Evolution Of Internal Structure Research Articles

Compaction Creep and Evolution of Transport Properties of Carbonate Fault Gouges During the Percolation of CO2‐Rich Fluids

AbstractTo investigate the impact of CO2‐rich fluids on compaction behaviors and transport properties in carbonate fault zones, we conducted compaction‐coupled fluid flow experiments with CO2‐rich fluids percolating precompacted calcite aggregates. Our findings reveal distinct responses among samples subjected to different fluid conditions. Specifically, samples exposed to dry conditions exhibited negligible compaction strain, while those under wet‐closed conditions displayed relatively minor strain. In contrast, samples subjected to flow‐through conditions demonstrated significant compaction strain, with strain rates higher by 2–3 orders of magnitude than closed conditions due to enhanced pressure solution, subcritical cracking, and chemical dissolution. Strain rate, permeability, and grain size distribution exhibited spontaneous variations in response to fluid flow and compaction. Microstructures and mechanical and transport data suggest that deformation during the initial infiltration of CO2‐rich fluids was dominated by subcritical cracking, followed by pressure solution as grain size evolved, which resulted in compaction and reduced permeability. The persistent infiltration of CO2‐rich fluids further enhanced inhomogeneous dissolution‐precipitation with preferred dissolution channels serving as fluid pathways. The re‐precipitations may cement fault rocks and form low permeability seals, resulting in anisotropic fluid flow and localized fluid pressure in fault zones. As applied to nature, our results provide experimental evidence for the evolution of internal structures and transport behaviors, shedding important light on the mechanisms and sealing potential of carbonate faults in response to the infiltration of CO2‐rich fluids during the post‐seismic and inter‐seismic processes.

Multi-scale study on the mechanical properties of concrete based on crack propagation under hygrothermal coupling

During the long-term service of structures, the environmental temperature and humidity fields can lead to the evolution of the pore structure within concrete materials, thereby affecting changes in mechanical properties. However, the mechanisms of their influence still need further clarification and are challenging to correlate for prediction. Addressing this specific issue, the current study focuses on evaluating 78 cubic concrete specimens with C30 and C40 strength grades. These specimens were subjected to a 95 % humidity environment at temperatures of 40 °C, 60 °C, and 80 °C for durations of 30, 60, and 90 days. The research involved conducting both microstructural and macroscopic mechanical tests to quantify the evolution of internal hydrates, pore structures, and pore distribution, as well as to assess the changes in the macroscopic mechanical properties of the specimens. To accurately predict the process, a multiscale finite element model (FEM) based on the bilinear continuum cohesive damage model was established. This model quantifies the changes in mechanical properties of mesoscopic three-phase concrete due to microcrack evolution under hygrothermal coupling. To facilitate multiscale simulations between the Representative Volume Element (RVE) and macroscopic scale models, relevant subroutines were written using Umath, and parallel computing was employed to reduce the computational burden. For the macroscopic model in this study, a homogenized concrete cube model with dimensions identical to the experimental specimens was selected. The effectiveness of the parallel computational model was validated by comparing and analyzing the simulation results under uniaxial compression with the experimental data. The findings indicate that hygrothermal action leads to the initiation and expansion of microcracks within the concrete, thereby impacting the alteration of its mechanical properties. In addition, the parallel computing-based multiscale FEM can more accurately quantify the typical damage process of crack propagation while reducing the computational load and may provide relatively accurate predictions of the mechanical characteristics of concrete under the influence of hygrothermal coupling, ensuring effective modeling of macroscopic structural specimens in various temperature and humidity environments.

Dynamics and Internal Structure Evolution during the Glass Transition of the Ethylene-Cyclic Olefin Copolymers: A Molecular Dynamics Simulation.

