Search Results (475)

Initially, the effects of sheet combinations for joining two sheets, including 780 MPa steel and A5052 aluminum alloy sheets, on the joined cross-sectional shapes of the sheets in a clinch-bonding process and the tension-shear load of joined sheets were investigated. The effect of an adhesive on the amounts of the interlock and the minimum thickness in the upper sheet was not large, whereas the effect of the sheet combination was observed. Subsequently, for joining the upper 980 MPa ultra-high-strength steel and lower aluminum alloy sheets in the clinch-bonding process, the effects of the die shape, punch velocity, and sheet holding force on the joinability were investigated. As a result, defect-free conditions were narrowly constrained. Finally, a method that involved controlling material flow using an adhesive with fine particles to increase friction between the sheets was introduced. The upper 980 MPa steel and lower aluminum alloy sheets were successfully joined using this approach. Full article

The pull-back method for determining dynamic tensile strength assumes one-dimensional stress wave propagation and material homogeneity. This study validates these assumptions for unidirectional reinforced concrete (UDRC) through experiments and numerical simulations. Split Hopkinson pressure bar tests were conducted on plain concrete, plain UDRC, and deformed UDRC specimens containing a central 6 mm steel bar. Ultra-high-speed digital image correlation at 500,000 fps enabled precise local strain rate measurements (3 s⁻¹ to 55 s⁻¹) at fracture locations. Finite element simulations revealed that while reinforcement induces localized multi-axial stresses near the steel–concrete interface, the bulk concrete maintains predominantly uniaxial stress conditions. Experimental results showed less than 1% variation in pull-back velocity between specimen types. Statistical analysis confirmed a unified strain rate-strength relationship: ${\sigma }_{\mathrm{spall}}=4.1+4.7{log}_{10}\left(\dot{\epsilon }\right)$$\mathrm{M}\mathrm{Pa}$, independent of reinforcement configuration (ANCOVA: $p=0.2182$ for interaction term). The dynamic tensile strength is governed by concrete matrix properties rather than reinforcement type. These findings are the first to experimentally and numerically validate the pull-back method’s applicability to UDRC systems, establishing that dynamic tensile failure is matrix-dominated and enabling simplified one-dimensional analysis for reinforced concrete under impact. Full article

This study looked at the performance requirements of repair materials for concrete structures in cold regions, systematically analyzing the effects of steel fiber dosage (0.7–2.1%), early-strength agent PRIORITY dosage (6–10%), and their coupling effects on the workability, interfacial bond strength, and freeze–thaw resistance of rapid-hardening ultra-high-performance concrete (UHPC). Through fluidity testing, bond interface failure analysis, freeze–thaw cycle testing, and pore analysis, the mechanism of steel fibers and early-strength agent on the multi-dimensional performance of fast-hardening UHPC was revealed. The results showed that when the steel fiber dosage exceeded 1.4%, the flowability was significantly reduced, while a PRIORITY dosage of 8% improved the flowability by 20.5% by enhancing the paste lubricity. Single addition of steel fibers decreased the interfacial bond strength, but compound addition of 8% PRIORITY offset the negative impact by optimizing the filling effect of hydration products. Under freeze–thaw cycles, excessive steel fibers (2.1%) exacerbated the mass loss (1.67%), whereas a PRIORITY dosage of 8% increased the retention rate of relative dynamic elastic modulus by 10–15%. Pore analysis shows that the synergistic effect of 1.4% steel fiber and 8% PRIORITY can reduce the number of pores, optimize the pore distribution, and make the structure denser. The study determined that the optimal compound mixing ratio was 1.4% steel fibers and 8% PRIORITY. This combination ensures construction fluidity while significantly improving the interfacial bond durability and freeze–thaw resistance, providing a theoretical basis for the design of concrete repair materials in cold regions. Full article

