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The Alessandrina Library was founded in 1667 by pope Alexander VII Chigi and is nowadays housed in the Campus of Sapienza University of Rome (Italy). Within its Ancient (mostly made of rag paper) and Modern (mostly made of contemporary paper) collections, it includes more than one million books produced from the XVI to the XXI century. In 2019, six thermo-hygrometers were deployed in its multi-storey repository to monitor temperature (T) and relative humidity (RH). Hourly T and RH data collected over 2 years allowed us to evaluate spatial and temporal thermo-hygrometric distributions and to carry out a comprehensive assessment of the climate-induced risks (mechanical, chemical, and biological deterioration mechanisms). Vertical temperature gradients associated with unstable conditions occurred in winter, resulting in upraising air flows up to the ceiling. The risky short-term RH fluctuations (EN 15757:2010) were determined to avoid mechanical stress in case of loans, relocation, and consultation. The Time Weighted Expected Lifetime (TWEL) index was used to evaluate the chemical risk for different paper-based collections as a function of their acidity and degree of polymerisation, also considering the typical response time of paper books to T and RH changes. The TWEL calculation estimated that the durability of acidic paper was around 300 years and highlighted that rag paper could be subject to cellulose hydrolysis only in summer and autumn, while contemporary paper was mostly at no risk. The risk of mould germination (Sedlbauer diagram) was possible on few days in Autumn, while the production of insect eggs (Brimblecombe empirical function) was favoured during approximately 42% of time over the year. In addition, illuminance and colorimetric measurements (performed on selected book covers) showed that light-sensitive objects could be exposed to the photodeterioration risk in the east-facing side of the repository. Although the investigation focussed on a specific case study, a similar approach could be effectively adapted to most library and archival repositories conserving paper-based collections. [ABSTRACT FROM AUTHOR]

The Journal of Materials Science awards the annual Robert W. Cahn Prize to recognize the best paper published in the journal each year. The winner of the 2023 Cahn Prize is a research paper titled "Molecular modelling of graphene nanoribbons on the effect of porosity and oxidation on the mechanical and thermal properties." The paper presents molecular simulation models of graphene nanoribbons and examines the impact of porosity and oxidation on their mechanical and thermal conductivity properties. The paper is well-written and provides valuable insights for researchers in this field. Two other papers were selected as runners-up for the prize. All the monthly finalists for the Cahn Prize can be downloaded for free on the journal's website. [Extracted from the article]

Mechanical fields over thin elastic surfaces can develop singularities at isolated points and curves in response to constrained deformations (e.g., crumpling and folding of paper), singular body forces and couples, distributions of isolated defects (e.g., dislocations and disclinations), and singular metric anomaly fields (e.g., growth and thermal strains). With such concerns as our motivation, we model thin elastic surfaces as von Kármán plates and generalize the classical von Kármán equations, which are restricted to smooth fields, to fields which are piecewise smooth, and can possibly concentrate at singular curves, in addition to being singular at isolated points. The inhomogeneous sources to the von Kármán equations, given in terms of plastic strains, defect induced incompatibility, and body forces, are likewise allowed to be singular at isolated points and curves in the domain. The generalized framework is used to discuss the singular nature of deformation and stress arising due to conical deformations, folds, and folds terminating at a singular point. [ABSTRACT FROM AUTHOR]

Mechanical fields over thin elastic surfaces can develop singularities at isolated points and curves in response to constrained deformations (e.g., crumpling and folding of paper), singular body forces and couples, distributions of isolated defects (e.g., dislocations and disclinations), and singular metric anomaly fields (e.g., growth and thermal strains). With such concerns as our motivation, we model thin elastic surfaces as von Kármán plates and generalize the classical von Kármán equations, which are restricted to smooth fields, to fields which are piecewise smooth, and can possibly concentrate at singular curves, in addition to being singular at isolated points. The inhomogeneous sources to the von Kármán equations, given in terms of plastic strains, defect induced incompatibility, and body forces, are likewise allowed to be singular at isolated points and curves in the domain. The generalized framework is used to discuss the singular nature of deformation and stress arising due to conical deformations, folds, and folds terminating at a singular point. [ABSTRACT FROM AUTHOR]

The Fenshui River Bridge is a steel–concrete composite girder bridge with a main span of 90 m. Due to the topographical constraints, the steel box girder was constructed without the use of auxiliary piers, employing incremental launching techniques. This article focuses on the construction technology used for the steel box girder of the Fenshui River Bridge. Firstly, using the influence matrix method, the cable force is determined based on the maximum cantilever state of the structure, with the vertical deformation of the front end of the guide beam and the horizontal deformation of the top of the tower as the control objectives. The unstressed cable length is then calculated based on the mechanical relationship between cable deformation and cable force. A calculation method for adaptive cable force is proposed, which is based on the variation of the stress-free cable length within the adaptive structural system. Next, the finite element analysis method was employed to determine the optimal layout position for the tower. The results indicate that during the incremental launching construction of the steel box girder, the calculated cable forces using the method proposed in this paper are in close agreement with the measured cable forces. At the maximum cantilever state of the structure, the calculated and measured values of the cable force resulted in a percentage difference of 3.96%. The calculated values of deformation and stress in key sections showed a percentage difference of 6.4% for deformation and 6.6% for stress. To maximize the effectiveness of the tower and cable, the tower should be positioned above the bridge pier when the guide beam crosses the maximum span. The findings of this paper can serve as a reference for the construction of similar types of bridges. [ABSTRACT FROM AUTHOR]

Deep rock are often in a true triaxial stress state. Studying the impacts of varying unloading speeds on their strain energy (SE) density is highly significant for predicting rock stability. Through true triaxial unloading principal stress experiments and true triaxial stress equilibrium unloading experiments on sandstone, this paper proposes a method to compute the SE density in a true triaxial compressive unloading principal stress test. This method aims to analyze the SE variation in rocks under the action of true triaxial unloading principal stresses. Acoustic emission is used to verify the correctness of the SE density calculation method in this paper. This study found that: (1) Unloading in one principal stress direction causes the SE density to rise in the other principal stress directions. This rise in SE, depending on its reversibility, can be categorized into elastic and dissipated SE. (2)When unloading principal stresses, the released elastic SE density in the unloading direction is influence by the stress path and rate. (3) The higher the unloading speed will leads to greater increases in the input SE density, elastic SE density, and dissipative SE density in the other principal stress directions. (4) The dissipated SE generated under true triaxial compression by unloading the principal stress is positively correlated with the damage to the rock; with an increase in unloading rate, there is a corresponding increase in the formation of cracks after unloading. (5) Utilizing the stress balance unloading test, we propose a calculation method for SE density in true triaxial unloading principal stress tests. [ABSTRACT FROM AUTHOR]