Amorphous ethylene-cyclic olefin copolymers (COCs) which can be used in cell phone lenses and prefilled syringes have attracted increasing attention due to their excellent and tunable thermal properties. In order to better explain the influence of COC microstructure (cyclic olefin types and content) on the glass transition mechanism, we used molecular dynamics (MD) simulations to track the evolution of free volume, diffusion coefficients, atomic mobility, trans conformation probabilities, and characteristic parameters of α-relaxation kinetics during the quenching process. MD results show that for the classic COC E-co-NB (ethylene-norbornene copolymer), an increase in cyclic olefin content from 25 to 50 mol % reduces atomic mobility, limiting the molecular chain movement at higher temperatures and improving Tg. Compared to NB, the more rigid rings in tricyclopentadiene (TCPD) and exo-1,4,4a,9,9a,10-hexahydro-9,10(1',2')-bridged phenylidene-1,4-bridged methylideneanthracene (HBM) have the following effects: (1) reducing the thermal expansion coefficient and overall chain mobility; (2) enhancing the diffusion energy barrier; (3) promoting the formation of local ordered structures; (4) accelerating α-relaxation dynamics at high temperatures and improving the dynamic fragility m. These lead to an upward shift in the temperature region where chain movement is limited and thus improve Tg and high-temperature dimensional stability. In this simulation, the correlation equation between Tg, m, and the microstructural parameters of COCs is established, which is of great significance for the development of COCs with high performance.

Porous asphalt mixture performance in cold regions: Case study of Chicago

Porous Asphalt Mixtures (PAMs) improve resilience of flexible pavements in the context of climate change. PAM application in coastal cities like Chicago is of interest; however severe winters compromise the durability of PAMs given their susceptibility to freeze-thaw. This work aimed to evaluate the to-date experience with PAMs in Chicago, exploring benefits and limitations to be considered for future projects in the Southern Lake Michigan region. Full-depth PAM cores were obtained from seven sections, constructed in different years. Tests were conducted to assess the evolution of surface texture, internal structure, and multi-scale material properties. Regarding surface texture, consistent change over time was observed, such as loss of the “channel” structure, and increase in mean profile parameters because of raveling. Also, binder content varies with time within different layer depths. An aging gradient in terms of stiffness from top to bottom within the layer does not necessarily occur. However, binder ductility shows drastic and consistent reduction throughout the layer as aging increases. Analysis of internal structure using a CT scan revealed that there is also a lack of homogeneity in the air void and particle size distributions. Finally, this study suggests that full-depth PAM viscoelastic behavior is sensitive to saturation level, and this is a relevant subject to be considered, since partial saturation may be a condition often observed in the field.

Investigation on unstable flow characteristics and energy dissipation in Pelton turbine

The internal flow scale of a Pelton turbine is variable, and the interaction between the jet and the bucket has strong transient characteristics, resulting in an incomplete understanding of its internal vortex structure evolution and energy dissipation mechanisms. In order to reveal the influence of vortices on the flow regime and energy dissipation mechanisms in the turbine, this paper establishes the correlation between the unsteady flow characteristics and energy dissipation inside the Pelton turbine based on the energy balance equation and quantifies the energy losses inside the turbine. The results indicate that vortex structures present a non-uniform distribution inside the jet, which disrupts the uniformity of the jet velocity distribution, resulting in an uneven distribution of high-vorticity zones on the bucket surfaces and intensifying flow interference among the buckets. The strong shear flow caused by the downstream flow detachment of the needle guide, the turbulent boundary layer on the jet surface, and the wake effect downstream of the needle tip are the main reasons for the dissipation of fluid kinetic energy. The distribution range of energy loss on the rotating bucket closely corresponds to the position of the high-vorticity zone. Energy dissipation inside the turbine is primarily in the form of turbulent kinetic energy, and the injectors and runner are the primary energy dissipation components. Moreover, inside the injectors, each form of energy loss remains relatively constant, whereas inside the runner, its rotation induces fluctuating energy losses attributed to Reynolds stress work. The results contribute to an enhanced understanding of the energy dissipation characteristics and complex flow mechanisms within the turbine, providing reference for the optimisation and efficient operation of the multi-nozzle Pelton turbine.