The utilization of steel slag (SS) in construction materials represents an effective approach to improving its overall recycling efficiency. This study incorporates SS into a conventional ground granulated blast-furnace slag (GGBS)–fly ash (FA)-based binder system to develop a ternary system comprising SS, GGBS, and FA, and investigates how this system influences the static mechanical properties of ultra-high-performance geopolymer concrete (UHPGC). An axial point augmented simplex centroid design method was employed to systematically explore the influence and underlying mechanisms of different binder ratios on the workability, axial compressive strength, and flexural performance of UHPGC, and to determine the optimal compositional range. The results indicate that steel slag has a certain negative effect on the flowability of UHPGC paste; however, with an appropriate proportion of composite binder materials, the mixture can still exhibit satisfactory flowability. The compressive performance of UHPGC is primarily governed by the proportion of GGBS in the ternary binder system; an appropriate GGBS content can provide enhanced compressive strength and elastic modulus. UHPGC exhibits ductile behavior under flexural loading; however, replacing GGBS with SS significantly reduces its flexural strength and energy absorption capacity. The optimal static mechanical performance is achieved when the mass proportions of SS, GGBS, and FA are within the ranges of 9.3–13.8%, 66.2–70.7%, and 20.0–22.9%, respectively. This study provides a scientific approach for the valorization of SS through construction material applications and offers a theoretical and data-driven basis for the mix design of ultra-high-performance building materials derived from industrial solid wastes. Full article

This paper assesses experimentally and theoretically the hydrogen-assisted cracking sensitivity of an ultrahigh-strength lath martensitic steel, recently used to manufacture tendon rods for structural engineering. The experimental values of the J-integral were obtained by tensile testing up to failure precracked SENT specimens in air, as an inert environment and in a thiocyanate aqueous solution, as a hydrogen-promoter medium. In parallel, the theoretical resources necessary to apply the Dugdale cohesive model to the SENT specimen were developed from the Green function in order to predict the J-integral dependency on the applied load and the crack size, with the cohesive resistance being the only material constant concerning fracture. The comparison of theoretical and experimental results strongly supports the premise that the cohesive crack accurately models the effect of the mechanisms by which the examined steel opposes crack propagation, both when in hydrogen-free and -embrittled conditions. The identification of experimental and theoretical limit values respectively involving a post-small-scale-yielding regime and unstable extension of the cohesive zone allowed for the value of the cohesive resistance to be determined, its condition as a material constant in hydrogen-free medium confirmed, and its strong decrease with hydrogen exposure revealed. Full article

Ultra-high-performance concrete (UHPC) reinforced with recycled tire steel fibers (RTSFs) was studied to evaluate its mechanical properties and cracking behavior. Using acoustic emission (AE) monitoring, researchers tested various RTSF replacement rates in compression and flexural tests. Results revealed a clear trend: mechanical properties initially improved then declined with increasing RTSF content, peaking at 25% replacement. AE analysis showed distinct patterns in energy release and crack propagation. Signal timing for energy and ringing count followed a delayed-to-advanced sequence, while b-value and information entropy changes indicated optimal flexural performance at specific replacement rates. RA-AF classification demonstrated that shear failure reached its minimum (25% replacement), with shear cracks increasing at higher ratios. These findings demonstrate RTSFs’ dual benefits: enhancing UHPC performance while promoting sustainability. The 25% replacement ratio emerged as the optimal balance, improving strength while delaying crack formation. This study provides insights into the mechanism by which waste tire steel fibers enhance the performance of UHPC. This research provides valuable insights for developing eco-friendly UHPC formulations using recycled materials, offering both environmental and economic advantages for construction applications. Full article

An ultra-high-strength press-hardening steel (PHS) and a high-strength dual-phase steel (DP) were butt-joined by the gas metal arc welding (GMAW) process, aiming to assess the effects of a high heat input welding process on the structure-property relationship and residual stress. The post-weld microstructure, the microhardness profile, the tensile behavior, and the experimentally obtained residual stresses (by x-ray diffraction) of the steels in dissimilar (PHS-DP) and similar (PHS-PHS, DP-DP) pair combinations have been analyzed. Results indicated that the ultimate tensile strength (UTS) of the dissimilar pair PHS-DP achieves a similar strength to the DP-DP joint, whereas the elongation was similar to that of the PHS-PHS weldment. The failure location of the tensile specimens was expected and systematically observed at the tempered and softer sub-critical heat-affected zone (SC-HAZ) in all welded conditions. Compressive residual stresses were consistently observed along the weldments in all specimens; the more accentuated negative RS were measured in the PHS joint attributed to the higher volume fraction of martensite; furthermore, the negative RS measured in the fusion zone (FZ) could be well correlated to weld restraint due to the sheet anchoring during the welding procedure, despite the presence of predominant ferrite and pearlite microstructures. Full article

