Generation Of Internal Stresses Research Articles

Investigation of the Impact of Defects on Internal Stress Generation in DO3 Crystals

Investigation of the Impact of Defects on Internal Stress Generation in DO3 Crystals

Insights into the Fragmentation of Aluminum Hydride and Its Effect on Combustion and Agglomeration of HTPB Propellant.

AlH3 has gained considerable attention as a fuel additive due to its ability to offer high specific impulse and superior combustion performance. However, few studies have focused on the fragmentation and agglomeration behavior of AlH3. This study investigated the effects of fragmentation of AlH3 and AlH3/PVDF particles on the thermal decomposition, ignition, agglomeration, and combustion of HTPB propellants. Thermal analysis indicated that AlH3 and AlH3/PVDF can accelerate the decomposition of ammonium perchlorate by abundant active sites for the adsorption of the decomposition intermediates. Single-particle combustion uncovered the mechanism behind the directional spray of molten Al from the AlH3 particle and the fragmentation of the AlH3/PVDF particle. The melting of porous Al induces particle shrinkage due to solid-liquid interfacial tension and the structural restoration of the oxide shell, which consequently results in the sealing of cracks in the oxide shell of AlH3. Additionally, the accumulation of internal tensile stress leads to the reopening of these cracks and the directional ejection of the molten Al. The flexible oxide shell contributes to a smaller minimum normalized diameter of the AlH3/PVDF particle, aiding in the generation of internal tensile stress, while the sublimation of AlF3 induced the fragmentation. Synchrotron-based X-ray imaging revealed the formation of aggregates promoted by molten Al, the splitting of AlH3 aggregates due to hydrogen explosion, and the enhanced fragmentation of AlH3/PVDF due to the synergistic effect of hydrogen explosion and the sublimation of AlF3. Compared to raw particles, the CCPs (condensed combustion products) of SP2 propellant display a 48% reduction in average size (D50 = 24.5 μm), whereas there is an over 89% decrease in particle size for the CCPs of SP3 propellant (D50 = 5.14 μm). This study contributes to understanding the fragmentation of AlH3 and AlH3/PVDF upon ignition and combustion, providing valuable insights for the development and optimization of propellants containing AlH3.

Effects of Interlayer on the Microstructure and Mechanical Properties of Resistance Spot Welded Titanium/Steel Joints: A Short Review

In this paper, the influence of interlayer on titanium/steel dissimilar metal resistance spot welding is reviewed from the aspects of macroscopic characteristics, microstructure and interface bonding properties of the joint. Previous studies have demonstrated that TiC, FeTi and Fe2Ti intermetallic compounds with high brittleness are formed in the joint during titanium/steel welding, which reduces the strength of the welded joint. Researchers proposed different interlayer materials, including Cu, Ni, Nb, Ta, 60%Ni-Cu alloy and BAg45CuZn. Firstly, adding an interlayer can weaken the diffusion of Fe and Ti. Secondly, the interlayer elements can combine with Fe or Ti to form solid solutions or intermetallic compounds with lower brittleness than Fe–Ti compounds. Finally, Cu, Ni, Ag, etc. with excellent ductility can effectively decrease the generation of internal stress, which reduces the formation of defects to improve the strength of the joint.

Characterisation of Curing of Vinyl Ester Resin in an Industrial Pultrusion Process: Influence of Die Temperature.

