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Gears fabricated using powder metallurgy technique (PMT) have the potential to replace traditional wrought gears due to their very high forming efficiency and relatively low cost. However, their practical applications are restricted due to the lack of understanding of their fatigue properties and failure mechanisms. This work investigates the contact fatigue performance and corresponding failure mechanism of a typical Fe-based small-module gears fabricated by PMT. The fatigue tests were conducted under grease lubrication condition, using a power-opened small-module gear testing rig. The experimental results show that the current PMT fabricated small-module gears exhibit contact fatigue properties comparable to those of traditional wrought steel gears. It has been found that the subsurface cracks induced from pores are the dominant cause of spalling of PMT fabricated small-module gears during rolling contact. The cracks tend to propagate between adjacent pores, playing a vital role in the overall fatigue life. Thus, this study suggests that PMT can be a successful method for fabricating Fe-based small-module gears with excellent contact fatigue property if formation of pores during fabrication is well inhibited.

Understanding the fatigue behaviors of weld joints is significant in engineering practice. Rotary friction welding (RFW) can join the additively manufactured polymer components. Until now, no research has focused on the fatigue behavior of polymer components jointed via RFW. This study investigates the fatigue life of ABS/PC dissimilar components fabricated via RFW and proposes the fatigue mechanism based on the failure structure. This work uses five different cyclic loads and rotational speeds to investigate the fatigue life. The fatigue life of the RFW of ABS/PC dissimilar rods is better compared with the pure ABS and pure PC specimens due to weld and integrity microstructural changes resulting from the combination of ABS and PC materials. The number of cycles until the rupture of RFW of ABS/PC dissimilar components (y) can be determined by the cyclic load (x) according to the prediction equation of y = −838.25x2 − 2035.8x + 67,262. The fatigue life of the RFW of ABS/PC dissimilar components increase with the increased rotational speed. The number of cycles until rupture (y) can be determined by the different rotational speeds (x) according to the prediction equation of y = 315.21x2 + 2710.4x + 32,124. [ABSTRACT FROM AUTHOR]

This research study investigates the fatigue properties of resistance spot welds (RSWs) made from selective laser‐melted (SLMed) maraging steel (MS1) and compares them with dual‐phase (DP) steel RSWs. The findings reveal that MS1 spot welds exhibit superior fatigue properties compared with DP steel RSWs, regardless of the printing orientation of the MS1 substrate. Furthermore, fatigue fracture occurs at the MS1 substrate, with a transition of fracture location observed from MS1 to itself and MS1‐to‐DP600 dissimilar welds. The results of structural stress analysis indicate that the weld nugget diameter is not the only factor affecting the fatigue life. The study also highlights the importance of considering materials inhomogeneity and fracture modes in fatigue life assessment. Based on the findings, the study recommends the development of a new fatigue life prediction curve for spot welds of additive manufacturing (AM) materials. Such a curve would be beneficial in promoting the further application of AM technology for functional parts of design verification vehicles (DCV) and production cars. Overall, the research contributes to the understanding of the fatigue properties of RSWs made from SLMed MS1 and DP steel, which has implications for the design and manufacturing of parts in the automotive industry. Highlights: SLMed MS1 RSWs showed superior fatigue properties compared with DP600‐DP600 spot welds.Softening occurred at weld nugget for MS1‐MS1 RSWs while at HAZ of MS1 for MS1‐DP600 RSWs.Fatigue fracture occurred in MS1 steel for MS1‐DP600 RSWs due to the defects in MS1 sheets.The lower constant m for MS1 than that of other steels explained different fatigue life. [ABSTRACT FROM AUTHOR]

The microstructural characteristics and mechanical properties of friction stir lap welding (FSLW) of thin Al-clad Al (1.5 mm) and ultra-thin Al-clad mild steel (0.7 mm) sheets are experimentally investigated. Optical microscopic observation confirms that the FSLW joints are successfully fabricated without noticeable joining defects, such as cracks and hooks. The material flow is correlated with the silicon (Si) element distribution in the stir zone (SZ). The distribution of Si indicates that material flow is asymmetrical between the advancing side (AS) and the retreating side (RS) of the SZ. The tensile test result shows that all the FSLW joints fracture from the Al-clad Al side of the joints. During the fatigue tests, two different crack propagation modes (vertical crack in the Al-clad Al loading side; horizontal crack in the Al-clad mild steel loading side) are observed simultaneously due to the combined effect of the asymmetric geometry of the FSLW joint and the different loading configurations. The results of fatigue tests suggest that the asymmetric microstructure of the joint retards the propagation of a vertical crack in the Al-clad Al loading side for the RS-loaded FSLW joint. As a result, the fatigue life of the RS-loaded FSLW joint becomes clearly higher than that of the AS-loaded FSLW joint.