CT-based investigation on internal structure evolution for high-speed railway graded aggregate materials during vibratory compaction

The research gap on the internal structure evolution (ISE) of the high-speed railway graded aggregate (HRGA) materials during vibratory compaction has resulted in challenges in exploring its compaction physical-mechanical properties. Based on a self-developed vibratory compaction instrument (SVCI), a new assessment framework was established to evaluate physical-mechanical evolution properties during vibratory compaction, including the revised dry density ρrd, the revised soil stiffness Krb, and the revised modulus of subgrade reaction K20. The X-Ray computed tomography (X-Ray CT) tests of an HRGA specimen at different compaction stages were carried out to characterize the physical-mechanical evolution properties. The results show that during vibratory compaction, the ρrd presents a rapidly increasing-slowly increasing-continuously stable evolution, while Krb and K20 shows a rapidly increasing-slowly decreasing characteristic. The compaction locking point Tlp is proposed to quantify the inflection point of mechanical properties during vibratory compaction, which is related to the gradation types of the HRGA materials. Before achieving the compaction locking point Tlp, coarse grains gradually deflect toward horizontal orientation to form a stable skeleton with insignificant crushing. Approximately 60% of coarse grains in the horizontal direction can achieve a stable skeleton under the action of the excitation force. After achieving Tlp, the coarse grains present the localized crushing phenomenon at the contact points due to further compaction, and it indicates the destruction of the stable skeleton. The overall shape of the coarse grains remains unchanged, but their surface corners are gradually rounded. The experimental results can make a significant contribution to the filed for further investigating the vibratory compaction mechanism and enhancing the compaction quality control system.

Internal structural evolution of conductive cementitious composites subjected to multi-step ohmic heating curing strategy under severely cold temperature

Ohmic heating (OH) curing is a novel method for winter concrete construction, a more effective strategy is required for the modification of this method to achieve the high-quality and rapid structure fabrication. In this work, a multi-step OH curing strategy was introduced to prepare Ni coated carbon fibers and nano carbon fibers reinforced conductive cementitious composites (NiCFs/NCFs-CCC) under −20 °C, detailed thermal strain evolution, mechanical strengths and microstructure of the samples were clarified. With the increasing OH curing step, NiCFs/NCFs-CCC exhibited lower thermal strain during the curing process. The thermal strains inside 1-step OH cured sample was larger than 4000 με, which was much larger than those of 2-step OH cured sample and 3-step OH cured sample (lower than 2500 με), highlighting the advantage of multi-step OH curing on reducing the potential internal damage. Moreover, 3-step OH cured sample exhibited higher compressive strength of 68.2 MPa, meeting an increase of 23.1% with that of 1-step OH cured sample. MIP analysis result showed the similar regularity that multi-step OH curing was beneficial for the densification of the pore structure with lower porosity and higher proportion of harmless pores. This work provides a new insight on the modification of OH curing method, which has been verified by the internal structural evolution of the samples, providing an effective curing strategy for high-quality winter concrete construction.

Mechanical and internal structural damage evolution in cold recycled mixture under fatigue loading

Cold recycling technology has the virtue of lower cost, fewer pollutant emissions, and less consumption of raw materials and fossil fuels in pavement maintenance and construction. This study aimed to investigate the evolution of fatigue damage in cold recycled mixtures and provide a theoretical basis for improving the service life of cold recycled pavements with high reclaimed asphalt pavement (RAP) utilization rates, ultimately contributing to energy and resource conservation. To achieve this objective, we conducted a study on the mechanical and internal structural damage evolution of cold recycled mixtures produced using different agents (cement and asphalt emulsion). This was accomplished by combining indirect tensile fatigue testing with X-ray CT scanning techniques. The results indicate that the mechanical response of cold recycled mixtures under fatigue loading is closely related to the evolution of their internal structure during various stages of fatigue. Furthermore, it is found that the type of stabilization agent used in these mixtures has a significant impact on their initial air void content and volume distribution, which ultimately affects both the rate and extent of their mechanical and structural damage evolution. Based on these results, recommendations have been proposed for enhancing the fatigue performance of cold recycled pavement through appropriate mixture design strategies that account for these factors mentioned above.

