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Void growth, as a critical stage in ductile fracture, could play some important role in the ductility of high entropy alloys (HEAs). Under a high-strain-rate tension (strain rate: 0.1~0.4/ns), our atomistic simulations reveal a severely suppressed growth of void (initial diameter: 4.0~9.1 nm) in HEAs at a low temperature, compared with that in Ni metal or Ni-based binary alloys. The emission of unclosed glide loops is found critical to void growth. Void growth is suppressed, once dislocation emission is suppressed due to the solute drag effect. Regarding the alloying elements, we find the void suppression relevance as Fe>Co and Cr>Fe>Mn.

In order to improve the wear resistance of 27SiMn steel substrate, Fe-based alloy coatings were prepared by laser cladding technology in the present study. In comparison to the conventional gravity powder feeding (GF) process, high-speed powder feeding (HF) process was used to prepare Fe-based alloy coating on 27SiMn steel substrate. The effect of diversified energy composition of powder materials on the microstructure and properties of coatings were systematically studied. X-ray diffractometer (XRD), optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the phase structure and microstructure of Fe-based alloy coatings, and the hardness and tribological properties were measured by the microhardness tester and ball on disc wear tester, respectively. The results show that the microstructure of conventional gravity feeding (GF) coatings was composed of coarse columnar crystals. In comparison, owing to the diversification of energy composition, the microstructure of the high-speed powder feeding (HF) coatings consists of uniform and small grains. The total energy of the HF process was 75.5% of that of the GF process, proving that high-efficiency cladding can be achieved at lower laser energy. The refinement of the microstructure is beneficial to improve the hardness and wear resistance of the coating, and the hardness of the HF coating increased by 9.4% and the wear loss decreased to 80.5%, compared with the GF coating. The wear surface of the HF coating suffered less damage, and the wear mechanism was slightly adhesive wear. In contrast, wear was more serious in the GF coating, and the wear mechanism was transformed into severe adhesive wear.

In a first effort, the dislocation structure and related mass transfer in a macroporous FCC metal is studied via atomistic simulation. The interaction between the void and its periodic image has been intense ever since the onset of dislocation emission. The accumulated mass transfer is due to the emission of multiple shear loops, which shapes the morphology of the macroporous metal. The removal of mass from the intervoid ligament triggers its breakdown, connecting the voids. The strengthening of intervoid interaction is shown by running multiple cases with atom count ranging from 7.1 million to 95.4 million. The critical stress for dislocation emission versus the size of void is in a good agreement to the Lubarda model adjusted by the Gibson-Ashby scaling law. Nevertheless, the critical stress of void with a short intervoid ligament distance (ILD) demonstrates a surprisingly insensitivity of strain rate compared with the void that can be deemed standalone.

In this research, we proposed a highly sensitive hydrogen sensor based on a new suspended structure of cross-stacked multiwall carbon nanotube (MWCNT) sheet. MWCNT sheet is a kind of CNT film which has a super-high CNT alignment and can be easily prepared by drawing from the spinnable CNT array in large scales. By stacking the sheets onto an electrode with a 1 × 1 cm hole in mutually perpendicular directions, sensors with suspended cross-stacked structure were realized. Afterwards, a two-side Pd functionalization was introduced. The effects of suspended structure, cross-stacked structure and two-side Pd functionalization were investigated respectively. It was observed that the sample with 2 + 1 layers of cross-stacked MWCNT sheet and two-side 3 nm Pd deposition showed the best gas sensing performance with a relative resistance change of 35.30% at 4% H2. This result indicates that the proposed sensor is one of the best among all reported MWCNT based hydrogen sensors. The method demonstrated in this research gives a potential solution for the mass production of CNT-based sensors with high sensitivity and reliability.

In nature, rock discontinuities, e.g., cracks, voids and joints, are usually three-dimensional, which to a large extent control the volumetric fracturing behaviors of rock masses. Understanding their growth behaviors and effects on rock volumetric fracturing properties is crucial for the stability assessment of rock engineering. In this study, three-dimensional printing (3DP) method was adopted to fabricate resin-based artificial rocks containing single flaw and double pre-existing penny-shaped 3D internal flaws. Static uniaxial compression tests were, subsequently, conducted on these samples to investigate the influence of flaw number, flaw angle (α) and ligament angle (β) on the volumetric fracturing behaviors of the 3DP artificial rocks. The results indicate that flaw geometry has remarkable influence on the mechanical and fracture behaviors of the flawed samples. The single flawed sample with α equal to 60° has the lowest compressive strength (σc) and axial strain at the peak stress (ea). σc and ea of the double flawed sample generally increase when β changes from 45° to 105°. When the flaw number increases from one to two, the initiation stress of the first wing crack, σc and ea decrease. With the aid of high-speed cameras, we studied 3D crack growth inside the transparent 3DP resin samples in real-time for the first time. Wing and anti-wing cracks wrapped around the flaw edge could only propagate for approximately 1–1.5 times the length of the initial flaw. Wing cracks generated at the inner tips of the flaws cannot coalesce, except for the sample with β of 105°. The maximum crack propagation velocity in single flawed specimens is higher than that in double flawed samples. The continuous propagation of the secondary cracks developed after the peak stress lead to the burst-like failure of the flawed samples. This study could enhance our understanding of volumetric fracturing behaviors of rocks.

This study evaluated the low-cycle fatigue (LCF) life of type 316 austenitic stainless steel to determine the effect of multiple high-density pulsed electric currents (HDPECs). Fatigue properties were analyzed with the fatigue crack growth (FCG) and LCF tests by considering different conditions of multiple HDPECs and its application. The HDPEC densities of 100, 150 and 200 A/mm2 with application numbers from 1 to 17 times were used. The LCF results were assessed by using fatigue models, and the effectiveness of the application methods was examined. Under the HDPEC density of 200 A/mm2, increasing the number of HDPECs during the period of crack propagation is the best way for delaying FCG. Multiple applications of HDPEC caused a decrease in the length and an increase in the depth of striations in the specimen’s fatigue fracture surface, and the degree of ductility was increased thereby leading to the delay in FCG. The proposed conditions pave the way toward improving the LCF life of the material.

