Development Of Internal Stresses Research Articles

Deterioration of fire resistance of ultra-high-performance polypropylene-fibre-reinforced concrete (UHPPFRC) under long-term acid rain corrosion action

The application of ultra-high-performance polypropylene-fibre-reinforced concrete (UHPPFRC) has become widespread due to its superior mechanical properties and non-flammable characteristic. Compared to UHPC, the fire resistance of UHPPFRC, which is crucial in ensuring structural integrity and safety during fire incident, has been improved by the addition of PP fibres. In recent years, the escalation of industrial emission has resulted in a more pronounced pollution of acid rain, a major type of sulphate attack on outdoor concrete structures, leading to the aging of UHPPFRC. Given the long service life of civil engineering structures, the fire performance of aging UHPPFRC should be given significant consideration, yet few studies have taken it into account. This study, for the first time, analyses the deterioration of fire resistance of acid-rain-induced aging UHPPFRCs and compares their probability of failure under fire condition across different corrosion durations, providing guidance for the maintenance of UHPPFRC in acid-rain-prone areas. A novel coupled chemo-thermal-mechanical model is developed, which is employed to investigate the sulphate penetration process and internal stress development within UHPPFRC during the corrosion stage, and to predict temperature and internal stress distribution during the subsequent heating stage. The effects of sulphate concentration, heating rate and fibre dosage on the probability of failure of aging UHPPFRC are then studied. The results obtained from the numerical investigation indicate that the aging effect can significantly compromise the fire resistance of UHPPFRC and augment the probability of failure at elevated temperature. Also, the probability of failure of aging UHPPFRC is more sensitive to changes in sulphate concentration than changes in heating rate.

Residual stresses prediction in transition metal nitrides sputtered coatings using artificial neural network and experimental evaluation of surface morphology

In the production of transition metal nitride hard coatings under specific deposition conditions, the resulting residual stresses play a critical role in determining the mechanical properties, adhesion, and overall performance of the coatings. In this work, an artificial neural network (ANN) that utilizes feed-forward architecture is used to predict the average residual stress in titanium nitride (TiN) coatings with different coating thickness. These coatings are manufactured under varying bias voltages and temperature on substrate. The impact of variations in coating thickness, bias voltage, and temperature on development of internal residual stresses and changes in microstructure within sputtered coatings have been gathered from existing literature. The data collected is within the context of fibrous (zone I) and mix columnar and fibrous structure (combined zone) as defined by an existing structure zone model. The model is trained with experimental data from literature. The model is trained with experimental data from literature. The validation of the ANN model is carried out by comparing the predictions with experimental results. X-ray diffraction is used to analyze the residual stresses, preferred orientation, and the structure (zone I and combined zone) of produced coatings under different sputtering parameters. The regression and performance curves are used to assess the efficacy of ANN model. The coefficient of determination of trained model and mean square error are 0.9918 and 0.35, respectively. The predicted results show good agreement with experimental and subsequently AFM grain morphology analysis captures the growth mechanisms involved in the residual stresses evolution.

Investigation into the impact of charging rates on the stress development within silicon composite electrodes.

Silicon, renowned for its remarkable energy density, has emerged as a focal point in the pursuit of high-energy storage solutions for the next generation. Nevertheless, silicon electrodes are known to undergo significant volume expansion during the insertion of lithium ions, leading to structural deformation and the development of internal stresses, and causing a rapid decline in battery capacity and overall lifespan. To gain deeper insights into the intricacies of charge rate effects, this study employs a combination of in situ measurements and computational modeling to elucidate the cyclic performance of composite silicon electrodes. The findings derived from the established model and curvature measurement system unveil the substantial alterations in stress and deformation as a consequence of varying charge rates. Notably, the active layer experiences compressive forces that diminish as the charge rate decreases. At a charge rate of 0.2, the active layer endures a maximum stress of 89.145 MPa, providing a comprehensive explanation for the observed deterioration in cycling performance at higher charge rates. This study not only establishes a fundamental basis for subsequent stress analyses of silicon electrodes but also lays a solid foundation for further exploration of the impact of charge rates on composite silicon electrodes.

