Induced Stress Concentrations Research Articles

Cyclic Performance of Prefabricated Bridge Piers with Concrete-Filled Steel Tubes and Improved Bracing Connection Detail

Concrete-filled steel tubes (CFTs) offer significant structural advantages in terms of stiffness, strength, and ductility. The concrete core enhances the stiffness and compressive strength of columns, whereas the steel tube serves as a reinforcement to resist tension and bending by confining the concrete. Moreover, CFT columns offer exceptional resistance, such as high strength, ductility, and energy absorption capacity. This study presents experiments focused on prefabricated bridge piers featuring multiple CFT columns. Commercial circular steel tubes were utilized to streamline fabrication efforts, with bracings employed to enhance structural performance by connecting the CFT columns. Component tests were conducted for different connection details to prevent premature failure owing to cyclic loading. A full-scale modular pier was designed to explore its cyclic behavior using the bracing connection details derived from the component test. The plastification location in a modular pier can be designed using the connection details, as validated experimentally. The results of this study indicate that CFT columns, as the main component of the bridge pier, can be protected by designing connection details to induce stress concentration in the braces, thereby achieving ductile behavior.

Effect of loading area on triaxial compression behavior of microcapsule-based self-healing concrete

Concrete is customarily subjected to a complex triaxial stress state in structures under service. True triaxial experiments using cubic specimens in laboratory settings often utilize shims marginally smaller than the specimen surface for loading purposes. However, the influence of shim dimensions on the triaxial mechanical characteristics of concrete remains obscure. Therefore, in this study, for urea-formaldehyde (UF)/epoxy microcapsule-based self-healing concrete, shims of varying sizes were designed to conduct true triaxial compression experiments, examining the effects of loading area size and microcapsule content on triaxial mechanical behavior. The crack damage morphology was observed using a scanning electron microscope (SEM), the rupture of microcapsules was analyzed through energy-dispersive X-ray spectroscopy (EDS), and the micropore structure was determined via nuclear magnetic resonance (NMR). The research findings reveal that reducing the loading area induces stress concentration near the shim edge, significantly decreasing the first peak strength with a maximum reduction of 37%; while also increasing the second peak strength, with a maximum increase of 52%. Notably, a “sleeve-like” delamination occurs when the loading area is below 95% of the specimen surface area. Incorporating microcapsules into concrete enhances ductility and porosity, achieving a maximum 1.9% increase in first peak strain and 1.98% in porosity. This study may offer valuable insights and data for the optimization of concrete materials and structural design, thereby contributing to the safety and durability of engineering structures.

High-performance flexible sensor with rapidly adjustable sensitivity for wearable electronics

Despite numerous advances in sensor structural design, existing sensors still struggle to achieve rapidly adjustable sensitivity. Here, we utilized a structural design with alternating hard and soft sensor connections, and a lockout device to construct a sensitivity-adjustable sensor (AHS-L). Silicone rubber foams (SF) served as the flexible matrix for fabricating sensors with different hardnesses. Among them, the soft sensor (SRC-ACPF) retained the foam structure, and constructed dual conductive networks both within and on the pore walls, enhancing its resistance sensitivity with a gauge factor (GF) of up to 561.9. In contrast, the SF used in the hard sensor (ASR-PPS) featured a fingerprint-like structure. A conductive network was introduced on the pores and backfilled with liquid silicone rubber to achieve a “soft and hard” distinction from SRC-ACPF. The fingerprint-like structure induced stress concentration during stretching, resulting in high resistance sensitivity for ASR-PPS (GF up to 711.5). This microstructure also amplified contact signals, enabling ASR-PPS to realize intelligent material recognition. Notably, the alternating soft and hard structure of the AHS-L allowed the tensile deformation to be concentrated in the SRC-ACPF. Meanwhile, due to the initial resistance difference, the resistance change in SRC-ACPF has less effect on the overall resistance fluctuation of AHS-L than ASR-PPS. Therefore, a lockout device was introduced to limit the deformation of the SRC-ACPF, allowing the deformation pattern of the AHS-L to be tuned and enabling the sensitivity to be quickly adjusted from low (GF of 3.7 within 0–22% strain, 63.8 within 22–38% strain) to ultra-high (GF of 444.7 within 0–20% strain, 821.2 within 20–46% strain).

