Increase In Fatigue Life Research Articles

Fatigue Assessment of Copper‐Brazed Stainless‐Steel Joints for Plate Heat Exchangers

ABSTRACTCyclic pressures can cause fatigue failure in the brazed joints and plates of the plate heat exchangers (PHEs). This study examines the fatigue behavior of PHEs made from 316L and 304L steels brazed with copper foils employing strain‐controlled fatigue tests to explore if 304L could replace 316L in the existing production line for cost reduction. Fatigue tests were conducted at four different load levels with a stress ratio of zero and a frequency of 5 Hz. Finite Element Analysis was used to assess strain distribution and estimate PHE lifespan based on generated strain versus number of cycles to failure curves. The microstructural analysis revealed that copper diffuses more easily into 316L than 304L, and using 50 μm thick foil causes more defects compared with 100 μm foil. It was shown that 316L joints have a significantly increased fatigue life compared with 304L. Both 316L and 304L met the 15‐year lifetime requirement set by manufacturers.

Assessment of multiaxially loaded safety components with uniaxial stress state in fatigue critical sites caused by design

AbstractThe focus of this study, using results obtained with hollow specimens of the alloy Al 2024 T3 (AlCuMg2 T3) with a one‐sided bore, was to determine if, under multiaxial loading, the fatigue strength in a notch with a uniaxial stress state can simply be assessed without the application of any multiaxial strength hypothesis. In the fatigue critical location, the bore edge, only a superimposition of normal stress components, resulting from axial loading or/and torsion, occurs. In the case of out‐of‐phase loading, the resulting local uniaxial stress is even lower than the stress under in‐phase loading and leads to an increase of fatigue life to crack initiation under the non‐proportional loading, which is not observed in the case of multiaxial stressing/straining of the material itself. As the local stress state in the crack initiation site is uniaxial, the application of a multiaxial strength hypothesis, for the failure criterion of crack initiation (l=0.2 mm), is not necessary. Furthermore, the benefit of non‐homogenous stress distributions, which allow locally higher stresses than the levels given by the Woehler‐line for unnotched specimens, was underlined by considering highly‐stressed, material‐volume‐related support factors.

Numerical investigation of patch geometry effect on the fatigue life of aluminum panels containing cracks repaired with CFRP composite patch using XFEM and CZM approach

This study investigated numerically the effect of using composite patches with various geometric shapes to improve the fatigue life of aluminum panels of AA7075-T6 and AA2024-T3 containing cracks and compared with the available experimental results. The Cohesive Zone Model (CZM) and Extended Finite Element Method (XFEM) were applied to static and fatigue analyses. Four types of rectangular, trapezoidal, left-oriented and right-oriented triangular composite patches were used in the numerical analysis. With the combination of XFEM and CZM methods, and using the Paris equation, the fatigue crack propagation and fatigue life of aluminum panels containing cracks can be reasonably predicted. A composite patch with a suitable shape and geometry, fatigue life of the repaired panel can be improved considerably. With a 204% improvement in fatigue life in comparison to the unrepaired panel, the repaired sample made of aluminum alloy AA2024-T3 alloy achieved the most significant improvement in fatigue life. In AA7075-T6 alloy samples repaired with composite patches, the samples repaired with rectangular and trapezoidal patches experienced the highest fatigue life increases, respectively with 342 and 290% compared to the unrepaired samples. The comparison of numerical simulation results with the available experiment results shows a good agreement.