The interior deformation or rupture of materials is usually derived from measuring the surface deformation of specimens. In this study, a set of uniaxial compression tests combined with in situ X-ray computed tomography (CT) was performed on siltstone specimens with a pre- existing three-dimensional (3D) surface flaw with various inclination angles (α = 30◦, 45◦, 60◦). The ratio of flaw depth to specimen thickness was 0.26. Based on the CT images of the specimens under different loads, the digital volumetric speckle photography procedure was applied to quantifying the 3D interior deformation of the specimens, and 3D displacement and major principal strain e1 distribution in different sections were determined. The characteristics of internal strain before specimen fracturing indicated strain localization, which could describe the propagation mechanism of the single 3D surface flaw.

In this study, we investigated the relationship between intermetallic compounds (IMCs) and electromigration (EM)-based metallic micro-/nanowire growth and concluded that IMC has a negative effect on the wire growth. One of the key features of the sample structure in EM-based wire growth is passivation, which mechanically suppresses the deformation of the metallic interconnect due to the accumulation of atoms diffused by EM. Thicker passivation allows for higher pressure generation for wire growth; thus, the wire can be extruded by the higher pressure. However, as the compressive residual stress of passivation (which causes delamination) increases with the thickness of passivation deposited by sputtering, it is implied that excessively thick passivation causes delamination, which in turn relieves the pressure essential for wire growth, by releasing the interconnect constraints. In EM-based wire growth, generally a sample structure consisting of thin-film multilayers was used, and it often resulted in the delamination due to interlayer separation between the metallic interconnect and the topmost passivation. Mitigation of delamination enables the stable EM-based growth of a wire. To prevent delamination, Ti was introduced between the metallic interconnect and passivation. The relation of IMC formation behavior to the wire growth was investigated. It was experimentally shown that IMC contributed to the prevention of delamination but inhibited the wire growth. Therefore, sandwiching the metallic interconnect with materials that do not form IMC is advantageous for EM-based wire growth.

Cold spray (CS) has significant technical advantages in additive manufacturing of aluminum matrix composites. In this study, Al with different content of Al2O3 composite deposits was successfully produced by CS. The effect of Al2O3 content and heat treatment temperature on the microstructure and mechanical properties of the deposits was comprehensively studied. The results show that as the content of Al2O3 increases, the hardness, Ultimate Tensile Strength (UTS), and elongation of the deposits gradually improve in the as-sprayed state. Due to the significant hammering effect of Al2O3 particles on Al particles, there is a local metallurgical bond between adjacent Al particles in Al-50 and Al-75 deposits, and dimples appear in the fracture morphology of the deposit. With the increase of the heat treatment temperature, the diffusion of atoms at the particle-particle interface transforms the bonding between Al particles from mechanical interlocking to metallurgical bonding, so that the deposit exhibits improved ductility. However, heat treatment also eliminates the residual compressive stress and introduces grain growth through recovery and recrystallization processes, resulting in the decrease of hardness and UTS of the deposits.

One of the challenges limiting the application of laser cladded Fe-based coatings is the coarse columnar grains formed during the process. In order to improve the microstructure homogeneity of laser cladded coatings, Fe-based alloy (Fe78.5Cr15.6Ni4.0Si0.7) composite coatings with different La2O3 additions were prepared on the surface of 45-steel substrate by laser cladding. The effect of La2O3 content on the microstructure and properties composite coatings were systematically studied. X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used to analyze the phase structure and microstructural evolution of Fe-based alloy composite coatings, and the hardness and tribological properties were measured by the microhardness tester and ball on disc wear tester, respectively. The results show that the addition of La2O3 plays the role of grain refinement, and certain amount of addition can completely eliminate the coarse columnar grains. Due to the fine-grain strengthening and dispersion strengthening of La2O3, the hardness and wear resistance of Fe-based alloy composite coatings have been improved. Especially, when the content of La2O3 is 1 wt%, the hardness increases to 3.1 times and the wear loss decreases to 27.7%, compared with the substrate material. The wear surface of composite coating with 1 wt% La2O3 suffered minor damage, and the wear mechanism is slightly abrasive wear. In contrast, higher amount of 2 wt% La2O3 addition has less effective grain refining where some columnar grains appear on the interface, and the hardness and wear resistance of composite coatings decrease as well. Meanwhile, the wear mechanism is transformed into a mixed mode of abrasive wear and adhesive wear.

Quantitative visualization of the full-field stress and strain in porous structures is of significance to reveal the mechanism of the damage and failure of engineering materials. To quantitatively characterize the full-field stresses and strains, optical measurement methods, such as photoelasticity for determining the stress fields, moire and digital image correlation (DIC) methods for measuring the strain fields, have been developed in previous works. However, it is challenging to combine these methods to measure stress and strain fields simultaneously because the speckles and gratings for strain measurement will disturb the interference fringes in the photoelasticity. In this study, a method incorporating photoelasticity and DIC techniques was developed to determine the stress and strain fields simultaneously in different porous structures which were fabricated by three-dimensional (3D) printed technique. The tested models were printed with a highly transparent, colourless material and the speckles on the front surface of the tested models were printed with a translucent, magenta material. Under compressive loads, the stress fields in these models were obtained based on the photoelastic patterns and the strain fields were determined by DIC method. The results indicate the proposed method can simultaneously determine the full-field stresses and strains and the comparison of the distribution of the principal stress and strain difference and the distribution of the shear stress and strain verified its good accuracy. In addition, the stress and strain distribution in different pore structures were compared and found that the optimal design of the pore distribution can greatly change the concentration of the stress and strain fields.

The distinct spalling performances of reactive powder concrete (RPC) specimens with various silica fume (SF) contents exposed to high temperatures were observed via high-resolution photography. The RPC microstructures and pore structures after high-temperature exposure were characterized using scanning electron microscopy and mercury intrusion porosimetry. The results provide experimental evidence of the high-temperature spalling mechanism of RPC. Increasing the SF content in RPC increases its compressive strength and compactness, offering greater mitigation of devastating spalling behaviour, but also producing more pulverized spalling remnants. This is attributed to the post-heating cracked microstructure and refined pores, which promote localized rather than entirely explosive spalling.