Microstructure evolution and mechanical properties of TC4/Ni metallic-intermetallic laminated composites: Formation and strengthening effects of Ti₂Ni phases

TC4/Ni metallic-intermetallic laminated (TN-MIL) composites were prepared by hot pressing at different temperatures and with varying Ni foils thicknesses. The phase formation and Ti₂Ni microstructure evolution were analyzed in detail using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and x-ray diffraction (XRD). The mechanical properties were evaluated through nanoindentation, room-temperature compression, and tensile tests, while fracture morphology was examined by SEM. The results show that as the hot-pressing temperature increases, the Ni₃Ti and TiNi phases at the interface gradually transform into the Ti₂Ni phase. Once the Ni is fully dissolved in the TC4 matrix, the intermetallic compound layer disappears. This diffusion process creates a concentration gradient in TC4. As the Ni concentration increases, the Ti₂Ni phase transitions from an island-like structure to a network, and ultimately forms a lamellar or mixed morphology. The TN-MIL composite prepared at 800°C has three intermetallic compound layers of Ni₃Ti, TiNi and Ti₂Ni, as well as lamellar, network and island-like Ti₂Ni structures in the TC4 matrix. This composite demonstrates excellent mechanical performance, achieving a maximum compressive strength of 1580 MPa and an ultimate tensile strength of 1010 MPa. The Ti₂Ni phase improves the material's deformation resistance due to its high strength and hardness, thereby increasing the overall hardness and strength. However, the inherent brittleness of Ti₂Ni and its propensity to induce stress concentrations at grain boundaries under strain conditions significantly elevate the risk of material fracture.

Three-dimensional peridynamic simulation on triaxial compression failure mechanical behavior of cylindrical marble specimen containing pre-existing fissures

A series of triaxial compression experiments and analyses were conducted out by using the three-dimensional bond-based peridynamics (BB-PD) model to investigate the mechanical behavior, deformability, and failure behavior of marble samples under different confining pressures. This study assessed the impact of confining pressure on mechanical performance of the intact marble, calibrated PD model micro-parameters using experimental data, and predicted the fissure behavior of marble specimens with single or parallel fissures. Through detailed analysis of crack evolution, crack pattern, displacement fields, and principal stress fields, the research revealed that the ductility of marble depended on the confining pressure, especially in the presence of fissures. Increasing confining pressure facilitated a transition from splitting failure to conjugate shear failure modes and induced stress concentration at fissure tips, leading to wing cracks and secondary cracks initiation. The presence and number of fissures were found to directly influence the mechanical behavior and failure process complexity of marble. A higher number of fissures reduced brittleness and strength while enhancing ductility and variety of failure modes. Seven distinct failure patterns of marble were identified under triaxial compression, illustrating the combined effect of confining pressure and fissures on marble specimen failure modes. This work can provide a theoretical and numerical basis for fissured rocks fracture in deep engineering.

Structural optimization and in vitro corrosion analysis of biodegradable Mg-Nd-Zn-Zr alloy clip

Magnesium (Mg) alloy which benefits from biodegradability and mechanical characteristics offers great potential for the development of degradable hemostatic clips. However, the deformation process induces stress concentration, which in turn accelerates the corrosion rate of Mg hemostatic clips. In this study, two types of R-shape clips based on Mg-Nd-Zn-Zr alloy were designed with structural features of no teeth and staggered teeth and simulated using finite element analysis (FEA), and the corrosion behaviors of the Mg clips were investigated by immersion test and electrochemical measurement. Furthermore, the clamping properties of the Mg clips were evaluated by burst pressure test. The simulation results showed that the R-shape clip with staggered teeth caused the minimum stress (1.237 MPa) to blood vessels. After the clamping deformation process, the closed clips remained intact without any signs of cracking. In vitro degradation analysis indicated that the corrosion rate of the closed clip was slightly faster than that of the open clip, and the Mg clip maintained its efficacy in achieving vascular closure even after a 4-week period of immersion, indicating a commendable performance in secure ligation closure. In addition, the burst pressure test results showed that the staggered teeth clip exhibited a higher burst pressure (88.73 ± 2.58 kPa) with less mechanical damage occurring to the ligated vessels compared with the toothless clip, meeting the requirements for clinical application. Therefore, the newly developed R-shape Mg alloy clip, featuring staggered teeth, has demonstrated excellent mechanical stability and shows great promise in the application of biodegradable tissue clips.

