Induced Stress Concentrations Research Articles

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.

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.

Tensile Failure Behaviors of Adhesively Bonded Structure Based on In Situ X-ray CT and FEA.

Adhesive bonding plays a pivotal role in structural connections, yet the bonding strength is notably affected by the presence of pore defects. However, the invisibility of interior pores severely poses a challenge to understanding their influence on tensile failure behaviors under loading. In this study, we present a pioneering investigation into the real-time micro-failure mechanisms of adhesively bonded structures using in situ X-ray micro-CT. Moreover, the high-precision finite element analysis (FEA) of stress distribution is realized by establishing the real adhesive layer model based on micro-CT slices. The findings unveil that pores induce stress concentration within the adhesive layer during the tensile process, with stress levels significantly contingent upon pore sizes rather than their specific shapes. Consequently, larger pores initiate and propagate cracks along their paths, ultimately culminating in the failure of adhesively bonded structures. These outcomes serve as a significant stride in elucidating how pore defects affect the bonding performance of adhesively bonded structures, offering invaluable insights into their mechanisms.

Microstructural characteristics and bending properties of extruded SEN9 and SEN11 magnesium sheets

Mg–Al-based alloys with small amounts of Ca and Y, referred to as stainless, environment-friendly, nonflammable (SEN) alloys, exhibit significantly improved resistance to corrosion and ignition compared to commercial Mg alloys. This study investigates the microstructural characteristics and bending properties of extruded sheets of Mg–(9, 11)Al–1Zn–0.3Mn–0.3Ca–0.2Y alloys (SEN9 and SEN11). These sheets contain particles containing Ca or Y, such as Al2Y, Al2Ca, Al8Mn4Y, and Al–Ca–Zn phases, along with dynamically precipitated Mg17Al12 particles. In the SEN9 sheet, Mg17Al12 particles are distributed in a band-like pattern along the extrusion direction, whereas in the SEN11 sheet, a more uniform distribution of abundant Mg17Al12 particles is observed. In both sheets, a macrocrack initiates on the bottom surface of a specimen and propagates into the specimen at an angle of approximately 35° during three-point bending tests. The SEN11 sheet exhibits a higher bending yield strength (by 27%) than the SEN9 sheet, primarily due to the stronger grain-boundary and particle hardening effects in the former, resulting from finer grains and a greater number of particles. In the SEN9 sheet, sharp deformation twins and subsequent cracks along these twins are formed during bending. Meanwhile, in the SEN11 sheet, the numerous Mg17Al12 particles induce stress concentration during bending, ultimately leading to premature bending failure before the formation of twins. As a result, the SEN11 sheet exhibits a 59% lower bending formability compared to the SEN9 sheet.

Development of shape memory alloy fiber reinforced debondable adhesive

This study introduces an innovative debondable adhesive approach by incorporating shape-memory alloy (SMA) fibers within an epoxy adhesive matrix. Employing meticulous techniques, such as winding machinery and other apparatuses, exceedingly thin Nitinol SMA fibers have been successfully integrated into the adhesive layer. Notably, this adhesive exhibits the dual functionality of a fiber-reinforced composite at operational temperatures, conferring heightened joint strength. Upon disassembly at elevated temperatures, the shape-memory effect of the fibers induces stress concentration within the adhesive layer, thereby facilitating effortless debonding and significantly enhancing the recycling efficacy.Various configurations of fiber volume fractions (2.5 %, 5.0 %, 10.0 %) and orientations (0°/0°, 0°/90°, 90°/90°) were explored to assess their impact on joint strength across ambient and elevated temperatures. The investigation reveals that the 0°/0° fiber orientation yields optimal debondable adhesive characteristics. Additionally, higher volumetric percentages generally correlate with improved adhesive strength and debonding capability. This study further elucidates the underlying mechanisms governing the observed effects on joint debonding strength.

Effect of machined surface integrity on fatigue resistance of aluminum-silicon alloy ZL109

In this paper, the fatigue resistance of aluminum-silicon (Al-Si) alloy specimens obtained by single-step feed forward cutting (PZ), step-by-step feed forward cutting (FZ) and step-by-step forward-reverse finish cutting (FZF) was investigated based on the characterization of surface integrity (roughness, residual stress, microhardness), using a combination of turning experiments, surface integrity analysis and fatigue performance analysis. The fatigue life of the specimens was tested at room temperature and high cycle under the three machining processes, and an empirical model of the fatigue life was established and analyzed for the fatigue fracture morphology. The results showed that the fatigue life of the specimens can be improved by changing the surface integrity such as increasing the microhardness, reducing the surface roughness, and increasing the surface residual compressive stress. Compared with PZ, the fatigue life of FZ and FZF samples increased by 61.8 % and 64.1 % respectively. And it is obtained by analyzing experimental result and fatigue fractures that the relatively poor surface topography (i.e., the larger surface roughness) could induce stress concentration and accelerate fatigue crack initiation, the compressive residual stress mainly inhibited the crack propagation process to improve fatigue strength, and the work hardening could restrain the initiation of fatigue cracks.