Shrinkage characteristics of sustainable mortar and concrete with alkali-activated slag and fly ash Binders: A focused review

Alkali-activated binders are emerging as promising sustainable alternatives to ordinary Portland cement (OPC) due to their lower environmental footprint. They offer competitive mechanical properties and enhanced durability, making them attractive options for a wide range of construction and infrastructure applications. This paper is a focused review of key findings related to the shrinkage characteristics of mortar and concrete with alkali-activated slag and fly ash binders. Shrinkage induces nonuniform deformations in mortar/concrete, leading to the development of internal tensile stresses and the formation of cracks. Such cracks may be sufficiently large to facilitate the penetration of harmful substances, which may compromise the integrity of structural elements. Several studies indicated that the combination of fly ash and slag precursors in appropriate proportions produces mortar with favorable shrinkage cracks when compared to mortar with individual binders. This article discusses the influence of the following factors on shrinkage characteristics: binder type, binder proportions, water/binder ratio, binder content, aggregate content, alkaline activator type, and concentration. The article compares the shrinkage of mortar/concrete prepared using mostly alkali-activated slag and fly ash binders and compares their shrinkage response to OPC. Shrinkage types evaluated include chemical shrinkage, autogenous shrinkage, and drying shrinkage. Shrinkage responses reported in this article include the sample curing method and environment as they are known to influence durability, strength, and shrinkage characteristics. In general, mortar/concrete with alkali-activated fly ash and slag experience higher shrinkage rates than conventional concrete with OPC binders. The study further explores shrinkage mechanisms and methods to mitigate shrinkage in alkali-activated binders.

SiO2 soot preform sintering process for the synthesis of large-size and high-purity silica glass

The indirect chemical vapor deposition (CVD) method is a promising method to synthesize high-purity silica glass. As a critical procedure of this method, the sintering process is rather complex due to the interactive physical phenomena, and an in-depth understanding is therefore required. In this paper, a comprehensive multiphase porous medium model is developed by introducing the shrinkage mechanism, which agrees well with the experimental results. With this model, the dynamic characteristics during the sintering process, including temperature distribution, volumetric deformation, hydroxyl content evolution, and internal stress development, are well predicted. A parametric sensitivity analysis is conducted then, which shows that the average porosity and hydroxyl concentration are mainly affected by the average particle radius, while the maximum effective stress is sensitive to both Young's modulus and the average particle radius. Finally, the effects of key operating parameters are explored, and it is found that a lower working pressure, an appropriate sintering temperature such as 1450 °C, or a proper initial porosity such as 80.0%, is more desirable for a sintered product with high optical performance. This study can deepen the understanding of the SiO2 soot preform sintering process, and provide a good basis for further optimization.

Internal stress development within wood during drying: A master curve concept and its application on drying stress evaluation

The generation of internal stress (IS) in flat-sawn and quarter-sawn rubberwood boards during drying has been investigated using an online restoring force (RF) technique, which restrained a half-split rectangular wood specimen. Particular attention was given to the longest IS reversal regime. The IS development proceeds faster in the flat-sawn specimens than that in the quarter-sawn specimens while the maximum IS magnitudes in both specimens are rather similar. By normalizing the IS reversal period, a master IS profile, the derivative of the measured RF to the IS reversal time ratio versus the IS reversal time ratio, is proposed. This master curve, exhibiting some degree of independence from wood orientation and drying temperature, shows variations correlated with the free water content in the wet zone and the dry/wet zone fractions. The process of IS reversal, unaffected by temporary unrestraint, advances as the dry zone expands inwards and ends when the wet zone disappears. Assuming mechanical equilibrium between the dry and wet zones, the IS in both zones can be estimated from the RF data. The maximum tensile IS in the dry zone, indicating a risk of surface checks, evolves at slightly lower magnitudes at higher drying temperatures and is lower in the quarter-sawn specimens. The IS relaxation in the dry zone, still largely taking place in the absence of the applied RF, highlights the main contribution of the dry/wet zone fractions, continuing to proceed without restraining, to the IS development. These findings, emphasizing the significance of dry and wet zones, should pave the way for a better understanding of the IS development within wood during drying.

