Four, hot-rolled, plain-carbon steels with varying carbon content were subjected to slow strain-rate tensile (SSRT) tests in a 95-MPa gaseous hydrogen environment at ambient temperature. The ...influence of pearlite volume fraction on the magnitude of hydrogen-induced degradation of the materials’ strength and ductility was thereby determined. Hydrogen was seen to significantly affect strain-to-failure and reduction-in-area in all four materials, wherein such a loss of tensile ductility was ascribed to the premature initiation and subsequent propagation of surface micro-cracks as revealed by the quantitative damage evolution analyses on the post-fractured specimens. The pearlite grains on sample surfaces manifestly served as the preferential origins of hydrogen-induced micro-cracks, resulting in more considerable embrittlement in materials possessing a higher percentage of pearlite, due to the rapid coalescence of discrete embryonic damage during tensile straining.
•Slow strain-rate tensile (SSRT) tests were performed in 95-MPa gaseous hydrogen.•Four carbon steels with different fraction of pearlite were comparatively studied.•Evolution of hydrogen-assisted cracks on the specimen surfaces was also elaborated.•Increasing pearlite fraction resulted in a severer hydrogen-induced ductility loss.•Emphasis was placed on crack initiation from pearlite and their mutual coalescence.
A numerical model is developed for surface crack propagation in brittle ceramic coatings, aiming at the intrinsic failure of rare-earth silicate environmental barrier coating systems (EBCs) under ...combustion conditions in advanced gas turbines. The main features of progressive degradation of EBCs in such conditions are captured, including selective silica vaporization in the top coat due to exposure to water vapor, diffusion path-dependent bond coat oxidation, as well as crack propagation during cyclic thermal loading. In light of these features, user-defined subroutines are implemented in finite element analysis, where surface crack growth is simulated by node separation. Numerical results are validated by existing experimental data, in terms of monosilicate layer thickening, thermal oxide growth, and fracture behaviors. The experimentally observed quasi-linear oxidation in the early stage is also elucidated. Furthermore, it is suggested that surface crack undergoes rapid propagation in the late stage of extended thermal cycling in water vapor and leads to catastrophic failure, driven by both thermal mismatch and oxide growth stresses. The latter is identified as the dominant mechanism of penetration. Based on detailed analyses of failure mechanisms, the optimization strategy of EBCs composition is proposed, balancing the trade-off between mechanical compliance and erosion resistance.
Ytterbium disilicate (Yb2Si2O7, YbDS) is an excellent top-layer material for environmental barrier coatings (EBCs) to protect ceramic matrix composites (CMCs) from harsh corrosion. However, the EBC ...surface cracking due to the corrosion transformation from YbDS to ytterbium monosilicate (Yb2SiO5, YbMS) during steam cycling is a bottleneck problem for its application. In this paper, four distinct geometric models of EBC with pre-cracks were developed to determine how the undulating and porous structure tuned EBC surface cracking. It was found that the pre-cracks in the valleys of the EBC surface were more likely to propagate than those at the peaks. Wavelength variation on the EBC surface had a more pronounced effect on crack propagation than amplitude variation. The porous structure not only alleviated EBC surface cracking, but also reduced the promotion of crack propagation caused by increasing wavelength. This indicates that optimizing the microstructure of EBCs will contribute to improving their performance.
The material removal and the surface formation mechanism of C-plane sapphire in multipass ablation using a nanosecond laser were investigated. The surface morphology and the width and depth of the ...grooves under different laser-processing parameters were characterised and investigated based on an analysis of the laser energy fluence distribution in time and space, which determines the obtained depth and width of the groove. The results show that the material removal mechanism in the sapphire processed using a nanosecond UV laser was a mixture of thermal and photochemical effects. The laser thermal effect caused sublimation, vaporisation, melt, and sputtering by plasma and was characterised by the resolidified bulk area and the deposition of fine particles. Surface cracking in a certain direction to the laser scanning path was also observed because of the gradient-temperature-induced thermal stress. In addition, the photochemical effect was revealed by the obtained X-ray photon spectra, in which the appearance of a metal Al peak and a decreasing O1s peak after laser ablation indicated the photolysis of Al2O3.