The physical visualisation of a three-dimension (3D) stress field is a promising method for quantitatively analysing and revealing the stress distribution and evolution of a porous solid, and it significantly contributes to the understanding of the governing effects of stress fields on the mechanical behaviours of complex porous solids. However, experimental limitations regarding the manufacture of complex porous models and the extraction of the stress distributions in matrices inhibit the accurate visualisation of the 3D stress fields of porous structures. This paper presents a method of experimentally visualising and elucidating the 3D structures and stress fields of porous solids using photopolymer materials, 3D printing, the frozen-stress method, and photoelastic tests. Transparent thick discs containing various randomly distributed pores were produced using photopolymer materials and 3D printing technology. Experimental measures, including the frozen-stress method, photoelastic testing, and the phase-shifting method, were applied to quantitatively characterise the 3D stress fields distributed throughout the porous discs under radial-direction compressive loads. The temperature for ‘freezing’ stresses in the photopolymer materials was experimentally determined. The effects of pore distribution and population on the stress-field characteristics were investigated. The experimental results were used to validate the numerical analysis of the stress-field characteristics of the porous models. The visualisation test results agreed well with those of the numerical simulations. The proposed method can be used to visually quantify the characteristics and evolution of the 3D stress fields of porous solids.

Comprehensive molecular dynamics (MD) simulations are performed to study the stress response and deformation mechanism in void-embedded, single-crystal and nanocrystalline, high-entropy-alloy (HEA) films under uniaxial tensile loading. Our results reveal that certain void-embedded HEA films can be, by far, superior to pure Ni in terms of tensile ductility and the resistance to crack propagation. The fracture strains of 10%Co CoCrFeMnNi and the equiatomic CoFeMnNi, respectively, double or triple that of the equiatomic CoCrFeMnNi, which still doubles that of pure Ni. Regarding the deformation mechanism, high tensile ductility of HEAs can be attributed to the formation of partial dislocations, nanotwinning and the impediment of the otherwise glissile dislocations due to the weak pinning effect. The ultimate tensile strength of HEA film shows better resistance against stress deterioration due to elliptical voids. The stress response of the void-embedded, nanocrystalline Ni films obeys the reverse Hall-Petch effect, while the void-embedded, equiatomic HEA film does not. The maximum stresses of nanocrystalline HEA films of different grain size are virtually the same. The void-embedded, finer-grain HEA film exhibits a better tensile ductility than the void-embedded, coarser-grain HEA film.

Since the fabrication technique for high-entropy alloy (HEA) nanowires/nanopillars is still in its infancy, neither experimental nor modeling analyses of their cold-welding performance have been reported. Based on insights accumulated in our previous experiments and simulations regarding cold-welded metallic nanowires, in this study, the cold-welding performance of HEA nanowires is probed by atomistic simulations. Among different materials, our simulations reveal that extensively twinned structures are formed in CoCrMnFeNi samples, but not in CoCrCuFeNi or Ni samples. The larger fracture strain in certain HEAs is due to the improved ductility around the fracturing area as well as multiple twinning. Unlike in Ni samples, the fracture strains in HEA samples, regardless of being cuboid or cylindrical, are improved by shrinking the sample size. Among different orientations, the [010]-direction monocrystalline nanowires fail at a strain over 0.6, which is almost double that of the [111] direction. The fracture strains in polycrystalline HEA samples are, on average, larger than those in polycrystalline Ni samples. Furthermore, fracture strains in randomly generated polycrystalline HEA samples are more predictable than those in polycrystalline Ni samples with identical grain configurations. As previously reported, dislocation emission is still a prerequisite to fracture in all cold-welded samples.

Quantitative visualization of the hidden full-field stresses in three-dimensional (3D) solids is crucial for solving various engineering problems; however, it is challenging to achieve using the conventional experimental techniques. Frozen stress technique is an effective method to characterize internal stress fields. However, difficulties in fabricating the complex 3D models impede the extension of this method. The method combining additive manufacturing or 3D printing, printable transparent photopolymers, and frozen stress techniques provides a new approach to overcome these challenges. However, the effects of continuous temperature rise and loading during the frozen stress tests on the stress birefringence of the thermo–sensitive photopolymers have not been studied thoroughly. In this study, the stress birefringence characteristics of the photopolymers were examined using 3D printed transparent disk models and frozen stress tests under various temperatures and radial loads. A reflection polariscope system incorporated with a high-temperature loading chamber was designed to freeze and capture the stress fringes in the printed model. The thermo–optic curve, stability of full-field fringes, and elasticity and plasticity of the photopolymer under high temperatures and remaining loads were investigated. The effects of high temperatures and remaining loads on the stability of the fringe orders were analyzed. The linear relationship between the fringes and stresses in the material was verified using the fringe orders in the central points of the disk models. The critical temperatures that discriminate between the three different thermo–stress states, i.e., glassy state, transition state, and freezing state, were discussed. Our results indicate that the lowest critical temperature for freezing stresses in the tested photopolymer is 80 °C.