With the continuous construction of urban traffic roads, more and more new roads are cut off by existing roads to form "dead end roads". There is an urgent need for a trenchless method suitable for urban ultra-shallow overburden to build the undercrossing tunnel. To solve this problem, this paper proposed the micro pipe jacking and joint assembly structure (MPJ & JAS) method, which has the characteristics of shallow burial depth, low cost, short construction time, flexible cross-section setting and high space utilization. The MPJ & JAS method construct a large cross-section tunnel through assembling small cross-section elements, quite different from traditional methods. Therefore, this paper designed a CT-shaped integrated joint, the mechanical performance of which was verified and clarified by tensile test. The bending test and finite element (FE) analysis proved the reliability of MPJ & JAS tunnel structure, and confirmed the structure performances such as the failure models, crack behaviors, load–deflection response and stress–strain distribution. Moreover, the influences of the steel plate thickness, concrete strength and shear connector spacing were determined by the FE analysis. On the basis of test results and reasonable assumptions, a theoretical design method considering the influence of the CT-shaped integrated joint was proposed, which can effectively predict the bending strength of the MPJ & JAS tunnel structure with an error of less than 10%. Finally, in view of the characteristics of the MPJ & JAS method, the suitable micro pipe jacking machine, soil reinforcement measure, hydraulic traction construction technology, high-precision guidance system and concrete construction quality detection method based on the phased array ultrasonic imaging technology were developed, supporting the accurate and efficient construction of the MPJ & JAS tunnel. [ABSTRACT FROM AUTHOR]

This paper introduces a metamodelling technique that employs gradient-enhanced Gaussian process regression (GPR) to emulate diverse internal energy densities based on the deformation gradient tensor F and electric displacement field D 0 . The approach integrates principal invariants as inputs for the surrogate internal energy density, enforcing physical constraints like material frame indifference and symmetry. This technique enables accurate interpolation of energy and its derivatives, including the first Piola-Kirchhoff stress tensor and material electric field. The method ensures stress and electric field-free conditions at the origin, which is challenging with regression-based methods like neural networks. The paper highlights that using invariants of the dual potential of internal energy density, i.e., the free energy density dependent on the material electric field E 0 , is inappropriate. The saddle point nature of the latter contrasts with the convexity of the internal energy density, creating challenges for GPR or Gradient Enhanced GPR models using invariants of F and E 0 (free energy-based GPR), compared to those involving F and D 0 (internal energy-based GPR). Numerical examples within a 3D Finite Element framework assess surrogate model accuracy across challenging scenarios, comparing displacement and stress fields with ground-truth analytical models. Cases include extreme twisting and electrically induced wrinkles, demonstrating practical applicability and robustness of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper aims to derive the motion equations for a small-scale flexoelectric-multilayer plate and move deep through its vibrational response to several determining variable variations. Flexoelectricity indicates the electrical polarization induction in response to the non-uniform strain gradient. As moving toward small scales, the importance and the role of this property comes more to the picture. The under-examination structure is fabricated of three layers; a mid-layer which is assumed to be composed of the porous exponential and power law-functionally graded materials, two flexoelectric face sheets, and the Vlasov foundation. The motion equations are derived mathematically based on the new EP of porous material, Hamilton's principle, and modified couple stress theory. Utilizing the new EP power law besides evaluating the influence of the variables, length scale parameter, materials composition, geometrical characteristics, porosity, foundation parameter, and boundary conditions (B.C.s) on the dimensionless natural frequency (DNF) of such a sandwich model is the novelty of this paper. It is revealed that the lateral ratio enhancement causes DNF and stiffness reduction in the model. Furthermore, among all considered B.C.s, simply support and clamped support refer to the lowest and the highest DNF, respectively. The content of the recent paper serves as a good source to provide a better understanding of the vibrational response of high stiffness-to-weight ratio structures to different variable variations. [ABSTRACT FROM AUTHOR]

The rail profile milling process allows considerable thickness to be removed in a single pass. However, the residual waviness formation law after milling and its subsequent effect on rail performance remains uncertain. In this paper, the wavelength and wave height of the residual waviness are identified as the key characteristics. The relationship between milling speed and residual waviness is determined, and a numerical model of the residual waviness formed on the surface after milling is established based on the commonly used three milling speeds of 300, 600, and 1000 m/h, and its accuracy is verified using the parameters of the residual waviness detected by the milling experiments. A three-dimensional finite element analysis model of wheel-rail contact was employed to analyze the contact stresses and low fatigue cycles at a wheel load of 11.5 tons with no residual waviness on the rail profile surface and with three types of residual waviness, respectively. The results show that the residual waviness changes the wheel-rail contact position and the morphology of the contact area, reduces the maximum contact stress and increases the strain fatigue cycles. The application of elevated milling speeds reduces the wheel-rail contact stresses after milling, thereby increasing the low fatigue cycles. [ABSTRACT FROM AUTHOR]

In this paper, we consider isotropic Mindlin–Toupin strain gradient elasticity theory, in which the equilibrium equations contain two additional length-scale parameters and have the fourth order. For this theory, we developed an extended form of Boussinesq–Galerkin (BG) and Papkovich–Neuber (PN) general solutions. The obtained form of BG solution allows to define the displacement field through a single vector function that obeys the eight-order bi-harmonic/bi-Helmholtz equation. The developed PN form of the solution provides an additive decomposition of the displacement field into the classical and gradient parts that are defined through the standard Papkovich stress functions and modified Helmholtz decomposition, respectively. Relations between different stress functions and the completeness theorem for the derived general solutions are established. As an example, it is shown that a previously known fundamental solution within the strain gradient elasticity can be derived by using the developed PN general solution. [ABSTRACT FROM AUTHOR]

Background: The revolutionary technology of smartphone-based retinal imaging has been consistently improving over the years. Smartphone-based retinal image acquisition devices are designed to be portable, easy to use, and cost-efficient, which enables eye care to be more widely accessible especially in geographically remote areas. This enables early disease detection for those who are in low- and middle- income population or just in general has very limited access to eye care. This study investigates the limitation of smartphone compatibility of existing smartphone-based retinal image acquisition devices. Additionally, this study aims to propose a universal adapter design that is usable with an existing smartphone-based retinal image acquisition device known as the PanOptic ophthalmoscope. This study also aims to simulate the reliability, validity, and performance overall of the developed prototype. Methods: A literature review has been conducted that identifies the limitation of smartphone compatibility among existing smartphone-based retinal image acquisition devices. Designing and modeling of proposed adapter were performed using the software AutoCAD 3D. For the proposed performance evaluation, finite element analysis (FEA) in the software Autodesk Inventor and 5-point scale method were demonstrated. Results: Published studies demonstrate that most of the existing smartphone-based retinal imaging devices have compatibility limited to specific older smartphone models. This highlights the benefit of a universal adapter in broadening the usability of existing smartphone-based retinal image acquisition devices. A functional universal adapter design has been developed that demonstrates its compatibility with a variety of smartphones regardless of the smartphone dimension or the position of the smartphone's camera lens. The proposed performance evaluation method generates an efficient stress analysis of the proposed adapter design. The end-user survey results show a positive overall performance of the developed universal adapter. However, a significant difference between the expert's views on the developed adapter and the quality of images is observed. Conclusion: The compatibility of existing smartphone-based retinal imaging devices is still mostly limited to specific smartphone models. Besides this, the concept of a universal and suitable adapter for retinal imaging using the PanOptic ophthalmoscope was presented and validated in this paper. This work provides a platform for future development of smartphone-based ophthalmoscope that is universal. [ABSTRACT FROM AUTHOR]