Pultrusion is a high-volume manufacturing process for Fibre-Reinforced Polymer (FRP) composites. It requires careful tuning and optimisation of process parameters to obtain the maximum production rate. The present work focuses on the correlation between the set die temperatures of 80 °C, 100 °C, 120 °C, and 140 °C and the resin cure state at constant pull speeds. Lab-scale oven trials were conducted to understand the thermal behaviour of the resin system and to provide a temperature range for the pultrusion trials. Dielectric Analysis (DEA) was used during pultrusion trials to monitor the effect of die temperature on the cure progression. The DEA results showed that, by increasing die temperature, the exothermic peak shifts closer towards the die entry. Moreover, the degree of cure for samples processed at 140 °C was 97.7%, in comparison to 86.2% for those cured at 100 °C. The rate of conversion and the degree of cure correspond directly to the set die temperatures of the pultrusion trials, contributing to understanding the effect of die temperature on cure progression. Mechanical and thermal material properties were measured. Samples cured at 120 °C showed the highest mechanical performance, exceeding those cured at 140 °C, linked to the generation of higher internal stress due to the higher rate of conversion. This work can be used as a guide for pultruded composite sections, to understand the cure behaviour of resin systems under various applied temperatures and the impact of the die temperature conditions on thermal and mechanical properties.

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.

Semi-analytical solution for thermo-poro-elastic stresses in a wellbore cement plug and implications for cement properties that minimize risk of failure

In high temperature and high pressure environments, inducing pore pressure changes in low permeability cement can be problematic. Despite the typical design goal of achieving very low permeability in cement plugs for better isolation, this work reveals through a new semi-analytical TPE stress solution that lower permeability is not always ideal. In fact, such materials are unable to rapidly drain excess pore pressure compared to the rate at which it builds due to thermal changes may lead to the generation of internal tensile stress and increased risk of damage. This work identifies the parameter groups linked to the “permeability penalty” by utilizing dimensional and pairwise bivariate analysis through Monte Carlo sampling. It proposes two dimensionless parameter groups that divide into three distinct regions based on the likelihood of producing tensile effective stresses in a plug. This work provides a new wellbore cement design concept focuses on achieving the lowest permeability without risking failure from pore pressure buildup, rather than simply achieving the lowest possible permeability. Furthermore, this work for the first time highlights the important role of cement's specific heat in mitigating pore pressure buildup, paving a new path for enhancing this property in future cement design.

Chemo-Mechanical Analysis of Lithiation/Delithiation of Ni-rich Single Crystals

Single crystal, Ni-rich layered lithium metal oxides are promising candidates for next-generation cathodes in lithium-ion batteries. However, these Ni-rich materials display anisotropic swelling and contraction during cycling, and this may lead to the generation of internal stresses and thereby to fracture and capacity loss. In this work, the spatio-temporal evolution of lithium concentration and stress state within a (NMC811) single crystal are predicted using a fully coupled chemo-mechanical model. The stress state in the crystal arises from a spatially non-uniform distribution of Li concentration, and from a non-linear dependence of intercalation strain upon lithium concentration. The peak tensile stress is greatest near top-of-charge, due to the high sensitivity of intercalation strain upon lithium occupancy at low concentrations, and the peak tensile stress increases with both cycling rate and particle dimension. Significantly, the predicted peak tensile stress is insufficient to cause basal plane fracture of single crystals when their diameter is below 2.5 μm and the charging and discharging rates are below 5C. This suggests that intraparticle fracture is not a significant degradation mode for well-designed NMC811 single crystals.

Lattice parameters of austenite and martensite during transformation for Fe–18Ni alloy investigated through in-situ neutron diffraction

Martensitic transformation is accompanied by the generation of microscale and macroscale internal stresses during cooling below the martensitic transformation start temperature. These internal stresses have been determined through X-ray or neutron diffraction, but the reported results are not consistent, probably because the measured lattice parameter is influenced not only by the internal stress but also by several factors, including solute elements and crystal defects. Therefore, in-situ neutron diffraction combined with dilatometry measurements during martensitic transformation and subsequent cyclic tempering were performed for an Fe–18Ni alloy. The phase strains calculated by lattice parameter variations show that a hydrostatic compressive strain in austenite and a tensile strain in martensite arose as the martensitic transformation progressed during continuous cooling or isothermal holding. However, the phase stresses of austenite and martensite estimated from these strains failed to hold stress balance law when dense crystal defects involved in the processes. After these crystal defects were removed by appropriate tempering, the stress balance law held well. Meanwhile, the phase stresses of austenite and martensite were changed to opposite, revealing their true identity. Furthermore, the lattice parameter of quenched full martensite and the specimen diameter were both decreased by tempering, demonstrating that there is a direct correlation among lattice parameter, specimen volume and dislocation density. As a result, various crystal defects in austenite and martensite, introduced by plastic accommodation, were suggested to affect their lattice parameters and then their phase stresses.

