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In this study, forged EA4T steel specimens are subjected to three quenching temperatures of 800, 860, and 900 °C. Microhardness tester, laser scanning confocal microscopy, and scanning electron microscopy (SEM) are employed to measure microhardness, metallographic structure, and fatigue fracture characteristics, respectively. Heat treatment is found to be necessary to improve mechanical properties and fatigue behavior of initial forgings, and the effects of microstructure and defect are significant. Hence, considering variations of grain types and roughness between the initial forgings and heat-treated specimens, the fatigue property with defects are modified for roughness-affected axles by an improved KT diagram. The results indicate that the samples quenching at 860 °C achieved better fatigue performance due to the presence of fine polygonal ferrite (PF), bainite, as well as martensite. The fatigue life prediction error for the roughness-affected axle is between 16 and 18%. All of the fracture mechanism, grain size, and dislocation slip with different quenching temperatures affecting the fatigue performance are analyzed. Because the volume fraction of the martensite and bainite is significantly higher reaching 57.48% and more refined grains are obtained, the dislocation slip to the surface is delayed at the quenching temperature of 860 °C. Thus, the fatigue behavior is superior. Moreover, the yield strength increased by 22% compared to that of the initial forgings. Studies on the effect of the heat treatment process on fatigue behavior of EA4T steel contribute with a practical reference to the engineering application of high-speed rail axles.

Selective laser melting (SLM), as a revolutionary technology for metal manufacturing, attracts tremendous attention because it can produce complex components to benefit the customised production. H...

The effect on curing deformation and strength of 90° plies in stringers for composite T-shaped stiffened panel were presented by numerical analysis and experimental measurement. In this work, the finite element model (FEM) for curing deformation based on cure kinetics formulation was utilized. The model for buckling and failure based on Tsai-Wu failure criteria and incremental/ Newton -Raphson mixed iteration equation was established. The curing, buckling and post-buckling analysis were performed on composite T-shaped stiffened panels to obtain the deformation, the critical load and mode of failure, with different 90° plies-ratios in stringers. Furthermore, the experimental investigation has been carried out for specimens with different plies-ratios of 90° plies in stringers. The good agreement between the numerical results obtained by finite element method and the experimental ones demonstrated that this kind of numerical analysis could estimate appropriately the behavior of the structure. In addition, it was found that the 90° plies in stringers play an important role in reducing the curing deformation, but it has little effect on the axial compression loading capacity for composite T-shaped stiffened panel, which is useful for application in civil aviation aircraft.

Two types of aluminum (Al) alloys, namely 2024-T3 and 7075-T6, were treated by micro-arc oxidation (MAO). Microstructure of as-fabricated ceramic coatings was investigated. Furthermore, the effect of MAO time on fatigue life of the coated samples with thin coating was systematically explored. Morphologies of the coating, second phase particles, and distribution of copper (Cu) and Zinc (Zn) in the coating were observed by scanning electron microscopy, confocal laser scanning microscopy, and energy dispersive spectrometry. Residual stress and phase compositions were analyzed by X-ray diffraction. More Cu in 2024-T3 Al alloy promoted formation of the coating; however, surface porosity of the coating was high. Residual stress induced in the coating deposited on AA2024 was compressive in nature. In contrast, the residual stress in the coating prepared on AA7075 was tensile due to the high content of Zn in the substrate. The results of fatigue test showed that fatigue life of the coated AA2024 increased; however, the MAO treatment was detrimental to fatigue performance of the AA7075 compared to uncoated samples. With prolonging oxidation time, the fatigue life of the coated samples first increased and then decreased. The changes of fatigue life were attributed to the residual stress, coating defects including micro-pores and micro-cracks, and second phase particles in the Al alloy substrate.

