Tensile Stress Research Articles

The selection mechanism of mineral bridges at the interface of stacked biological materials for a strength-toughness tradeoff

The strength-toughness tradeoff in biological materials such as nacre and bone is essentially due to their stacked microstructures formed by hard and soft phases. In some of these materials, purely soft phase acts as interface layers linking hard phases (platelets), while in some others, hard-phase bridges exist in the soft phase to form a hybrid interface. In order to disclose the selection mechanism of such different interface structures in biological materials, a novel shear-lag model with an interface consisting of alternatively distributed elasto-plastic (soft) and brittle-elastic (hard) segments is proposed. Using this model, solutions of tensile stress and tensile displacement in hard platelets and shear stresses in soft and hard interfacial segments are analytically achieved. Effects of the hybrid interface on the effective mechanical performances of the composite are analyzed, the results of which are well consistent with the existing experimental observations in biocomposites and bio-inspired composites. The most important finding is that the fracture strain of the soft phase has a decisive effect on the selection of a purely soft-phase interface or a hybrid interface of hard and soft phases in stacked biological materials in order to realize a tradeoff between strength and toughness. When the failure strain of the soft phase is relatively small, such as nacre, the purely soft-phase interface is too weak to transfer enough load to the platelet, and hard bridges are necessarily required to reinforce the interface and guarantee an efficient load transfer. When the soft phase has a sufficiently large failure strain, such as bone, the purely soft-phase interface is tough enough to sustain a large shear deformation, realizing an efficient load transfer and adequate utilization of all constituents, while an additional hard bridge is not conducive to the composite toughness due to its reducing effect on the interfacial shear deformation. The results not only help people gain a deeper understanding of the secrets behind the construction of different interfaces in biological materials, but also provide useful guidance for interface optimization design in strong and tough artificial materials.

Surface integrity characterization of third-generation nickel-based single crystal blade tenons after ultrasonic vibration-assisted grinding

Machined surface integrity of workpieces in harsh environments has a remarkable influence on their performance. However, the complexity of the new type of machining hinders a comprehensive understanding of machined surface integrity and its formation mechanism, thereby limiting the study of component performance. With increasing demands for high-quality machined workpieces in aerospace industry applications, researchers from academia and industry are increasingly focusing on post-machining surface characterization. The profile grinding test was conducted on a novel single-crystal superalloy to simulate the formation of blade tenons, and the obtained tenons were characterized for surface integrity elements under various operating conditions. Results revealed that ultrasonic vibration-assisted grinding (UVAG) led to multiple superpositions of abrasive grain trajectories, causing reduced surface roughness (an average reduction of approximately 29.6%) compared with conventional grinding. After examining the subsurface layer of UVAG using transmission electron microscopy, the results revealed that the single-crystal tenon grinding subsurface layer exhibited a gradient evolution from the near-surface to the substrate. This evolution was characterized by an equiaxed nanocrystalline layer measuring 0.34 μm, followed by a sub-microcrystalline grain-forming zone spanning 0.6 μm and finally, a constituent phase-twisted distorted deformation zone over 0.62 μm. Under normal grinding conditions, the tenon exhibited low surface hardening (not exceeding 15%), and residual compressive stresses were observed on its surface. In cases where grinding burns occurred, a white layer appeared on the tenon’s surface, which demonstrated varying thicknesses along the teeth from top to root due to thermal-force-structural coupling effects. Additionally, these burns introduced residual tensile stresses on the tenon’s surface, potentially substantially affecting its fatigue life. This paper enhances our understanding of UVAG processes and establishes a foundation for their application in manufacturing single-crystal turbine blades for next-generation aero-turbine engines.