The method of segmented hydraulic fracturing in the coal seam roof has proven to be an efficient technique for coalbed methane exploitation. However, the behavior of hydraulic fractures in multilayer formations with significant differences in mechanical properties is still unclear. This paper studied the variation in hydraulic fracture width at the coal-rock interface by employing experimental method with a true triaxial hydraulic fracturing experimental system and numerical simulation method. Results revealed that the hydraulic fracture more likely to expanded along the coal-rock interface instead of break through it with the small horizontal stress difference and low flow rate injection condition. And improving the injection flow rate lager than a critical value, the hydraulic fracture tends to break through the coal-rock interface. Hydraulic fractures in both mudstone and coal beds exhibited a trend of increasing and then decreasing of fracture width at the interface. Since the strength of the coal seam was lower compared to that of the mudstone, maintaining high pressure was no longer necessary when the hydraulic fracture crossed the interface and entered the coal seam, leading to a reduction in fracture width within the mudstone. During the later stages of fracturing, the entry of proppant into the coal seam became challenging, resulting in a phenomenon characterized by excessive fluid but insufficient sand. The time required for the fracture width to traverse the proppant was found to be inversely proportional to the difference in horizontal ground stress and the flow rate of the fracturing fluid. And it was directly proportional to the modulus of elasticity, permeability of the coal seam, and interface strength. The interface strength has the greatest influence on the width of hydraulic fractures. In conclusion, this study provides valuable insights into the behavior of hydraulic fractures in multilayer formations with varying mechanical properties. The findings contribute to a better understanding of the factors affecting hydraulic fracture width within coal seams, which can ultimately enhance the efficiency of coalbed methane exploitation. [ABSTRACT FROM AUTHOR]

The stability of rock pillar is crucial for ensuring the construction safety of twin tunnels with small clearance, especially when transitioning from the traditional left–right tunnel layouts to the up-down configurations due to complex and variable site constraints. However, there are limited researches on the evaluation and comparative study of the stability of these two types of rock pillars in twin tunnels. This paper introduces the yield approach index (YAI) as a measure to assess the stability of rock pillar in twin tunnels with small clearance, and various influencing factors including side pressure coefficient (SPC), stress release rate (SRR), and the thickness of rock pillar (characterised by the ratio of rock pillar thickness to tunnel diameter, RPT/TD) are considered in the analysis. The study compares and analyzes the stability differences of the rock pillar in different situations. It is observed that the two sides of up-down tunnels pose a higher risk while the rock pillar in the left–right configuration being the most vulnerable. The stability of the rock pillar between the up-down tunnels is significantly higher than that of the left–right tunnels under similar conditions. Moreover, the up-down tunnels exhibit greater sensitivity to SPC, whereas the left–right tunnels are more sensitive to SRR. Additionally, the study reveals that increasing the RPT/TD can effectively improve the stability of the rock pillar within a specific range (1/4 to 2/3). The research method and obtained results of this paper can provide some important references for the stability evaluation and design of twin tunnels with small clearance. [ABSTRACT FROM AUTHOR]

In a recent paper Abeyaratne et al. (J. Mech. Phys. Solids 167:104958, 2022) concerning the stability of surface growth of a pre-stressed elastic half-space with surface tension, it was shown that steady growth is never stable, at least not for all wave numbers of the perturbations, when the growing surface is traction-free. On the other hand, steady growth was found to be always stable when growth occurred on a flat frictionless rigid support and the stretch parallel to the growing surface was compressive. The present study is motivated by these somewhat unexpected and contrasting results. In this paper the stability of a pre-compressed neo-Hookean elastic half-space undergoing surface growth under plane strain conditions is studied. The medium outside the growing body resists growth by applying a pressure on the growing surface. At each increment of growth, the incremental change in pressure is assumed to be proportional to the incremental change in normal displacement of the growing surface. It is shown that surface tension stabilizes a homogeneous growth process against small wavelength perturbations while the compliance of the surrounding medium stabilizes it against large wavelength perturbations. Specifically, there is a critical value of stretch, λ cr ∈ (0 , 1) , such that growth is linearly stable against infinitesimal perturbations of arbitrary wavelength provided the stretch parallel to the growing surface exceeds λ cr . This stability threshold, λ cr , is a function of the non-dimensional parameter σ κ / G 2 , which is the ratio between two length-scales σ / G and G / κ , where G is the shear modulus of the elastic body, σ is the surface tension, and κ is the stiffness of the surrounding compliant medium. It is shown that (a) λ cr → 1 as κ → 0 and (b) λ cr → 0 + as κ → ∞ , thus recovering the results in Abeyaratne et al. (J. Mech. Phys. Solids 167:104958, 2022) pertaining to the respective limiting cases where growth occurs (a) on a traction-free surface and (b) on a frictionless rigid support. The results are also generalized to include extensional stretches. [ABSTRACT FROM AUTHOR]

In this paper, dynamic stability of a smart sandwich nanotube is investigated. The core of the nanostructure is made from a single-walled carbon nanotube (SWCNT), which is coated with two ZnO piezoelectric layers. The inner piezoelectric layer is taken as a sensor and the outer layer as an actuator. The sandwich nanotube is embedded in an elastic visco-Pasternak medium. The modified couple stress theory is utilized to consider the small-scale effect, and the Kelvin-Voigt model is used to model the carbon nanotube and the ZnO layers. The surface tension is taken into account using the Gurtin–Murdoch theory. The nonlinear governing equations, derived by the Hamilton principle, are solved using differential quadrature method (DQM), to investigate dynamic stability range, and to calculate natural frequencies. The results of this study are compared against existing similar data in the literature, presented for some simpler cases. Then, the effect of various parameters, including the small scale parameter, the ratio of piezoelectric layer thickness to nanotube thickness, surface stresses, the visco-elastic foundation effect, the magnetic field effect, and the electrical polarity are scrutinized on the system dynamic stability range. The results indicate that consideration of the small scale can reduce the transverse displacement and frequency while increasing the thickness of the piezoelectric layer leads to a higher frequency. Moreover, incorporating the surface stresses into the model can increase the stability. Using the modified couple stress theory can be considered the main novelty of this paper. [ABSTRACT FROM AUTHOR]

Slab track is a recent technology used to cope up with the train high axle loads and speed, it has replaced the ballast material in classical ballasted track with either reinforced concrete slab or asphalt layer in order to increase both stability and durability of the railway lines. This paper aims to propose a new slab track design model which can be used to design/analyze any slab track systems under vertical loads using AREMA and EN specifications for high-speed systems (300 kmph). This model has been validated through experimental work held in Heriot-Watt University then applied to the most common slab track systems (BÖGL, Shinkansen, and RHEDA 2000) in the world. The standard section of RHEDA 2000 slab track has shown the best structural performance and efficiency compared with BÖGL and Shinkansen standard sections regarding the rail deflection, stresses of rails, and stress of replacement soil layer and subgrade soil. This paper has concluded the rail deflection is the most critical factor for the slab track design regarding EN specifications while the subgrade stresses is the vital criterion concerning AREMA specifications. Furthermore, EN-Specifications are found to be more conservative than AREMA specifications for the design or analysis of all the slab track types. [ABSTRACT FROM AUTHOR]