Evolution of the strain localization and shear-zone internal structure in the granular material: Insights from ring-shear experiments

Shear zone is widely observed in natural faults and landslides as well as laboratory experiments on granular material. Understanding the shear-zone evolution process in granular materials is crucial in studying the dynamics of the landslides and faults. However, it is still not well understood. To this end, we conducted a series of ring-shear experiments to investigate the evolution of strain localization and shear-zone internal structure in cohesive and non-cohesive granular materials. The quantitative evaluation was conducted by using high-solution X-ray computed tomography (CT). The analyses included visualization of shear-zone internal structure and quantification of particle shapes, orientations, and grain-size distributions at different shear strain levels. We found that with the increase of shear displacement, the large particles inside the shear zone became more and more rounded, but without an orientation, wear and attrition resulted in an abundance of nanoparticles within the shear zones. We also found fine-particle layers (nanoparticle layers) formed on either side of shear zones during shear localization, implying that shear development progressed from a shear zone to an interface. These findings offer some new understanding of the evolution of shear zones in granular materials.

4D printing of biomimetic anisotropic self-sensing hydrogel actuators

Organisms in nature, from plants, insects to mammals, have anisotropic and regularly arranged tissue structures, which are key to rapid and precise response to environmental stimuli. In this study, nature-inspired anisotropic hydrogel actuators are fabricated by 4D printing thermo-responsive composites of short carbon fibers (SCFs) and poly(N-isopropylacrylamide) (PNIPAM). The SCFs could be well aligned and programmed in hydrogel matrix, resulting in diverse precisely controlled anisotropic structures. The intrinsic anisotropy causes unbalanced swelling and shrinking upon thermo-stimulation, leading to reversible biomimetic shape morphing. Finite element analysis (FEA) simulations are used to estimate the internal anisotropic stress distribution and structure evolution. Biomimetic anisotropic butterfly-like, flytrap-like and starfish-like hydrogel actuators are fabricated by 4D printing. On the other hand, the anisotropic SCFs/PNIPAM hydrogels are conductive and could be used as wearable strain sensors with high sensitivity. The conductive anisotropic hydrogels can generate sensing signals when they are motivated by temperature changes. This self-monitoring soft actuator is promising for the development of soft robots with stimulation-sensing-actuation feedback loop, which has broad application prospects in artificial intelligent devices.

Study on internal erosion and structural evolution mechanism of soil–rock mixture

Study on internal erosion and structural evolution mechanism of soil–rock mixture

Liquefaction behavior of inherently anisotropic sand under cyclic simple shear: Insights from three-dimensional DEM simulations

Inherent fabric anisotropy has been widely recognized to have significant effects on the liquefaction behavior of sand. A discrete element method (DEM) study was conducted to investigate the liquefaction behavior of inherently anisotropic sand under cyclic simple shear conditions. In the DEM model, the under-compaction method was modified to generate uniform and inherently anisotropic specimens. A series of DEM simulations of undrained cyclic simple shear tests were performed to investigate the effects of bedding plane angles with a full spectrum ranging from −90° to +90°. The relative magnitudes of the three effective normal stress components were compared and analyzed. The internal structure evolution was quantified through a contact-normal-based fabric tensor that could describe the load-bearing structure of granular specimens. The simulation results show that the effect of bedding plane angle on the liquefaction resistance of anisotropic sand is not monotonic. For example, as the bedding plane angle of specimen increases from 0° to 90°, the liquefaction resistance first decreases and then increases, with the most detrimental scenario occurs at 45°. This could be explained by correlating the magnitudes of shear modulus with the discrepancy between the loading direction and the direction of fabric during each cycle.

