Search Results (194)

This study investigates the hydrogen embrittlement susceptibility of laser melting deposition (LMD)-produced Ti-6Al-4V alloy with different build orientations (0°, 45°, 90°) through electrochemical hydrogen charging, slow strain rate testing, and microstructural characterization. Ti-6Al-4V alloys are widely used in marine and offshore engineering, where cathodic protection and corrosion reactions can generate hydrogen, leading to hydrogen ingress and potential embrittlement. Results show that prolonged hydrogen charging induces hydride formation, α-phase fragmentation, and β-phase dissolution, significantly degrading corrosion resistance and mechanical properties. Hydrogen embrittlement susceptibility exhibits notable anisotropy: elongation reductions for 0°, 45°, and 90° specimens are 40.1%, 40.8%, and 29.4%, respectively. The relatively superior resistance observed in the 90° orientation may be associated with its single-layer structure and more uniform dimple distribution. In contrast, the multilayer interfaces in other orientations are likely to serve as preferential sites for hydrogen accumulation, which may contribute to the increased embrittlement susceptibility. This research reveals the failure mechanism of LMD Ti-6Al-4V in hydrogen environments and supports its application in marine engineering. Full article

Copper-containing steel is widely used in ship plates and other marine engineering fields due to its excellent mechanical properties and good weldability. However, in hydrogen-containing media environments, ship plate steel is prone to hydrogen embrittlement during service. Existing research primarily focuses on steel grades with copper content below 3 wt.%, while the diffusion and trapping behavior of hydrogen in ultra-high-copper steel with copper content exceeding 3 wt.% remains unclear. Therefore, this study designed an ultra-high-copper-content steel with a copper content of 6.01% and investigated the diffusion behavior of hydrogen in the test steel under different hydrogen charging current densities through microstructure characterization, slow strain rate tensile testing, electrochemical hydrogen permeation, and internal friction tests. The results indicate that with an increase in hydrogen charging current density, accompanied by a slight degradation in mechanical properties, the irreversible hydrogen trap density increases by 50.7%. A large number of microstructures, such as phase boundaries, grain boundaries, and dislocations, have formed inside the material, which have reversible trapping effects on hydrogen, effectively suppressing the migration of hydrogen in the crystal structure and reducing the embrittlement phenomenon caused by hydrogen. This study expands the application potential of copper-containing steel in the field of ocean engineering, providing an important reference for the future development of high-strength, hydrogen embrittlement-resistant copper steel with ultra-high copper content. Full article

Silane sol was applied to seal the pores in a micro-arc oxidation coating, with the results proving that the treatment increased the anti-corrosion characteristics of aluminium alloy. Moreover, an electrochemical workstation was employed to test the open-circuit voltage, polarisation potential, and polarisation current of the samples. According to the results, after the aluminium alloy was treated with the micro-arc oxidation coating and underwent subsequent sealing treatment, the open-circuit potential increased from −0.64 to −0.44 V, the corrosion potential from −0.54 to −0.31 V, and the corrosion current density from 56.23 × 10⁻⁷ to 7.76 × 10⁻⁷ A. However, when samples were corroded by 1 mol/L HCl, the corrosion potential and corrosion current density decreased to −0.34 V and 20.42 × 10⁻⁷ A, respectively, proving that sealing the pores on the micro-arc oxidation coating only prevented substrate corrosion for a short time. In addition, slow-strain-rate stretching experiments were conducted to explore the mechanical performances of the samples, determining that the surface treatment had an insignificant effect on the stress of the aluminium alloy but had an important effect on its elongation, and when the surface of the alloy was treated with micro-arc oxidation coating, its elongation decreased from 28% to 26%. Full article