Twinning is an important deformation mechanism for magnesium alloys, which has a significant impact on the texture evolution and mechanical properties. Pre-twinning deformation has been considered as an effective method for texture regulation. For each twinning mode, there are six crystallographically equivalent variants. Properly identifying the activated twin and understanding its variant selection criteria are essential for the development of high-performance magnesium alloys. As summarized in this paper, the observed twin variant by the commonly employed electron backscatter diffraction technique can be identified by several approaches, including misorientation analysis, trace analysis and matrix method. Schmid factor analysis was commonly performed to explain the selection of twin variants. To broaden the application scope under complex stress state, a generalized Schmid factor was derived by introducing a stress tensor. The efficiency of Schmid criteria to assess twin variant selection was confirmed to be influenced by the type of the applied stress. To consider the local effect on twin activation, in particular twin-twin transfer and slip-induced twinning, displacement gradient tensor calculation and geometrical compatibility factor analysis have been employed. It was demonstrated that local strain accommodation played a critical role in selecting the variants of cross-boundary twins in magnesium alloys. Assisted with crystal plasticity finite element modeling, the resolved shear stress on twinning and a composite Schmid factor combining the global Schmid factor and geometrical compatibility factor were obtained to better explain the activated twin variants in magnesium alloys. All the above-mentioned Schmid law based criteria and some energy based criteria as well are summarized in this paper. Their applications in evaluating twin variant selection in magnesium alloys are critically reviewed and discussed. [ABSTRACT FROM AUTHOR]

In this paper, we have studied the behaviour of two symmetric mode-III collinear cracks in a functionally graded material (FGM). The fundamental goal of this paper is to provide insight on the interaction of two cracks in FGMs with the strain gradient effect. To assess the influence of gradient elasticity, we have considered two key parameters ℓ and ℓ ′ , which describe the size scale effect caused by the underlying microstructure and are related to volumetric and surface strain energy, respectively. The crack boundary value problem have been solved by the approach involving Fourier transforms and the innovative hyper-singular integro-differential equation method, where the integral equation contains the two terms in integrals for the both cracks. A system of equations has been constructed by employing the Chebyshev polynomial expansion and then by choosing the suitable collocation points the system of equation have been solved. Our investigation involves the determination of stress intensity factors at both crack tips. These factors are vital for understanding the material's fracture behavior and structural integrity. Furthermore, we explore the variations in the displacement profile when the distance between the cracks is reduced to close proximity. This particular scenario is of significant interest as it provides insights into how the interaction between the cracks impacts the overall structural response. [ABSTRACT FROM AUTHOR]

The mechanical properties of rockfill materials are not only influenced by microscopic factors such as particle morphology and gradation, but also closely related to different loading stress paths. It is of great significance to study the microscopic mechanical properties of rockfill materials under different stress paths for revealing the macroscopic mechanical properties as well as the microscopic deformation and failure mechanisms of rockfill materials. In this paper, based on the results of triaxial tests, a series of numerical triaxial simulation tests under different stress paths were carried out using the discrete element particle flow method, and the deformation, strength change rules, and fine structure evolution mechanism under three stress paths were explored. The results demonstrated that there were significant differences in the effects of stress paths on the stress–strain and strain-volume change characteristics of the rockfill materials. Stress paths exhibited little effect on the strength characteristics. The anisotropy of strong contact number and strong contact force was the microscopic source of macroscopic strength. The contact situation between the particles was the main microscopic factor affecting the macroscopic deformation. The intrinsic mechanism of macroscopic deformation properties could be revealed by the average coordination number and porosity. The stress path affected the growth rate of the number of bond failures and the total number of failures. The relationship between macroscopic mechanical properties and microstructural evolution under different stress paths was also discussed. The findings can provide meaningful insights into the deformation control and stability analysis of rockfill engineering. [ABSTRACT FROM AUTHOR]

Background: The modelling of the sheet metal forming operations requires accurate and precise data of the material plastic behaviour along non-proportional strain paths. However, the buckling phenomenon severely limits the compressive strain range that could be used to deform thin metal sheets. Objective: The main aim of this paper was to propose an effective device, that enables to determine of accurate stress-strain characteristics of thin metal sheet specimens subjected to axial deformation without buckling and with a special emphasis on friction correction. Methods: In this paper, an anti-buckling fixture was proposed to assess the deformation characteristics of X10CrMoVNb9-1 (P91) power engineering steel, and DP500 and DP980 dual-phase steels, under compression loading. The fixture enables monitoring of the friction between the specimen and supporting blocks during the test, and thus the precise stress response of the material could be determined. Results: The effectiveness of the fixture was evaluated under tension–compression cyclic loading and during the compression tests in which high-strength thin metal sheets were successfully deformed up to 10% without specimen buckling. Furthermore, the successful determination of a friction force variation between supporting blocks and the specimen during tests enabled to determine an actual force acting on the specimen. Conclusions: The proposed testing fixture was successfully assessed during the compression and cyclic tension–compression of high-strength thin metal sheets as no buckling was observed. Its advantage lies in adapting to change its length with specimen elongation or shrinkage during a test. The friction force generated from a movement of both parts of the device could be effectively monitored by the special strain gauge system during testing and thus its impact on the stress-strain characteristics could be successfully eliminated. [ABSTRACT FROM AUTHOR]

In this paper, for the first time, the role of manufacturing parameters of fused deposition modeling (FDM) on the shape memory effect (SME) is investigated by design of experiments. PLA-TPU blend with a weight composition of 30:70% is processed by melt mixing and then extruded into 1.75 mm filaments for 3D printing via FDM. SEM images reveal that TPU droplets are distributed in the PLA matrix, and the immiscible matrix-droplet morphology is evident. Box-Behnken design (BBD), as an experimental design of the response surface method (RSM), is implemented to fit the model between variables and responses. The shell, infill density, and nozzle temperature are selected as variables, and their effects on loading stress, recovery stress, shape fixity, and shape recovery ratio are studied in detail. An analysis of variance (ANOVA) is applied to estimate the importance of each printing parameter on the output response and assess the fitness of the presented model. The ANOVA results reveal the high accuracy of the model and the importance of the parameters. Infill density and nozzle temperature had the greatest and least roles on shape memory properties, respectively. Also, the values of shape fixity and shape recovery were obtained in the ranges of 58–100% and 53–91%, respectively. Despite many researches on 4D printing of PLA, low ductility at room temperature and high stress relaxation rate are its weakness, which are covered by adding TPU in this research. Due to the lack of similar outcomes in the specialized literature, this paper is likely to fill the gap in the state-of-the-art problem and supply pertinent data that are instrumental for FDM 3D printing of functional shape memory polymers with less material consumption. [ABSTRACT FROM AUTHOR]

In Recognition of the 90th Birthday of Millard Beatty Professor Beatty has contributed a wide variety of research papers and book articles on topics in finite elasticity, continuum mechanics and classical mechanics, including some fundamental experimental work. Finally, Professor Beatty's passion for teaching is exhibited in a number of his didactic related articles on classical mechanics and is emphasized further in two volumes on the principles of engineering mechanics. In a series of fundamental papers, the first of which, Part 1, appeared in 1992 in this journal, Beatty, together with Michael Hayes, developed the constitutive theory of an isotropic Bell constrained material. While this three-part work concluded their joint research on Bell materials, later, in 2005, Beatty and Hayes got together again and arranged for the production, and served as co-editors of, the special volume: "Mechanics and Mathematics of Crystals - Selected Papers of J. L. Ericksen". Along the way, Beatty also developed a number of special mathematical results which dealt with: determinants having complex conjugate elements; general integrals arising in plane finite elasticity; and an integral identity applicable to continuum mechanics. [Extracted from the article]