Polymer gels are widely applied to increase the sweep efficiency in mature oil fields facing severe water channeling problems after years of water flooding. The formation of water channeling and successful polymer gel treatment are rooted in complex pore-scale displacement and diversion. Thus, accurate characterization of the pore space and direct visualization of the interior displacement processes in real reservoir rocks such as water flooding, polymer gel injection, and chase floods are crucial in determining the controlling mechanisms of water channeling and polymer gel diversion. In this study, 3D X-ray micro-computed tomography (μCT) was used to characterize the porous structures of three distinct levels in water-flooded reservoir sandstones. A heterogeneous digital model was designed, which comprised representative layers of the obtained porous structures. Then, 3D-printing technology was used to manufacture a transparent model based on the digital prototype. A series of displacement experiments were conducted in the printed model to directly visualize the flow behaviors in the actual pore geometries. In the experiments, the model was first saturated with white oil and flooded by formation water until the outlet discharged only the water phase. Then, an immediately prepared polymer gel was injected. After three days of gelation, an antidilution polymer was injected to sweep the residual oil in the model. Finally, a surfactant flooding agent was injected designed to further increase the oil recovery. The entire four-step displacement process was directly visualized, and the saturation of each injection and the oil recovery were calculated. During water flooding, preferential flooding channels formed in the highest water-flooded porous layers due to the low viscosity ratio between the injected formation water and the displaced white oil, as well as the permeability contrast in the heterogeneous model. The injected polymer gel mainly followed the preferential flooding channels because of the low resistance force in these channels. After gelation, the polymer gel blocked the main preferential channels and diverted the chase floods to nearby pores. A desirable diversion of the polymer gel highly depends on its propagation path during injection and effective blocking after gelation. Experimental visualization and displacement mechanism analysis indicate the efficacy of the flow diversion by the polymer gel treatment in the heterogeneous porous model. The results provide useful information on the displacement behavior in heterogeneous porous structures, and the conclusions apply to similarly scaled systems at the reservoir scale.

In order to enhance effectively permeability of coal seams and increase the efficiency of gas extraction, the pulse gas fracturing is proposed as a new stimulation method. In this study, the pulse gas fracturing experiment of low permeability coal on the laboratory scale was executed and sequential 267 pulse times were preformed. The mercury intrusion porosimetry, scanning electron microscope and permeability measurements were conducted to investigate the changes in the pore structure and permeability caused by the pulse gas injection. The results show as follows. (1) The volume of the coal specimen repeats the swelling-shrinkage process during the pulse gas fracturing which contributes to improve the size and shape of pores and generate micro-cracks in the coal. The deformation of the coal sample has a fatigue threshold in the pulse gas fracturing and its value is near 100 pulse times. (2) The pulse gas fracturing promotes the transfer of smaller pores to larger pores and improves the pore size distribution. After the pulse gas fracturing, the average incremental pore volume of transition pores, mesopores and macropores increases 25.85%, 117.86% and 105.07%, respectively. The total pore volume of the coal sample has an increase of 80.65%. The macropores, mesopores and transition pores volumes increase by 165.22%, 438.33% and 27.27%, respectively. The results indicate that pulse gas fracturing can improve the pore space and the pore distribution, and ultimately increase the permeability of the coal. The change that micropores transfer into larger pores also results that the cumulative pore specific surface of transition pores, mesopores and macropores has an increase of 10.18%, contributing to coalbed methane (CBM) sorption/desorption and diffusion. (3) The crossover network cracks are formed in the coal during the pulse gas fracturing. The porosity and permeability of the coal are obviously improved by the pulse gas fracturing, indicating that the pulse gas fracturing can be used to effectively enhance the permeability of CBM reservoirs. It is worth noting that there is a critical pulse time for the increase of the coal permeability and its value is about 100 pulse times under the stress condition and pulse gas injection method of this research.

Heat injection into coal seams can stimulate coalbed methane reservoirs to enhance the recovery of coalbed methane. However, the effects of temperature change induced coal-gas interactions on gas production are still unclear, although several studies have taken the temperature sensitive coal-gas interactions into considerations. In this study, thermal coal-gas interactions, such as thermal expansion, alteration of the gas sorption capacity, thermal fracturing and thermal volatilization, were firstly observed and validated using experimental data from the literature. Secondly, these interactions were conceptualized into the porosity and permeability models of a dual-porosity coal seam. Thirdly, a fully coupled thermo-hydro-mechanical finite element model was developed to consider comprehensively the interactions among coal deformation, gas transport in the coal matrix and the fracture network, thermal volatilization and heat conduction/convection. Finally, the stimulation effects of heat injection into a coal seam reservoir with one heat injection well and one production well were numerically investigated and compared with the effects observed with the conventional coalbed methane recovery method. These results indicate that (1) heat injection can enhance the cumulative gas production by 70% over 30 years; (2) with heat injection, the gas composition decreased preferentially in the vicinity of the production well and the temperature affected area between the production well and the heat injection well; (3) the porosity and permeability of the coal matrix were greatly increased by both temperature enhanced gas desorption and thermal fracturing but slightly decreased by thermal expansion; (4) the porosity and permeability of the fracture network were linearly increased by temperature enhanced gas desorption but slightly decreased by thermal fracturing and thermal expansion; (5) the enhancement of gas production by thermal fracturing was due to the great increases of the porosity and permeability in the coal matrix, but the porosity and permeability in the fracture network decreased slightly. Therefore, heat injection induced enhancement was due to the increased coal matrix permeability for gas transport.

This paper provides a method for design of high-temperature dual-band radome walls with graded porous structure. Both of the dual-band performances and the thermo-mechanical properties under loading-free and loading conditions of the proposed structures are taken into account. Results demonstrate the effectiveness of the method for design of high-temperature dual-band structures, of which the design frequency can be arbitrarily selected and subsequently the pass-bands are selectable, rather than design of a structure with specific dimensions and pass-bands. Results also show that the dual-band structures with graded porous structure can achieve better thermo-mechanical properties compared to traditional A-sandwich structures, and assembly stress can enhance the thermal and mechanical bearing capabilities of the high-temperature dual-band structures as well.

BaTi2O5 (BT2) films were deposited on Pt-coated MgO(110) single-crystal substrates by laser chemical vapor deposition. The single-phase BT2 thick films were deposited at high deposition rates. The BT2 thick films consisted of elongated grains with columnar cross-section. With increasing the deposition temperature (Tdep), the orientation of BT2 thick films changed from (112) to (511), and the grain size increased. The (112)-oriented BT2 thick film deposited at Tdep=956 K had dielectric constant (e′) of 67 and dielectric loss (tanδ) of 0.015, while the (511)-oriented BT2 thick film deposited at Tdep=964 K had e′ of 74 and tanδ of 0.019 at 300 K and 1 MHz.