Welded joints in API 5L X70 pipeline steel represent critical locations for pipelines intended for hydrogen service because welding can create microstructural inhomogeneity, stress concentrations, and uneven mechanical properties that can promote hydrogen-assisted degradation. In hydrogen-containing environments, these effects may manifest as reduced ductility, loss of fracture resistance, and increased cracking susceptibility, particularly in the weld metal and heat-affected zone. Therefore, welding procedures for X70 intended for hydrogen applications must be evaluated using systematic mechanical testing and microstructural characterization under defined hydrogen exposure conditions. The study investigates the detrimental effects of hydrogen on the mechanical integrity of pipeline materials, focusing on welded joints of the API 5L X70 steel, a candidate material for use in hydrogen-containing environments. The weldability and structural performance of the X70 pipeline steel joints in hydrogen environments, produced using automated multi-pass metal active gas (MAG) welding, was experimentally studied. Welding was performed on a DN800 pipe with precise control over welding parameters. Comprehensive analyses were conducted on the welded joints, including microstructure examinations, hardness measurements, slow strain rate testing in high-pressure gaseous H₂ with a N₂ baseline and fracture toughness testing. In high-pressure hydrogen SSRT showed a moderate reduction in ductility relative to nitrogen, with reduction of area decreasing from 81.2% (N₂) to 69.1 and 71.5% (H₂), while time-to-failure remained comparable (475 min in N₂ vs. 497 and 496 min in H₂) Ultimate tensile strength was not reduced (579 MPa in N₂ vs. 609 and 597 MPa in H₂). Secondary surface cracks were observed only on specimens tested in hydrogen. Fracture mechanics testing after hydrogen exposure yielded K_IH values of 58–59 MPa√m in the weld metal and 57–61 MPa√m in the HAZ, exceeding the 55 MPa√m acceptance threshold applied in this study. The results highlight the necessity of optimized welding techniques and targeted material analyses to ensure the safety and durability of pipelines in hydrogen-rich environments, thereby contributing to the development of reliable infrastructure for sustainable energy systems. Full article

In this study, we investigated the effect of heat treatment-induced grain size on the hydrogen embrittlement (HE) resistance of 30MnB5 steel, focusing particularly on the variation in prior austenite grain (PAG) size. As the heat treatment time increased, the PAGs coarsened, leading the martensite packets, blocks, and lath sizes to also coarsen. As the microstructure became more refined, the boundary density of the packet–block–lath structure increased along with a significant increase in the low-angle grain boundary (LAGB) fraction. The microstructure refinement accelerated the initial permeation rate of hydrogen, while the high density of LAGBs and trap sites effectively suppressed its long-term diffusion/localization. The slow strain rate tensile test confirmed that the tensile strength and elongation of 30MnB5 steel in a hydrogen environment were lower than those in air, indicating HE. Furthermore, the results showed that the HE sensitivity decreased in the fine microstructure condition, as evidenced by the smaller reduction in elongation compared to the coarse microstructure. The study results will enhance the understanding of hydrogen-induced degradation in hot-stamped automotive steels and offer fundamental insights for optimizing heat treatment strategies applied to 30MnB5 steel for mitigating HE. Full article

Stress corrosion cracking (SCC) is a critical failure mechanism that arises from the synergistic interaction between tensile stress and corrosive environments, leading to sudden and often catastrophic failures in structural components across various industries, including aerospace, nuclear energy, oil and gas, and marine engineering. This review synthesizes current understanding of SCC mechanisms, including film rupture and anodic dissolution, hydrogen embrittlement, and adsorption-induced cleavage, and evaluates material susceptibility across steels, aluminum alloys, nickel-based alloys, titanium, and emerging high-entropy alloys. Environmental factors such as aqueous chemistry, temperature, pressure, pH, and dissolved gases are examined for their roles in SCC initiation and propagation. Advanced testing methodologies, including slow strain rate testing, bent-beam configurations, electrochemical monitoring, and high-resolution microscopy, are discussed for characterizing SCC behavior. Engineering mitigation strategies are presented, encompassing material selection, stress reduction, surface treatments, and environmental control. Case studies illustrate real-world SCC failures and inform best practices. Emerging trends highlight the potential of machine learning for predictive maintenance and the development of SCC-resistant materials through additive manufacturing and microstructural engineering. This comprehensive review provides mechanical engineers with actionable insights for designing, maintaining, and safeguarding components against SCC in demanding service environments. Full article