Surface condition driven fatigue performance of laser powder bed fusion H13 steel

Surface variation is an inherent imperfection in the laser powder bed fusion (LPBF) processed samples, comprising of primary roughness (PRR) from the melt pool solidification and secondary roughness (SER) from adhered powder particles. Components under cyclic loading are sensitive to surface defects, making surface finishing crucial for fatigue-bearing parts. However, this undermines LPBF’s advantage of creating high-performance and geometrically complex components. This study examines the effect of surface roughness on fatigue crack initiation in LPBF H13 steel, a high-strength alloy with a tensile strength of 1800 MPa. The focus is on scenarios where contour scanning strategies are not employed. Vertically deposited samples were subjected to three surface conditions: as-built (AB) with PRR + SER, half-polished (HP) with only PRR, and well-machined (M) with no PRR or SER. Optical and digital surface roughness characterization and stress-controlled fatigue testing were performed. Results revealed that while AB-samples showed significantly rough surface compared to HP-samples, their fatigue performance did not reduce. In contrast, M-samples had a slight improvement in roughness but exhibited a fatigue life increase by a factor greater than 17. This was attributed to the PRR, specifically the solidification valleys between the neighboring melt pools. Factographic analysis showed that the PRR-driven fatigue crack initiation is a critical determinant of fatigue performance.

Fatigue performance of repair-welded and HFMI-treated transverse stiffeners

AbstractLarge portions of infrastructure buildings, for example, highway and railway bridges, are steel constructions and reach the end of their service life due to an increase of traffic volume. Repair welding can restore the current welded constructional detail with a similar fatigue strength. However, due to the increase of fatigue loading (traffic), an increase of fatigue strength is needed in such bridge structures. For this reason, the combination of repair welding and high-frequency mechanical impact (HFMI) treatment was investigated in this study in order to quantify the increase of fatigue life by combining both methods. For this, transverse stiffeners made of steel grade S355J2 + N were subjected to fatigue loading until a pre-determined crack depth was reached. The cracks were detected by non-destructive testing methods. Weld repair was realized by removing the material containing the crack and re-welded by a gas metal arc welding (GMAW) process, following that post weld treated was applied by HFMI-treatment and the specimens were subjected to fatigue loading again. Hardness profiles, weld geometries, and residual stress states were investigated for both the original and the repaired condition. In the repaired condition without additional HFMI treatment, a similar fatigue life than in the original condition is observed for the specimens. The repair-welded and HFMI-treated specimens reach a significant higher fatigue life compared to the repaired ones in the as-welded condition.

Utilizing Spatial Statistical Bounds on Residual Stress Fields From Cold Expansion: Effects on Fatigue Crack Growth Behavior

ABSTRACTThis paper assesses the impact of utilizing statistically defined residual stress fields from cold expansion (Cx) in linear elastic, multi‐point fracture mechanics analyses using the spatial analysis of residual stress (SpARS) methodology. There is significant value and interest in leveraging the increased fatigue life afforded by Cx, but it is imperative to quantify the variability of the residual stress to understand the expected variability in benefit due to Cx. Comparisons of the predicted fatigue lives from SpARS‐produced statistical residual stress fields are made to fatigue test data. Results demonstrated that the less compressive 95% upper bound from the mean residual stress would be a reasonable strategy as it supplies a compromise between safety and inherent material and process variability while still producing a sizable improvement in predicted fatigue life. In this study, using SpARS to incorporate statistically representative residual stress fields in fatigue crack growth analyses demonstrates a methodology to aircraft structural engineers for improved fleet management and allow increased aircraft availability through fewer inspections.

Mitigating environmental assisted cracking in heterogeneous welds by laser peening without coating

In this work Laser Peening without Coating (LPwC) was applied underwater on heterogeneous weld joint made of P265GH and X6CrNiTi18-10 steel tubes to prevent Environmental Assisted Cracking on the weld/P265GH steel fusion boundary. Special laser head was designed to precisely deliver 200 mJ green laser beam on the centrally located weld interface on the inner surface of a pipe 76 mm wide and 420 mm long. Residual stress analysis revealed that the LPwC treatment led to generation of compressive residual stresses in the critical fusion boundary area with the magnitude of −168 MPa despite the absence of a protective layer in the peening process. The depth of compressive stresses reached up to 0.8 mm. Plastic deformation induced by LPwC led to grain refinement in the microstructure and improved hardness by 16 %. Samples sectioned from the treated pipe were subjected to accelerated corrosion-mechanical testing which showed more than 2.5x increase in cycles to failure after LPwC. Fractography analysis showed shorter length and decreased number of corrosion cracks after LPwC as well as formation of protective oxide layer on top of the treated surface. 3-point bend testing further showed more than 8x increase in corrosion fatigue life.

