dislocations, second phases and mechanical properties of Al–Cu–Mg alloy hemispherical components were evaluated under a temperature field. From the edge to the central part of the component, the grain size first decreased and then increased, while the hardness and residual stress first increased and then decreased. With the extension of exposure time, the dislocation density decreased and the coarsened Al2CuMg phases appeared, while the grain size of the component remained stable. Furthermore, the dislocation density declined faster and Al2CuMg phases precipitated rapider in the die fillet area in comparison with the situation at the bottom of the hemisphere. The higher dislocation density could provide more nucleation sites for Al2CuMg phases. Thereafter, more dislocations were consumed, thus accelerating the reduction of stored energy and lattice distortion under the temperature field. As a result, investigating the microstructural stability and mechanical properties of Al–Cu–Mg alloy hemispherical components can make a contribution to fabricating highly stabilized navigation devices to be further applied in the field of space exploration.

An experimental investigation into the influence of incorrect root gaps in welded-in transverse stiffeners on fatigue performance

The paper presents a comprehensive set of special experimental fatigue tests designed to determine the fatigue strength sensitivity of steel components, under circumstances where fabrication quality levels deviate from regulatory standards. Precise monitoring of the manufacturing process of test specimens containing internal imperfections facilitates accurate documentation of the phenomena that contribute to the eventual shape of the imperfection. Welded-in stiffeners with incorrect root gaps of varying sizes were subjected to fatigue testing, as such studies have never been carried out in experimental form before, as the background data to EN 1993-1-9 disclosed. The experimental investigation confirms numerical estimations, which predicted that even large root gaps do not result in lower fatigue strength values, when compared to stiffeners that were properly fitted. Moreover, details in welded beams that feature an intersection of three welds at the stiffener exhibit no discernible decrease in fatigue performance compared to rolled sections.

Exceptional fatigue strength of a microstructurally stable bulk nanocrystalline alloy

The long-term safety of structural elements in many advanced applications, such as power generation and transportation infrastructures, is paramount, especially under cyclic damage conditions. In most coarse-grained (CG) materials (with an average grain size > 1000 nm), the damage tends to localize under cyclic loading conditions, resulting in low fatigue strength. On the other hand, earlier studies on nanocrystalline (NC) alloys (average grain size below 100 nm) show an improved fatigue limit compared to the CG counterparts. However, cyclic deformation mechanisms in such materials remain elusive. Here, we use a microstructurally stable NC Cu-3at.%Ta alloy as a model material to demonstrate exceptional fatigue strength compared to other NC materials in literature and an endurance limit on par with tool steels. Through postmortem characterization combined with atomistic simulations, the underlying mechanism was found to be related to diffuse damage accumulation. During cyclic loading, many grains participate in the plasticity processes (such as the nucleation of dislocations at grain boundaries), resulting in homogenous rather than localized damage accumulation typically observed in CG materials. Atomistic simulations reveal a greatly suppressed damage accumulation in the Cu-3at.%Ta alloy and its more uniform distribution across the microstructure compared with NC Cu. The paper highlights a design strategy to develop advanced structural alloys with exceptional fatigue resistance.

Mechanical properties of sinter hardened sintered steels prepared by hybrid alloying

For PM steel precision parts, high mechanical loading is very common in service. In particular for automotive applications, fatigue up to very high loading cycle numbers may be encountered. Advanced alloying concepts for sintered steels are required, involving also alloy elements with high oxygen affinity. In the present study, hybrid alloyed sintered steels based on prealloyed steel powder Fe-1.8%Cr were prepared by the press-and-sinter route. Mn and Ni, respectively, were admixed as well as a masteralloy containing Mn and Si, the alloy element content being varied, and the combination hardness vs. impact energy was assessed. From the most promising variants, fatigue test specimens were prepared, sinter hardened and characterized, fatigue testing being done by ultrasonic resonance up to Nmax = 10E10 cycles. Ni alloying proved to be positive both for the impact energy and the gigacycle fatigue strength while Mn resulted in high hardenability but low impact and fatigue strength, in part because of intergranular embrittlement. The grade with Mn-Si masteralloy was slightly less ductile than the Ni alloyed variant, but in particular at N > 10E8 the fatigue endurance strength was similar to that of the Ni alloyed type, however with significantly lower scatter.