•Embedded PZT sensor developed and used for Electrical impedance (EI) measurements from concrete.•Internal cracking increases compliance and damping of concrete.•EI signature changes with stress ...level in concrete.•Applied stress and damage produce counteracting effects on EI measurement from sensor.•Internal damage detected by PZT sensor significantly before visual observation on surface.
An embedded PZT-based sensor for monitoring internal damage in concrete is presented. An experimental program for producing controlled increments in levels of stress and damage in the concrete is developed. Formation and coalescence of microcracks are evaluated using digital image correlation. The electrical impedance (EI) measurements are recorded from the embedded sensor for different levels of stress and damage in the concrete. Changes in the EI signature associated with the resonant conditions of the PZT patch produced by stress and damage in the concrete are identified. The EI measurements provide a sensitive indication of the applied stress. Applied stress and damage in the concrete produce two counteracting effects on the EI resonant frequency. The internal damage in the concrete is detected from the EI measurement significantly earlier than the occurrence of surface cracking. Embedded sensors are more sensitive to the internal cracking in concrete compared to surface mounted PZT patches.
Polypropylene (PP) pellets exposed to solar radiation, ultraviolet B (UVB) radiation and heat in four stimulated treatments: dry-air, seawater-air, seawater-darkness, and dry-darkness for 0.5–1.5 ...years to investigate morphology and chemical change under various environmental conditions. Scanning electron microscopy and infrared spectroscopy were employed to characterize the virgin and degraded pellets. The degraded PP pellets under solar and UVB irradiation revealed 35% and 12% cracks, respectively. Moreover, carbonyl and hydroxyl groups formed on the surface gradually extended to the interior. However, under photo-irradiation, PP pellets floating in seawater showed less degradation than those in a dry environment. The formation of biofilm may retard the photo-degradation of PP pellets in the seawater when biocides are absent. Results also indicated that the photo oxidation dominated over thermal oxidation during the degradation process in the terrestrial and marine environments.
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•Dry polypropylene exposed to solar and UVB radiation degraded as cracked surfaces.•In seawater the formation of biofilm slows polypropylene photodegradation.•Biological activity in seawater led to fibrous structures on the plastic.•Photo-oxidation dominated thermal oxidation in the production of CO and OH groups.
The morphology and chemical properties of polypropylene pellets degraded in the terrestrial and marine environments reveal their interaction between marine pollutant and biota and help us understand the fate of plastic debris.
Optical devices with tunable specular optical transmittance have recently attracted great interest due to their wide range of applications. However, the reported methods of realizing tunable optical ...transmittance suffer from complex fabrication processes, high cost, unstable materials, or low tuning range. In this study, a simple, cheap, and highly effective approach to achieve large tuning range of optical transmittance through harnessing surface wrinkling–cracking patterns on polydimethylsiloxane (PDMS) films is reported. The surface wrinkling–cracking patterns are induced by stretching ultraviolet/ozone‐treated PDMS films and can effectively scatter the light transmitted through the films. With moderate 50% uniaxial tensile strain, the optical transmittance can be tuned between 92 and 9.2%. The films can be reversibly tuned between transparent and opaque for 1000 cycles without losing structural integrity and optical performance, with promising application in smart windows. By patterning the surface wrinkling–cracking patterns, an elastomeric switchable display is also demonstrated, which can be turned “ON” and “OFF” by mechanical strain. The material design demonstrated in this work offers a promising means to dynamically tune the optical properties of functional materials via strain‐controlled surface topography. This study can find applications in various optical devices and systems that require tunable optical properties.
Large optical transmittance modulation (83%) via surface wrinkling–cracking patterns on elastomeric polydimethylsiloxane films is reported. Such films can be reversibly tuned between transparent and opaque for 1000 cycles without losing structural integrity and optical performance. By patterning the surface wrinkling–cracking patterns, an elastomeric switchable display is also demonstrated, which can be turned “ON” and “OFF” by mechanical strain.