Fatigue notch strengthening effect of nickel-based single crystal superalloys under different stress ratios

Nickel-based single crystal superalloys (Ni-SXs) are widely used in turbine blades of aircraft engines. With the increasing demand for higher temperature-bearing capacity, the introduction of film cooling holes, impingement holes, and trailing edge slots for cooling induces stress concentration, which compromises the structural integrity of the blades, generates complex multiaxial stress states, and adversely affects their operational performance. In this study, Ni-SXs, DD6, was utilized to investigate the influence of stress ratios on fatigue performance under complex stress states. Low cycle fatigue (LCF) tests were carried out at different stress ratios with stress concentration coefficient Kt = 1.0, 2.0 and 3.0. The results indicate that the notched specimens exhibit a significant fatigue notch strengthening effect under high stress ratios. The fracture surface and microstructure also indicate that under high stress ratios loading, the notches exhibit significant creep failure characteristics. This means that the main cause of fatigue notch strengthening is similar to creep notch strengthening effect. A macroscopic anisotropic constitutive model coupled with damage was developed and applied to finite element analysis of notched specimens. The results demonstrate that as the stress ratio rises, the stress relaxation effect at the notch becomes more pronounced, yet the level of damage diminishes. Additionally, a stress-equivalent model based on Bridgman stress was proposed, which effectively unifies the lifetime trends of notched and smooth specimens, predicting lifetimes within a threefold error band.

Contribution of spatially variable pitting corrosion on fatigue life prediction of RC beams

Pitting corrosion induces stress concentration in steel bars, significantly reducing the fatigue life of reinforced concrete (RC) beams. Moreover, spatially variable pitting corrosion can cause fatigue fractures in tensile steel bars at various cross-sections of beams, not just at the mid-span section. This will further increase the failure probability of beams but is rarely considered by existing studies. Thus, this study proposes a fatigue life prediction method for RC beams experiencing spatially variable pitting corrosion. First of all, the spatial variability of pitting corrosion in tensile steel bars in RC beams is simulated through the three-dimensional laser scanning method and the spectral representation method, in which the steel bar diameter, length, distance, corrosion level, and the correlation between corrosion pits are systematically considered. Subsequently, the fatigue crack initiation and propagation of pitting corroded steel bars are modeled based on fracture mechanics. The spatial analysis is then conducted on corroded RC beams until fatigue failure occurs. The proposed prediction method is verified with results from existing references. The findings highlight that neglecting the spatial variability of pitting corrosion leads to an overestimation of the fatigue life of corroded RC beams, especially under uniformly distributed loadings and high corrosion levels. This study provides a reasonable prediction for the lifespan of corroded RC beams.

A Novel Flow‐Balanced Friction Stir Channeling Technology: High Rectangularity Design and Channel Formation Mechanisms

Friction stir channeling (FSC) presents a promising approach for achieving continuous channels in heat exchangers, which are considered as leading candidates for high‐efficiency thermal dissipation. However, the irregular cross‐sectional shape of prepared channels induces stress concentration and compromises the load‐bearing capacity and structural integrity. Herein, flow‐balanced FSC (FB‐FSC) technology is proposed to fabricate channels with enhanced rectangularity. This technique allows for precise control over the shape, path, position, and arrangement of the channels. The channel formation in FB‐FSC is affected by material overflow–reflux behaviors and stick–slip conditions. Material overflows onto the shoulder due to shearing and extraction then refluxes under axial pressure to close the channel. The material stuck to the probe rotates accordingly and is pulled out into the channel cavity to form a deposition layer. By regulating channel formation mechanisms, channel rectangularity of 90.8%, continuous formation of 14 m, minimum spacing of 2 mm, variable arc radius, and gradient distribution are directly achieved via FB‐FSC. The FB‐FSC technology enables the efficient formation of cooling channels with high quality, which is significant for the design of heat exchangers to realize desired performances.