Variations of VHCF Characteristics by Microstructure of a Spring Steel to UNSM Treatment

<i>The bainitic structure resulting from the austempering of spring steel exhibits high strength and ductility. On the other hand, there appears to be no study on the effects of very high cycle fatigue (VHCF) and ultrasonic nanocrystal surface modification (UNSM) of this bainite structure. Therefore, this study compared the fatigue properties of VHCF using spring steel with bainitic and martensitic structures and bearing steel data. This study analyzed the characteristics of microstructure transformation associated with the heat treatment cycles and studied and evaluated the fatigue strength characteristics because of the UNSM in terms of fracture mechanics method and fracture surface analysis through electron backscatter diffraction, scanning electron microscopy fracture analysis, and energy-dispersive spectroscopy analysis. The fatigue limit of UNSM-treated spring steel was improved significantly by approximately 33% to 50% compared to the fatigue test results of the untreated material in the VHCF. In the long life range of bainized spring steel, fish-eye cracks appear in the form of the fine granular area, and fatigue cracks occur in the form of fish-eye cracks that occur in the bainite facet and matrix, resulting in a significant increase in fatigue strength and fatigue life.</i>

Fatigue Life and Stress Analysis of a Single Cylinder Four Stroke Crankshaft

This paper focuses on the optimization of a crankshaft using ANSYS software in terms of weight and strength. The initial designs of the crankshaft, piston, and connecting rod were created using SolidWorks. The force generated by the gas during the combustion process was calculated to be 12017 N. Next, the SolidWorks assembled system was imported into ADAMS View software for simulation, which revealed a time-variant force of the crankpin of 14049 N. The calculated value was verified with results obtained from analytical calculations, showing a deviation of 0.23%. Finite element analysis was done for the crankshaft using ANSYS transient structural after applying loadings and boundary conditions. The optimization process aimed to minimize the crankshaft's weight while maintaining its strength and durability. The results of the ANSYS simulations showed a weight reduction of 2.5% from the original 2.983 kg to 2.907 kg, while maintaining the required strength and durability. The optimized crankshaft was compared to its original design in terms of fatigue life, weights, and stresses. The maximum von Mises stress was reduced by 16%, shear stress by 3.5%, and deformation by 3.5%, which were validated through analytical calculations. The crankshaft analysis resulted in a significant increase in fatigue life, calculated to be infinite under the given conditions. To conclude, the objective to optimize the crankshaft for performance and efficiency was achieved, demonstrating a 2.5% weight reduction and substantial improvements in fatigue life and stress distribution, proving the effectiveness of ANSYS software for the design optimization process.

Fatigue damage analysis and mitigation for steel structures using UHM-CFRP prestressed technique

Fatigue stresses in steel bridges can lead to crack propagation, gradually decreasing the structural stability of the bridge. Conventional strengthening techniques for structures have shown limited effectiveness in preventing crack growth. The objective of this research is to enhance the fatigue life of cracked steel beams through the application of ultra-high modulus carbon fiber reinforced polymer (UHM-CFRP) plates. Two sets of tests, including quasi-static testing and fatigue testing, were conducted on both cracked and uncracked steel beams. All beams were tested under identical loading conditions, with a total of five beams examined. Control beams were used to compare fatigue life under different loading ranges. One beam was reinforced with UHM-CFRP plates, another utilized prestressed UHM-CFRP plates, and another employed UHM-CFRP plates with end clamps. The experimental study revealed a significant 8 times increase in fatigue lives for cracked steel beams repaired using the UHM-CFRP compared to the control beam. Additionally, the (10 %) prestressed UHM-CFRP plate not only doubled the fatigue life of identical beams compared to UHM-CFRP plates but also significantly reduced residual deflection during fatigue crack growth, highlighting its efficacy in fatigue resistance.