Laser metal deposition (LMD) is a common manufacturing technique of laser additive manufacturing (LAM) which belongs to coaxial powder feeding method. The LMDed SS304 specimens are fabricated by the same combination of the process parameters so that they have the similar properties. The microstructure is similar to that of the selective laser melted (SLMed) specimen. The grain sizes of the LMDed specimen are compared with those of the specimens manufactured by the traditional method and SLM. Through the calculation of Ni and Cr equivalents in the LMDed SS304 specimen and the analytical results of electron backscattered diffraction (EBSD) and energy dispersive spectroscope (EDS), the phase compositions of the LMDed SS304 specimen are evaluated. The hardness of the LMDed specimen is compared with that of the conventionally manufactured wrought SS304 and SLMed specimens. And due to the differences of the grain size and phase composition on the different directions, the hardness of the LMDed specimen is anisotropic. The static tensile properties (ultimate tensile strength (UTS), yield strength (σ0.2) and elongation (EL)) and fatigue strength (FS) of the LMDed specimen are compared with those of the specimens made by the traditional method and SLM. The S N curve is established by the multiple experiments. Scanning electron microscopy (SEM) is applied to observe the fracture morphology of the static tension and tension-compression fatigue tests. And EDS is applied to analyze the chemical compositions of the particles on the fracture surface. As expected, LMD has the higher static tensile properties and fatigue strength than those of the specimen manufactured by the traditional method.

Due to the stratified nature of the manufacturing process, material extrusion (ME) parts have lower mechanical properties than those fabricated by traditional technology. This is one of the most significant defects hindering the development and application of this rapid prototyping technique. In this paper, vibration was applied to the ME process by using piezoelectric ceramics for the first time to improve the mechanical properties of the built parts. The vibrating ME equipment was established, and the specimens processed in different build directions were individually fabricated without applied vibration and with different applied vibrations. To quantify the effect of applied vibration on their mechanical properties and to summarize the influencing rule, a series of experimental tests were then performed on these specimens. A comparison between the testing results shows that the tensile strength and plasticity of the specimens, especially those processed in the Z direction, can be obviously improved by applied vibration. The orthogonal anisotropy is decreased obviously. The improvement becomes greater with increasing vibration frequency or amplitude. From the microscopic point of view, it can be seen that applied vibration can reduce the part’s defects of porosity and inclusion as well as separation between layers and, thereby, improve the bonding strength.

In this paper, the static compressive mechanical behaviors of carbon fiber reinforced composites (CFRP) single-lap bolted joints under anti-buckling test fixture were studied. Acoustic emission (AE) technology was used to monitor the damage evolution of specimens. X-ray micro-computed tomography (X-ray μCT) was used to evaluate the damage of CFRP single-lap bolted joints. The surface morphology of the holes was studied by field emission scanning electron microscopy (FESEM). An image processing and analyzing method was proposed, which allows the damage of the specimens to be accurately visualized, separated and quantified in 3D. The results show that X-ray μCT characterization technique greatly improves the ability to characterize the damage of CFRP single-lap bolted joints. The squeezing damage of the composite holes caused by the inclination of the bolt is an important factor affecting the bearing capacity of CFRP single-lap bolted joints.

Ceramic coatings were produced on AA7075-T6 alloy by micro-arc oxidation (MAO). The MAO treatment was performed at different duty cycles (8, 10, 15, and 20%). Effects of duty cycle on microstructure and fatigue behavior of the coated samples were investigated. Surface features and cross-section morphology, and fatigue fracture were observed by SEM and laser microscope. Phase structure and residual stress in the coatings were analyzed by X-ray diffraction. The nature and relaxation mechanism of the residual stress were revealed. Finally, the influence of the residual stress relaxation on the fatigue life at high and low cyclic stresses was explored.

In this paper, thin micro-arc oxidation (MAO) coatings (5–10 μm) were produced on 2024-T3 aluminum (Al) alloy with duty cycles of 8, 10, 15, and 20%. The objective was to investigate the fatigue properties of the MAO coated and uncoated samples under high cycle fatigue (HCF) and low cyclic fatigue (LCF) conditions. Surface morphology of coatings, interface between the coating and substrate, and fatigue fracture were observed by scanning electron microscopy and metallographic microscope. Three-dimensional (3D) surface morphology of ceramic layers and surface roughness were examined by a 3D laser microscope. Coating phase structure and residual stress were analyzed by X–ray diffraction. In addition, mechanical properties of the coated and uncoated samples were evaluated by static tensile testing. The test results showed that the MAO treatment improved fatigue properties of Al alloy substrate. Residual compressive stress in MAO coating was the reason of increasing fatigue life. However, duty cycles didn't affect the mechanical properties of the substrate. At LCF conditions, the fatigue life of the coated sample at the 20% duty cycle was lower due to the large cracks. In contrast, samples treated with the 8 and 15% duty cycles had inferior fatigue properties at HCF conditions. The concentrated distribution of micro-pores and the cracks on the coating cross-sections were detrimental to HCF life.