Cold dwell fatigue of titanium alloys used in aero-engine components has threatened flight safety for over five decades. This phenomenon was encountered in Al-containing titanium alloys, occurring at relatively low temperatures, where load hold at applied peak stress result in reduction in number of cycles by an order of magnitude or more (the so-called dwell debit). The present paper reviews the recent advances in dwell-fatigue of titanium alloys with the primary motivation being to understand the dwell-fatigue damage mechanism relating microstructural configurations, dislocations and local strain rate sensitivity. Numerous studies have focused on the identification of fatigue-critical microstructural configurations, such as microtextured regions, rogue grain pairs and (0001) twist grain boundaries. As an example of dwell resistant alloy design, Timetal 575 is a candidate alloy for aero-engine fan disc applications with both superior static and cyclic strengths and low dwell sensitivity.

The microstructure, tensile properties, and fatigue behavior of friction stir welding (FSW) joints of multilayer AA3003-clad AA6013 are experimentally investigated. Linear butt welding is performed using a concave tool equipped with a columnar threaded pin at a rotating speed of 600 rpm and a transverse speed of 200 mm/min. The microstructures of FSW joints were observed using a field emission scanning electron microscope equipped with an electron backscatter diffraction system and an energy dispersive spectrometer. The microstructural characterization in the stir zone (SZ) reveals grain refinement, precipitate refinement, zigzag line, and the AA3003-clad layer fragments due to material mixing. The tensile test result of all-weld joints shows that the tensile strength of the SZ is significantly higher than that of base metal, while the cross-weld joints show a typical ductile fracture in the base metal. The result of fatigue tests shows that all the cross-weld joints fracture from the SZ. Analysis of the fatigue failure mechanism indicates that the crack causing the fatigue fracture originates from the AA3003-clad layer fragments in the advancing side of the SZ. The fatigue analysis also confirms that surface roughness and zigzag line are not the cause of the final fatigue fracture of the cross-weld joint for the present study.

Understanding the fatigue behaviors of weld joints is significant in engineering practice. Rotary friction welding (RFW) can join the additively manufactured polymer components. Until now, no research has focused on the fatigue behavior of polymer components jointed via RFW. This study investigates the fatigue life of ABS/PC dissimilar components fabricated via RFW and proposes the fatigue mechanism based on the failure structure. This work uses five different cyclic loads and rotational speeds to investigate the fatigue life. The fatigue life of the RFW of ABS/PC dissimilar rods is better compared with the pure ABS and pure PC specimens due to weld and integrity microstructural changes resulting from the combination of ABS and PC materials. The number of cycles until the rupture of RFW of ABS/PC dissimilar components (y) can be determined by the cyclic load (x) according to the prediction equation of y = −838.25x2 − 2035.8x + 67,262. The fatigue life of the RFW of ABS/PC dissimilar components increase with the increased rotational speed. The number of cycles until rupture (y) can be determined by the different rotational speeds (x) according to the prediction equation of y = 315.21x2 + 2710.4x + 32,124.

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 109 cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

A multilayer overlay coating system containing an intermediate intermetallic layer is an architecture expected to show good fatigue resistance. Experimental characterization and modeling simulations were carried out to classify the different crack initiation mechanisms occurring during fatigue of this coating system (2IML coating system) and to reveal how changes in the layer architecture lead to fatigue improvement. Surface crack initiation seems to play the dominated role in the fatigue failure of this coating system as over 55% of observed cracks were surface cracks. Fatigue improvement is achieved by increasing surface initiation resistance via decreasing the IML‐Top layer thickness. However, subsurface initiation mechanisms inhibit the improvement (dominated by surface initiation mechanism) achieved by locating the IML‐Top layer closer to the top surface. Optimizing the design of 2IML multilayer coatings based on the fatigue performance should eliminate the unreacted nickel layer in the intermediate IML‐Top layer, increase the subsurface interface strength, and decrease the density of voids occurring in the annealing process. Highlights: Mechanical properties of materials in layered coatingsFatigue failure mechanisms in layered coatings composed by harder and softer materialsEffect of each mechanism on fatigue performance with varying architectural parameters [ABSTRACT FROM AUTHOR]