Due to the constraints of early mining conditions in some coal mines in China, a large number of pillar-type coal pillars remain in the mined-out areas. During the upward mining above the underlying pillar-type goaf, it is usually necessary to backfill the underlying goaf to form a backfill–coal pillar synergistic bearing structure, which jointly bears the load during the upward mining process. In this paper, a combination of laboratory mechanical tests and numerical simulations is used to study the failure characteristics of coal pillars, stress–strain curve characteristics, force chain transmission characteristics, and the number and distribution of fractures under the influence of backfill strength and filling ratio. The critical strength and critical filling ratio of coal pillars with different widths under the coordinated action of different backfill strengths and filling ratios are analyzed. The results show that the composite with a backfill filling ratio of 90% exhibits a stepwise change after coal pillar failure, while the composites with filling ratios of 70% and 50% show a cliff-like drop after coal pillar failure. The composite with a filling ratio of 50% completely loses its bearing capacity after coal pillar failure; the backfill is limited by its height and cannot bear the load repeatedly with the failed coal pillar, and the bearing stage lacks the common bearing stage in which the backfill wraps the failed coal pillar. The number of fractures in the coal pillar decreases with the increase in backfill strength. High-strength backfill can provide higher lateral restraint for the coal pillar through its own anti-deformation capacity. Increasing the backfill filling ratio can reduce the propagation rate of internal fractures in the coal pillar, slow down the deformation time of the coal pillar, and prevent the coal pillar from impact failure. When the coal pillar width is 8 m, the critical filling ratio of the backfill decreases from 84% to 70% as the backfill strength increases from 2 MPa to 6 MPa; when the coal pillar width is 11 m, the critical filling ratio decreases from 69% to 62%; when the coal pillar width is 14 m, the critical filling ratio decreases from 58% to 55%. The research results provide important on-site guiding significance for the safe implementation of upward mining. Full article

To investigate the synergistic reinforcement mechanism of single and combined incorporation of carbon nanotubes (CNTs) and graphene oxide (GO) on the dynamic mechanical properties of alkali-activated recycled aggregate concrete (AARAC), 81 cylindrical specimens were designed with varying dimensions, recycled coarse aggregate (RCA) replacement ratios, and single/combined nanomaterial incorporation schemes. Uniaxial compression tests were conducted to obtain the stress–strain curves of AARAC under different strain rates (10⁻⁵ s⁻¹, 10⁻³ s⁻¹, and 10⁻¹ s⁻¹), and a dynamic constitutive model for AARAC was established. The results indicate that under static conditions (strain rates of 10⁻⁵ s⁻¹ and 10⁻³ s⁻¹), the coupling law between the RCA replacement ratio and nanomaterial dosage is determined by the balance between the defect degree of recycled aggregates and the improvement effect of nanomaterials. Specifically, at a 50% RCA replacement ratio, the single incorporation of 0.1% CNTs can enhance the mechanical properties of AARAC; at a 100% RCA replacement ratio, the synergistic effect of the combined incorporation of 0.1% CNTs and 0.05% GO can mitigate the defects of fully recycled aggregates. In contrast, under dynamic conditions (strain rate of 10⁻¹ s⁻¹), Nanomaterials (CNTs and GO) optimize load transfer efficiency and slow down the process of crack propagation, leading to a much greater improvement in the mechanical properties of AARAC compared to static conditions, with the combined incorporation achieving better performance at a 100% RCA replacement ratio. As the specimen size increases from 75 mm to 150 mm, the increase in static peak strain is relatively small, which is attributed to the more uniform deformation distribution and stronger deformation coordination capability of larger specimens under static loading. Under dynamic loading, the influence law of peak strain and elastic modulus is consistent with that of peak stress. Based on these findings, a dynamic constitutive model for AARAC modified by single and combined incorporation of CNTs/GO was established. The predicted curves of the model are in good agreement with the experimental curves, with an error range of 2.3–7.1%, which can well describe the constitutive relationship of the tested material. Full article

Previous research showed that a strain of Leuconostoc mesenteroides, isolated from goat’s raw milk cheese, was effective in slowing down the growth and reducing the maximum concentration of L. monocytogenes when evaluated in a milk model; furthermore, the extent of inhibition was dependent on the milk’s initial pH. The objectives of this study were as follows: (1) to determine whether the growth of L. monocytogenes in goat’s pasteurized milk cheese during maturation could be approximated from growth data obtained in the milk model medium, either in monoculture or in coculture with L. mesenteroides, and if so, (2) to model a milk-to-cheese conversion factor (Cf) for L. monocytogenes growth rate. Challenge tests were conducted by inoculating L. monocytogenes in monoculture and in coculture with L. mesenteroides in goat’s pasteurized milk adjusted at initial pH levels of 5.5, 6.0, and 6.5. The process of cheesemaking continued, and cheeses were ripened at 12 °C for 12 days. Each experimental growth curve was adjusted to a pH-driven dynamic model where the microbial maximum growth rate is a function of pH. As observed in the milk model medium, in coculture with L. mesenteroides, the optimum growth rate (μ_opt) of L. monocytogenes in maturing cheese was affected by the initial pH of milk: the lowest rate of 0.863 ± 0.042 day⁻¹ was obtained at the initial pH 5.5, in comparison to 1.239 ± 0.208 and 1.038 ± 0.308 day⁻¹ at pH 6.0 and 6.5, respectively. Regardless of the milk’s initial pH, L. mesenteroides did not reduce the maximum load of L. monocytogenes in maturing cheeses, as it did in the milk medium. On the contrary, at the milk’s initial pH of 5.5, 6.0, and 6.5, L. mesenteroides was able to decrease, on average, 2.2-fold, 1.5-fold, and 1.9-fold the μ_opt of L. monocytogenes in both milk medium and cheese, without significant differences between matrices. Following such validation in goat’s cheese, the square root of milk-to-cheese Cf for L. monocytogenes was estimated as 0.751 (SE = 0.0108), and the type of culture (monoculture and coculture) was not found to affect Cf (p = 0.320). In conclusion, this work validated the pre-acidification of milk as an efficient strategy that, when combined with the use of a protective culture, can synergically enhance the control of L. monocytogenes in cheese. Full article