Surface integrity characterization of third-generation nickel-based single crystal blade tenons after ultrasonic vibration-assisted grinding

Machined surface integrity of workpieces in harsh environments has a remarkable influence on their performance. However, the complexity of the new type of machining hinders a comprehensive understanding of machined surface integrity and its formation mechanism, thereby limiting the study of component performance. With increasing demands for high-quality machined workpieces in aerospace industry applications, researchers from academia and industry are increasingly focusing on post-machining surface characterization. The profile grinding test was conducted on a novel single-crystal superalloy to simulate the formation of blade tenons, and the obtained tenons were characterized for surface integrity elements under various operating conditions. Results revealed that ultrasonic vibration-assisted grinding (UVAG) led to multiple superpositions of abrasive grain trajectories, causing reduced surface roughness (an average reduction of approximately 29.6%) compared with conventional grinding. After examining the subsurface layer of UVAG using transmission electron microscopy, the results revealed that the single-crystal tenon grinding subsurface layer exhibited a gradient evolution from the near-surface to the substrate. This evolution was characterized by an equiaxed nanocrystalline layer measuring 0.34 μm, followed by a sub-microcrystalline grain-forming zone spanning 0.6 μm and finally, a constituent phase-twisted distorted deformation zone over 0.62 μm. Under normal grinding conditions, the tenon exhibited low surface hardening (not exceeding 15%), and residual compressive stresses were observed on its surface. In cases where grinding burns occurred, a white layer appeared on the tenon’s surface, which demonstrated varying thicknesses along the teeth from top to root due to thermal-force-structural coupling effects. Additionally, these burns introduced residual tensile stresses on the tenon’s surface, potentially substantially affecting its fatigue life. This paper enhances our understanding of UVAG processes and establishes a foundation for their application in manufacturing single-crystal turbine blades for next-generation aero-turbine engines.

Numerical study of voids geometry effects on sandstone’s mesoscopic fracture Mechanism: Insights from acoustic emission and moment tensor inversion

In the field of rock mechanics, the study of void shape on the mechanical properties and fracture mechanisms of sandstone is crucial for the progression of rock engineering practices. In this study, PFC3D is utilized to conduct numerical simulations of uniaxial compression on intact rock and brittle rocks containing five different shapes of voids (including circular, inverted U-shaped, trapezoidal, rectangular, and square). By employing acoustic emission and moment tensor inversion techniques, the microscopic fracture mechanisms of rocks with different void shapes are revealed. Our key findings reveal: (1) The PFC3D’s Soft-Bond Model is adept at simulating acoustic emissions with high accuracy, facilitating an authentic reproduction of the uniaxial compression process in sandstone, including the nonlinear compaction stage. (2) The presence of hole defects markedly undermines the structural strength and deformability of the sandstone, with circular holes having minimal impact, whereas rectangular holes exert the most significant detriment. (3) A thorough analysis of fracture mechanisms demonstrates that specimens with hole defects primarily undergo initial tensile, remote cracks, V-shaped notch, and shear fractures, with shear fractures being pivotal in leading to specimen failure by rapidly evolving into macroscopic fractures that merge with V-shaped notches. (4) Acoustic Emission (AE) analysis highlights the distribution of tensile stress around the upper and lower extremities of the holes and shear stress along the left and right sidewalls, establishing shear stress as the dominant factor in the failure of specimens with hole defects.

Multiple-stage dynamic responses and failure behaviors of surrounding rocks subjected to development blasting: Exponential and triangular paths

During the development blasting of circular tunnels, the detonation of multiple blastholes arranged on concentric circles induces a complex dynamic response in the surrounding rocks. This process involves multiple blast loadings, static stress unloadings, and stress redistributions. In this study, the dynamic stresses of the surrounding rocks during development blasting, considering multiple blasting-unloading stages with exponential paths and triangular paths (linear simplified paths of exponential paths), are solved based on the dynamic theory and the Fourier transform method. Then, a corresponding discrete element model is established using particle flow code (PFC). The multiple-stage dynamic stress and fracture distribution under different in situ stress levels and lateral coefficients are investigated. Theoretical results indicate that the peak compressive stresses in the surrounding rocks induced by both triangular and exponential paths are equal, while the triangular path generates greater additional dynamic tensile stresses, particularly in the circumferential direction, compared to the exponential path. Numerical results show that the exponential path causes less dynamic circumferential tensile damage and forms fewer radial fractures than the triangular path in the first few blast stages; conversely, it exacerbates the damage and instability in the final blast unloading stage and forms more circumferential fractures. Furthermore, the in situ stress determines which of the two opposite effects is dominant. Therefore, when using overly simplified triangular paths to evaluate the stability of surrounding rocks, potential overestimation or underestimation caused by different failure mechanisms should be considered. Specifically, under high horizontal and vertical stresses, the static stress redistribution with layer-by-layer blasting suppresses dynamic circumferential tensile and radial compressive damage. The damage evolution of surrounding rocks in multi-stage blasting under different in situ stresses is summarized and classified according to the damage mechanism and characteristics, which can guide blasting and support design.