Impact of DLC Coating Deposition on the Fatigue Strength of Al-7075-T6 Aluminum Alloy

Abstract: Al-7075 has interesting mechanical properties but is susceptible to corrosion. Physical Vapor Deposition (PVD) of coatings results in good corrosion resistance and compressive stresses of the order of 1 GPa on the surface of metallic components. However, the impact of PVD films on the strength of Al-7075-T6 is uncertain. This paper provides a summary of the findings of the Authors’ research group in recent years on the fatigue behavior of Al-7075-T6 with and without PVD Diamond-like Carbon (DLC) coating. The results indicated that DLC-coated specimens have lower fatigue strength than uncoated specimens for lives up to about 10000000 cycles. The failure mechanism was determined by observation of the fracture surfaces of the failed specimens. The stress analysis performed confirms the experimental observation, with crack nucleation expected below the surface of coated specimens, where the highest tensile stresses occur during fatigue loading.

Fatigue cracking criterion of high-strength steels induced by inclusions under high-cycle fatigue

Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a large scatter of fatigue lives. In this work, four S–N curves and more than 200 fatigue fracture morphologies were comprehensively investigated with a special focus on the size and type of inclusions at the fatigue cracking origin in GCr15 steel with a wide strength range by different heat treatments after high-cycle fatigue (HCF). It is found that the percentage of fatigue failure induced by the inclusion including Al2O3 and TiN gradually increases with increasing the UTS, while the percentage of failure at sample surfaces decreases conversely and the fatigue strength first increases and then decreases. Besides, it is interestingly noted that the inclusion sizes at the cracking origin for TiN are smaller than that for Al2O3 because the stress concentration factor for TiN is larger than that for Al2O3 based on the finite element simulation. For the first time, a new fatigue cracking criterion including the isometric inclusion size line in the strength-toughness coordinate system with specific physical meaning was established to reveal the relationship among the UTS, fracture toughness, and the critical inclusion size considering different types of inclusions based on the fracture mechanics. And the critical inclusion size of Al2O3 is about 1.33 times of TiN. The fatigue cracking criterion could be used to judge whether fatigue fracture occurred at inclusions or not and provides a theoretical basis for controlling the scale of different inclusion types for high-strength steels. Our work may offer a new perspective on the critical inclusion size in terms of the inclusion types, which is of scientific interest and has great merit to industrial metallurgical control for anti-fatigue design.

Comprehensive fatigue assessment for different types of aluminum fillet welded joints by local approaches

This paper investigates different types of MIG-welded fillet joints (lap welded joints, T welded joints, non-load-carrying cruciform welded joints, load-carrying cruciform welded joints) of 7N01 aluminum alloys under fully reversed pulsating axial loading. Fatigue assessments of the as-welded test results based on nominal, and local approaches are performed considering geometrical factors in different types of welded joints. The fatigue strength for these joints was compared with the international standard codes and those of other materials. Subsequently, the geometric parameters on the fatigue characteristic indicators in different types of welded joints were studied systematically by combining finite element calculated results and analytical methods, such as weld length, notch angles, incomplete penetration length, et al. The results show that the 7N01 lap welded joint owns the lowest fatigue strength (29 MPa) at 2*106 cycles. The corresponding geometrical analytical solutions of local fatigue indicator Strain Energy Density (SED) were established for these welded joints, which could quantitatively describe the fatigue characteristic relationships among different types of welded joints. The theoretical predictions based on SED notch geometric semi-analytical models are relatively higher than the recommended design curves of other local approaches.

Effect of the Position in the Build Chamber on the Fatigue Strength of Additively Manufactured Maraging Steel MS1

The quality of additively produced parts and the achievable mechanical response may be affected by several factors, such as build orientation, heat treatment, or machining. A further rarely investigated factor is the position of the built part in the chamber with respect to inert gas flow. Previous studies have highlighted that the interaction between gas flow and laser track may induce an intense vaporization with consequent lack of fusion, particle entrainment, drop in density and denudation of the produced part, which is likely to detrimentally affect mechanical properties. This study addresses the effect of part position on the fatigue strength of heat-treated maraging steel MS1 produced by an EOSINT M280 machine in a nitrogen environment. Novelty arises from the lack of studies in this field, especially under fatigue. A factorial plan with subsequent statistical analysis highlighted that positioning the part upstream with respect to the gas flow leads to a slightly lower fatigue strength; however, no significant differences are observed. The failure mode, involving initiation from subsurface porosities of the same size, is also unaffected. Finally, a fatigue limit of 26% of the ultimate tensile strength is found, which is consistent with previous outcomes.