• The multi-scale computational model for the long-term service performance degradation of movable MEMS devices is established by integrating finite element and molecular dynamics methods. • The modeling location for molecular dynamics needs to select an appropriate unit to ensure temporal and spatial consistency with the finite element method. • The developed method can discern the influence patterns of microstructural changes on long-term service performance, providing optimized strategies for material processing and design. The rapid advancements in AI and 5G/6G communication technologies have intensified the demand for MEMS devices characterized by exceptional performance and diversity. Simultaneously, the imperative for enduring stability in these devices has become increasingly critical. Unforeseen reliability issues pose substantial challenges to the full-scale application of these advanced MEMS devices, underscoring the importance of understanding their failure mechanisms. Presently, there is a scarcity of comprehensive research in reliability computation and analysis. In this study, we adopt cross-scale damage variables as a nexus, linking responses across three scales: material, structural, and performance. We implement a synergistic finite element-molecular dynamics approach for multiscale analysis. Using a prototypical MEMS resonator structure as a case study, we establish quantitative interrelations among these three scales, enabling precise depiction of atomic failure behavior and its impact on resonant frequency degradation. This approach offers an atomically grounded explanation for the fundamental material processes underlying frequency degradation. Our numerical analyses explore the failure mechanisms responsible for the frequency degradation in MEMS resonators. Additionally, we propose several optimized strategies for the design and preparation phases aimed at enhancing fatigue performance. These findings are particularly relevant in light of the growing necessity for devices that maintain long-term stability. [ABSTRACT FROM AUTHOR]

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 109 cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data.

• Subsurface failure is in the form of oversized facet formed by α p phase cluster cleavage. • The oxide shedding causes the surface failure at high temperature. • Higher temperature and stress make the oxide shedding failure mechanism dominant. The failure mechanism of Ti60 titanium alloy in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) at different temperatures was investigated. The results showed that, at different temperatures, there are two different fatigue failure mechanisms, i.e., the oversized facet and oxide shedding failure mechanisms. At room temperature, most of the fatigue cracks initiate at the oversized facet. At 300℃, the crack initiates at the oversized facet for the test with small applied stress, while the crack initiates at the oxide shedding site for the test with large applied stress. At 400℃, all the fatigue cracks initiate at the oxide shedding site. Higher temperature and stress will be conducive to the failure mechanism of oxide shedding to become dominant. [ABSTRACT FROM AUTHOR]

• The COD, CSD, crack width, and the crack propagation length for mixed mode I–II fatigue crack have been continuously captured with the DIC method. • The tortuous mixed mode I–II fatigue crack propagation path has been visualized using the DIC method. • The DIC method is an effective way to measure the complete mixed mode I–II fatigue crack propagation in concrete. The propagation of fatigue cracks leads to the degradation of the load-carrying capacity of concrete structures and even fatigue failure. An in-depth investigation of the fatigue crack propagation process is essential to reveal the fracture mechanism and further assess the safety of concrete structures. This paper presents an experimental study of mixed mode I–II fatigue crack propagation in concrete. Fatigue tests are conducted on three-point bending (TPB) beams with an off-center initial crack with different fatigue load levels. The digital image correlation (DIC) method is employed to observe the complete fatigue crack propagation process, and the method is verified to be applicable. The results indicate that the mixed mode I–II fatigue crack propagation process can be divided into three stages, namely, the rapid propagation in the initial stage, a stable propagation stage, and the final fast propagation until unstable failure. The mixed mode I–II crack propagation path under static loading is a good approximation of that under fatigue loading. The results also show that the crack tip opening displacement (CTOD), crack tip sliding displacement (CTSD), mode I stress intensity factor (SIF), and mode II SIF corresponding to the unstable failure of the TPB beam are approximately constant and independent of the fatigue load level. In particular, the negligible CTSD and mode II SIF demonstrate that the mixed mode I–II fatigue failure of concrete is dominated by the mode I component. [ABSTRACT FROM AUTHOR]