Unraveling kinking: A plasticity enhancing failure mode in high strength nano metallic laminates

Kinking is an important and plasticity-enhancing deformation/failure mode in numerous mechanically anisotropic materials including high-strength nano metallic laminates (NMLs). However, our current limited understanding of the mechanics of kinking and its dependence on microstructural attributes is insufficient for thoroughly comprehending and eventually being able to control failure behaviors of materials. In this study, we investigate kinking dependencies on microstructural attributes in NMLs via in situ micropillar compression, multiscale microstructure characterization, dislocation dynamic simulations, and crystal plasticity modeling. By examining several NML systems (Cu/Fe, Ag/Fe, Al-4Mg/Fe), we demonstrate that the development of internal stresses during loading activates local layer-parallel glide triggering kinking in NMLs. Furthermore, this work reveals the effect of key microstructural features including layer thickness, layer waviness, interface barrier strength, and work hardening capacity on kink band formation in NMLs. More broadly, our efforts represent a generically applicable approach for probing large-strain deformation behavior of complex materials via synergetic modeling and experimental efforts.

A generalised diffusion equation corresponding to continuous time random walks with coupling between the waiting time and jump length distributions

This work outlines a transiently coupled continuous time random walk framework. The coupling is between the displacement probability density function (PDF) and the elapsed waiting time, and is of the form . Coupling of this kind generates larger displacements for longer waiting times, however, decouples on longer timescales. Such coupling is proposed to be physically relevant to systems in which diffusion is driven by the development of internal stresses which release and develop cyclically. This article outlines the associated generalised diffusion equation (GDE), which describes the time evolution of the position PDF, . The solution for is obtained using the properties of the Fox H function, both in terms of its transform properties but also its expansion theorems. The second moment and the asymptotic features of the solution are extracted. The relaxation of back to the solution of a decoupled-type GDE is highlighted.

Temperature effect on elastic and fracture behaviour of lead-free piezoceramic BaTiO3

Ferroelectric ceramics, especially piezoceramics, are widely used in the field of sensors, micromanipulators, and capacitors. Barium titanate and its derivates are the first choices between lead-free materials. The main aim of the paper is to clarify fundamental processes taking place in the vicinity of Curie temperature with a focus on the fracture behaviour of pure polycrystalline barium titanate. The explanation of observed changes in the mechanical behaviour of this material is based on the experimental approach supported by numerical simulations utilising features of the real microstructure on the grain level. Several model materials with various grain microstructures were manufactured from the submicron barium titanate powder sintered at various temperatures. Two resulting materials with a suitable distribution of grains were selected for further investigation. The grain size influenced not only the exact position of the temperature of the Curie point but also the kinetics of the lattice transformation, elastic, and fracture properties. The significant drop observed in the fracture resistance was attributed to the development of localised internal thermal stresses, which was supported by the results of the performed numerical simulations. The coincidence of the volume change of neighbouring grains due to lattice transformation together with a significant variation in elastic properties can lead up to a 20% decrease in the measured fracture toughness. Understanding this behaviour is essential for the processing and correct application of lead-free barium titanate materials.

A Manufacturing Process Simulation of Toughened Cyanate-Ester-Based Composite Structures with Respect to Stress Relaxation.

The objectives of this study were to experimentally determine the effects of the stress relaxation of a cyanate-ester-based composite, derive and integrate constitutive equations into commercial FEM software, and apply this approach to understand the formation of residual stress in a typical aerospace structure—namely, a stiffened panel. In preliminary studies, a cyanate-ester-based composite with increased fracture toughness for high-temperature applications was developed. High curing temperatures up to 260 °C will inevitably lead to high process-induced stresses. To assess the magnitude of impact on the development of internal stresses, the relaxation behavior of the neat resin was measured and characterized. The system was toughened, and the effect of stress relaxation increased as the temperature got closer to the glass transition temperature of the toughener, which was approximately 240 °C. With the use of an incremental linear viscoelastic model, the relaxation behavior was integrated into a process model with a holistic approach. A stiffened panel was manufactured and used as the validation use case. The displacement field was validated with an optical 3D measuring system, and good agreement was found between the simulated and experimental results. The maximum difference between the elastic and the viscoelastic solution was found to be 15%. Furthermore, the stress magnitude in the transverse material direction resulted in a more critical value higher than the material strength.