Investigations of Metallurgical Differences in AISI 347 and Their Influence on Deformation and Transformation Behaviour and Resulting Fatigue Life.

Due to variations in chemical composition and production processes, homonymous austenitic stainless steels can differ significantly regarding their initial microstructure, metastability, and thus, their fatigue behavior. Microstructural investigations and fatigue tests have been performed in order to evaluate this aspect. Three different batches and production forms of nominally one type of steel AISI 347 were investigated under monotonic tensile tests and cyclic loading under total strain and stress control in low and high cycle fatigue regimes, respectively. The deformation induced α'-martensite formation was investigated globally by means of in situ magnetic measurements and locally using optical light microscopy of color etching of micrographs. The investigation showed that the chemical composition and the different production processes influence the material behavior. In fatigue tests, a higher metastability and thus a higher level of deformation induced α'-martensite pronounced cyclic hardening, resulting in significantly greater endurable stresses in total strain-controlled tests and an increase in fatigue life in stress-controlled tests. For applications of non-destructive-testing, detailed knowledge of a component's metastability is required. In less metastable batches and for lower stress levels, α'-martensite primarily formed at the plasticization zone of a crack. Furthermore, the formation and nucleation points of α'-martensite were highly dependent on grain size and the presence of δ-ferrite. This study provides valuable insights into the different material behavior of three different batches with the same designation, i.e., AISI 347, due to different manufacturing processes and differences in the chemical composition, metastability, and microstructure.

Post-Processing of Inconel 718 Alloy Fabricated by Additive Manufacturing: Selective Laser Melting

The review analyses Inconel 718 (IN718) alloy, which is the nickel-based superalloy and has great application in industries due to its superior mechanical properties even at elevated temperatures by means of the solid-solution strengthening and precipitation strengthening. However, because of the tool over-wear, poor part surface integrity, high hardness and low thermal-conductivity properties, it is difficult to manufacture finished products with using conventional machining methods. It is especially urgent for the products of complex designs. In this regard, justification is given for the widespread use of modern additive manufacturing (AM) for the fabrication of the products from IN718. The most popular is AM based on the selective laser melting (SLM) technique, which can fabricate complex geometries with superior material properties. At the same time, the metal parts fabricated by SLM suffer from excessive residual porosity, residual tensile stress in the near-surface layer, and the formation of a relatively rough surface. In addition, the SLM-inherited surface defects can cause stress concentration to initiate cracks, reducing the fatigue strength of the printed parts. The review focuses on identifying potential solutions to the surface-finish complex additive manufactured to improve the surface properties to meet the industry requirements. Therefore, the improvement of the IN718-alloy-parts’ surface properties printed by the SLM becomes especially relevant. Currently, different surface post-processing technologies are being developed to obtain the expected surface quality of the SLM-components. As demonstrated, the finish surface enhancement treatments led to significant improvement in the wear resistance, corrosion resistance, increase in fatigue life, and tensile strength of the metallic materials. Therefore, adapting surface post-processing technologies has become a growing area of interest as an effective tool for improving the functionality and service lifetime of SLM IN718-alloy components. The review aims to analyse the main results of the most systematic studies of the currently developed surface post-treatments aimed to improving the surface-structure quality and properties of the IN718 parts fabricated by SLM. These results contribute to a better understanding of the role of the various-parameters’ effects on the surface improvements during the surface post-processing and changes in the structure–phase state, and physical, chemical and mechanical properties. Examples of the effects of a series of surface post-processing methods are presented: laser polishing, mechanical magnetic polishing, cutting finish-machining operations, shot peening, sandblasting technique, ultrasonic-impact treatment, and electrochemical polishing.