Compact multi-rotor unmanned aerial vehicles (UAVs) can be operated in many challenging environmental conditions. In case the UAV requires certain considerations in designing like lightweight, efficient propulsion system and others depending upon the application, the hybrid UAV comes into play when the usual UAV types cannot be sufficient to meet the requirements. The propulsion system for the UAV was selected to be coaxial rotors because it has a high thrust-to-weight ratio and to increase the efficiency of the propulsion system, a unique propeller was proposed to achieve higher thrust. The proposed propeller was uniquely designed by analyzing various airfoil sections under different Reynolds's number using X-Foil tool to obtain the optimum airfoil section for the propellers. Since the design with duct increases efficiency, the Hybrid UAV presented in this paper has the modified novel convergent–divergent (C–D)-based duct which is a simplified model of a conventional C–D duct. The yawing and rolling maneuverings of the UAV could be achieved by the thrust vectoring method so that the design is simpler from a structural and mechanical perspective. The use of UAVs has risen in recent years, especially compact UAVs, which can be applied for applications like surveillance, detection and inspection, and monitoring in a narrow region of space. The design of the UAV is modeled in CATIA, and its further performance enactment factors are picked from advanced computational simulations relayed bottom-up approach. The predominant computational fluid dynamics (CFD) and fluid structure interaction (FSI) investigations are imposed and optimized through Computational Analyses using Ansys Workbench 17.2, which includes analysis of structural behaviour of various alloys, CFRP and GFRP based composite materials. From the structural analysis Titanium alloy came out to be the best performing materials among the others by having lower total deformation and other parameters such as normal and equivalent stress. The dynamics control response is obtained using MATLAB Simulink. The validations are carried out on the propeller using a thrust stand for CFD and on the duct through a high-jet facility for structural outcomes to meet the expected outcome. [ABSTRACT FROM AUTHOR]

In this paper, we compute holographic Euclidean thermal correlators of the stress tensor and U(1) current from the AdS planar black hole. To this end, we set up perturbative boundary value problems for Einstein's gravity and Maxwell theory in the spirit of Gubser-Klebanov-Polyakov-Witten, with appropriate gauge fixing and regularity boundary conditions at the horizon of the black hole. The linearized Einstein equation and Maxwell equation in the black hole background are related to the Heun equation of degenerate local monodromy. Leveraging the connection relation of local solutions of the Heun equation, we partly solve the boundary value problem and obtain exact two-point thermal correlators for U(1) current and stress tensor in the scalar and shear channels. [ABSTRACT FROM AUTHOR]

Creep failure of hierarchical materials is investigated by simulation of beam network models. Such models are idealizations of hierarchical fibrous materials where bundles of load-carrying fibers are held together by multi-level (hierarchical) cross-links. Failure of individual beams is assumed to be governed by stress-assisted thermal activation over local barriers, and beam stresses are computed by solving the global balance equations of linear and angular momentum across the network. Disorder is mimicked by a statistical distribution of barrier heights. Both initially intact samples and samples containing side notches of various length are considered. Samples with hierarchical cross-link patterns are simulated alongside reference samples where cross-links are placed randomly without hierarchical organization. The results demonstrate that hierarchical patterning may strongly increase creep strain and creep lifetime while reducing the lifetime variation. This is due to the fact that hierarchical patterning induces a failure mode that differs significantly from the standard scenario of failure by nucleation and growth of a critical crack. Characterization of this failure mode demonstrates good agreement between the present simulations and experimental findings on hierarchically patterned paper sheets. [ABSTRACT FROM AUTHOR]

The freezing damage of rock tunnels in cold region involves ice-water phase change and complicated interaction of Thermo–Hydro–Mechanical (THM) field. Taking the fractured rock mass of cold region tunnels as research subject, the THM coupling model of cold region tunnels was established, which is based on the seepage mechanics, heat transfer theory, damage mechanics and equivalent continuum theory. This model could reflect the anisotropic properties of deformation, water migration and heat transfer caused by the initial fracture of rock mass. The construction and operation processes of a rock tunnel in cold region were simulated, and results were compared with the measured value and predecessor's achievements. It shows that proposed model could reflect the anisotropic property of surrounding rock and the simulated deformation and stress are not symmetrical. Compared with the literature, the calculated results in this paper are closer to the measured values. The insulating layer has a significant effect on the stress of the supporting structures. The maximum tension stress of the lining is 4.5 times as that without insulating layer, and the lining will be destroyed for the overlarge tension stress. [ABSTRACT FROM AUTHOR]

The paper deals with numerical analysis of the effect of stress state and loading direction with respect to the rolling direction on damage and fracture behavior of anisotropic metals. The continuum damage model has been enhanced to take into account the influence of production-induced anisotropies and loading direction on damage criteria and on evolution equations of damage strains. Constitutive parameters are determined using experimental results taken from tests with uni- and biaxially loaded specimens. The focus of the paper is on three-dimensional micro-mechanical numerical analyses of micro-defect-containing representative volume elements covering a wide range of stress states. These calculations lead to more insight in the different damage and failure processes on the micro-scale and their influence on the macroscopic damage laws. With the obtained numerical results it is possible to detect general trends, to propose governing equations for the damage criteria, to develop evolution equations for the damage strains, and to identify constitutive parameters of the anisotropic material model. It is shown that the anisotropic behavior and the loading direction with respect to the principal axes of anisotropy affect the evolution of damage mechanisms on the micro-level as well as the corresponding damage strains. [ABSTRACT FROM AUTHOR]

The most common coated structure contact problems include spherical, conical and cylindrical contact. The evaluation of mechanical performance for coating structures has always been a very important issue in the field of mechanics and materials; due to the too small proportion between the thickness of the coating and the substrate, the deviation of traditional evaluation methods becomes unacceptable. Indentation technology is the basis for analysis, measurement, and the standardized application of coating structures. The research object of this paper is cylindrical contact, which is one part of building the theoretical framework of contact mechanics of coating structures. In the paper, an accurate and efficient general theory of the frictional cylindrical contact problem for the coated structure is presented. The general solutions are expressed in the form of harmonic functions. 3D exact solutions of a transversely isotropic elasticity coated structure under frictional cylindrical punch contact are derived, based on the general solution and the boundary conditions. The theory is proposed in two cases, including the frictionless contact and the frictional contact. By contrast with existing theories (obtained in this paper by degradation), the numerical calculations show the good convergence, high accuracy, efficiency, and stability. The difference of stress singularity analysis between Finite Element Method (FEM) and the presented theory is explained, which shows the theory's validity and applicability. This analytical theory plays an important role in boundary stress singularity analysis. The finite element comparison shows that the analytical theory plays an important role in boundary stress singularity analysis and further, proves the validity and applicability of this theory. In the numerical analysis, the influence of coating thickness on interface stress is shown, the distribution forms of stress and displacement are given, and the influence of material parameters and coating thickness on interface failure is investigated. The analytical expressions are presented with the elementary functions. In the era of highly developed computer intelligence, the presented theory will be the basis of the interface failure problem analysis and material parameters' determination, also for future use deep learning to solve the problem of contact mechanics provides the basis of large precise sample. [ABSTRACT FROM AUTHOR]