Curing-induced internal stress in epoxy coatings: Effects of epoxy binder, curing agent, filler, initial solvent concentration, curing temperature, and relative humidity

Curing-induced internal stresses in epoxy coatings are highly influenced by the type and concentration of product ingredients and the conditions applied. In this work, the effects on the curing-dependent modulus and the internal stress development of the epoxy/crosslinker chemistry, curing temperature, relative humidity, filler conditions, and initial solvent concentration, are studied. Analytical methods include the attenuated total reflection-Fourier transform infrared (ATR-FTIR) technique, dynamic mechanical thermal analysis (DMTA), a 3D optical profilometer, and beam deflection.An elevated curing temperature (35 or 45 °C) resulted in a smaller elastic modulus and, despite an accelerated curing reaction and a higher final reactant conversion, only a slight increase, around 0.2 MPa, in the average internal stress. An increased relative humidity (from 35 to 60 %), also resulted in a smaller elastic modulus and less volumetric shrinkage and internal stress. However, at 90 % relative humidity, the internal stress, due to an enhanced final reactant conversion, was higher than observed at 60 % RH.The presence of either BaSO4 or CaCO3 filler in the formulation reduced the final reactant conversion and volumetric shrinkage, but resulted in a higher elastic modulus and internal stress.When the solvent concentration increased from zero to 20 vol%, the final conversion, although extra volumetric shrinkage was introduced by the solvent evaporation, increased from 0.72 to 0.85, while the internal stress decreased from 1.25 to 0.6 MPa.Guidelines for how to optimize coating formulations and curing conditions, to avoid internal stress, are presented.

On the design optimisation of direct energy deposited support structures to repair aero-engine turbine segments

A novel approach for down-selection of a repaired support structure design produced using Laser Blown Powder – Direct Energy Deposition (LBP-DED) and filled with interstitial Ni-Al powder (∼0.75 area fraction) in a turbine segment was investigated. Simulation of flattening and un-flattening of the segment with implications to degradation of the support structure was quantified using a four-point bend test to identify the role of axial Young’s modulus in out-of-plane flexure. Two markedly different LBP additive structures; Diamond Lattice (DL) - nodal and Continuous Path (CP) – non-nodal, were produced and compared with the un-repaired condition. At room temperature, the forward and rear walls and internal nodes of the original equipment (OE) and DL support structures were found to contribute significantly to the Young’s modulus, with significantly reduced stiffness observed in the CP structures. Oxidation plays a key role in the development of internal compressive stresses within the abradable, with a two-fold increase in elastic modulus in the CP structure, but a smaller increase occurred in OE and DL support structures. A decrease in elastic modulus and concomitant increase in radius of curvature (flattening) occurred with an increasing number of flexural cycles. Cracking is most prominent in the nodal design within the front and rear walls and cracks propagate either to the surface or towards the base of the abradable lattice. No such degradation was observed for equivalent flexural cycles in the original and CP support structures, even up to a significant number of cycles. A criterion for catastrophic failure of the abradable was deduced from a steep decrease in flexural elastic modulus accompanied with a marked change in curvature. A non-nodal design support structure is optimum to counter in- service flattening/un-flattening.