Combined effects of initial static shear and continuous principal stress rotation on the liquefaction behavior of granular materials

Extensive efforts have been made separately to investigate the effects of the initial static shear and the continuous principal stress rotation on the liquefaction behavior of sandy soils. However, the interplay and the combined effects of these two important factors have not been adequently studied, which may simply be attributed to the difficulty of implementing such complex loading path in the labrotory tests without introducing severe stress consentration. In this study, a newly-developed discrete element approach that avoids stress concentration and can apply arbitrary undrained loading path was implemented. Based on this approach, the cyclic resistance of both inherently anisotropic and isotropic granular materials under continuous principal stress rotation were studied considering different initial static shear levels. The internal structure evolution of the specimens were quantified through a contact-normal-based fabric tensor that describes the load-bearing structure of the granular specimens. The simulation results show that under otherwise identical condition, the specimen with higher initial static shear stress level is more prone to plastic shear strain accumulation and has weaker cyclic resistance under continuous principal stress rotation, which can be well explained by the fabric evolution.

Internal architecture and structural evolution of a horst relay zone from the northern Gulf of Suez rift, Egypt: Implications for syn-rift sedimentation

A remarkable outcrop of an extensional NW-oriented horst block takes place between conjugate normal fault segments at the Akheider-El Ramliya fault block (ARFB), along the northwestern termination of the Gulf of Suez rift. Detailed field geological mapping is accomplished to identify the architecture, linkage style as well as evolution of the encountered normal fault system at the ARFB. The resultant structural model revealed several geometries of normal faults interaction that form soft- and hard-linkage relay zones affecting pre- and syn-rift rock units. Such relay zones are well developed between the boundary faults of the ARFB. Additionally, three E-W oriented fault belts are mapped at the northern, central and southern parts of the ARFB enclosing left-stepping en echelon normal faults, where several relay zones are formed between the overlapped fault segments. Structural analysis and evolution of the mapped area revealed that the overall architecture of the ARFB representing a broad horst relay zone between the conjugate divergent boundary normal faults. Also, two sub-basins have been recognized at the downthrows of the border faults comprising syn-rift Oligocene and Miocene sediments. The results show that these faults are rejuvenated by a continued NE-SW extension through Late Eocene, Oligocene and Miocene times. Additionally, the formed relay zones via growth and propagation of such faults have main controls on syn-sedimentation at the northern Gulf of Suez rift. The results of the present study offer constraints on horst relay zones including, internal structural architecture, geometries and interaction of boundary faults as well as role of inherited structures. Also, it provides an outcrop model for architecture and evolution of similar structural styles (e.g., the Gulf of Suez rift) as well as for subsurface mapping and exploration in extensional relay zones.

Multiphase deformation, fluid flow and mineralization in epithermal systems: Inferences from structures, vein textures and breccias of the Kestanelik epithermal Au-Ag deposit, NW Turkey