The effects of fiber load and frictional work on fatigue behavior of water hydraulic artificial muscles in underwater environment

Water Hydraulic Artificial Muscles (WHAMs) exhibit characteristics of high pressure and substantial load capacity. In previous work, the fatigue life of WHAMs under elastic load has been investigated, and experimental results have indicated that the fatigue life is related to both fiber load and frictional work. To individually investigate the impact of fiber load and frictional work on the fatigue life of WHAMs, an experimental scheme was designed to test the fatigue life under different levels of fiber load and frictional work. Meanwhile, the working pressure and contraction range were determined for various working conditions, in order to assess their effects on the fatigue life of WHAMs. The W-test confirms that the fatigue life of WHAMs under each working condition follows a normal distribution. Based on the experimental results, the influence of fiber load and frictional work on fatigue life was analyzed. And the experimental results indicate that the impact of fiber load on the fatigue life of WHAMs is approximately three times greater than that of frictional work. Furthermore, substituting aramid 1414 fiber with aramid 3 fiber in the fiber sleeve led to a substantial increase in fatigue life, elevating the average fatigue life to more than 4 times. Based on the power-law model, a fatigue model of WHAM is established with fiber load and frictional work as factors. The relative error between the fatigue model and experimental results in previously published work demonstrates that the new fatigue model can accurately predict the fatigue life of WHAMs.

INCREASING FATIGUE LIFE OF NEW WELDED AND REPAIRED JOINTS BY HFMI TREATMENT

AbstractThere is a growing number of welded steel bridges in the world that are reaching the end of their design life. Repair of fatigue damage (cracks) or replacement with new modern designs is becoming an increasing issue. Fatigue damage is now one of the main design and construction risks associated with the construction of new dynamically stressed steel structures. Fatigue damage is now one of the main design and engineering risks associated with the construction of new dynamically stressed steel structures. Increasing fatigue life can be achieved by HFMI treatment that can be applied to new weld as well as existing welded joints. This paper presents the results of experiments focused on HFMI treatment to new and repaired butt welds of higher thicknesses.

Enhancing Fatigue Resistance of Polylactic Acid through Natural Reinforcement in Material Extrusion.

This research paper aims to enhance the fatigue resistance of polylactic acid (PLA) in Material Extrusion (ME) by incorporating natural reinforcement, focusing on rotational bending fatigue. The study investigates the fatigue behavior of PLA in ME, using various natural fibers such as cellulose, coffee, and flax as potential reinforcements. It explores the optimization of printing parameters to address challenges like warping and shrinkage, which can affect dimensional accuracy and fatigue performance, particularly under the rotational bending conditions analyzed. Cellulose emerges as the most promising natural fiber reinforcement for PLA in ME, exhibiting superior resistance to warping and shrinkage. It also demonstrates minimal geometrical deviations, enabling the production of components with tighter dimensional tolerances. Additionally, the study highlights the significant influence of natural fiber reinforcement on the dimensional deviations and rotational fatigue behavior of printed components. The fatigue resistance of PLA was significantly improved with natural fiber reinforcements. Specifically, PLA reinforced with cellulose showed an increase in fatigue life, achieving up to 13.7 MPa stress at 70,000 cycles compared to unreinforced PLA. PLA with coffee and flax fibers also demonstrated enhanced performance, with stress values reaching 13.6 MPa and 13.5 MPa, respectively, at similar cycle counts. These results suggest that natural fiber reinforcements can effectively improve the fatigue resistance and dimensional stability of PLA components produced by ME. This paper contributes to the advancement of additive manufacturing by introducing natural fiber reinforcement as a sustainable solution to enhance PLA performance under rotational bending fatigue conditions. It offers insights into the comparative effectiveness of natural fibers and synthetic counterparts, particularly emphasizing the superior performance of cellulose.