Stanovenie únavovej pevnosti zvarových spojov ocele S960MC zváraných laserovým lúčom

The work deals with determining the fatigue strength of butt-welded joints of S960MC steel. The fatigue strength of two versions of laser beam welded joints as well as the base material was assessed. To reduce the notch effect of the weld beam, the assessed weld joints were produced in a variety of methods utilizing filler material and additional dressing of the weld beam. The measurements determined variations in each type. The value of the fatigue strength decreased by 50 % to 67 % with respect to the base material, indicating that the notch effect of the weld joint had an impact on the value of the fatigue limit. Despite having relatively low fatigue strengths, both values meet normative standards and can be used in practical circumstances.

Dynamic photoelastic study of flexure hinges produced by additive manufacturing technology

There are many challenges in compliant mechanism redesign for additive manufacturing (AM) technologies, and in order to assess their applicability and propose adequate recommendations, the behavior of compliant elements has to be examined. Plastic materials are often used in AM technologies and are well suited for large volume compliant mechanisms due to their flexibility to strength ratio. However, they also have many disadvantages, notably a low fatigue strength and complex material behavior. Therefore, in this paper a photoelastic observations of corner-filleted flexure hinges manufactured from the transparent acrylic based photopolymer using the Digital Light Processing (DLP) AM technology subjected to the dynamic loading have been conducted in order to assess the fatigue strength and potentials of the photoelastic methods for the material behavior modeling. Three-point flexural tests on rectangular beam samples with the standardized dimensions printed in vertical and in horizontal orientation have been performed and the obtained data has been used for numerical simulations of corner-filleted flexure hinges. The specimens printed in different orientations exhibit significantly different anisotropic properties; the horizontally printed samples exhibit brittle behavior, while the samples printed vertically yields and consequently have much higher deformations, the calculated flexural moduli are also very different. Corner-filleted flexure hinges printed in vertical orientation and subjected to dynamic cantilever beam bending tests at relatively high stress amplitudes proved sufficient for the low dynamic compliant mechanisms applications. The qualitative photoelastic observations have shown that the photoelasticity is applicable in dynamic tests and useful in residual stress determination and stress concentration investigations.

Implementation of a nominal stress approach for the fatigue assessment of aluminium naval ships

Fatigue analysis is an important part of the structural design of weight-optimised naval ships, because they are often constructed from aluminium alloys or high tensile steel. Due to the low fatigue strength of these metals, the welded joints are vulnerable to cracking. Furthermore, naval ships can be required to operate in demanding environments, and remain in service longer than assumed during design. Thus, ship designers and in-service managers require efficient fatigue evaluation approaches. One industry-accepted and relatively quick method, documented in design codes, to assess the fatigue strength of welded joints is the nominal stress or S-N curve approach. However, there are uncertainties associated with using a design code; if all of the conditions for its applicability are not met, the analyst must reliably interpret the code. Further, different specifications for the use of S-N curves leads to different fatigue life estimates. In this paper, a refinement of the nominal stress approach for joints, typical of aluminium welded ship details, is proposed. The goal is to inject rigour and practicality into the fatigue analysis of naval ships. The refinement process leverages both in-service maintenance information and long-term strain measurements of a 56 m aluminium patrol boat. The sensitivity of the predicted fatigue life of welded details to the choice of S-N curve, available from a structural design code, and stress parameter extraction is investigated. Finally, recommendations for future work are provided.