Innovative digital twin with artificial neural networks for real-time monitoring of structural response: A port structure case study

Port structures face the constant threat of deterioration due to exposure to dynamic and seismic forces, as well as harsh environmental condition prevalent in a maritime setting. These challenges lead to induced stress concentrations within the structures, resulting in potential catastrophic failures. Despite significant effort, expenses, and time consumption associated with traditional structural health monitoring systems (SHM), regular inspections remain the norm. This study introduces innovative methods to replace traditional approaches, leveraging Artificial Neural Networks (ANNs) as surrogates for real-time finite element (FE) modelling. The goal is to develop FE-based real-time digital twins for port structure by using sensor data, visualizing structural behaviour, and enabling a comprehensive monitoring of its structural response. An ANN model is trained using a validated FE model of the structure, generating immediate deformations based on sensor readings. By implementing this approach, stress variations are efficiently obtained and visualized throughout the structure. Unlike traditional methods that follow an inverse approach in estimating the entire structural response based on sensor values, the ANNs demonstrate high efficiency in addressing ill-conditioning issues inherent in such processes. This integrated methodology showcases the effectiveness of ANNs in providing real-time insights into the structural response of port infrastructure, offering a viable addition to the conventional SHM practices. The trained ANN model generates results in a high accuracy where the testing error is in the order of 10−5 and it generates data within 15 milli seconds demonstrating the near real-time conditions. The development of digital twins, facilitated by ANNs, demonstrates a promising solution for continuous monitoring, predictive maintenance, and risk mitigation in the face of dynamic operational and environmental challenges. The study contributes to the advancement of smart infrastructure by harnessing the capabilities of artificial intelligence and digital twin technology.

Molecular dynamics study of axial tensile response of crystalline ultra-high molecular weight polyethylene under different loading conditions

Through molecular dynamics simulations, we investigated the effects of temperature on the axial tensile behavior of ultra-high molecular weight polyethylene (UHMWPE) crystals, considering the combined effects of transverse compression, strain rate, and molecular weight. The effects of temperature over the range of 100 K to 450 K is shown to reduce the mechanical properties. Existing chain end defects facilitate chain sliding and induce stress concentration in adjacent molecules, thereby reducing the strength and modulus of crystals. Lower molecular weight and higher temperatures promote chain sliding, while higher transverse compression and increased strain rates inhibit chain sliding, resulting in higher properties. Additionally, higher temperatures increase stress concentration due to thermal vibrations, which induce localized high stress conditions within polyethylene chains. The transition of the failure mode from chain sliding to bond breakage occurs at strain rates between 1012 s−1 and 1013 s−1, and is found to be independent of temperature, pressure, and molecular weight. The results are compared to the response of crystals without chain end defects. These insights contribute to a deeper understanding of the behavior of UHMWPE crystals under extreme loading conditions.

The role of mining-induced seismicity amplitude and frequency in gob-side roadway rib spalling

During underground coal mining, Mining-induced seismicity is a crucial factor leading to roadway deformation. Based on a case study of rib spalling caused by mining-induced seismicity in China, this paper utilizes the Universal Distinct Element Code to investigate the mechanism of rib spalling. It analyzes the stress evolution process of roadway surrounding rock and the damage process under the influence of mining-induced seismicity. Additionally, it examines the relationship between seismic wave amplitude, frequency and the damage characteristics of roadway surrounding rock. Findings show that seismic waves induce stress concentration zones in the deeper surrounding rock, causing shear failure, while stress concentration zones emerge in the top, bottom and shoulders of roadway, leading to tensile failure in the shallower surrounding rock. With increasing seismic wave amplitude, the critical crack length of surrounding rock decreases, resulting in a linear increase in crack count and distribution area. The roadway exhibits a ‘n'-shaped damage profile primarily driven by shear failure. With increasing seismic wave frequency, the crack count in the surrounding rock of the roadway exhibits a trend of initially increasing and then decreasing. This trend becomes more pronounced with larger amplitudes. Rib spalling damage is influenced by seismic wave frequency and amplitude: frequencies of 20–25 Hz and 80–100 Hz require amplitudes exceeding 3 m for damage, while 25–30 Hz and 55–80 Hz require amplitudes over 2 m, and 30–55 Hz necessitate amplitudes exceeding 1.5 m.