Experiments have observed that coal permeability experiences significant change when temperature varies from room temperature up to 100 °C. However, current permeability models have not well described this change so far. This paper proposes a thermally sensitive permeability model to describe the coal-gas interactions under variable temperatures. This model includes matrix permeability and fracture permeability. It describes the impacts of thermal expansion, thermal fracturing, the change of matrix sorption capacity, and the thermal volatilization of fracture surfaces on coal permeability. Particularly, the change of temperature in coal matrix may initiate, nucleate and grow up micro-pores and cracks, forming a crack cloud and changing the porosity of this matrix block. The fractal of cracks and pores is linearly evolved with temperature change. Second, the gas sorption capacity of coal matrix is modified by temperature change through an exponential function. Third, the thermal expansion of matrix is linearly related to temperature change but thermal volatilization occurs on the fracture surfaces and widens gas flow channels. Through the thickness change of volatile membrane, the change of fracture aperture is described by a quadratic function. This model is verified by three series of experimental data from either literature or our newly conducted tests. It is found that the permeability evolution with temperature has four stages which are dominated by one of four primary factors: thermal expansion, thermal volatilization, thermal fracturing and crack coalescence. In each stage, secondary factors may modify the evolution of permeability into sub-classes: linear curve, upward bending curve, or concave bending curve. These verifications confirm that this thermally sensitive permeability model can well describe the permeability evolution of different coals at either thermal expansion stage, or volatilization stage, or their transition.

Sorption-induced strain of coal is vital to the prediction of coal bed methane production and stimulation. Previous studies have addressed the relationship between strain and equilibrium pressure and the relationship between strain change and permeability evolution. However, the strain variations with respect to time and the comparison between the asynchronous difference of dynamic characteristics of adsorption swelling and mechanical compression were rarely explored. In this study, we investigated the anisotropy characteristics of strain change over time as well as the influence of the gas species, gas pressure, and pore structure on the dynamic strain variations by using the low temperature liquid nitrogen adsorption method and a self-designed coal strain measurement apparatus. Our results show that gas diffusion rate has a strong influence on sorption-induced strain of coal. Both mechanical compression and sorption-induced strain exhibited an asynchronous effect with respect to saturating time. We then propose a time-depended volumetric deformation model for coal to analyze the mechanism of the asynchronous effect during sorption. This study sheds light on the dynamic process of coal bed methane production and reveals restrictions of current permeability models.

The artificial freezing method is an effective method of shaft construction in water-rich soft rock beds. Engineering frozen walls involves the use of intact rock material, flaws and ice. To better understand the mechanical properties of frozen walls, explore the failure laws of frozen fractured rock, and solve the problem of complex ice-rock coupling, it is necessary to carry out laboratory triaxial tests on frozen fractured rock. Herein, X-ray diffraction and mesostructure observations are used to analyse the mineral composition and microstructural characteristics of red sandstone specimens taken from the Shilawusu coal mine. Then, the strength, deformation and crack evolution behaviour of red sandstone containing two pre-existing ice-filled fissures are obtained under triaxial compression using the self-developed DRTS−500 subzero rock triaxial testing system. The experimental results show that the peak strength and elastic modulus of the specimens with pre-existing flaws are all much lower than those of the intact specimens, and the strength parameters are distinctly related to the flaw combinations. For the frozen red sandstone containing two intermittent ice-filled fissures, the peak strength, elastic modulus and shear strength parameters all have a linear relationship with the confining pressure and temperature, whereas the shear strength (σ1 − σ3) has a nonlinear relationship with the confining pressure. The deviatoric stress-strain curve of the samples is divided by four characteristic stresses (compacting stress, crack initiation stress, dilatancy stress and peak stress) into five stages from loading to failure. The lower the confining pressure or temperature is, the faster the curve falls after reaching the stress-strain peak value, indicating that the sample exhibits increasingly brittle characteristics. The ultimate failure modes of the specimens containing two pre-existing ice-filled flaws include mixtures of several types of cracks (shear cracks, main cracks, secondary cracks, tensile cracks, coplanar cracks, slip cracks and wing cracks), which depend on the flaw combinations. An obvious crack coalescence failure mode can be observed in the GG and GS specimens, but no obvious crack coalescence phenomenon can be observed in the SS specimens.

In this paper, the high chromium Fe-based composite coatings with different mass fractions of MoS2 powders were prepared by supersonic plasma spraying technology. The influence of MoS2 addition on the microstructure, phase composition, hardness, coating-substrate bonding strength, and friction & wear properties was studied. The results show that the interface between the deposited coatings and the substrates exhibited no obvious detachment, and the porosity was reduced with the addition of MoS2. However, the hardness of composite coatings and bonding strength between coatings and substrates were decreased with the increase of MoS2 content. Attributed to the fact the Cr element in Fe-based powder played a role of stabilizing the ferrite phase and deoxygenation, no oxide and phase transformation of α-(Fe, Cr) were detected. The XRD results also revealed that partial decomposition of MoS2 occurred during the spraying process, leading to the formation of Mo. The worn surface morphology analysis showed that the wear mechanism of the Fe-based coating without MoS2 was mainly oxidative wear. Through the evolution of wear mechanism of three composite coatings, it is found that with the increase of MoS2 content, the phenomenon of abrasive & fatigue wear was intensified, owing to the reason that the bonding between Fe-based matrix material and MoS2 in the coating was poor. Therefore, the content of MoS2 should be rationally controlled in order to improve the friction & wear properties.

Surface-mount techniques primarily depend on soldering. However, soldering techniques have encountered some challenges in recent years. These challenges include rare metal recycling, thermal problems, and Pb toxicity. We recently developed a metallic nanowire surface fastener (NSF) to resolve the abovementioned problems. This fastener can be used to connect electronic components on a substrate at room temperature using the van der Waals force between each nanowire. This study demonstrates a 64-pin NSF that behaves like a ball grid array (BGA) for application to actual electronic devices. The adhesion strength and electrical properties of the NSF were investigated by adjusting the nanowire parameters, such as diameter, length, density (number per area), preload, and shape. The shape control of the nanowires greatly contributed to the improvement of the properties. A maximum adhesion strength of 16.4 N/cm2 was achieved using a bent, hook-like NSF. This strength was 4–5 times the value of the straight NSF. The contact resistivity was 2.98 × 10−2 Ω∙cm2. The NSF fabricated through the simple template method showed the room temperature bonding ability and adaptability to a highly ordered electrode like the BGA.