Improving the interfacial bonding strength and suppressing shrinkage porosity of 6061Al/GFRTP hot-pressing joints via interfacial modification

The application of metal-plastic hybrid structures is of great significance for structural lightweight. The metal surface treatment is critical for metal-plastic joints. In this study, nanosecond laser was used to construct different microscale grooves on 6061 aluminum alloy (6061Al) surface to realize stable joints with glass fiber reinforced thermoplastic composites (GFRTP) by hot-pressing joining. The impacts of nanosecond laser processing on interfacial bonding of 6061Al/GFRTP were studied using analytical methods and finite element simulations. The results showed that the mechanical interlocking effectively improved the strength of connections. Moreover, the laser texturing patterns had different effects on promoting the wettability of metal surface and improving the heat conduction path. During the hot-pressing joining process, the poor wettability of molten GFRTP on untextured metal surface and different solidification rates at various molten positions led to the formation of shrinkage, which reduced the bonding area and induced stress concentration. When the pitch distance of textured grooves was 0.3 mm, the porosity was reduced to 1.15 %, and the corresponding maximum bonding strength was 9.78 MPa.

Effect of Stress Ratio K due to Varying Overburden Topography on Crack Intensity of Tunnel Liner

A structural health assessment of the tunnel lining is carried out to investigate and mitigate the cause of defects in the tunnel lining to ensure user safety. Many factors affect the tunnel’s integrity, causing a wide range of defects. Identifying the possible cause and its relation with the degree of associated defect is essential for tunnel stability. In conjunction with defects of tunnel failure, liner cracks were found to be most critical because they could trigger other defects to form and jeopardize tunnel stability. Thus, this research acquires visual inspection followed by comprehensive mapping and quantification of liner cracks to enhance our understanding of the effect of varying overburden on the intensity of liner cracks. The numerical analysis was further performed to analyze the influence of varying topography (overburden) on the crack intensity of the liner using the stress ratio, K as a parameter in terms of the state of stress using induced stress (IS) and stress concentration factor (SCF) as a performance evaluation criterion. The work demonstrates that the shift in magnitude and position of liner cracks on the different portions of the tunnel liner implies the effect of a regionally external factor (varying overburden depth) on tunnel degradation. For advances in tunnel assessment based on the regional diversity of cracks along the longitudinal profile of the tunnel, the tunnel should be divided into zones based on the overburden to identify the varying intensity of the liner crack. This study shows the variable crack density of 575 m to 628 m, 650 m to 700 m, and 704 m to 724 m for shallow, intermediate, and deep zone, respectively. The findings of this study can provide scientific guidance to tunnel inspectors and engineers regarding the tunnel liner health assessment and identifying the subsequent method of repair.

The Interconnection of Carbon Active Addition on Mechanical Properties of Hybrid Agel/Glass Fiber-Reinforced Green Composite.

Nowadays, the hybridization of natural and glass fiber has promised several advantages as a green composite. Nevertheless, their different characteristics lead to poor mechanical bonding. In this work, agel fiber and glass fiber was used as reinforcements, and activated carbon filler was added to the polymer matrix of a hybrid composite to modify its characteristics and mechanical properties. A tensile and bending test was conducted to evaluate the effect of three different weight percentages of activated carbon filler (1, 2, and 4 wt%). Vacuum-assisted resin infusion was used to manufacture the hybrid composite to obtain the high-quality composite. The results have revealed that adding 1 wt% filler yielded the most optimum result with the highest tensile strength, flexural strength, and elastic modulus, respectively: 112.90 MPa, 85.26 MPa, and 1.80 GPa. A higher weight percentage of activated carbon filler on the composite reduced its mechanical properties. The lowest test value was shown by the composite with 4 wt%. The micrograph observations have proven that the 4 wt% composite formed agglomeration filler that can induce stress concentration and reduce its mechanical performance. Adding 1 wt% filler offered the best dispersion in the matrix, which can enhance better load transfer capability.