Cold sintering constructed in situ drug-loaded high strength HA-PLA composites: Potential bone substitution material

Hydroxyapatite (HA) is widely used as bone substitute material in repairing bone defects and injuries. However, the high brittleness and low fatigue strength of HA make it difficult to satisfy the long-term service requirements of implants. In general, the mechanical properties of ceramics can be improved by sintering which usually operated above 1000 °C. The high temperature not only brings huge energy consumption and environmental pollution, but also loss the surface-active sites on HA, which greatly limits the biological functions of the material, such as drug loading and degradation. A method that can achieve the densification of HA at a lower temperature is particularly important to improve mechanical and biological properties. In this work, a promising drug-loaded HA-polylactic acid (HA-PLA) composite with high densification and mechanical properties was prepared by cold sintering process at 80 °C, in addition, the relatively low operating temperature enables the material to realize in-situ loading of drugs. The cold-sintered composite with HA/PLA was 7/3 showed the best performance with significant biodegradation (16.8%) and drug release (95%) ability. Therefore, this study provides a simple and universal method for the preparation of biological composite material, which provides a useful platform for efficient in situ drug loading biomaterials.

Investigations into Gas-Pore Effects on Fatigue Strength with a Peridynamic Approach

Fatigue evaluations are one of the main challenges to applying additively manufactured primary structural elements onto aircraft, especially for the gas-pore effects on fatigue strength. In this work, a bond decomposition strategy (BDS) in peridynamics (PD) is proposed; combined with our previously proposed model for fatigue damage, numerical simulations were performed to study the effect of Gas Pore (GP) on fatigue strength. Compared with the strategies in original paradigm of peridynamics, BDS achieves more elaborate description for bond status, predicts deformation fields around discontinuities with improved accuracy, and makes the spacing of material points become independent of discontinuity geometries. Two initiation modes are found in PD simulations, which exert an obvious impact on the final fatigue lifetimes; furthermore, it is revealed that GP not only leads to lower fatigue strength but also results in dispersity of fatigue strength data, in which dispersity is more severe if the GP size is larger, and the decline of fatigue strength is the most severe if the GP is located at subsurface for the same GP size.

Improved fatigue strength of Cr-electroplated 7075-T6 Al alloy by micro-shot peening

The effects of micro-shot peening (MSP) on the rotating bending fatigue resistance of Cr-electroplated AA 7075-T6 Al alloy were investigated. Fe-based amorphous particles with sizes of 50–80 μm were used as the shot balls and peened under 100 % surface coverage. The shot-peen intensity, determined by the height of the N-type Almen specimen, was 0.110 mm. The Cr-electroplated layer consisted of a network of microcracks in the as-plated condition. Without the necessity of micro-crack initiation, the Cr-electroplated substrate had the shortest fatigue life and lowest fatigue strength among the tested samples. MSP significantly increased the fatigue strength/life of 7075-T6 Al alloy with or without Cr-plating. The introduction of high residual compressive stress combined with the formation of a nanograined structure in the MSP zone was responsible for the high resistance to fatigue crack growth of the MSP sample.

Effect of Hydrothermal Degradation on Flexural Fatigue Strength of Various Cubic-Containing Translucent Zirconia.

The aim of this study is to investigate the fatigue and hydrothermal degradation behavior among the cubic-containing translucent yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP). Four groups of commercial Y-TZP (T, ST, XT, and P), containing different amount of cubic crystal, were examined. Artificial aging was accomplished by autoclaving at 122°C under 2 bar pressure for 8 hours. Fatigue simulation parameters were set using an axial 50 N load, a total of 240,000 cycles. Flexural fatigue strength was evaluated. Two-way analysis of variance with Tukey's honestly significant difference test was used to determine the difference in fatigue strength values between various type of zirconia materials within control and aging with a significant level of 5%. Weibull analysis was conducted on the fatigue strength data. All groups showed the mean flexural fatigue strength had no significant difference in hydrothermally aged specimens compared with specimens without aging (p = 0.154). Group T showed significantly higher flexural fatigue strength followed by group ST, XT, and P. Group XT and P showed no significant difference in strength value (p > 0.05). Group T demonstrated a lower probability of failure than group ST, XT, and P whether with or without aging. Weibull modulus in group T and ST with aging condition was increased and higher than all the experimental groups. Cubic-containing zirconia materials (group ST, XT, and P) exhibited lower flexural fatigue strength than non-cubic 3Y-TZP zirconia (group T). However, various cubic-containing translucent zirconia was assumed to have high resistance to hydrothermal degradation.