MICROSTRUCTURAL CHANGES OF THE PEO COATING INDUCED BY SILANE-BASED SOL–GEL TREATMENT

In this paper, the silane-based sol–gel treatment process was applied to the plasma electrolytic oxidation (PEO) ZM6 Mg-alloy. The microstructural characteristics and corrosion resistance of the PEO/organic composite coatings were studied by means of SEM and XRT in conjunction with electrochemical measurements. The results showed that the microdefects in the PEO coating could be effectively filled with the silane compounds, reducing the porosity of the PEO coating. The treated PEO coating had a hydrophobic property, which may be related to the siloxane network (Si–O–Si) formed on the PEO coating surface. The corrosiveness of the silane agent to the PEO coating and the generation of internal stress in the PEO coating resulted in the damage of microstructures of the PEO coating. The global and local electrochemical measurements showed that the corrosion-resistant performance of the PEO Mg-alloys after the silane-based sol–gel treatment was improved and its service life would be shortened by the balance of micropores without sealing and the damaged microstructure.

Oxygenation-Controlled Collective Dynamics in Aquatic Worm Blobs.

Many organisms utilize group aggregation as a method for survival. The freshwater oligochaete, Lumbriculus variegatus (California blackworms) form tightly entangled structures, or worm "blobs", that have adapted to survive in extremely low levels of dissolved oxygen (DO). Individual blackworms adapt to hypoxic environments through respiration via their mucous body wall and posterior ciliated hindgut, which they wave above them. However, the change in collective behavior at different levels of DO is not known. Using a closed-loop respirometer with flow, we discover that the relative tail reaching activity flux in low DO is ∼75x higher than in the high-DO condition. Additionally, when flow rate is increased to suspend the worm blobs upward, we find that the average exposed surface area of a blob in low DO is ∼1.4x higher than in high DO. Furthermore, we observe emergent properties that arise when a worm blob is exposed to extreme DO levels. We demonstrate that internal mechanical stress is generated when worm blobs are exposed to high DO levels, allowing them to be physically lifted off from the bottom of a conical container using a serrated endpiece. Our results demonstrate how both collective behavior and the emergent generation of internal mechanical stress in worm blobs change to accommodate differing levels of oxygen. From an engineering perspective, this could be used to model and simulate swarm robots, self-assembly structures, or soft material entanglements.

Analysis of factors influencing on performance of solid tires: Combined approach of Design of Experiments and thermo-mechanical numerical simulation

Excessive loading conditions and internal heat build-up are the major causes of failures in solid resilient tires. Both operational and design related factors can affect tire performance including internal stress generation and heat build-up. In spite of their limitations, experimental techniques are still the most common method of identifying the factors that affect tire performance. This study explores a combined approach of numerical modelling and Design of Experiments (DoE) to identify the impact of several key design and operational factors on the performance of solid tires under different temperature levels. First, 3D Finite Element (FE) static and thermal models of the solid resilient tires are developed by incorporating suitable temperature dependent hyperelastic material models and relaxation properties. The temperature-dependent mechanical behaviours of rubber material are obtained by using Dynamic Mechanical Analyzer (DMA) and the William Landel Ferry (WLF) parameters. The developed FE thermal model is used to predict the tire performance, including the tire blasting region, which is then validated with experimental data. Secondly, a two-level fractional factorial DoE analysis is developed using response readings of maximum stresses, deformations and contact pressure obtained from the validated FE thermal model. The factors used in the DoE include the number of rubber layers, applied load, rolling speed, ramp angle and aperture level. The results provide optimal operational and design parameters of the solid resilient tire at different environmental temperatures and show the potential for considerable stress reduction with minimal changes to tire deformation and contact area. These results highlight the capability of the combined FEM and statistical design approach to drastically reduce the number of physical experiments required for obtaining optimal tire performance thus saving significant time and cost.