The freezing method is an effective method for shaft construction in water-rich strata. To determine the mechanical properties of frozen walls that contain ice, study the fracture and instability of frozen rock walls, and solve ice-rock coupling problems, it is necessary to research frozen rock masses under triaxial compressive stress. In this paper, using X-ray diffraction and mesostructure observation, the microstructural characteristics and deformation mechanism of frozen red sandstone specimens taken from the Shilawusu coal mine at a depth of 300 m in the Ordos Basin were studied. A series of tests were performed on frozen red sandstone samples under triaxial compression at 4 temperatures and 4 confining pressures using self-developed DRTS−500 subzero rock triaxial test system. The strength and deformation characteristics of the frozen red sandstone under three-dimensional loading were examined. Test results showed that the bonding water formed by charge adsorption on the surface of the mineral particles and the freezing of pore gravity water enhance the cementation of the mineral particles and the flow plasticity under high stress. At constant temperature, the peak strength and elastic modulus of the frozen red sandstone increase with increasing confining pressure. At constant confining pressure, the peak strength and elastic modulus of the frozen red sandstone increase with decreasing temperature, but temperature has little effect on the peak strain. The internal friction angle of the frozen red sandstone is between 28° and 35° and decreases linearly with decreasing temperature. The cohesion is between 4 and 9 MPa and increases linearly with decreasing temperature. At high freezing temperatures, with an increase in confining pressure, the main factors controlling of the triaxial strength of the rock samples change from the cohesive force to the biting force caused by the mineral particles and ice crystals and the friction between them, and the failure mode of the rock samples changes from tension to shear failure. When the temperature is low, the main controlling factors for sample failure are the cohesive force and friction bite force, and the swelling failure and tension-shear composite failure are the main factors. Based on the parabolic strength criterion and the statistical damage theory derived from the strain equivalence principle, a statistical damage constitutive model that considers the void compaction stage and residual strength deformation stage is established, and the analytical expressions for the model parameters are given, which verify the rationality and validity of the model.

Polypropylene fibers are embedded to prevent reactive powder concrete (RPC) from spalling failure under high temperatures. This paper probes the influence of embedded fibers at various volumetric dosages on the thermomechanical properties of polypropylene-fibered reactive powder concrete (PPRPC) exposed to high temperatures up to 350 °C and on the spalling performance and characteristics up to 600 °C. The thermomechanical properties include the characteristic temperature for spalling, and residual strengths, such as the compressive strength, split tensile strength, and flexural tensile strength. A high-definition charge-coupled device camera and scanning electron microscope technology were employed to capture the spalling processes and to detect the microstructural changes in the materials with various fiber dosages. To understand and characterize the mechanism by which polypropylene fibers influence the thermal spalling of RPC, a numerical model to determine the moisture migration and vapor pressure transmission during spalling was developed in this paper. It showed that there was an optimal volumetric dosage of fibers to prevent PPRPC from explosive spalling. The relationships between the mechanical characteristics of PPRPC and the fiber dosages were derived based on experimental data.

At millimeter dimensions or less, the conventional bonding technology tends to suffer from severe performance and reliability degradation. Moreover, the high heating temperature is usually needed. Here, we report a room-temperature electrical surface fastener based on copper/polystyrene core/shell nanowire (NW) arrays. Uniquely, this electrical surface fastener exhibits high macroscopic adhesion strength (∼44.42 N/cm 2 ) and low electrical resistance (∼0.75 × 10 −2 Ω cm 2 ). Furthermore, it was found that the adhesion strength of this surface fastener can be mediated by the shell thickness and the molecular weight of polystyrene. Finally, the contact mechanics theory was used to explain the adhesion mechanism.

A novel method using microwaves is proposed to detect and evaluate defects in glass fiber reinforced plastic (GFRP) laminates. A thin circular plastic film simulating delamination was inserted in GFRP laminates and was detected using microwave reflectometry. A focusing mirror sensor consisting of a horn antenna and two metal mirrors was used to improve the measurement resolution. In addition, the thickness of the film was calculated using a proposed model based on microwave propagation theory. Measuring the variation in the amplitude of the microwave reflectivity, a 7.5-μm-thick film was successfully detected in a 3-mm-thick GFRP laminate. Moreover, the calculated results of the inserted film thickness had a high degree of accuracy.

In this study, the effect of residual stress in a film on the growth behavior of a free-standing metallic micro/nanowire due to electromigration (EM) is examined. The growth of a wire is accompanied by atomic diffusion, accumulation of atoms, and release of compressive EM-induced localized hydrostatic stress due to the accumulation of atoms. Hence, the growth of the wire dominantly depends on the EM-induced localized stress caused by the accumulation of atoms. Because rigid passivation generates a strong localized stress field in the metallic interconnect, with greater accumulation of atoms, the EM-induced localized stress state for wire growth is influenced by passivation conditions, including the thickness and residual stress associated with passivation. Two samples with different passivation thicknesses, resulting in different levels of residual stress, were used to elucidate the influence of passivation conditions on the growth performance of Al microwires. The growth rate was experimentally measured. An x-ray diffraction system was used to obtain the value of residual stress in passivation, demonstrating that a higher absolute value of compressive residual stress results in a lower growth rate. In contrast, a lower absolute value increases the growth rate of the wire and can decrease the delamination risk of the topmost passivation, deposited by sputtering. Contrarily, a passivation that is too thin, resulting in a lower absolute value of compressive stress, increases the risk of passivation crack due to the accumulation of atoms by EM. A suitable passivation thickness for a desired wire growth must be determined based on this finding., ファイル公開：2021/01/13

This paper aims to explore the failure modes and crashworthiness characteristics of double hat shaped tubes made of weave carbon fiber reinforced plastic (CFRP) subjected to quasi-static axial crushing and transverse bending. Experimental investigations were carried out into three different thicknesses of the composite tubes fabricated by the bladder molding process. Three distinct failure modes, classified as progressive end crushing (I), unstable local buckling (II) and mid-length collapse (III), were observed in the axial crushing tests, whereas only one similar progressive collapse mode was observed in transverse bending tests. It is shown that the thickness is a critical parameter affecting the failure mode and energy absorption capability, leading to the increase in the peak load and specific energy absorption (SEA) during the tests. The SEA of the tested double hat shaped tubes under axial crushing ranges from 60 to 90 J/g, which is marginally higher than that of regular sectional CFRP tubes but over 2 times of that of conventional metallic tubes. By comparison, the load bearing and energy absorption capabilities of the tubes under transverse bending are much lower than those of the axially compressed tubes (less than 10% and 1%, respectively).