• The fatigue failure process and composite action of steel-concrete composite deck slab are experimentally studied. • Effects of load amplitude and loading sequence on fatigue response are discussed. • The multiaxial stress state of the weld region is revealed using a verified finite element model. • Fatigue crack initiation position is assessed by the critical plane approach. • Effects of structural parameters and shear-span ratio on fatigue damage parameters are clarified. This paper aims at investigating the variable-amplitude fatigue behavior and failure mechanism of the steel-concrete composite (SCC) deck slab with perforated ribs. Firstly, two identical full-scale specimens were tested under different fatigue loading procedures. Test results reveal that the fatigue failure process starts with the initiation of cracks at the welding part between the steel plate and perforated rib, after which the slab could sustain tens or hundreds of thousands of load cycles until the crushing of concrete. As load amplitude increases by 54%, the crack propagation rate is accelerated by 300%, and the fatigue life from the crack initiation to the crushing of concrete is reduced by 79%. Higher load amplitude applied in the earlier service stage leads to the irreversible and a higher level of degradation on the stiffness, deformation recovery, and composite action. Further, the finite element (FE) model is established and verified, through which the stress state of the welding part is analyzed. It reveals that owing to the shear transfer between the steel members and concrete layer, the welding part is in the multiaxial stress condition where longitudinal normal stress, vertical normal stress, and shear stress coexist. Subsequently, the crack initiation position is predicted using the critical plane approach based on the SWT parameter, which shows good consistency with experimental results. Finally, by conducting the parametric analysis, the effects of geometric parameters and shear-span ratio on the SWT parameter of the welding part are clarified. [ABSTRACT FROM AUTHOR]

• Passivation treatment of abrasive belt can effectively improve surface integrity of aero-engine blades. • The passivated abrasive belt grinding can promote the grain refinement and microstructure homogenization of the machined surface. • Blade of passivated abrasive belt grinding can suppress the initiation of fatigue cracks on the surface and prolong fatigue life of aero-engine blades. • An anti-fatigue grinding method with blunt grinding and slow feed was proposed. As precision machining process of aero-engine blades, the surface integrity after abrasive belt grinding closely related to its fatigue resistance. In order to explore new anti-fatigue abrasive belt grinding methods, reveal the influence mechanism of blade surface integrity on anti-fatigue performance, and provide guidance for the anti-fatigue grinding, vibration fatigue tests were carried out on blades processed by different grinding process, and the effects of abrasive belt passivation treatment and feed speed on blade surface integrity and fatigue life were studied. Fatigue failure behavior of blades processed by different processes were also discussed from the fracture morphology and cross-section analysis. Results showed that blades processed by blunt grinding have different fatigue failure mechanisms, and the passivation treatment of abrasive belt and low feed speed can improve the surface integrity and fatigue life of aero-engine blades. The combination process of abrasive belt passivation treatment and slow feed speed were proposed for anti-fatigue grinding processing of aero-engine blades. In this research, compared with blades processed by new abrasive belt with feed speed 1000 mm/min, the fatigue life of blades processed by passivated abrasive belt at feed speed 100 mm/min was increased by 3.1 times. [ABSTRACT FROM AUTHOR]

• A 3D-FINV model of tubing strings is established and verified by a similar experiment. • A cumulative damage theory is used to establish the fatigue life prediction method. • The fatigue failure mechanism of tubing string in 3H curved wells is elucidated. • The safety control methods of tubing string are proposed. In view of the fatigue failure of tubing strings by flow-induced nonlinear vibration (FINV) in high-temperature, high-pressure and high-yield (3H) curved gas wells, a three-dimensional (3D) FINV model of the tubing string is established using the micro-finite method, energy method and Hamilton variational principle, and verified by a similar experiment of tubing vibration. Meanwhile, the cumulative damage theory is used to establish the fatigue life prediction method of tubing string combined with the stress response which was determined by the proposed FINV model and the S-N curve of the tubing material (13Cr-L80) which was measured by fatigue test. Thus, the influences of production rate, well trajectory parameters (inclination angle and well section length), as well as the installation position of downhole tools (packer and centralizer) on the fatigue life of the tubing string are systematically analyzed. The results obtained demonstrate that, first, with the increase of production rate, the fatigue life of the tubing string decreases, and the field production allocation should be far away from the sudden production rate which can be determined using the proposed analysis method. Second, in the well trajectory design, the vertical section length should be reduced, while that of build-up section and the angle of stable inclined section should be increased. Third, the packer and centralizer should be placed in the optimal position, which depends on the well structure, downhole tool size, etc. and can be determined using the proposed analysis method. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving the fatigue life of tubing string in 3H curved gas wells. [ABSTRACT FROM AUTHOR]