Negative Thermomechanical Effects in Granular Composites with Incompatible Thermomechanical Parameters of Their Components

Within the framework of the theory of thermoelasticity, the problem of the generation of additional internal thermal stresses in a composite material with granular fillers under the influence of temperature is posed. Using the spherical inclusion model applied in the theory of composites, constitutive equations are formulated, and their analytical closed-form solutions are constructed. It is shown that the internal thermal stresses initiated in the system have a localized character, are proportional to the values of the elastic moduli of the materials of each phase and the difference between their coefficients of linear thermal expansion, and decrease in inverse proportion to the cube of the radial coordinate. With the help of specific applied examples, it has been established that even at relatively moderate values of temperature changes, these stresses can reach limit values that contribute to the initiation of internal latent localized defects and cracks in the system. The cases of thermally stressed states of a spherical inclusion coated with a layer of another material, an inclusion with a cavity, and a spherical pore are also considered.

Lack of Functional Trehalase Activity in Candida parapsilosis Increases Susceptibility to Itraconazole.

Central metabolic pathways may play a major role in the virulence of pathogenic fungi. Here, we have investigated the susceptibility of a Candida parapsilosis mutant deficient in trehalase activity (atc1Δ/ntc1Δ strain) to the azolic compounds fluconazole and itraconazole. A time-course exposure to itraconazole but not fluconazole induced a significant degree of cell killing in mutant cells compared to the parental strain. Flow cytometry determinations indicated that itraconazole was able to induce a marked production of endogenous ROS together with a simultaneous increase in membrane potential, these effects being irrelevant after fluconazole addition. Furthermore, only itraconazole induced a significant synthesis of endogenous trehalose. The recorded impaired capacity of mutant cells to produce structured biofilms was further increased in the presence of both azoles, with itraconazole being more effective than fluconazole. Our results in the opportunistic pathogen yeast C. parapsilosis reinforce the study of trehalose metabolism as an attractive therapeutic target and allow extending the hypothesis that the generation of internal oxidative stress may be a component of the antifungal action exerted by the compounds currently available in medical practice.

Elaboration and mechanical properties of monolithic and multilayer mullite-alumina based composites devoted to ballistic applications

Both monolithic and multilayer mullite-alumina ceramic composites dedicated to ballistic applications were produced by uni-axial pressing and tape casting before being sintered at 1510 °C. The compositions combining andalusite and kaolin in addition to α-alumina exhibit the more promising mechanical properties, reaching the performances of pure ballistic alumina. However, for a given bulk density value, a significant improvement of the performances (Young's modulus and ballistic impedance) only by varying the composition of the monoliths is limited. The second approach reported in this work has consisted in the development of judicious multilayer materials based on the generation of internal residual stresses in thinner layers. Compared to the monolithic materials, the most efficient multilayer configurations exhibit a failure stress improved by +35% and a fracture energy increased by +60%. Indeed, the layers subjected to compressive stress promote a significant crack bifurcation during rupture.

Effect of microstructure and internal stress on hydrogen absorption into Ni thin film electrodes during alkaline water electrolysis