Flexible dual-mode pressure sensor for simultaneous dynamic-static sensing with wide-range detection and ultra-fast response speed

Flexible pressure sensors play a vital role in advancing the Internet of Things (IoT). However, piezoelectric sensors are limited in static pressure detection and suffer in achieving high flexibility and high sensitivity simultaneously, while piezoresistive sensors typically have slow response speeds and insensitive high-frequency detection ability. Here, we select a novel basalt fiber fabric (BFF) that can withstand high temperatures exceeding 1000 °C as a new substrate to enable one-step fabrication of piezoelectric layer based on coherently grown Pb(Zr0.52Ti0.48)O3 (PZT) thin films via a simple dipping method. The high piezoelectric coefficient g33 of 362.5 mV·m·N−1 and the unique core-sheath structure induced stress concentration on the PZT sheath enable the flexible piezoelectric layer (bending radius < 1.45 mm) to exhibit high sensitivity (voltage sensitivity of 2.766 V/kPa within the range of 0.11–11.11 kPa) and fast response speed (11 ms). The piezoresistive layer consisting of a three-dimensional porous elastic carbon nanotube-polyurethane sponge provides a detection range comparable to piezoelectric layer (0.11–66.67 kPa). A dual-mode flexible pressure sensor (DMFS) has been developed by integrating the piezoelectric layer with the piezoresistive layer. The DMFS exhibits an extended detection range compared to previous dual-mode pressure sensors, rapid response speed, and high sensitivity, which enables simultaneous detection of both dynamic and static pressures. The DMFS can be used for keyboard and movement posture recognition, which has significant application prospects in human-machine interaction.

In-situ intragranular multiple phases reinforced lamellar SiCp/Al-Cu joint induce excellent strength-plasticity without heat treatment

The intragranular second phase is expected to overcome the bottleneck of plasticity in metal matrix composites caused by grain boundary distribution induced stress concentration. In this study, Al3Ti fine second–phase-reinforced SiCp/Al joint was successfully prepared using the laser–friction stir technique. The mechanical properties and deformation mechanism of laminar s SiCp/Al joint was studied in detail. The second-phase particles were encapsulated inside the grains by the rapid solidification characteristic of laser metallurgy. Subsequently, the grains and the second phase underwent further refinement and violent deformation under the friction stir technology. Finally, Al3Ti and α-(Al,Mn) intragranular reinforcement phases were prepared. Tensile property tests results show that the maximum tensile strength of SiCp/Al joint attained is 325 ± 12 MPa and the fracture elongation is 4.49 % ± 0.44 %, which exceeds the reported properties of SiCp/Al joint. This study presents a new strategy for preparing fine second–phase-reinforced metal matrix composites.

Study on removal mechanism of polycrystalline diamond wafer by grinding containing transition metals

Polycrystalline diamond wafers valued for their unique optical properties and thermal conductivity, are optimal for high-technologies applications like optical windows and thermal spreaders. They require a very smooth surface finish, demanding to reach nanometer-level roughness. Their hardness and corrosion resistance, however, make surface grain removal difficult, and large grains can cause stress concentration, which can result in cracking. Traditional CMP faces limitations due to slow oxidizer reactivity and contamination risks. This study exploits the reaction between transition metals and diamond wafers to improve removal rates and grain elimination. Adding Fe, Ti, and Ni reactive abrasives in resin diamond wheels, material removal rate by weighing the weight change and surface roughness was measured using a ZYGO 3D profiler. Fe notably increased removal efficiency and surface quality, indicating that transition metals enhance the transformation of surface grains into amorphous carbon for superior processing.

Effects of precipitation on the stress corrosion crack initiation of alloy 718 in simulated pressurized water reactor primary environment

The effect of precipitation on the stress corrosion cracking (SCC) initiation of Alloy 718 in high temperature hydrogenated water was investigated through slow strain rate tensile (SSRT) test. To clarify the roles of different secondary phases, different heat treatments were applied to separate the precipitation of those phases. There is a tradeoff between the mechanical strength and SCC resistance. Both intergranular δ phase and intragranular γ′/γ′′ phases can enhance the mechanical strength but decrease the SCC resistance of alloy. That is because the precipitates (especially the intergranular δ phase) can be preferentially oxidized due to the high content of active elements. Hence, the grain boundary decorated with δ phase would be significantly degraded. Moreover, both precipitates (especially the intragranular γ′/γ′′ phases) can aggravate the degree of localized deformation. Such enhanced deformation localization can induce stress concentration and promote SCC initiation at grain boundary.