The successful completion of the Zhengzhou–Xi’an high-speed railway project has greatly improved the construction level of China’s large-section loess tunnels, and has resulted in significant ...progress being made in both design theory and construction technology. This paper systematically summarizes the technical characteristics and main problems of the large-section loess tunnels on China’s high-speed railway, including classification of the surrounding rock, design of the supporting structure, surface settlement and cracking control, and safe and rapid construction methods. On this basis, the key construction techniques of loess tunnels with large sections for high-speed railway are expounded from the aspects of design and construction. The research results show that the classification of loess strata surrounding large tunnels should be based on the geological age of the loess, and be determined by combining the plastic index and the water content. In addition, the influence of the buried depth should be considered. During tunnel excavation disturbance, if the tensile stress exceeds the soil tensile or shear strength, the surface part of the sliding trend plane can be damaged, and visible cracks can form. The pressure of the surrounding rock of a large-section loess tunnel should be calculated according to the buried depth, using the corresponding formula. A three-bench seven-step excavation method of construction was used as the core technology system to ensure the safe and rapid construction of a large-section loess tunnel, following a field test to optimize the construction parameters and determine the engineering measures to stabilize the tunnel face. The conclusions and methods presented here are of great significance in revealing the strata and supporting mechanics of large-section loess tunnels, and in optimizing the supporting structure design and the technical parameters for construction.
Continuous casting of hypo-peritectic steel was conducted with a pilot slab caster. Such experimental data as local heat flux, thickness of solidified shell or mold flux film, and dendrite primary ...arm spacing were obtained. On the basis of these experimental results, influence of mold flux on initial solidification in the mold was discussed. With mild cooling by crystallization of mold flux, local heat flux and solidification rate decreased in the mold. The changes in them quantitatively correspond to each other. Dendrite primary arm spacing increased with the mild cooling. Relationship between the arm spacing and cooling rate was established and cooling rate on quite initial stage of solidification was stimated. Cooling rate at 1 mm thickness of solidified shell was estimated as about 10000–17000 K/min and changed by mold flux. Unevenness of the solidified shell thickness becomes remarkable when the shell grows to be 1 mm thick. Relation between the unevenness and the cooling rate was discussed, and critical cooling rate against the uneven solidification was observed around 17000 K/min. Thermal resistance of mold flux film was also evaluated and it was clarified that thermal resistance in the film is larger than that by air gap, and Crystallization in the film contributes to increase of both resistances. It is also considered that increase of casting speed makes air gap thinner, so reduction of radiation by crystallization of mold flux becomes more important in high speed continuous casting.
This study investigates the cyclic freeze-thaw (F-T) effect on crack patterns in saline clayey soil. Samples with different NaCl concentrations (0 M, 1 M, 2 M, and 3 M) are prepared and subjected to ...cyclic F-T processes. Each cycle involves freezing the samples at a constant temperature of −20 °C for 24 h and thawing at room temperature (21 °C) for 24 h. Additionally, a parallel experiment is conducted to observe the effects of desiccation at room temperature. The weight and surface crack patterns of the samples are continuously monitored. The image processing technique quantifies crack morphology and determines crack area, surface crack ratio, crack width, crack length, and fractal dimension. The results reveal that as the salt concentration increases, the F-T and desiccation samples exhibit decreased crack area and width. This decrease is attributed to the reduced saturation vapor pressure caused by higher salt concentrations, which slows down water evaporation and delays crack formation. The final crack pattern in the F-T samples is closely linked to the initial pattern of ice crystal formation. Lower salt concentrations result in the formation of larger ice crystals, leading to larger cracks during thawing. In the experiment, where the temperature was above the NaCl eutectic point, only ice crystals were formed, and the salt remained isolated as the salt concentration increased in the unfrozen solution. Conversely, during the freezing process, samples with higher salt concentrations with higher unfrozen solution content experience upward movement of the solution due to temperature gradients and water evaporation. This study provides new insights into understanding the soil water and salt movement and the phenomenon of surface clay soil cracking during the freezing-thawing process.
•Investigates the cyclic freeze-thaw (F-T) effect on crack patterns in saline clayey soil.•The image processing technique is applied to quantify soil cracks.•The evaporation rate decreased with increased salt content and delayed the appearance of cracks.•Ice crystal size and volume are significant at low salt concentrations.•The unfrozen water content of saline soil is related to the electric double layer.
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