This study investigates the influence of steel fiber content (0~3% by volume) on the workability and mechanical properties of ultra-high-performance concrete (UHPC) incorporating slag or fly ash. Although UHPC exhibits excellent strength and durability, its brittleness and high cost hinder broader applications. Steel fibers are known to improve mechanical performance and toughness, but their interaction with mineral admixtures remains underexplored. Flowability, compressive strength, flexural behavior, impact resistance, and microstructure were evaluated. Results show that increasing fiber content significantly reduces workability, with fly ash-based mixes showing better flowability than slag-based ones. Mechanical properties improved with higher fiber volume, and an optimal content of 2% achieved the best balance between workability and strength. Impact testing and stress–strain analysis revealed that steel fibers enhance energy absorption and toughness under dynamic loading. SEM observations at 3 days indicated initial bonding between the fibers and matrix, with minor microcracks, suggesting further strength development over time. This work offers a fresh understanding of the combined influence of steel fibers and mineral admixtures in UHPC and supports their effective application in durable and cost-efficient high-performance concrete design. Full article

This study investigates the effect of surface oxide control on the phosphatability of ultra-high-strength steel (UHSS) for automotive applications. Surface oxides were manipulated by adjusting the dew point to −50 °C and 0 °C during the annealing process, and the corresponding changes in phosphating behavior were examined. The surface characteristics of the samples were analyzed using X-ray photoelectron spectroscopy (XPS) and field-emission transmission electron microscopy (FE-TEM), while the phosphatability of the samples was evaluated through electrochemical measurements. The sample annealed at a dew point of −50 °C formed continuous Si and Mn oxide films (~10 nm), which significantly suppressed the phosphatability. In contrast, when annealed at 0 °C, internal oxidation occurred along the grain boundaries to a depth of about 3 μm, resulting in the formation of discontinuous Si and Mn oxides on the surface, which greatly enhanced phosphatability. This difference was also supported by OCP measurements: the −50 °C specimen showed a gradual increase in potential, whereas the 0 °C specimen rapidly reached −0.59 V and then stabilized. The findings of this study demonstrate that optimizing the annealing atmosphere provides an effective approach to enhance the phosphating performance of UHSS without the need for additional surface treatments. Full article

Predicting the flexural behavior of fiber-reinforced ultra-high-performance concrete (UHPC) remains a significant challenge due to the complex interactions among numerous mix design parameters. This study presents a machine learning-based framework aimed at accurately estimating the modulus of rupture (MOR) of UHPC. A comprehensive dataset comprising 566 distinct mixtures, characterized by 41 compositional and fiber-related variables, was compiled. Seven regression models were trained and evaluated, with Random Forest, Extremely Randomized Trees, and XGBoost yielding coefficients of determination (R²) exceeding 0.84 on the test set. Feature importance was quantified using Shapley values, while partial dependence plots (PDPs) were employed to visualize both individual parameter effects and key interactions, notably between fiber factor, water-to-binder ratio, maximum aggregate size, and matrix compressive strength. To validate the predictive performance of the machine learning models, an independent experimental campaign was carried out comprising 26 UHPC mixtures designed with varying binder compositions—including supplementary cementitious materials such as fly ash, ground recycled glass, and calcium carbonate—and reinforced with mono-fiber (straight steel, hooked steel, and PVA) and hybrid-fiber systems. The best-performing models were integrated into a hybrid neural network, which achieved a validation accuracy of R² = 0.951 against this diverse experimental dataset, demonstrating robust generalizability across both material and reinforcement variations. The proposed framework offers a robust predictive tool to support the design of more sustainable UHPC formulations incorporating supplementary cementitious materials without compromising flexural performance. Full article

To address the issues of high carbon emissions from concrete and high energy consumption in the manufacturing of traditional steel fibers, this study investigates the feasibility of replacing industrial steel fibers (ISF) with recycled tire steel fibers (RSF) in high-performance cement-based materials. The study examines the effects of fiber type and dosage on the mechanical properties within the systems of ultra-high-performance concrete (UHPC) and slurry-infiltrated fiber concrete (SIFCON) and analyzes the carbon emission levels using the Life Cycle Assessment (LCA) method. Research results indicate that the compressive and tensile strengths of SIFCON are significantly higher than those of UHPC. Under the same conditions, RSF has little difference in tensile performance when compared with ISF, suggesting a great substitution potential. Carbon emission analysis shows that although the total carbon emissions of the SIFCON system are relatively high, its performance improvement is remarkable. Both the carbon emission per tensile strength and carbon emission per compressive strength are lower than those of UHPC, demonstrating a high degree of environmental friendliness. Overall, this study shows that RSF can not only effectively enhance the performance of high-performance cement-based materials but also reduce carbon emissions, making it a reinforcing material with both excellent performance and sustainability. Full article