Zwitterionic modification: A strategy to enhance the mechanical properties, lubricity and hemo- and biocompatibility of silicone poly(carbonate urethane urea)

Incorporating zwitterions into the blood-contacting materials has been demon-strated an effective and convenient strategy to minimize protein adsorption and platelet deposition, but is frequently detrimental to the mechanical properties. Herein a feeding 29.0 mol% sulfobetaine (SB) siloxane poly(carbonate urethane urea) (SSiPCUU) was synthesized and evaluated. Although the equilibrium water uptake ratio was increased to 1.20% markedly depressing the hydrolytic degradation resistance and mechanical properties after the zwitterionic modification, its maximum tensile stress and tear strength in the wet state still kept the highest among all the tested samples because the urea groups created therefrom gave rise to the formation of bifurcating hydrogen bonds making a significant contribution to the mechanical properties alongside the unique ion clusters formed from pendant SB species. As a consequence of zwitterionic incorporation, it exhibited an extremely low friction coefficient indicating an excellent lubricity in a wet environment without any post-treatment. Accordingly, both the nonspecific protein adsorption and platelet adhesion were evidently decreased. Moreover, it also depicted a good transparency. The high lubrication performance is desirable for lessening the damage to human blood vessels and a good transparency is favorable to the visualization of the flow of liquid in the catheter. These results illustrated the great potential of the zwitterionic SSiPCUU used in the preparation of balloon catheters, artificial blood vessels, stent coverings and other blood-contacting medical implants and devices.

Influence of shielding gas flow on the TIG welding process using stainless steel 304 material

A common issue encountered with main heat exchanger equipment is improper operation, which can lead to the development of cracks in the stainless-steel pipes. The welding process alters the metal microstructure in the heat-affected zone, thereby affecting the mechanical properties of the welded joint. To mitigate this issue, TIG welding with argon shielding gas is employed. This method helps prevent oxidation and ensures the formation of a stable welding arc in 304 stainless steel, which is renowned for its excellent mechanical properties and corrosion resistance. The objective of this study is to evaluate the impact of variations in shielding gas flow on the mechanical properties of 304 stainless steel plates during the TIG welding process. The aim is to determine the optimal settings for producing robust and long-lasting welded joints. To assess the hardness of the welded joints, we employed a Brinell-type Hardness Tester FB-3000LC machine. A Brinell steel ball indenter measuring 5 mm on the HBW scale and applying a load of 125 Kgf was utilized. At a protective gas flow rate of 8 L/min, the average tensile stress was 44.72 N/mm², strain was 0.177, modulus of elasticity was 2518 MPa, and hardness was 99.712 HBW. Increasing the gas flow rate to 13 L/min resulted in an average tensile stress of 47.50 N/mm², strain of 0.189, elastic modulus of 2525 MPa, and hardness of 105.522 HBW. Further increasing the gas flow rate to 18 L/min led to an average tensile stress of 49.69 N/mm², strain of 0.192, modulus of elasticity of 2597 MPa, and hardness of 106.704 HBW. Based on the research findings, it was observed that the weld area exhibited an increase in hardness values due to the heat generated during the welding process. The use of protective gas flow during welding is deemed effective in producing well-formed welded joints, as it prevents fractures from occurring within the weld area during the tensile test process. The choice of protective gas is determined by the dimensions of the material plate.