The influence of temperature on the fracture behavior of siltstone is investigated in detail by mode I fracture toughness tests under three-point bending in situ SEM observations. A total of 27 specimens subjected to thermal pre-treatment have been tested. Experimental results indicate that effects of temperature on siltstone fracture behavior are obvious, not only on failure mechanism, but also on mechanical parameters like peak failure loads, fracture toughness and modulus of elasticity. The failure mechanism changes from intergranular fracture to mixed intergranular and transgranular fractures, and finally to intergranular fracture and thermal cracking with temperature from 25 to 60 °C. Fracture toughness KIC decreases slightly from room temperature 25 to 100 °C, and then increases significantly from 100 to 125 °C, and then gradually declines from 125 to 600 °C. A new numerical elastic modulus estimation method is proposed, considering a series of fluctuated experimental data. The variation of the elastic modulus with the temperature is similar with that of fracture toughness.

Theoretical and experimental studies have revealed that energy dissipation and release play an important role in the deformation and failure of coal rocks. To determine the relationship between energy transformation and coal failure, the mechanical behaviors of coal specimens taken from a 600-m deep mine were investigated by conventional triaxial compression tests using five different confining pressures. Each coal specimen was scanned by microfocus computed tomography before and after testing to examine the crack patterns. Sieve analysis was used to measure the post-failure coal fragments, and a fractal model was developed for describing the size distribution of the fragments. Based on the test results, a damage evolution model of the rigidity degeneration of coal before the peak strength was also developed and used to determine the initial damage and critical damage variables. It was found that the peak strength increased with increasing confining pressure, but the critical damage variable was almost invariant. More new cracks were initiated in the coal specimens when there was no confining pressure or the pressure was too high. The parameters of failure energy ratio β and stress drop coefficient α are further proposed to describe the failure mode of coal under different confining pressures. The test results revealed that β was approximately linearly related to the fractal dimension of the coal fragments and that a higher failure energy ratio corresponded to a larger fractal dimension and more severe failure. The stress drop coefficient α decreased approximately exponentially with increasing confining pressure, and could be used to appropriately describe the evolution of the coal failure mode from brittle to ductile with increasing confining pressure. A large β and small α under a high confining pressure were noticed during the tests, which implied that the failure of the coal was a kind of pseudo-ductile failure. Brittle failure occurred when the confining pressure was unloaded—an observation that is important for the safety assessment of deep mines, where a high in situ stress might result in brittle failure of the coal seam, or sudden outburst.

This study aims to identify the impact responses and residual flexural properties of narrow carbon fiber reinforced plastic (CFRP) laminates using the energy profile diagram (EPD). A serial of impact tests was performed with different thicknesses of narrow CFRP laminates to examine the damage process and extent from initiation to complete breakage. The quasi-static three-point bending tests were carried out to characterize the degradation of residual flexural properties induced by the impact. The overall impact energies are clearly separated into three regions by the penetration threshold and crack threshold respectively determined by the EPD. The main damage modes are matrix cracking for lower impact energies, delamination and fiber breakage for middle impact energies and complete breakage for higher impact energies. It is also found that the degradation of normalized residual flexural properties can be divided into three regions using these abovementioned two thresholds, of which the greatest loss occurs in the middle energy region, whilst the maximum reductions of normalized flexural strength and flexural modulus are smaller than 20% in the first region. By comparison, the 6-ply specimens are of lower impact resistance and lower residual flexural properties than the 9-ply specimens under same impact energy; and the residual flexural strength is more sensitive to impact loading than flexural modulus.

Fatigue cracks were healed by controlling a high-density electric current. The changes in the displacement distribution around the crack tip and the stress intensity factor before and after crack healing were evaluated quantitatively with a digital image collation method. According to the results, it was determined that the cracks were closed by approximately 2 to 7 µm in this study. On the other hand, the stress intensity factor decreased or increased depending on the conditions of the crack and the current applied. The physical restriction between the crack surfaces, such as bridging, is important with respect to lowering the stress intensity factor after healing.

With the ongoing miniaturization in electronic packaging, the traditional solders suffer from severe performance degradation. In addition, the high temperature required in the traditional solder reflow process may damage electronic elements. Therefore, there is an increasing urgent need for a new kind of nontoxic solder that can afford good mechanical stress and electrical contact at low temperature. This paper presents a method of fabricating nanowire surface fastener for the application of microelectronic packaging bonding at room temperature. This surface fastener consists of copper core and polystyrene shell nanowire arrays. It showed an adhesive strength of ∼24 N/cm2 and an electrical resistance of ∼0.41 × 10−2 Ω·cm2. This kind of nanowire surface fastener may enable the exploration of wide range applications, involving assembly of components in the electronic packaging.

To investigate the effect of high-density electric current on healing of the fatigue damage, the recovery of residual plastic strain was quantitatively evaluated with the digital image correlation method. The microhardness was measured within the plastic zone at the root of notch. Furthermore, the dislocation structures before and after the application of electric current were investigated by transmission electron microscopy to further understand the mechanics of the healing effect. It was concluded that the fatigue damage was healed by a decrease in dislocation density. As a result, fatigue crack initiation was delayed by the healing of fatigue damage.