The failure characteristics of geotechnical materials are quite complex, among which there are obvious differences between soil materials and rock materials. In order to describe the failure characteristics of soil materials and rock materials uniformly, a new failure and yield criterion, called VML criterion, is proposed in this paper. This strength criterion has the following characteristics. (1) On the deviatoric plane, the criterion can be expressed as a power parameter expression of von Mises criterion and spatial mobilized plane (SMP) criterion, so it can be used to describe cohesive materials such as metals or frictional materials such as sand. (2) On the meridian plane, another power parameter expression can be used to consider tensile stress strength. On the meridian plane, the material strength envelope can be expressed as a curve, so the hydrostatic pressure effect of soil material can be reasonably considered. (3) In order to consider the volumetric compression yield properties of soil materials under isotropic compression, a yield criterion considering the volumetric compression yield properties is also proposed on the deviatoric plane. Since geotechnical materials are often in plane strain condition in engineering, a VML criterion expression considering plane strain condition is proposed in this paper. The criterion shows that the internal friction angle of soil under plane strain is usually 1.1 times of that under conventional triaxial compression. The comparison between the predicted and tested data shows that the VML criterion proposed in this paper can be widely applied to the failure simulation of rock and soil materials, and the plane strain VML criterion can also be applied to the failure prediction with plane strain state. [ABSTRACT FROM AUTHOR]

Masoud and Maas present an energy-based approach to model the nonlinear dynamic behavior of soft material structures comprising dielectric elastomer transducers suspended with flexible elastic structures. The approach can be easily utilized to design and control soft transducer systems based on dielectric elastomer transducers, which is successfully illustrated for a buckled beam structure actuated by a multilayer dielectric elastomer transducer. Smart structures employ multifunctional materials, which take advantage of a wide spectrum of constitutive couplings ranging from thermomechanical to electro-magneto-mechanical to chemo-mechanical material behavior. [Extracted from the article]

Buckling analysis of functionally graded porous nano conical shell subjected to axial compression is studied in this paper. The porous material properties vary across the thickness by a special function. First order shear deformation theory and modified couple stress theory are used to obtain the governing equation. Shells' nonlinear equilibrium and linear stability equations are obtained by Euler's relations and Treftz principles. To solve the stability equations, the differential quadrature method (DQM) is used. The accuracy and the reliability of this paper are compared with reported studies. Finally, the effect of shell's porous parameters, boundary condition, vertex angle and length scale on the amount of critical buckling force is discussed. [ABSTRACT FROM AUTHOR]

Rock mechanical property is significant for shale gas development and exploitation. Shale compressive strength, tensile strength, elastic deformation and so on, are necessary parameters for drilling, completion and fracturing work in shale formation. Among all these shale mechanical parameters, brittleness is a tricky and significant rock property, which has been widely used to hydraulic fracturing design. Currently, although so many works have been conducted to investigate shale brittleness, there is no precise definition of brittleness. In particular, there is no consensus on which method is the most reliable for shale brittleness evaluation. It is vital to figure out how to evaluate shale brittleness in a reliable method. Thus, this paper presents an experimental study on shale mechanical properties, analyzing mechanical features in stress strain curve, relation between mineral content and strength, mechanical parameters at varying confined stress. Based on shale mechanical characteristics and its brittle exhibition, stress strain curve from triaxial compression test is divided into 3 stages, namely, elastic stage, plastic stage and post peak stage. In combined with brittle characteristics in 3 stages of axial and radial stress–strain curves, a new brittleness index has been established for assessing shale brittleness. In order to prove the applicability of new brittleness index, its result is compared with shale failure sample after triaxial test and existing brittleness indexes based on mineral content, elastic deformation, energy, stress and strain, showing a good consistency and proving its practicability. Based on this brittleness index, influence factors of shale brittleness have been discussed. It is shown that elastic module is the most important factor of shale brittleness. Bedding plane makes shale brittleness have strong anisotropy. Brittleness is not only relied on its structure and mineral (like bedding plane, silicate and clay mineral content), but is also highly affected by external stress. Large confined pressure is able to impair shale brittleness. Outcome in this study can offer theoretical guidance for shale exploitation. [ABSTRACT FROM AUTHOR]

The paper presents analytical approach to electrostatic field for dielectric spheroids problem. For both oblate and prolate spheroids fields distributions are derived. The variable separation method is applied. Electrostatic force is evaluated with the help of the Maxwell stress tensor generalized method, material force density, coenergy and equivalent dipole. Levitation forces for both oblate and prolate dielectric spheroids versus axis permittivities and spheroids height are presented. [ABSTRACT FROM AUTHOR]

Background: This paper presents a numerical modelling procedure of dynamic loading due to traffic on a two-story residential building. In developing countries cities and towns, poor quality narrow road sections with high traffic density are common. In such cases, ground vibration due to traffic could be higher and lightweight buildings located closer are exposed to traffic-induced dynamic loading. Design codes require a proper assessment of such vibrations. However, a clear and definite procedure of assessment is not usually provided. This research presents an assessment procedure of dynamic loading due to traffic on a soil foundation system of light weight buildings based on numerical modeling. Traffic induced ground vibration acceleration amplitudes, frequencies and durations were measured, and the dynamic loads were calculated from measured vibration accelerations and vibrating mass of the vehicle. A two-story residential building with flexible square shallow footings was modelled together with the foundation soil using PLAXIS-2D. The dynamic load was modelled as harmonic loading considering the highest amplitude of vibration measured. Results: On the calculation stage, static loading analysis, dynamic loading analysis and free vibration dynamic analysis were carried out. A maximum increase in extreme total displacement of the soil to 22.03 mm was observed after the dynamic loading from 18.96 mm extreme total displacement due to static loading. Extreme effective mean stress in the soil increased to 112.81 kPa from 110.5 kPa, due to the dynamic loading. In addition, a differential settlement of 3.14 mm between two adjacent footings was observed after the traffic induced ground vibration. Furthermore, the Mohr–Coulomb plastic points were observed to be concentrated to the side of the soil-foundation system which the dynamic load was acting on. Conclusion: The regular exposure to traffic-induced vibrations may cause frequent change in stress and deformation response of the foundation-soil system. In areas where lightweight buildings are exposed to regular traffic induced vibrations, proper assessment of the effect should be carried out and measures should be taken to mitigate the problem. Improving road surfaces and limiting vehicle speed are possible remedial measures to reduce ground vibrations due to traffic. [ABSTRACT FROM AUTHOR]

This paper presents a study on the temperature dependent photo-elastic coefficient in single-crystal silicon with (100) and (110) orientations at a wavelength of 1550 nm. The measurement of the photo-elastic coefficient was performed using a polarimetric scheme across a wide temperature range from 5 to 300 K. The experimental setup employed high-sensitivity techniques and incorporated automatic beam path correction, ensuring precise and accurate determination of the coefficient's values. The results show excellent agreement with previous measurements at room temperature, specifically yielding a value of d n / d σ = - 2.463 × 10 - 11 1/Pa for the (100) orientation. Interestingly, there is a significant difference in photo-elasticity between the different crystal orientations of approximately 50 % . The photo-elastic coefficient's absolute value increases by approximately 40% with decreasing temperature down to 5 K. These findings provide valuable insights into the photo-elastic properties of silicon and its behavior under varying mechanical stress, particularly relevant for optomechanical precision experiments like cryogenic gravitational wave detectors and microscale optomechanical quantum sensors. [ABSTRACT FROM AUTHOR]