Efforts to improve the cell efficiency of hydrogen production by water electrolysis continue to address the electrochemical kinetics of the oxygen and hydrogen evolution reactions in detail. The objective of this work is to study a parasitic reaction occurring during the hydrogen evolution reaction (HER), namely the absorption of hydrogen atoms into the bulk electrode. Effects of the electrode microstructure and internal stress on this reaction have been addressed as well in this paper. Ni thin film samples were deposited on a Si substrate by sputter deposition with different deposition pressures, resulting in different microstructures and varying levels of internal stress. These microstructures were first analyzed in detail by Transmission Electron Microscopy (TEM). Cathodic chrono-amperometric measurements and cyclic voltammetries have then been performed in a homemade electrochemical cell. These tests were coupled to a multi-beam optical sensor (MOS) in order to obtain in-situ curvature measurements during hydrogen absorption. Indeed, since hydrogen absorption in the thin film geometry results in a constrained volume expansion, internal stress generation during HER can be monitored by means of curvature measurements. Our results show that different levels of internal stress, grain size and twin boundary density can be obtained by varying the deposition parameters. From an electrochemical point of view, this paper highlights the fact that the electrochemical surface mechanisms during HER are the same for all the electrodes, regardless of their microstructure. However it is shown that the absolute amount of hydrogen being absorbed into the Ni thin films increases when the grain size is reduced, due to a higher grain boundaries density which are favourite absorption sites for hydrogen. At the same time, it was concluded that H2 evolution is favoured at electrodes having a more compressive (i.e. a less tensile) internal stress. Finally, the subtle effect of microstructure on the hydrogen absorption rate will be discussed as well.

Evaluation of The Electrochemo-Mechanically Induced Stress in All-Solid-State Li-Ion Batteries

The mechanical degradation of all-solid-state Li-ion batteries (ASSLBs) is expected to be more severe than that in traditional Li-ion batteries with liquid electrolytes due to the additional mechanical constraints imposed by the solid electrolyte on the deformation of electrodes. Cracks and fractures could occur both inside the solid electrolyte (SE) and at the SE/electrode interfconce. A coupled electrochemical-mechanical model was developed and solved by the Finite Element Method (FEM) to evaluate the stress development in ASSLBs. Two sources of volume change were considered, namely the expansion/shrinkage of electrodes due to lithium concentration change and the interphase formation at the SE/electrode interface due to the decomposition of SEs. The most plausible solid electrolyte decomposition reactions and their associated volume change were predicted by density functional theory (DFT) calculations. It was found that the stress associated with a volume change due to solid electrolyte decomposition can be much more significant than that of electrode volumetric changes associated with Li insertion/extraction. This model can be used to design 3D ASSLB architectures to minimize their internal stress generation.

In-situ monitoring of hydrogen absorption into Ni thin film electrodes during alkaline water electrolysis

Although hydrogen production by water electrolysis has already been studied intensively, research efforts continue to be directed towards further improving the electrochemical kinetics and the overall cell efficiency. The objective of the current work is to address an often overlooked parasitic reaction step during the hydrogen evolution reaction (HER), namely the diffusion and absorption of hydrogen adatoms from the electrode surface into the bulk. Chrono-amperometric measurements have been realised in a homemade electrochemical cell on Ni thin film samples. The latter were coupled to a multi-beam optical sensor to obtain high resolution in-situ curvature measurements during the HER. In the thin film geometry, hydrogen absorption into the electrode results in a constrained volume expansion. This in turn leads to internal stress generation, which can be monitored in-situ by means of curvature measurements. Our results on Ni thin film electrodes during alkaline water electrolysis show that such in-situ curvature measurements allow to study both the kinetics and the amount of hydrogen absorption in great detail. In particular, we observed that a higher fraction of hydrogen is being absorbed into the electrode at low overpotential, and that the rate of absorption increases significantly with increasing overpotential. We also discuss the impact of these observations on the identification of the rate-determining steps during a typical HER polarisation curve.

In-Situ Observation of Martensitic Transformation in a Fe–C–Mn–Si Bainitic Steel During Austempering