Under the influence of engineering disturbances, the loading rate of surrounding rock is in a state of continuous adjustment. This study conducts experimental investigations on the mechanical response characteristics under different strain rates (10⁻⁵ s⁻¹, 10⁻⁴ s⁻¹, and 10⁻³ s⁻¹). During the uniaxial loading process of coal–rock composite specimens, multi-parameter monitoring was implemented, and a systematic study was carried out on the ring-down count induced by microcracks, the energy values of acoustic emission (AE) events, the stage-dependent strain characteristics on the specimen surface, and the surface temperature variation characteristics. Additionally, the stress–strain curve characteristics under different strain rates were comparatively analyzed in stages. The loading process of the coal–rock composite specimens was reproduced using the Particle Flow Code (PFC3D 6.0) simulation software. The simulation results indicate that the stress–strain results obtained from the simulation are in good agreement with the laboratory test results; based on these simulation results, the energy accumulation and dissipation characteristics of the coal–rock composite specimens under the influence of strain rate were revealed. Furthermore, a microscopic damage model considering strain rate was constructed based on the Weibull probability statistics theory. The results show that strain rate has a significant impact on the strength, elastic modulus, and failure mode of the coal–rock composite specimens. At low strain rates, the specimens exhibit obvious progressive failure characteristics and strain localization phenomena, while at higher strain rates, they show brittle sudden failure characteristics. Meanwhile, the thermal imaging results reveal that at high strain rates, the overall temperature rise in the composite specimens is rapid, whereas at low strain rates, the overall temperature rise is slow—but the temperature rise in the coal portion is faster than that in the rock portion. The peak temperature at high strain rates is approximately 2 °C higher than that at low strain rates. The PFC simulation results demonstrate that the larger the strain rate, the faster the growth rate of plastic energy in the post-peak stage and the faster the release rate of elastic energy. Full article

The long-term stability of deep underground excavations near aquifer-bearing strata is primarily controlled by the time-dependent deformation and permeability changes in the surrounding rock mass under the combined effects of mechanical loading and groundwater seepage. This study experimentally investigates the creep behavior and permeability evolution of tuff specimens subjected to stepwise reductions in confining pressure under coupled seepage and stress conditions. Conventional triaxial compression tests were conducted to determine the peak strength at confining pressures of 10, 15, and 20 MPa. Subsequently, triaxial creep tests were performed, maintaining axial stress at 70% of the previously established peak strength, with a constant seepage pressure of 4 MPa, while progressively decreasing the confining pressure. The results clearly reveal a three-stage creep process—with instantaneous, steady-state, and accelerated phases—with the radial strain exceeding axial strain and ultimately dominating at failure. This indicates that failure is characterized by significant volumetric expansion. At the specified initial confining pressures of 10 MPa, 15 MPa, and 20 MPa, the tuff specimens exhibited volumetric strains of −1.332, −1.119, and −0.836 at failure, respectively. Permeability evolution depends on the creep stage, showing a pronounced increase during the accelerated creep phase that often surpasses the cumulative permeability changes observed earlier. The specimen’s permeability at failure increased by factors of 3.97, 3.21, and 3.61 compared to the initial stage of the experiment, respectively. Additionally, permeability evolution exhibits a strong functional relationship with volumetric strain, which can be effectively modeled using an exponential function. The experimental findings further indicate that, as the confining pressure is gradually reduced, the permeability evolves following a clear exponential trend. Additionally, a higher initial confining pressure slows the rate at which permeability increases. These findings clarify the three-stage creep behavior and the associated evolution of the permeability index in tuff under coupled seepage–stress conditions. Additionally, they present a quantitative model linking permeability to volumetric strain, offering both a theoretical foundation and a new approach for assessing the long-term stability risks of deep underground engineering projects. Full article