Cells within connective tissues are routinely subjected to a wide range of non-uniform mechanical loads that regulate many cell behaviors. In the present study, the relationship between cell orientation angle and strain value of the membrane was comprehensively investigated using an inhomogeneous strain field. Additionally, the cellular axial strain threshold, which corresponds to the launching of cell reorientation response, was elucidated. Human bone marrow mesenchymal stem cells were used for these experiments. In this study, an inhomogeneous strain distribution was easily created by removing one side holes of an elastic chamber in a commonly used uniaxial stretching device. The strains of 2D stretched membranes were quantified on a position-by-position basis using the digital image correlation method. The normal strain in the direction of stretch was changed continuously from 2.0 to 15.0 %. A 3D histogram of the cell frequency, which was correlated with the cell orientation angle and normal strain of the membrane, made it possible to determine the axial strain threshold accurately. The value of the axial strain threshold was 4.4 ± 0.3 %, which was reasonable compared with previous studies based on cyclic uniaxial stretch stimulation (homogeneous strain field). Additionally, preferential axial strain of cells, which was a cell property firstly introduced, was also achieved and the value was −2.0 ± 0.1 %. This study is novel in three respects: (i) it precisely and easily determined the axial strain threshold of cells; (ii) it is the first to suggest preferential axial strain of cells; and (iii) it methodically investigated cell behavior in an inhomogeneous strain field.

A technique to detect delamination in composite materials by noncontact, rapid and high sensitive microwave reflectometry with a focusing mirror sensor was proposed. The focusing mirror sensor, which has high sensitivity and resolution, is expected to detect delamination sensitively. In this paper, the ability of microwave inspection to detect delamination in glass fiber reinforced plastic (GFRP) and carbon fiber reinforced plastic (CFRP) was verified. As the results, the existences of 100 μm thick delamination in 3 mm thick GFRP laminate and 2 mm thick CFRP laminate were detected.

To investigate the effect of high-density electric current on the delay of fatigue crack initiation, the dislocation structures before and after the application of electric current were investigated by transmission electron microscopy. Dislocation density was quantitatively characterized before and after the application of electric current to further understand the mechanics of the healing effect. Atomic force microscope results showed that the slips disappeared locally and the slip height decreased on the surface of the specimens. Furthermore, the delaying effect of the crack initiation due to the application of electric current was evaluated by the fatigue crack-initiation model in which the accumulation of the dislocation density was considered. [doi:10.2320/matertrans.M2013198]

Reactive powder concrete(RPC) is vulnerable to explosive spalling when exposed to high temperature.The characteristics of micro pore structure and vapor pressure of RPC are closely related to the thermal spalling.Applying mercury intrusion porosimetry(MIP) and scanning electron microscopy(SEM) techniques,the authors probed the characteristics of micro pore structures of plain RPC200 when heated from 20-350℃.The pore characteristics such as specific pore volume,threshold pore size and most probable pore size varying with temperatures were investigated.A vapor pressure kit was developed to measure the vapor pressure and its variation inside RPC200 at various temperatures.A thin-wall spherical pore model was proposed to analyze the thermo-mechanical mechanism of spalling,by which the stresses varying with the vapor pressure q(T) and the characteristic size of wall(K) at any point of interest were determined.It is shown that the pore characteristics including specific pore volume,average pore size,threshold pore size and most probable pore size rise significantly with the increasing temperature.200℃ appears to be the threshold temperature above which the threshold pore size and the most probable pore size climb up dramatically.The increase in the specific pore volume results from the growth both in quantity and in volume of the transition pores and the capillary pores.The appearance of the explosive spalling in RPC200 is mainly attributed to being unable to form pathways in favor of releasing water steam in RPC and to the rapid accumulation of high vapor pressures as well.The thin-wall sphere domain where the vapor pressure governs the spalling is bounded through the pore model.

The explosive spalling of high-strength concrete due to fire is a problem that has garnered increasingly widespread attention, particularly the explosive spalling of reactive powder concrete (RPC). For years, based on the vapor pressure mechanism, the addition of fibers has been demonstrated to be somewhat effective in protecting against spalling. However, relevant experiments indicate that fibers are not effective for dense concrete, which is a challenge for the simple vapor pressure mechanism in providing spalling resistance for RPC. The authors found that silica fume plays an important role in the explosive spalling of RPC. Thus, four classes of RPCs with different ratios of silica fume were prepared, and the spalling phenomena and the inner temperature distribution during heating were investigated. The results show that silica fume content has a prominent effect on the spalling process of RPC.

The pore structure characteristics of reactive powder concrete (RPC) were investigated by means of the mercury injection method at seven temperature levels, namely, 20°C, 100°C, 150°C, 200°C, 250°C, 300°C, 350°C, respectively. The characteristic parameters such as porosity, pore volume, average pore size and threshold aperture varied with temperatures were analyzed. The results indicate that the porosity, pore volume, threshold aperture and other characteristic parameters of RPC increased with the temperature increasing.

Corroded cables from a cable-stayed bridge in China that had been in service for 18 years were employed to investigate the basic mechanical properties and residual fatigue life of wires and cables. First, the wires were randomly selected from the cables near the bottom anchorages and cut into segments as test specimens. The extent of corrosion of the wires was experimentally investigated. A tensile loading test was conducted on the wires to obtain the mechanical properties of the corroded single wires. The fatigue life of the corroded single wires was experimentally studied, and a dramatic degradation in fatigue life was observed. This phenomenon was interpreted using SEM images. Fatigue tests on two corroded cables were also conducted, and the test results indicated that the fatigue life of the cables had also decreased dramatically. A Monte Carlo simulation was conducted to obtain the fatigue life of cables. The simulation results indicated that the fatigue life of a cable was controlled by the small fraction of wires in the cable with the shortest fatigue lives. The fatigue life of a cable at a certain failure probability was dependent on the number of wires in the cable, but the mean fatigue life of a cable was not affected by the number of wires in the cable. DOI: 10.1061/(ASCE)BE.1943-5592.0000235. © 2012 American Society of Civil Engineers. CE Database subject headings: Cable-stayed bridges; Fatigue; Corrosion; Monte Carlo method; Simulation; Experimentation. Author keywords: Stay cable; Corrosion; Fatigue; Monte Carlo simulation.