Internal stress development within wood during drying: regime and kinetics

Internal stress (IS) development in juvenile and mature rubberwood during drying has been studied using an online measurement of a restoring force (RF) required to restrain a half-split rectangular wood specimen. According to an abrupt change in the restoring force rate, the evolution of IS can be divided into four distinct regimes, i.e., (i) zero IS, (ii) accelerating IS, (iii) reversing IS and (iv) decelerating IS. In stage I, the wood moisture content (MC) is still above fiber saturation point (FSP) with no drying shrinkage. In stage II, the negative RF constantly accelerates. IS mainly develops due to drying shrinkage with the activation energy of 19 kJ/mol. In stage III, the negative RF sharply decelerates and later reverses to the positive value. This regime takes place as the dried zone moves inwards and ends when there is no wet zone left. The stress reversal phenomenon having the activation energy of 36 kJ/mol contains two negative maxima, which occur faster in mature wood than in juvenile wood. Possible creep relaxation due to shear deformation of hemicellulose/lignin matrix between the cellulose fibrils is discussed. Finally, the positive RF decelerates in stage IV with the RF rate gradually declining and remaining at zero up until the end of drying. The corresponding activation energy of stage IV is 52 kJ/mol for both juvenile and mature rubberwood.

Evaluation and mechanisms of internal stress development in high-build epoxy protective coatings during thermal cycling

Long-term protection in highly corrosive environments has advanced to use high-solid coatings with high thickness, known as high-build coatings. However, these coatings were found to be more susceptible to cracking and delamination failures in-service with temperature variations. To understand how a high-build film is stressed at different temperatures, we developed an efficient measurement method by employing imaging scanning device and image analysis to quantitatively evaluate internal stress in two purposely formulated epoxy coatings with different elasticities through 30 thermal cycles between −25 to 50 °C. The results showed that the magnitude of internal stress was dependent on the coating elasticity, film thickness, and the difference in coefficient of linear thermal expansion (CTE) between the coating and its substrate apart from the service temperature and the thermal cycles. Moreover, thicker coating develops a significantly higher internal stress, which explains why cracking or delamination is typically associated with excessively thick coatings. The investigated mechanisms of the stress development during thermal cycling will be a useful guide to formulate mechanically durable high-build coatings.

Mesoscopic simulation of crack propagation and bond behavior in ASR damaged concrete with internal/external restraint by 3D RBSM

One of the most serious serviceability concerns for reinforced concrete (RC) structures is expansion and cracking resulting from the alkali-silica reaction (ASR), which has a negative effect on material properties as well as the bond between reinforcement and surrounding concrete. In a typical RC member, ASR induced concrete expansion and cracking are restrained by internal reinforcement as well as the boundary conditions. The mechanism is complex and difficult to understand through experimental study, so predicting the residual capacity of a damaged RC structure is not easy. In this study, the authors use a 3D Rigid Body Spring Model (RBSM) comprising mortar, aggregate and steel elements and which is able to simulate ASR expansion in reinforced concrete. To study the complex interactions among multiple parameters and quantify the effect of ASR damage on structural behavior, previously reported experiments on ASR induced expansion under internal and external restraint are simulated, along with experiments on pullout behavior after ASR expansion. The effect of restraints on macroscopic ASR expansion is well modeled in each case, and how the development of internal stresses and concrete cracking influenced by the restraint can be explained from the simulations. The simulations enable discussion of the number of ASR-induced cracks and the internal stress condition in various cases. The peak bond strength of concrete with different ASR damage levels, as reported in pullout experiments, is predicted accurately and the load-displacement curves of ASR damaged concrete are discussed.

A non-local model for the description of twinning in polycrystalline materials in the context of infinitesimal strains: application to a magnesium alloy

A polycrystalline plasticity model, which incorporates the contribution of deformation twinning, is proposed. For this purpose, each material point is treated as a composite material consisting of a parent constituent and multiple twin variants. In the constitutive equations, the twin volume fractions and their spatial gradients are treated as external state variables to account for the contribution of twin boundaries to free energy. The set of constitutive relations is implemented in a spectral solver, which allows solving the differential equations resulting from equilibrium and compatibility conditions. The proposed model is then used to investigate the behavior of a AZ31 magnesium alloy. For the investigated loading conditions, the mechanical behavior is controlled by the joint contribution of basal slip and tensile twinning. Also, according to the numerical results, the development of crystallographic texture, morphological texture and internal stresses is consistent with the experimental observations of the literature.