Effect of multi-pass overlapping friction stir processing on fatigue behavior of Al-Cu alloy

More than half of mechanical breakdowns stem from fatigue failure, often occurring suddenly and without warning. Friction stir processing (FSP) enhances the material's toughness and resilience to fatigue by refining its grain structure. This study investigates how multi-pass overlapping technique impacts the fatigue crack growth rate of FSPed Al-Cu alloy. Microstructural analysis revealed that the stir region exhibited uniformly dispersed and fragmented precipitates and finely recrystallized ultrafine grains. The hardness and strength were reduced, and ductility was enhanced after FSP due to high thermal cycling. Fatigue testing demonstrated a significant increase in fatigue life and reduced fatigue crack growth rate attributable to the combined effects of precipitation and grain refinement during cooling-assisted FSP. SEM examination of fatigue fracture surfaces revealed dimples indicative of ductile failure in the rapid crack propagation zone, while the steady-state propagation region displayed striation markings and secondary fractures.

Surface integrity and fatigue performance of Ti6Al4V alloy peened by sinking bead abrasive jet

Exploring a green, low-cost peening process to improve the surface integrity of titanium alloys, increasing fatigue life, and breaking through the limitations in large-scale applications is an urgent need at present. The study presents a novel peening method using sinking beads as the shot, combined with the low dust pollution and water film cushioning advantages of waterjet peening. The impact of the process on the surface integrity of titanium alloys is meticulously discussed to elucidate the microstructural evolution. Constant stress fatigue tests are performed to evaluate the effectiveness of the proposed method. It was found that the jet pressure and feed rate determined the energy density and the number of peening for sinking bead abrasives, which were the key factors affecting the surface material properties of the peening specimen. By optimizing the process parameters, the maximum surface residual compressive stress can reach 875.5 MPa, the microhardness can attain 481.7 HV, and the surface roughness can be reduced to 1.594 μm. The surface of the peened specimen is covered with impact craters, exhibiting a plastic deformation layer with evident grain refinement and lattice distortion. The ductility of the peened specimen decreased, with axial deformation reducing from 0.278 mm to 0.260 mm under constant cyclic stress. Due to the increase in microhardness and the introduction of residual compressive stresses, the fatigue cycles of the specimen were increased from 20692 to 33615, improving the fatigue life by 62.45%.

Effect of scanning strategy and laser peening on microstructure and fatigue properties of laser-directed energy deposition-built 15-5 PH stainless steel

PurposeThe aim of this paper is to study the effect of laser shock peening (LSP) on mechanical behaviour of the laser-directed energy deposition (LDED)-based printed 15-5 PH stainless steel with U and V notches. The study specifically concentrates on the evaluation of effect of scan strategy, machining and LSP processing on microstructural, texture evolution and fatigue behaviour of LDED-printed 15-5 PH steel.Design/methodology/approachFor LSP treatment, 15-5 PH steel was printed using LDED process with bidirectional scanning strategy (XX [θ = 0°) and XY [θ = 90°]) at optimised laser power of 600 W with a scanning speed of 300 mm/min and a powder feed rate of 3 g/min. Furthermore, LSP treatment was conducted on the V- and U-notched fatigue specimens extracted from LDED-built samples at laser energy of 3.5 J with a pulse width of 10 ns using laser spot diameter of 3 mm. Post to the LSP treatment, the surface roughness, fatigue life assessment and microstructural evolution analysis is performed. For this, different advanced characterisation techniques are used, such as scanning electron microscopy attached with electron backscatter diffraction for microstructure and texture, X-ray diffraction for residual stress (RS) and structure information, Vicker’s hardness tester for microhardness and universal testing machine for low-cycle fatigue.FindingsIt is observed that both scanning strategies during the LDED printing of 15-5 PH steel and laser peening have played significant role in fatigue life. Specimens with the XY printing strategy shows higher fatigue life as compared to XX with both U- and V-notched conditions. Furthermore, machining and LSP treatment led to a significant improvement of fatigue life for both scanning strategies with U and V notches. The extent of increase in fatigue life for both XX and XY scanning strategy with V notch is found to be higher than U notch after LSP treatment, though without LSP samples with U notch have a higher fatigue life. As fabricated sample is found to have the lowest fatigue life as compared to machines and laser peened with both scan strategies.Originality/valueThis study presents an innovative method to improve the fatigue life of 15-5 PH stainless steel by changing the microstructure, texture and RS with the adoption of a suitable scanning strategy, machining and LSP treatment as post-processing. The combination of preferred microstructure and compressive RS in LDED-printed 15-5 PH stainless steel achieved with a synergy between microstructure and RS, which is responsible to improve the fatigue life. This can be adopted for the futuristic application of LDED-printed 15-5 PH stainless steel for different applications in aerospace and other industries.Graphical abstract