New grades of dual-phase (DP) steels with ultimate tensile strength (UTS) up to 1500 MPa have been developed using a continuous annealing process. This study investigates the effects of over-aging temperature and NbC precipitates on the microstructure and hydrogen embrittlement of these DP steels. Increasing the over-aging temperature promotes carbide coarsening, which reduces tensile strength, but simultaneously stabilizes retained austenite by inhibiting martensite transformation and enhances ductility through the TRIP effect. Compared to the reference DP steel, the Nb-added DP steel exhibits further strength enhancement due to fine-grain strengthening and precipitation strengthening. Results from slow strain rate tensile (SSRT) and thermal desorption spectroscopy (TDS) tests demonstrate that the Nb-added DP steel possesses superior resistance to hydrogen embrittlement. This improvement is primarily attributed to the hydrogen trapping effect of NbC precipitates, complemented by their grain refinement capability. Full article

The operational integrity of L415 pipeline steel, a critical component of China’s energy network, is severely threatened by the unique acidic red soil environments prevalent in Southern China. A significant knowledge gap exists regarding its specific failure mechanisms, particularly the interplay between Anodic Dissolution (AD) and Hydrogen Embrittlement (HE) in driving Stress Corrosion Cracking (SCC). This study systematically investigates the corrosion and SCC behavior of L415 steel in a simulated environment that replicates the typical soil chemistry of the Gannan region in Southern China. Results revealed that corrosion kinetics are highly dependent on environmental chemistry, with corrosion rates escalating nearly four-fold from 0.0505 mm/a to a severe 0.1949 mm/a, driven by the synergy of low pH and high SO₄²⁻ concentration. This behavior is governed by the integrity of the corrosion product film, where aggressive environments form porous, unprotective layers with low charge transfer resistance. Slow strain rate tensile (SSRT) tests confirmed that the steel’s susceptibility to SCC is strongly promoted by acidity. Critically, the dominant SCC mechanism was environment-dependent, transitioning from Hydrogen Embrittlement (HE) to intergranular cracking in the most acidic environment, and a mixed AD-HE mechanism causing transgranular cracking in high-chloride conditions. These findings provide a direct mechanistic link between soil chemistry and failure mode, offering a crucial scientific basis for developing environment-specific integrity management strategies for pipelines in these challenging terrains. Full article

X80 pipeline steel is a key material in the field of oil and gas transportation. Its damage behavior in a hydrogen-filled environment directly affects pipeline safety. In this study, through hydrogen permeation experiments and slow strain rate tensile tests, the electrochemical responses and hydrogen-induced cracking behaviors of X80 base metal and welded joints under hydrogen filling conditions in both AC and DC were systematically compared. The results show that when the base material is filled with hydrogen at 20 mA/cm² AC, the hydrogen permeation flux is the largest, and the overall hydrogen permeation parameter of the welded joint is lower than that of the base material. High-frequency polarization promotes hydrogen permeation, but anodic corrosion products at high current densities can impede hydrogen entry. The slow strain rate tensile test further confirmed that the mechanical properties of the material declined more significantly under direct current hydrogen charging, and the sensitivity to stress corrosion cracking was higher. Under alternating hydrogen charging conditions, due to the alternating effects of hydrogen charging at the cathode and corrosion at the anode, a relatively low hydrogen embrittlement sensitivity is exhibited. Full article

This work studies the hydrogen embrittlement (HE) behaviour of Dual-Phase steels with varying martensite content. Steels with martensite contents of 25 ± 5, 50 ± 4 and 78 ± 7% were realised by intercritically annealing an as-received DP steel. These steels were charged with hydrogen and consequently subjected to an in situ slow strain rate tensile test to characterise the embrittlement. It was found that the steel with 50% martensitic content showed the most ductility in air, but the highest embrittlement of 86 ± 10%. The extent of embrittlement does not increase further from the point that martensite forms a continuous network in the microstructure. The presence of martensite on the surface is linked to the formation of brittle crack initiation sites in these steels. Furthermore it was found that the anisotropic banded structure in the annealed steels promotes brittle crack propagation along the direction of banding, which originates from rolling process. This research shows that anisotropic martensite distributions as well as surface martensite should be avoided when developing rolled steels, to maximise HE resistance. Full article