Investigation of microstructure evolution and its effect mechanism on high temperature tensile properties of 304L stainless steel processed by laser powder bed fusion

Laser Powder Bed Fusion (LPBF) 304L stainless steel has garnered significant attention due to its unique blend of strength and ductility at room temperature. However, there is limited understanding of its high-temperature tensile properties. In this study, we conducted tensile testing on LPBF 304L at temperatures ranging from 623K to 1023K in its as-built state. Additionally, we performed a comparative tensile test on traditionally hot-rolled (at 1433K) 304L stainless steel under equivalent test conditions. The results revealed that LPBF 304L steel exhibited high ductility at 973K (ultimate tensile stress (UTS) is 257.3 ± 3.6 MPa)-1023K (UTS is 218.7 ± 3.8 MPa), but its strength (yield strength (YS) decreased from 346.0 ± 4.5 MPa to 182.7 ± 3.8 MPa) and plasticity (UTS decreased from 405.7 ± 5.3 MPa to 218.7 ± 6.4 MPa) decreased slowly as the testing temperature increases. We also delved into the microstructure and its impact on high-temperature tensile deformation. The fine grains and ensuing dynamic recovery were found to be responsible for the reduced strength and ductility of LPBFed 304L samples at elevated temperatures. Furthermore, a sawtooth fluctuation emerged in the stress-strain curve at 873K, which could be attributed to dynamic strain aging (DSA) and the interaction between solute elements and dislocations.

Synchrotron X-ray diffraction studies of the internal load transfer in Ni–CrC metal matrix composites

Strengthening in metal matrix composites (MMCs) is primarily due to the load transfer from the compliant matrix to the stiff reinforcement. While internal load transfer has been studied for conventionally manufactured MMCs, the extent to which it may be affected by the preexisting defects in additively manufactured MMCs remain elusive. In this study, we performed uniaxial compression loading on cold sprayed Ni–CrC particulate-reinforced MMC. We observe significantly enhanced strength and ductility compared to the brittle behavior previously reported in tension for the same MMC. In contrast to the absence of load transfer in tension, the presence of initial defects did not preclude the internal load transfer in the composites under compressive loading. In-situ high-energy X-ray diffraction analysis revealed a relatively constant load partitioning in the elastic regime, which aligned well with predictions by the Eshelby's inclusion model. Furthermore, an internal load transfer was observed from the Ni matrix to the CrC reinforcement upon plastic deformation of the Ni matrix. Finite element modeling further confirmed this and demonstrated that localized tensile stress at the interface in the transverse direction resulted in the interfacial debonding and partial relaxation of the matrix. We also report reinforcing particle fracture upon further compression which in turn triggered the eventual failure of the MMC.

Research on the damage mechanism of the rolling contact fatigue of a wheel–rail contact system under the influences of surface defects

Severe wheel–rail rolling contact fatigue (RCF) can be affected by the surface defects, which threaten the operational safety of rail vehicles. To explore the RCF damage mechanisms of the wheel–rail under the influences of surface defects, an indentation device was used to prepare surface defects. Twin-disc RCF and wear tests and finite element analyses (FEAs) were conducted. The influence of surface defects on wheel–rail RCF damage were explored by observing the surfaces and cross-sections of defects. The results show that as the number of cycles increases, an RCF crack is generated in the maximum stress concentration area of the defect, i.e., the region near the middle edge, and it expands in the form of a “C” shape. The leading edge of the defect spalls first due to the large number of laminar cracks. As the cracks continue to propagate, large spalls can be seen. The FEA results show that the residual stress distributed on the shoulder around the defect is the largest. The maximum residual tensile stress is located on the lower surface of the defect and the largest stress concentration is located on both sides of the middle edge of the defect when the wheel–rail rolls.

Sustainable microwave processing and surface characterization of powdered tungsten reinforced copper metal matrix (Cu-Wx) castings

The present work focuses on in-situ melting and casting of powdered copper metal matrix composites using a microwave hybrid heating approach. The composites of copper metal matrix are developed in such a way that the conductivity is least affected and related mechanical properties like flexural strength, tensile strength and hardness can be improved. Present work reports the processing and characterization of tungsten (W) powder reinforcement copper metal composites with weight percentages of W varied from 1%, 3% and 5%. The complete mechanism of interactions of Cu-Wx (x = 1%, 3% and 5% weight of copper powder) based composite powders with microwave heating is explained. The processed composite castings are characterized metallurgically through metallurgical microscope, SEM/EDS and XRD analysis; which reveals even dispersion of W particles in Cu matrix phase with some agglomerations. Some intermetallic formations are observed due to intense heating characteristics of microwaves. Further, mechanical and electrical characterizations of developed castings are carried out through Vicker's microhardness testing and electrical conductivity tests respectively. Addition of reinforced particles in metal matrix leads to an increase in microhardness on the expense of conductivity of composite. The 5% W addition led to 1.5 times increase in microhardness and 18.25% increase in resistivity. The mechanical testing revealed that the 5% W reinforced casting led to ∼22% higher ultimate tensile stress in comparison to pure copper casting. However, there was a significant decrease in ductility of metal matrix composites.