Twin and phase boundaries synergistic effect on multiscale dynamic fracture in Ni-based deformed superalloy

A multi-scale model of dynamic impact fracture of Ni-based deformed superalloy was proposed based on molecular dynamics and finite element methods, and verified by experiments to reveal the synergistic effect of twin and phase boundaries on dynamic fracture properties and behavior from nanoscale to macroscale. The results show that the synergistic effect of twin and phase boundaries can effectively suppress the unfavorable brittle propagation in the nanoscale owing to the dispersed distribution of strengthening phase-induced reduction of the stress concentration on the twin boundary. The nucleation and emission of perfect dislocations, Shockley dislocations, and Frank dislocations promote ductile propagation, while the pile-up of stair-rod dislocations induced stress concentration leads to brittle propagation. In the macroscale, the synergistic effect of twin and phase boundaries can effectively inhibit the loss of dynamic fracture toughness, and impede crack initiation and crack propagation through the reduction of Mises stress concentration and the decrease of maximum equivalent strain caused by the dispersed Ni3Al precipitates. The physical mechanism of the multi-scale crack propagation and fracture, and the influence law of dynamic fracture toughness of Ni-based deformed superalloy, can provide a guide to the failure prediction, protection, and development of aviation turbine blades.

In-situ solid-state deformation-driven rapid reaction towards higher strength-ductility Al-CuO composites

The strength-ductility dilemma of metallic matrix composites is mainly attributed to the mismatched phase/matrix interfaces, which induce stress concentration as dislocation piles up. Here, coherent nano-Al2O3 phases were in-situ formed in Al-CuO composites via rapid diffusion of neoteric deformation-driven metallurgy. The complex dislocation configuration was also promoted to induce the formation of about 14.5 % incoherent twin boundaries. The mean free path for dislocation movements was expanded, as dislocations could cut through the coherent nano-Al2O3 phases and interact with the incoherent twin boundaries without piled-up dislocations. The yield strength and elongation of the strength-ductility synergy Al-5CuO composites were 201 ± 14 MPa and 16.1 ± 0.5 %, indicating that in-situ Al2O3-Al coherent interface and incoherent twin boundaries exert remarkable ability of dislocation transmission.

The Role of Solidified Phases on the Hot Cracking of a Large-Size GH4742 Superalloy Ingot.

The effect of solidified phases on the hot cracking behaviour of a large-size GH4742 superalloy ingot produced using vacuum induction melting (VIM) is investigated in order to improve the quality of the final product. The results show that the solidification order of the ingot is γ matrix, MC carbide, η phase and γ' phase. Among them, the MC carbide and the η phase solidified in the mushy zone. The volume fraction of both the η phase and the MC carbide in the cracked zone is higher than that in the non-cracked zone, and a significant number of η phases are distributed near the hot cracks. The formation of solidified phases not only induces stress concentration at η phase/γ matrix interfaces but also reduces the ability of liquid feeding during solidification, thus promoting hot crack formation. It is believed that by controlling the segregation degree of both Nb and Ti, the volume fraction of η phases and MC carbides can be reduced to prevent hot cracking of the GH4742 superalloy VIM ingot.

Electrochemical shock and transverse cracking in solid electrolytes

Ceramic solid electrolytes are crucial for electrochemical devices, including emerging solid-state batteries. However, they are susceptible to degradation and failure under harsh conditions, leading to dendrite growth, cracking and short circuits. While longitudinal lithium dendrites have been identified as a primary degradation mechanism, recent experiments have revealed transverse reduction fronts and bowl-shaped cracks that differ significantly from the longitudinal picture. We propose an electrochemical shock model to explain these transverse degradation modes in solid electrolytes (SE) and mixed ionic-electronic conductors (MIEC), where SE is taken to be the very weakly electronic leaking limit of MIEC. The model describes a transverse layer with an abrupt oxygen potential jump over a short distance, caused by the electronic transport bottleneck on the Brouwer diagram. Using Li7La3Zr2O12 as an example, we demonstrate that even minor nonuniform lithium distribution associated with an electrochemical shock can induce stress concentrations, resulting in electrolyte cracking and bowl-shaped cracks. The electrochemical shock model highlights the significance of finite electronic conductivity in the degradation of SE and MIEC, providing insights for the design of durable solid electrolytes.