The CASTRIP process is an innovative method for producing flat rolled low-carbon and low-alloy steel at very thin thicknesses. By casting steel close to its final dimensions, enormous savings in time and energy can be realized. In this paper, an ultra-high-strength low-alloy corrosion-resistant steel was produced through the CASTRIP process. Microstructure and properties were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser confocal microscopy (LSCM), electron backscattered diffraction (EBSD), and tensile testing. The results show that the microstructure is mainly composed of polygonal ferrite, bainite ferrite, and acicular ferrite. The bainite ferrite forms parallel lath bundles nucleating at austenite grain boundaries, propagating perpendicularly into the parent grains. The acicular ferrite exhibits a cross-interlocked morphology preferentially nucleating at oxide/sulfide inclusions. Microstructural characterization confirms that the phase transformation of acicular ferrite and bainite ferrite introduces high-density dislocations, identified as the primary strengthening mechanism. Under the CASTRIP process, corrosion-resistant elements such as Cu, P, Sb, and Nb are completely dissolved in the matrix without grain boundary segregation, thereby contributing to solid solution strengthening. Full article

A novel ultrahigh-strength steel with Co and strengthened through nanoscale precipitation was developed. We found that the Co element had a synergistic effect on the precipitation process. The simulation results indicate that adding Co to steel can suppress the tracer diffusion coefficients of all the elements in the steel, hindering the atomic self-diffusion rate and long-range diffusion effect. A decrease in the atomic diffusion rate of precipitations will affect the nucleation, distribution, and growth of precipitations. The Atom probe tomography (APT) results indicate that the Co element not only dispersed uniformly in the matrix itself but also induced the uniform distribution of the precipitation phases. During the nucleation process of the precipitation, the rejected Co atoms formed small regions of high Co concentrations around the precipitation, inhibiting the coarsening of the precipitation. Under the synergistic effect of Co, the high number density of nanoscale NiAl and M₂C enhanced the strength of the steel. Full article

To develop an ultra-high performance concrete (UHPC) wall structure suitable for nuclear power plant applications, this study establishes a finite element model to evaluate the ultimate bearing capacity of UHPC walls under eccentric compression with single-sided thermal loading during accident conditions. The accuracy and reliability of the finite element analysis (FEA) method were rigorously validated by simulating and replicating experimental results using the same modeling approach adopted in this study. Based on the validated model, the influence of single-sided thermal loading on the ultimate bearing capacity of UHPC walls under nuclear power plant accident conditions was thoroughly investigated. Key parameters—including the reinforcement ratio, steel fiber volume fraction, temperature, eccentricity, and concrete strength grade—were systematically analyzed to determine their effects on the ultimate bearing capacity of UHPC wall specimens. The results demonstrate that the reinforcement ratio, steel fiber volume fraction, temperature, eccentricity, and concrete strength grade significantly affect the degradation rate of the ultimate load of UHPC walls as the temperature increases. Additionally, this paper proposes a calculation method for the normal section bearing capacity of rectangular cross-sections in UHPC large eccentric compression members under single-sided thermal loads. These findings provide theoretical support and scientific evidence for the design of new UHPC structural specimens in nuclear power plants. Full article

In this study, a strategy was adopted to promote the formation of NiAl precipitates with the aim of enhancing strength by incorporating a 0.2 wt.% Al into a traditional ultra-low carbon bainitic (ULCB) steel alloy. By integrating thermo-mechanical control processing (TMCP) and a tailored tempering process, a new-generation steel with an outstanding combination of properties has been successfully developed for shipbuilding and marine engineering equipment. It features a yield strength of 880 MPa, a yield ratio of 0.84, and an impact toughness of 175 J. The precipitation characteristics of nanoscale particles in this steel, including NiAl intermetallics and carbides, were systematically investigated. The results show that the alloy with low Al addition formed NiAl precipitates during tempering. The high-density distributions of NiAl, (Mo, V)C, and (Ti, V, Nb)C precipitates, which exhibit slow coarsening kinetics, played a dominant role in enhancing the strength of the tempered steel. In addition to precipitation, the microstructure before and after tempering was also analyzed. It was observed that a granular bainite morphology was favorable for decreasing the yield ratio. Additionally, the formation of reverse-transformed austenite during tempering was critical for retaining toughness despite substantial strength gains. Finally, theoretical modeling was employed to quantitatively assess the contributions of these microstructural modifications to yield strength enhancement of thermo-mechanical controlled processing (TMCP) and tempered steel. This study establishes a fundamental basis for subsequent industrial-scale development and practical engineering applications of novel products. Full article