Study of damage mechanism on single crystal 4H-SiC surface layer by picosecond laser modification (PLM)

The stable structure, high hardness, and brittleness of single-crystal 4H-SiC present challenges in achieving efficient and damage-free polishing processing. Ultra-short pulsed laser-induced surface modification provides a new solution to enhance the manufacturability of 4H-SiC. This paper aims to investigate the underlying mechanism of the picosecond laser-induced surface structural changes in 4H-SiC and the process of material removal from solid to vapour, elucidating the interaction mechanism between picosecond laser and 4H-SiC. A temperature gradient distribution model for picosecond laser irradiation on 4H-SiC was built to reveal the formation mechanism of subsurface crack damage. The results show that a combination of phase explosion and thermal effects controlled picosecond laser-modified 4H-SiC surfaces deposited spherical SiO2 particles and the process. The study demonstrates that the cracking behaviour in the subsurface of picosecond laser-modified SiC predominantly occurs in the recast region. The fundamental cause of cracking is attributed to the tensile stresses generated by thermal effects and the volumetric changes in the molten material resulting from the overlapping of neighbouring spots in alternating hot–cold cycles. The nanoindentation demonstrated that laser modification can effectively enhance the machinability of SiC. The findings of this study can provide a theoretical basis for efficient non-destructive machining of hard and brittle materials.

Study on the Tensile Failure Characteristics and Energy Calculation Model of Coal Seam Hard Roof Considering the Mining Speed

To reveal the influence mechanism of mining speed on roof fracture-type rockburst, the Brazilian split technique combined with acoustic emission monitoring technology was employed to study the effects of loading rates on the tensile failure characteristics and acoustic emission parameters of coal series sandstone. The linear relationship between the tensile strength of the samples and the change rate of tensile stress was determined. The mining speed was introduced into the mechanical model of initial and cyclic fracture of the hard roof, and the quantitative relationship between the maximum rate of change of tensile stress within the hard roof and the mining speed was derived. Based on this, a computational model for the bending elastic energy of the hard roof during initial and cyclic fractures, considering the mining speed, was established. The main findings are as follows: As the loading rate increases, the distribution range of acoustic emission energy in sandstone Brazilian split samples before failure widens, with a significant rise in acoustic emission ring-down counts and energy at failure. At lower loading rates, acoustic emission events primarily occur near sample failure, whereas at higher rates, they mostly happen in the early loading stage. The higher the mining speed, the less opportunity there is for internal micro-fractures to develop and expand before the hard roof fractures, which macroscopically results in increased tensile strength and a larger amount of energy released at the moment of fracture. Bending elastic energy rises approximately linearly with mining speed, and the thicker the hard roof, the more sensitive the bending elastic energy is to changes in mining speed. This effect is even more pronounced during cyclic fractures. Optimizing mining speed is crucial for preventing roof fracture-type rockbursts, especially in mining workfaces with thick and hard roofs.

3D Printing and Implementation of Engineered Cementitious Composites - A Review

While 3D printing of concrete (3DCP) has gained increasing interest in the construction industry, steel reinforcement remains a significant obstacle to 3D printing (3DP) construction. To address this concern, Engineered Cementitious Composites (ECC), also recognized as Strain-Hardening Cementitious Composites (SHCC), can provide structural performance and integrity, safety, durability, and strength without steel reinforcement. The article reviews scientific works on 3DCP using ECC and proposes further investigations to lead to better development. As a result, generally, Poly-Ethylene (PE) fibers are used more frequently because of their strength. Mix design parameters have been extensively examined in relation to fresh ECC rheological characteristics. Due to the printing process, fiber orientation may affect ultimate tensile strain. As compared to casted ones with random fiber orientation, fiber orientation aligned with tensile stress resulted in a higher ultimate tensile strain. Additionally, research showed that ECC including up to 2 % fiber can be mixed, extruded, and built. Morovere, results highlighted the comparison between printed ECC containing PVA and PE fibers, the influence of mix design parameters on extrudability, and the impact of fiber length and volume fraction on strain-hardening properties. The text also covers the effects of fiber orientation and nozzle distance on tensile performance and ultimate tensile strain, as well as the anisotropic properties of 3DP-ECC. As well as this, there are some areas that require further research, such as durability and response to a variety of loading conditions, such as seismic loading.