Based on the nonlocal strain gradient theory and flexoelectricity theory, a new model of transversely isotropic piezoelectric rectangular nano-plate subjected to a distributed load is proposed. The nonlocal effect, strain gradient effect and flexoelectric effect are all considered in the novel model. The governing equation and different boundary conditions for a clamped nano-plate under closed circuit and open circuit states are derived by Hamilton principle. Based on the new model, the electro-mechanical responses of piezoelectric nano-plate are numerically analyzed. When the dimensionless nonlocal scale coefficient is smaller than 3, both the central deflection and the central electric potential show significant size dependence. The nonlocal effect causes the nano-plate to be softened while the strain gradient effect makes it stiffened. Compared with the piezoelectric effect, the impact of the flexoelectric effect on the central electric potential plays a dominant role. In addition, the electric potential increases from each lateral edge to the center symmetrically, reaching a maximum at the central point. This paper elucidates the competitive relationship among the influencing effects and concurrently discovers that these effects have a comparable magnitude of impact on the electro-mechanical responses of nano-plate. [ABSTRACT FROM AUTHOR]

This paper provides an overview of technical enhancements of current encapsulant materials (epoxy molding compound and underfill) and future deployment of graphene materials for better thermal and mechanical stress management in thin and complex memory packaging. Also, with the aim to study the key technical barriers, influencing of each epoxy molding compound (EMC) and underfill (UF) materials properties and its reliability performance of impacts on materials used in semiconductor device packaging, this paper lays out the importance of future graphene of its superior materials thermal dissipation and performances especially for future higher memory density and low power memory devices. This article briefly reviews key aspects of encapsulant materials reliability concerns, factors impacting package strength, package warpage and thermal dissipation for future miniatured memory device packaging. [ABSTRACT FROM AUTHOR]

This paper presents a size-dependent study on the free vibration behaviour of the functionally graded (FG) porous curved microbeams. Based on the different higher-order shear deformation models and the modified strain gradient theory, the governing equations are derived using Hamilton's principle. Then, the isogeometric analysis approach is employed to solve these equations. Besides, the material properties and the material length scale parameters (MLSPs) vary along the thickness direction of the FG curved microbeams according to the rule of mixture scheme. Also, two types of porosity distributions across the thickness, including even and uneven, are considered. By increasing the order of the non-uniform rational b-spline (MLSPs) basis functions, the C2-continuity requirement can be easily achieved. To establish the validity of the proposed method, the present results are compared with those from the previous studies. Finally, the effects of the variable MLSPs, porosity parameter, material gradient indices, curvature and different boundary conditions on free vibration response of circular, elliptical and parabolic FG porous microbeams are investigated. The obtained results reveal that the size-dependent effects increase the natural frequency as well as porosity decreases it because of decreasing the stiffness. [ABSTRACT FROM AUTHOR]

The manufacturing processes of electrical machines may lead to significant degradation of the magnetic properties of their magnetic core (stator, rotor) performances and, as a consequence, to a decrease of their energy efficiency. While the effects of some processes (cutting, welding ...) are widely discussed in the literature, this is not the case with the compaction process although it is systematically used to maintain the assembly of electrical steel sheets that compose the magnetic circuits. In addition to the conventional one, a specific compaction process exists for high-power electrical machines. After an introduction, the paper firstly deals with the two studied processes (conventional, specific). Then, an experimental mock-up to study the impact of the two configurations on the magnetic properties (iron losses, normal magnetization curve) is presented. This mock-up is the first, in the literature, that allows to study the effect of a controlled compaction mechanical stress on magnetic properties. Obtained results in both configurations highlight a magneto-mechanical effect that is not reported in the literature where these effects are commonly considered following in-plane mechanical stresses. This paper presents a magneto-mechanical model, taking into account the compaction stress effect, as well as a modelling protocol to model the effect of 3D mechanical stress on magnetic properties, which has never been done in the literature. [ABSTRACT FROM AUTHOR]

Compared to homogeneous beams, metamaterial-based sandwich beams are known to be lighter and provide better mechanical characteristics. However, their application has mainly been limited to large-scale structures, and their potential for granting small-scale structures enhanced characteristics has not been fully explored yet. The present manuscript aims to investigate nonlinear vibration in size-dependent metamaterial-based sandwich microbeams with hexagonal and triangular lattice cores. To do so, well-known homogenization techniques based on modeling the representative volume element (RVE) of the lattice have been utilized to find the equivalent properties of the nonlinear cores. Moreover, the modified strain gradient theory is used to obtain the governing equation of motion for a small-scale microbeam and the multiple scales method is adopted to investigate the free vibration and primary resonance of the discretized system. Further, size effects and the effect of geometry on the free and forced nonlinear vibration of lattice sandwich beams are examined. In addition to highlighting the size effects, the results indicate that properly implementing nonlinear metamaterials in microbeams can enhance and tune vibrational performance across various length scales. The results of the present paper can provide a more detailed perception of the concept of nonlinear metamaterial-based microbeams and their application in MEMS/NEMS devices. [ABSTRACT FROM AUTHOR]

For predicting casting deformation by FEM (finite element method) thermal stress analysis, this paper experimentally constructed and validated the Drucker–Prager (DP) model and its revised model (RDP) for the condensation zone of bentonite-bonded sand molds. The condensation zone has been known as a remarkably low-strength region in the mold, and its mechanical response should be dominant to the casting deformation. To construct the models through uniaxial and triaxial compression tests, the test pieces reproducing the condensation zone were prepared by permeating water vapor through a test piece of bentonite-bonded sand. The constructed models were then applied to the FEM stress analysis of the triaxial test to validate them by comparing the analytical stress–strain curves with the experiment. The experimental stress–strain curve after permeating water did not show any distinct yielding point; however, the analytical curve with the original DP model clearly exhibited a yielding point due to the yield criterion of the model. The distinct yielding criterion should be essentially fatal to reproduce the stress–strain relationship of the condensation zone. Contrarily, the RDP model reproduced the experimental curve with less than 10 % error and was found to be advantageous for modeling the smooth stress–strain relationship of the condensation zone. [ABSTRACT FROM AUTHOR]

The dispersion and reflection characteristics of coupled waves in the piezomagnetic solid with flexomagnetic microstructure effect are studied in this paper. The flexomagnetic effect, strain gradient effect and micro-inertia effect are introduced into the constitutive equation and the governing equation to establish the wave propagation theory in the piezomagnetic solid with flexomagnetic microstructure effect. The secular equation of wave propagation is derived, and the properties of five coupled waves are analyzed. The dispersion characteristics of coupled waves and the reflection properties of coupled waves at the free boundary are solved numerically and checked by the energy conservation law. The influences of the flexomagnetic effect, strain gradient effect and micro-inertia effect on the wave propagation are discussed. It is found that the flexomagnetic effect mainly enhances the mechanical-magnetic coupled property, strain gradient effect mainly enhances the elastic property, and micro-inertia effect not only enhances the inertia property, but also essentially changes the dispersion characteristics of coupled waves. [ABSTRACT FROM AUTHOR]