Evolution of temperature stress and tensile properties of concrete during steam-curing process

Nowadays, steam curing is widely used in the production of prefabricated components. However, studies have shown that steam curing can lead to micro or macro-cracks in concrete caused by the curing regime which can affect the performance of the concrete in all aspects. Herein, the internal stress and tensile strength development of concrete during the steam-curing process was studied, so as to evaluate the crack risk of concrete during steam curing. To evaluate the internal stress during the steam curing, the internal temperature and thermal deformation of the concrete were tested. Considering the difficulty of measuring the tensile properties of early-age cement-based materials, a mortar tensile test device combined with digital image correlation (DIC) was designed in the paper. Meanwhile, thermal analysis tests were done to analyse the CH content of the concrete during steam curing. The results indicated that the mortar tensile test device in combination with the DIC designed in the paper was an effective method for testing the early tensile properties of cement-based materials. In the four stages of steam curing, micro-cracks in the concrete were most likely to occur in the cooling process. The addition of supplementary cementitious material is an effective method for restraining surface cracking in steam-cured concrete, and to ensure the tensile strength, sufficient water vapour is needed in the steam-curing process for a concrete mixture with large mineral admixtures, otherwise, a reduction in the tensile strength of more than 50% will occur.

Nucleating role of P2O5 in nepheline‐based transparent glass‐ceramics

AbstractThe nepheline‐based transparent glass‐ceramics are promising candidates for cover glass applications in electronic displays owing to their superior mechanical properties (than glasses) and ability to be chemically strengthened. However, our poor understanding about the kinetic and thermodynamic drivers controlling their crystallization processes usually results in their opacification and development of large internal stresses. The present work focuses on the development of nepheline‐based nanocrystalline transparent glass‐ceramic designed in the Na2O–Al2O3–SiO2 ternary system nucleated with P2O5. The temporal evolution of the phosphate and nepheline nanocrystal formation has been followed using X‐ray diffraction, scanning/transmission electron microscopy, and energy‐dispersive spectroscopy. The incorporation of P2O5 in the glass structure leads to the phase separation resulting in the crystallization of nanocrystalline Na3PO4 as an intermediate phase; thus, acting as a nucleating site for volume crystallization of nepheline. The optimization of nucleation and growth profile in the designed composition results in the formation of a transparent glass‐ceramic with high optical transmittance (91.5 ± 0.1%).

Thermally Induced Gradient of Properties on a Superhydrophobic Magnesium Alloy Surface

Fabrication of superhydrophobic coatings for magnesium alloys is in high demand for various industrial applications. Such coatings not only extend the service life of metal structures, but also impart additional useful functional properties to the coated surface. In this study, we show that nanosecond laser processing of long, thin stripes of magnesium alloys followed by the deposition of a hydrophobic agent onto the magnesium oxide layer is a simple, convenient, and easily reproducible method for obtaining superhydrophobic surfaces with property gradient along the sample. The mechanism of the gradient in wettability and electrochemical properties of the magnesium alloy surface is discussed based on the high-temperature growth of magnesium oxide and its following degradation. The latter is related to the development of internal stresses and the formation of cracks and pores within the oxide layer at prolonged exposure to high temperatures during the interaction of a laser beam with the substrate. The effect of heating during laser processing of magnesium materials with limited sizes on the protective properties of the forming coatings is elucidated.

Mechanisms of Stress Generation in Thin Films and Coatings

Analysis of the literature data on the reasons for the development of mechanical stresses in epitaxial, polycrystalline, and amorphous films during their formation and under various external influences is carried out. The mechanism of the appearance of internal stresses during heteroepitaxial growth of films caused by the mismatch of the crystal lattice constants of the film and substrate is described. The relationship between the development of mismatch stresses in heteroepitaxial films and changes in the nature of their growth is shown. Models of the occurrence of compressive and tensile stresses in polycrystalline films due to the formation and coalescence of islands at the initial stage of their growth are considered. The regularities of the evolution of internal stresses in continuous films are discussed depending on the conditions of their deposition, as well as their chemical composition, structure, and mechanical properties. The mechanisms of development of internal stresses in thin films associated with the formation of point defects in them, the incorporation of impurities, and phase transformations occurring in the deposition process are reviewed. External factors that lead to the appearance of stresses in thin films during their storage and operation are considered in detail.