We investigate the multiphase deformation, fluid flow, and mineralization processes in epithermal systems by presenting a detailed study of vein textures and breccias of the Kestanelik epithermal Au-Ag deposit, NW Turkey. The mineralization in the deposit is associated with several quartz veins. Fault-hosted veins and mode I veins share many textural and breccia characteristics owing to (i) overprinting of tectonic breccias formed during coseismic rupturing by subsequent coseismic hydrothermal brecciation and (ii) reworking of earlier vein breccia phases by repeated rupturing and hydraulic fracturing events. The spatial distribution of breccias at fault-hosted veins proposes that power of coseismic hydrothermal brecciation is controlled by the distance to the level of boiling within a vein. The brecciation affects the entire vein proximal to the level of boiling; however, it is limited to the footwall contact of the vein more distally at the upper levels of a vein. Varying number of mineralization events for the veins suggests that any individual earthquake event reopened only one or more sealed vein, but not all at once. Fewer mineralization events in fault-hosted veins compared to the mode I veins is either linked to (i) focusing of high fluid flux into the conduits of mode I veins that accommodate more dilation or (ii) reopening of mode I veins owing to the driven of extensional failure under low differential stress. Although fault-hosted veins record fewer mineralization events, they have higher average Au grade (4.106 g/t) compared to that of mode I veins (2.736 g/t). On the other hand, fewer mineralization events in wall rock structures compared to the adjacent faults is attributed to (i) absence or poor development of the damage zone structures in earlier seismic events or (ii) deactivation of them after clogging due to the rotation of the optimum stress field or (iii) their formation as hydraulic extension fractures. This study emphasizes the importance of detailed studies of vein infill for understanding the internal structural evolution of the veins in epithermal deposits that is interest to the geologists within both industry and academic fields.

The spatial pattern and influencing factors of tourism eco-efficiency in Inner Mongolia, China.

Tourism eco-efficiency is a performance basis for evaluating green total factor productivity and sustainable development. The objective of this study was to measure tourism eco-efficiency in Inner Mongolia and explore its influencing factors. The aim was to provide an accurate reference for improving the quality and efficiency of tourism in Inner Mongolia and promoting the sustainable development of the regional economy and society. Tourism eco-efficiency in Inner Mongolia from 2009 to 2019 was calculated using a super-slacks-based measure (SBM) model with an undesirable output. The spatial variation function was used to explore the spatial evolution pattern of tourism eco-efficiency in Inner Mongolia, and the influencing factors of the spatial evolution were analyzed by geographically weighted regression. Tourism eco-efficiency in Inner Mongolia is relatively low. Eco-efficiency values among cities in Inner Mongolia vary, and their distribution is not balanced. The structural eco-efficiency of tourism in Inner Mongolia has been consistent from 2009 to 2019. The degree of homogenization in the overall direction is relatively good. Furthermore, its spatial distribution form and internal structure evolution show a certain regularity and continuity. The pattern evolution of tourism eco-efficiency in Inner Mongolia is jointly driven by the economic level, environmental regulation, industrial structure, traffic conditions, resource endowment, and tourism reception facilities. These influencing factors show obvious spatial heterogeneity. From the perspective of Inner Mongolia, the difference in the tourism eco-efficiency value from 2009 to 2019 was relatively large, but the number of effective areas in the efficiency frontier generally showed a fluctuating growth trend. The range parameters of tourism eco-efficiency showed a decreasing trend, and the spatial correlation effect of tourism eco-efficiency in Inner Mongolia showed a decreasing trend under the influence of structural and spatial differentiation.

Analysis of Stress Relaxation in Bulk and Porous Ultra-High Molecular Weight Polyethylene (UHMWPE).

The reported study was devoted to the investigation of viscoelastic behavior for solid and porous ultra-high molecular weight polyethylene (UHMWPE) under compression. The obtained experimental stress curves were interpreted using a two-term Prony series to represent the superposition of two coexisting activation processes corresponding to long molecular (~160 s) and short structural (~20 s) time scales, respectively, leading to good statistical correlation with the observations. In the case of porous polymer, the internal strain redistribution during relaxation was quantified using digital image correlation (DIC) analysis. The strongly inhomogeneous deformation of the porous polymer was found not to affect the relaxation times. To illustrate the possibility of generalizing the results to three dimensions, X-ray tomography was used to examine the porous structure relaxation at the macro- and micro-scale levels. DIC analysis revealed positive correlation between the applied force and relative density. The apparent stiffness variation for UHMWPE foams with mixed open and closed cells was described using a newly proposed three-term expression. Furthermore, in situ tensile loading and X-ray scattering study was applied for isotropic solid UHMWPE specimens to investigate the evolution of internal structure and orientation during drawing and stress relaxation in another loading mode.