In the present work, ceramic coatings are deposited on the surface of 2024-T3 aluminum (Al) alloy by micro-arc oxidation (MAO). The Surface roughness of the Al alloy substrate is different in the range from 0.2 μm to 1.6 μm. This paper investigates the effect of surface roughness of the Al alloy substrate and surface treatment on the fatigue properties of 2024-T3 Al alloy. The morphology, microstructure, and phase composition of the oxide layers are examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Surface roughness of the coatings was measured using a 3D measuring laser microscope. The fatigue life of the Al alloy is evaluated under high and low cyclic stress. The results of the investigation have shown that the fatigue life of the coated samples with Ra = 0.2 μm is higher than others. The fatigue life of coated samples with Ra = 0.8 μm is very close to that with Ra = 1.6 μm under high cycle fatigue conditions. The reason is that the roughness of the interface between substrate and coating affects the fatigue life of coated samples. In contrast, the fatigue property changes when the coated samples were applied to high alternating stress. The fatigue performance of the coated Al alloy with Ra = 0.8 μm is the worst, followed with Ra = 1.6 μm. The rougher surface and the concentrating distribution of hills and valleys lead to this result.

The stress and displacement of the coating and substrate were analyzed according to the constructive relations of the micro-arc oxidation (MAO) coated aluminum alloy. Effect of the yielding of the interlayer and substrate on the constructive relations was explored. This study revealed the changes in displacement relationship between the coating and substrate under high and low tensile stresses. The yielding of the interlayer resulted in the displacement of the coating to be lower than that of the substrate. The results provided a theoretical basis for the residual stress relaxation, the larger plastic deformation zone found in the substrate adjacent to the MAO coating, and the disappearance of pores and the closure of cracks on the coating surface. The strain of the coating surface was measured using the YE2539 high-speed strain gauge and BX120-5AA strain gauge. The theoretical stress of the coating surface was compared with the measured results, exhibiting good consistency. Furthermore, the results of the in-situ observation test in previous study verified the rationality of the theoretical model analysis. In addition, the effect of the residual stress on the mechanical properties of the MAO coated samples was discussed. The residual compressive stress in MAO coating was beneficial to the improvement of the mechanical properties.

This study carried out the fatigue tests of selective laser melting (SLM) 304L austenitic stainless steel (SS) made by different scanning speeds. The purpose is to deeply understand the microstructure, fatigue failure process and fatigue failure mechanism of SLMed 304L SS. For this purpose, the microstructure, surface roughness and porosity were tested by field emission scanning electron microscope (FESEM) and electron backscatter diffraction (EBSD), while the residual stress was obtained by μ-X360 residual stress analyzer. In addition, the fatigue failure process of SLMed 304L SS was in-situ monitored through acoustic emission (AE) technique and thermal imager. The results show that SLMed 304L SS has excellent fatigue performance. The ‘critical’ value is defined to distinguish the critical influence of surface roughness and porosity. In addition, the temperature rise caused by the stored energy deriving from the active dislocation motions is affected by the loading direction and scanning direction by regulating the dislocation motions during fatigue testing.

An EA4T steel billet was forged at the different initial forging temperatures (1050, 1150, and 1250 °C). The microstructure and fatigue fracture of forged samples were observed by laser scanning confocal microscopy and scanning electron microscopy, respectively. The fatigue performance of samples forged at an initial forging temperature of 1250 °C was better owing to the presence of fine polygonal ferrite (PF). The difference in yield strength and ultimate tensile strength can be attributed to the average grain size of PF and acicular ferrite (AF) content, respectively. A relationship between the tensile strength and fatigue limit of EA4T steel is proposed. The mechanism of grain size and dislocation slip affecting the fatigue property was elucidated. Because the fine PF grains delayed the dislocations moving to the surface and inhibited the formation of intrusion and extrusion, premature cracks were not easily produced. This contributed to excellent fatigue behavior. Additionally, the pearlite colonies and fine PF delayed the crack growth rate and changed the crack propagation path. Studies on fatigue behavior provided a foundation for quality control and engineering application of EA4T steel.