Wire and arc additive manufacturing for strengthening of metallic components

Wire and arc additive manufacturing (WAAM), also known as wire arc directed energy deposition (WA-DED), offers valuable capabilities not only for manufacturing but also for strengthening and repairing aging components. This paper employs the finite element (FE) method to investigate the influence of deposition parameters on the strengthening efficiency of damaged steel plates strengthened by WAAM material. The study calibrates the inherent strain method (ISM) with thermo-mechanical analysis to accurately predict residual stresses (RS) in manufactured samples, demonstrating that the ISM enhances computational efficiency while effectively predicting RS. It thoroughly examines key parameters such as the deposition direction, maximum thickness, and the geometric configuration of the WAAM material, including shapes and in-plane dimensions. The results indicate that deposition perpendicular to the loading direction provides better performance compared to deposition along the loading direction as it induces less normal and through-thickness stresses. Furthermore, this research determines the optimal maximum thickness for the WAAM material, showing that an increase in thickness can lead to higher maximum tensile stresses at the interface between the newly WAAM material and the underlying base plate. The study also establishes the optimal in-plane dimensions for the WAAM material. The results suggest placing the maximum thickness of WAAM material near the damaged area and gradually decreasing it in two directions to ensure sufficient stiffness around the cracked area, while avoiding an abrupt change in stiffness. This approach generates appropriate compressive stresses around the crack tip and decreases maximum tensile stresses in the plate. The study further illustrates that employing a proper printing strategy without a subsequent machining process can effectively reduce the maximum tensile stress in the steel plate while minimizing material usage.

Biodegradable Polyurethane Derived from Hydroxylated Polylactide with Superior Mechanical Properties.

Developing biodegradable polyurethane (PU) materials as an alternative to non-degradable petroleum-based PU is a crucial and challenging task. This study utilized lactide as the starting material to synthesize polylactide polyols (PLA-OH). PLA-based polyurethanes (PLA-PUs) were successfully synthesized by introducing PLA-OH into the PU molecular chain. A higher content of PLA-OH in the soft segments resulted in a substantial improvement in the mechanical attributes of the PLA-PUs. This study found that the addition of PLA-OH content significantly improved the tensile stress of the PU from 5.35 MPa to 37.15 MPa and increased the maximum elongation to 820.8%. Additionally, the modulus and toughness of the resulting PLA-PU were also significantly improved with increasing PLA-OH content. Specifically, the PLA-PU with 40% PLA-OH exhibited a high modulus of 33.45 MPa and a toughness of 147.18 MJ m-3. PLA-PU films can be degraded to carbon dioxide and water after 6 months in the soil. This highlights the potential of synthesizing PLA-PU using biomass-renewable polylactide, which is important in green and sustainable chemistry.

Highly-Aligned All-Fiber Actuator with Asymmetric Photothermal-Humidity Response and Autonomous Perceptivity.

Soft robots adapt to complex environments for autonomous locomotion, manipulation, and perception are attractive for robot-environment interactions. Strategies to reconcile environment-triggered actuation and self-powered sensing responses to different stimuli remain challenging. By tuning the in situ vapor phase solvent exchange effect in continuous electrospinning, an asymmetric highly-aligned all-fiber membrane (HAFM) with a hierarchical "grape-like" nanosphere-assembled microfiber structure (specific surface area of 13.6m2g-1) and excellent mechanical toughness (tensile stress of 5.5MPa, and fracture toughness of 798KJm-3) is developed, which shows efficient asymmetric actuation to both photothermal and humidity stimuli. The HAFM consists of a metal-organic framework (MOF)-enhanced moisture-responsive layer and an MXene-improved photothermal-responsive layer, which achieves substantial actuation with a bending curvature up to ≈7.23cm-1 and a fast response of 0.60cm-1s-1. By tailoring the fiber alignment and bi-layer thickness ratio, different types of micromanipulators, automatic walking robots, and plant robots with programmable structures are demonstrated, which are realized for self-powered information perception of material type, object moisture, and temperature by integrating the autonomous triboelectric effect induced by photothermal-moisture actuation. This work presents fiber materials with programable hierarchical asymmetries and inspires a common strategy for self-powered organism-interface robots to interact with complex environments.