Densification behavior and properties of Li4SiO4 ceramic breeder with the addition of SiC as a sintering aid

Lithium orthosilicate (Li4SiO4) is regarded as a vital tritium breeder candidate due to its favorable properties, such as high lithium contents, superior tritium release behavior, and irradiation stability. However, the poor crushing load and low thermal conductivity of the Li4SiO4 ceramic pebbles fabricated via existing methods will lead to the blocking of the tritium release channels, giving rise to thermal stress concentration. To overcome the above shortcomings, the SiC was employed as sintering aid to improve Li4SiO4 ceramic pebbles via simple wet method in this paper. The effects of SiC content on the shrinkage, internal structure evolution, and crushing load of the Li4SiO4 ceramic pebbles are comprehensively investigated, and the densification process is analyzed in detail. It was found that the addition of SiC improves the crushing load (54 N) significantly owing to decreasing the densification activation energy. Moreover, the Li4SiO4 ceramic pebbles with excess Li2CO3 further displays excellent properties at 800 °C, such as the relative density of 90.3%, the crushing load of 65.3 N, and the thermal conductivity of 1.97 W/(m·K) respectively, which is much higher than that of traditional methods (at least 1000 °C). The Li4SiO4 ceramic pebbles with homogeneous micro-pore distribution, the porosity of 7.3% were further measured by using the X-ray computer tomography technique. Therefore, this proposed tactic has great prospects for the service performance of tritium breeding materials in the blanket.

The kinematics and micro mechanism of creep in sand based on DEM simulations

In this study, biaxial shear and creep numerical simulations were carried out on both dense and loose sands based on discrete element method to examine the kinematics and micro mechanism of creep. The simulations qualitatively reproduced the macroscopic behavior of sand during shearing and creep. Contact forces, contact normals and voids were used to describe the evolution of internal structure. And three independent modes of contact motion, i.e., contact deformation (normal and tangential), Type4 rolling and rigid-body motion, were utilized to explore the contributions of contact motions to sample deformation. It is found that (1) the change of creep stage can be charactered by the anisotropy evolution of internal structure, and the increase of internal structure anisotropy would induce the rise of creep rate (i.e., the creep rupture); (2) contributions from different modes of contact motion to macro sand deformation during creep and shearing process is complex, that is, the contact deformation is the dominant mode in both shearing and creep process, whereas the contributions from tangential contact deformation during creep is complete opposite to its effect in shearing process, which leads to contract during creep; the possible reason for this is that the creep behavior is deformation-path dependent; the creep rupture can occur only under the condition which the strain localization is developed; (3) the contributions of normal and tangential contact deformation to volumetric stain are also different in strong and weak contact force chains; the deformation in weak contact force chain is more similar to the macro deformation, which means that the macro deformation is controlled by the weak contact force chain.

Characterization of the stress distribution and evolution inside a thermal barrier coating after initial thermal cycles

AbstractThe residual stress introduced inside a thermal barrier coating (TBC) during manufacturing and service processes is one of the main causes of thermal barrier failure. The formation and evolution of internal stress in the TBC begin at the early stage of service, but studies on the mechanism of the distribution and evolution of the stress in the TBC during the initial thermal cycle are still lacking. To explore the evolution mechanism of the stress in the TBC interior, an experimental study on the regulation mechanism of the initial thermal cycle on the TBC internal stress was carried out in this paper. First, the internal stress of TBC specimens after thermal cycles was characterized based on photoluminescence spectroscopy (PL) and terahertz time‐domain spectroscopy (THz‐TDS) technologies, in which the homogenization of the near‐interface stress field was observed during the initial thermal cycle. Then, the evolution of the microstructure and phase structure of the TBC specimens was characterized. Finally, the phenomenological model of the evolution of the TBC internal structure was established, revealing that the initial thermal cycle regulated the microstructure of the top coating (TC) through phase transformation to realize the homogenization of the near‐interface stress field.