The martensitic transformation in a Fe–C–Mn–Si bainitic steel was examined by in situ high-temperature laser scanning confocal microscopy (LSCM) and dilatometry. The phenomenon of continuous martensitic transformation during austempering was firstly dynamically observed by LSCM. Differing from the commonly accepted viewpoint on martensite formation in bainitic steels, the martensitic transformation in the conventional medium-carbon bainitic steel was not instantaneous and proceeded gradually when the sample was austempered below martensite starting temperature (MS). It can be attributed to the generation of internal stresses, thermal activation, stimulating nucleation, and the segregation of Mn. In addition, apart from the continuous martensitic transformation, the bainitic transformation was also directly observed by LSCM during austempering below MS. Moreover, it was clear from the results of dilatation during austempering that the inflection point in the dilatation curve against time was not the demarcation point between martensitic and bainitic transformation, and in situ observations confirmed that martensite was still formed after the inflection point. Therefore, the obtained results could be an excellent reference to further understand the mechanism of bainitic and martensitic transformations in Fe–C–Mn–Si bainitic steel during austempering below MS. Graphical abstract presents a series of frames that reveals the rapid growth of some black units during austempering at 240 °C, and these rapidly growing units (marked by arrows) were not present in the previous frame of the video. These black growing units (martensite laths) shown in Graphical abstract appeared over a time interval of ~ 420 s from 837.39 to 1257.37 s during austempering. This phenomenon differs from the previously held viewpoint where martensitic transformation is expected to be finished in a very short period of time after a sample is cooled to a certain temperature below MS because of the diffusionless nature of martensite transformation in conventional medium-carbon bainitic steels. The growth of martensite laths during the austempering process at 240 °C.

Sintering temperature dependent micro and macro mechanical properties of Si3N4f/SiO2 composite materials

Wave-transparent materials for the high temperature use require highly reliable mechanical properties and excellent anti-oxidation performance. This study reports the densification, interfacial evolution, micro and macro mechanical properties of Si3N4f/SiO2 composites sintered at 800–1200 °C. The results show that significant shrinkage of silica matrix at 1100–1200 °C helps improve the density of Si3N4f/SiO2 composites, but also leads to an obvious reduction of mechanical properties. The bending strength of composites sintered at 800–1000 °C reaches 166.5–174.0 MPa, but linearly decreases to 102.5 MPa at 1100 °C and 19.8 MPa at 1200 °C. Similarly, fracture toughness also experiences a linear decline, decreasing from 7.0 to 7.2 MPa m1/2 at 800–1000 °C to around 3.7 MPa m1/2 at 1100 °C and 1.0 MPa m1/2 at 1200 °C. As to the high-temperature performance, the in-situ bending strength at 1200 °C of the 800–1000 °C sintered composites is lower than 75Mpa. Interfacial diffusion cooperated with the uneven shrinkage of silica matrix is regarded as major reasons to the degradation of composites sintered or tested at over 1100 °C, for that they can lead to an increase of fiber-matrix interfacial bonding and trigger the generation of internal stresses and even visible cracks.

Distribution of Magnetic Field Parameters in the Surface Layer of the Material of Reaction Furnace Coils after Operation Period

One of the main reasons for the limited service life period of the reaction furnace coils is the carburization of the surface layers, which leads to a decrease in the performance characteristics of the pipe material, decrease in plasticity, generation of internal stresses, change in the metal structure. Therefore, monitoring the state of coils surface in order to detect critical parameters of the carburized layer thickness, using non-destructive methods of control is relevant. The results of the distribution of magnetic parameters over the depth of the carburized layer in the fragments of pipes made of steel 20Х25Н20C2, operated under furnace conditions at high temperatures, for 1300, 6000, 8000, 10000 hours are presented in the article. Analysis of the results showed that the magnetic properties are manifested only in the surface layers of the reaction furnace tubes. At the same time, the longer the service life period, the deeper is the layer exercising the magnetic properties and the higher in this layer the values ​of the constant magnetic field intensity. Analysis of magnetic properties distribution in all studied pipe fragments, both from the inner and from the outer side, showed the non-uniformity of the constant magnetic field intensity distribution, while zones of extremely high values ​are observed. The layer-by-layer surface removal in these zones with the determination of the resultant constant magnetic field intensity showed that there are critical values of the carburization depth, after which a sharp increase of this parameter is registered. These results can be used as a method for carburization depth determination, and also used to develop criterion for rejecting coils of reaction furnaces.