Study of Phase Transformations and Interface Evolution in Carbon Steel under Temperatures and Loads Using Molecular Dynamics Simulation

Carbon steel materials are widely used in mechanical transmission. Under different temperature and pressure service conditions, the microscopic changes of stress and strain that are difficult to detect and analyze by experimental means will lead to failure deformation, thus affecting their operational stability and life. In this study, the molecular dynamics method is used to simulate the heating–cooling phase transition process of common carbon steel materials. Austenite transformation temperatures of 980 K (0.2 wt.%) and 1095 K (0.5 wt.%) are acquired which is determined by the volume hysteresis before and after transformation, which is consistent with the results of JMatPro phase diagram analysis. The internal stress state of the material varies between compressive stress and tensile stress due to the change of phase structure, and the dislocation characteristics during the phase transition period are observed to change significantly. Then, an α/γ two-phase interface model is constructed to study the migration of the phase interface and the change of the phase structure by applying a continuously changing external load. At the same time, the transition pressure of α→ϵ is obtained with a value of 37 GPa under three different initial loads showing the independence of the initial load and the historical path. Based on the molecular dynamics simulation and the phase diagram calculation of the carbon steel, the analysis method for the microstructure transformation and the stress–strain behavior of the phase interface under the external load can provide a reference for the design of microstructure and mechanical properties of alloy steel in the future.

A case study on early-age cracking of high-strength concrete construction by coupled thermal-mechanical analysis and field monitoring

The use of high-strength concrete (HSC) in constructing high-rise reinforced concrete buildings and their deep foundations is becoming more popular because of the significant reduction in member size and, thus, self-weight in structural members. However, the higher cement content in HSC may result in increased heat of hydration and autogenous shrinkage, growing concerns about forming early-age cracks in HSC structural members. This case study assessed the early-age cracking of HSC in bored pile construction of high-rise buildings. A nonlinear transient coupled thermal-mechanical response analysis was performed to evaluate the potential of early-age cracking caused by the heat of hydration generated in the HSC using ABAQUS and two subprograms, USDFLD and UEXPAN. A 3-D finite element model was developed considering thermal effects, time-dependent evolution of early-age strength and stiffness of HSC, crack strain development, autogenous shrinkage, and stress-relaxation in the analysis. Field measurements were carried out on a 3-m diameter bored pile constructed using Grade C50/60 HSC (fck,cube = 60 MPa) for a high-rise building project in Hong Kong to evaluate the validity of the numerical model. Optical fiber sensors were used to monitor the temperature changes of the pile for approximately 380 hours after casting. The temporal and spatial temperature distributions measured in the field measurements agree well with the simulations. Large thermal gradients and high tensile stress zones were observed across the horizontal cross-sections of the pile, resulting in the formation of vertical annular cracks. Based on the results of the analysis, the maximum crack widths were predicted and compared with the durability crack width limits.

Effect of stress control by growth adjustment on the edge thread dislocation density of AlN grown on the sapphire

The relationship between stress and dislocation density in MOCVD epitaxial AlN was studied. It has been found that the aluminum nitride (AlN) epitaxial layer generates tensile stress when the crystal islands are merged. By controlling the size and density of crystal islands at the end of 3D growth, the tensile stress generated during epitaxy can be effectively reduced. Mechanical calculations show that there is a linear relationship between the edge thread dislocations density of AlN and the tensile stress during growth. By controlling the stress during AlN growth below 0.1 Gpa, a high-quality AlN sample with an edge thread dislocation density of 6.31 × 107 cm−2 was obtained.