This study investigated the effect of countersunk hole depths (0.65, 0.90, and 1.20 mm) on the fatigue performance of the riveted AA2024-T3 alloy. The fatigue fracture of the samples was observed by scanning electron microscopy. The strain at the countersink hole was analyzed by finite element (FE) method. The stress intensity and stress concentration factor of the countersunk holes were calculated. The rationality of the FE analysis and theoretical calculation was verified by measuring the strain near the countersunk holes. The high cycle fatigue life of the samples with a countersunk depth of 1.20 mm was far less than the other samples. In contrast, the low cycle fatigue life changed insignificantly.

With the increasingly wide application of fused filament fabrication (FFF) technique, the built products are inevitably exposed to dynamic mechanical loading and vibration. However, there has been no systematic study in the literature on understanding and characterization of dynamic mechanical performance for FFF products. In this paper, the dynamic characteristics of FFF plates are quantified, with the effect of different extrusion width taken into account. A dynamic model of the built plate with cantilever boundary conditions is established, and the inherent characteristics are predicted. Modal tests are then performed on these samples to obtain the measured data. Through the comparison between predictions and measurements, the theoretical model is validated. Different extrusion width makes the material properties of the plates different, resulting in different dynamic characteristics. The scanning electron microscopy (SEM) analysis on the samples confirms that the dynamic characteristic is deteriorated as the extrusion width decreases. This present work provides theoretical basis and technical support for further research in improving the dynamic performance of FFF products and helps extend the applications of this technique.

The micro-arc oxidation (MAO) coatings were produced on AA7075-T6 alloy with a surface roughness (Ra) of 0.2, 0.8, and 1.6. The effects of the Ra of substrates on the morphologies of MAO coating and fatigue behavior of the coated samples were investigated. The structural characteristics of the MAO coating were analyzed by X-ray diffraction, scanning electron microscopy, and laser scanning confocal microscopy. Moreover, the residual stress in the oxide films was measured using an X-ray stress tester. The surface and cross-section analyses of the coatings revealed the mechanism of the oxide layer growth. The phase composition and the mechanism of residual stress were further investigated according to the heat transfer in the MAO process. The relationship between the nature of residual stress in MAO coating and the surface porosity as well as the temperature of the substrate and the coating was revealed. The fatigue test results showed a significant effect of the Ra of the substrate on the high cycle fatigue life at low cyclic stress levels. The residual stress relaxation attributed to the insignificant changes in the fatigue life of the coated samples with Ra = 0.2, 0.8, and 1.6 at high cyclic stress levels. Besides, the mechanism of the residual stress affecting the fatigue performance was explored.

Medium-carbon steel is commonly used for the rail, wire ropes, tire cord, cold heading, forging steels, cold finished steel bars, machinable steel and so on. Its fatigue behavior analysis and fatigue life estimation play an important role in the machinery industry. In this paper, the estimation of fatigue life of medium-carbon steel with different surface roughness using established S-N and P-S-N curves is presented. To estimate the fatigue life, the effect of the average surface roughness on the fatigue life of medium-carbon steel has been investigated using 75 fatigue tests in three groups with average surface roughness (Ra): 0.4μm, 0.8μm, and 1.6μm respectively. S-N curves and P-S-N curves have been established based on the fatigue tests. The fatigue life of medium-carbon steel is then estimated based on Tanaka-Mura crack initiation life model, the crack propagation life model using Paris law, and material constants of the S-N curves. 6 more fatigue tests have been conducted to validate the presented fatigue life estimation formulation. The experimental results have shown that the presented model could estimate well the mean fatigue life of medium-carbon steel with different surface roughness