In this paper, the stress fields in nonlinear sandwich and laminated composite shells subjected to displacement-dependent loads are analyzed. To solve the problem, the geometrically exact (GeX) hybrid-mixed four-node solid-shell element is proposed. The term GeX means that the middle surface is described by analytical functions, i.e., the parametrization of the surface is known. The laminated solid-shell element formulation is based on the choice of an arbitrary number of sampling surfaces (SaS) parallel to the middle surface and located at the Chebyshev polynomial nodes within the layers, in order to introduce the displacements of these surfaces as unknown functions. The outer surfaces and interfaces are also included into a set of SaS. Due to analytical integration utilized to evaluate the tangent stiffness matrix, the developed GeX solid-shell element under non-conservative loading shows excellent performance in the case of coarse meshes and makes it possible to use only one load step in most of the considered benchmark problems. It is shown that the difference between the second Piola–Kirchhoff and Cauchy stress tensors in some non-conservative problems for sandwich and laminated shells undergoing arbitrarily large rotations can be significant. [ABSTRACT FROM AUTHOR]

The thixotropic structural build-up is crucial in extrusion-based three-dimensional (3D) concrete printing. This paper uses a theoretical model to predict the evolution of static and dynamic yield stress for printed concrete. The model employs a structural kinetics framework to create a time-independent constitutive link between shear stress and shear rate. The model considers flocculation, deflocculation, and chemical hydration to anticipate structural buildability. The reversible and irreversible contributions that occur throughout the build-up, breakdown, and hydration are defined based on the proposed structural parameters. Additionally, detailed parametric studies are conducted to evaluate the impact of model parameters. It is revealed that the proposed model is in good agreement with the experimental results, and it effectively characterizes the structural build-up of 3D printable concrete. [ABSTRACT FROM AUTHOR]

The recycled powder (RP) from construction wastes can be used to partially replace cement in the preparation of reactive powder concrete. In this paper, reactive powder concrete mixtures with RP partially replacing cement, and natural sand instead of quartz, are developed. Standard curing is used, instead of steam curing that is normally requested by standard for reactive powder concrete. The influences of RP replacement ratio (0, 10%, 20%, 30%), silica fume proportion (10%, 15%, 20%), and steel fiber proportion (0, 1%, 2%) are investigated. The effects of RP, silica fume, and steel fiber proportion on compressive strength, elastic modulus, and relative absorption energy are analyzed, and theoretical models for compressive strength, elastic modulus, and relative absorption energy are established. A constitutive model for the uniaxial compressive stress-strain relationship of reactive powder concrete with RP is developed. With the increase of RP replacement ratio from 0% to 30%, the compressive strength decreases by 42% and elastic modulus decreases by 24%. [ABSTRACT FROM AUTHOR]

The functional relations between the modified couple stress theory (MCST) and the classical theory (CT) are derived and analyzed in detail for closed-form solutions of Timoshenko micro-beam. The results show that phenomenon of sharp change exists in the internal force fields at two ends of the micro-beam. This phenomenon is related to dimensionless material scale parameter l/h. The phenomenon at fixed end is different from that at free end because the shear force Q at fixed end cannot be degenerated from the MCST to the CT, which is proved from a mathematical point of view. What's more, the exponential functions in the closed-form solutions of the MCST are higher-order terms and can be ignored for the deflection and rotation when Poisson's ratio ν = 0–0.45, h/L = 1/100–1/8 and the l/h = 0–1. After exponential functions are omitted, except rotation of cantilever beam under bending moment M0, the concise solutions are acquired and error is very small. The size effects of the deflection and rotation can be calculated quickly without additional computing tools by the concise solutions. Some results obtained in this paper are compared with the published data in literatures, which prove the correctness of the results in this paper. [ABSTRACT FROM AUTHOR]

Background: Raman spectroscopy has become an effective experimental stress analysis method because of its high spatial resolution, nondestructive and noncontact, etc. However, the stress confidence of single point and single time detection is far from accurate. Especially for the component decoupling analysis of complex stress states, the characterization results of the stress components often have difficulty reflecting the real stress states. Objective: In this paper, based on stress characterization using polarized micro-Raman spectroscopy, we focus on the two main factors that affect the confidence of the Raman spectrum, namely, the frequency shift repeatability of the instrument and the polarization control accuracy. Methods: Through a combination of physical experiments and numerical ones, the influence of random error in the Raman measurement system, initial error and random error in the polarization angle on the decoupling characterization of the stress component is quantitatively analyzed. Results: The results show that due to the existence of various error factors and the influence of polarization angle combinations, the error levels for the characterization results for each stress component are not only difficult to ignore but also the difference is obvious, and the true stress state of the sample surface cannot be accurately analyzed. In particular, the influence of the random error in the polarization angle on the characterization results obtained for the stress component is dominant. Conclusions: Although various experimental error sources are difficult to eliminate, the influence of the above factors on the characterization of stress components can be effectively weakened by improving the experimental design and analysis method. [ABSTRACT FROM AUTHOR]

In this paper, a new laser shock microforming method, laser shock sheet bulk microforging (LSSBMF), is put forward on the basis of the merit of sheet bulk forming and laser shock microforming. In this study, AZ31B magnesium alloy workpieces with different grain sizes, workpieces with different thicknesses, and microgroove dies with different widths are selected to evaluate the size effects on the forming properties. In the experiment, the influences of laser loading on grain size, grain size on depth of forming, and microhardness are studied. In the numerical simulation, the die geometry and workpiece size effects on the material forming process are analysed. The research results show that under the same laser energy loading, the larger the grain size is, the more obvious the grain size refinement phenomenon is. The forming depth increases with growing grain size, whereas the microhardness decreases. Numerical simulation results indicate that under the same laser energy loading, workpiece forming depth decreases with growing die feature groove width, whereas the equivalent plastic stress increases. The equivalent plastic stress increases with increasing workpiece thickness, whereas the forming depth decreases. [ABSTRACT FROM AUTHOR]

Hot gas forming (HGF) is an advanced technique for fabricating complex-shaped hollow tubular parts. Practically, multi-step pre-forming involving pre-deformation is often necessary prior to HGF. This paper performs an experimental investigation to simulate the pre-forming operation evaluating the effect of pre-strain on the subsequent HGF. Firstly, the dislocation density was accumulated with uniaxial pre-stretching with different strains (5%, 10%, and 15%) at room temperature, simulating the pre-forming operations. Secondly, the sub-sized specimen from the pre-stretched sample was characterized at different temperatures (350, 400, and 450 °C) to evaluate the effect of pre-strain on successive hot deformation for simulating practical HGF. The experimental results proves the coupled influence of pre-strain and temperature on the flow stress and stress-strain variations. To thoroughly understand the micro-mechanisms, EBSD analysis of grains and grain boundary angles was carried out under different pre-deformation levels and temperatures, which shows the recrystallization phenomenon at 15% pre-strain and 450 °C temperature. Finally, a physical mechanism constitutive model is established based on the determined macro and micro results where the pre-strain effect shows the accurate modeling of stress flow behavior of the material. [ABSTRACT FROM AUTHOR]