Rheological behavior and microscopic characteristic of electromagnetic thermal activated crumb rubber and SBS modified asphalt

Rubber activation provides a feasible approach for the preparation of high-content rubber modified asphalt without increasing the complexity of the production process. The electromagnetic thermal activation method was used for crumb rubber. The high dosage (30 % mass ratio of neat asphalt) activated crumb rubber and SBS (1.2 % mass ratio of neat asphalt) modified asphalt (ARMA) was prepared. The neat asphalt (NA) and 20 % non-activated crumb rubber modified asphalt (RMA) constituted the control group. Storage stability, rheological behavior, and microscopic properties tests were conducted to evaluate the performance differences of them. The rheological behavior of RMA and ARMA was superior to NA. The electromagnetic thermal activation method fragmented rubber particles into numerous disordered small particles, thereby expanded the contact area with asphalt. This resulted in the storage stability of ARMA significantly better than that of RMA. The cross-linked wrinkles were generated by the combined effect of SBS modifier and activated crumb rubber, which enhanced the rheological, anti-fatigue, and elastic behavior. Both RMA and ARMA exhibited high-temperature PG grades of 82˚C. At 70°C, ARMA retained the capability to endure Extremely Heavy Traffic Grade conditions. At a strain level of 2.5 %, ARMA exhibited a 178 % increase in fatigue life compared to RMA. The crumb rubber was dispersed within the RMA and ARMA. Both ARMA and RMA did not exhibit the obviously bee-structure. However, ARMA exhibited a stronger polymer network. Moreover, both the carbonyl index and sulfoxide index of ARMA were lower than those of RMA and NA. The activation process improved the compatibility between rubber and asphalt, and realized the high value utilization of waste rubber.

Effect of surface high-density twinned structure on the fatigue crack initiation of Ti–6Al–4V

Fatigue crack initiation sites would transfer from surface to subsurface of metal specimens through shot peening (SP) or laser shock surface strengthening. This phenomenon is typically attributed to the introduction of compressive residual stress on the surface of the specimen during the surface strengthening process. In this study, Ti–6Al–4V specimens featuring a high-density twinned structure on their surfaces, which eliminated compressive residual stress, were fabricated. Fatigue test result revealed that fatigue crack initiation sites in specimens with twinned structures were predominantly located in the subsurface, near the junction area between the twinned structure and the matrix. This phenomenon was accompanied by an increase in fatigue life. The microstructural features in the fatigue crack source were analyzed using focused ion beam (FIB) coupled with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). It was observed that primary crack formed in badly oriented grains that are difficult to initiate dislocation slip according to the Schmid factor (SF). Nonetheless, TEM observations revealed the activation of basal <a> dislocations in these grains, potentially leading to crack nucleation along basal slip bands and subsequent propagation along the basal plane. Our results suggest that a high-density twin structure on the surface can enhance the fatigue strength of Ti–6Al–4V by transferring the fatigue crack source to the subsurface.