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To understand the relationship between the damage mechanism of sapphire and the anisotropic characteristic of its crystal structure, in-situ scratch tests under the confocal laser scanning microscope (CLSM) were performed along different directions of C-plane sapphire wafer. The relation between in-situ images and normal/lateral forces was established. Craters in front of the contact region (CR) between the indenter and the material, and subsurface special cracks separated from CR were observed. As the scratch direction changed, these special cracks and craters initiated and propagated on the left front, front, and right front of the CR, respectively. The scratch phenomena in different scratch directions exhibited triple symmetric characteristic around the c-axis. The probability of slip/twinning activation and the theoretical stress field in front of the indenter were calculated to investigate the fracture mechanism of sapphire. Craters and those special cracks appeared in the region with the highest probability of rhombohedral twinning and basal twinning activation, respectively. According to the theoretical stress field, the material ahead of the indenter axis was separated into regions I and II. Craters in front of the CR and those special cracks separated from the CR appeared in these two regions, respectively.

Numerical modeling of aircraft composites dynamic failure is a relevant and wide subject. This research concentrates on the barely visible impact damage that is caused by a low-velocity impact with a steel striker. The one-side crack model is verified on experimental data for short energy pulse on glass. An impact of steel striker and unidirectional composite rib was modeled, several sizes of striker and values of impact energy from to are considered. Several failure criteria were considered: Tsai–Hill, Tsai–Wu, Drucker–Prager, Hashin, Puck, LaRC03, LaRC04. Analysis of crack directions for the largest principal stress criterion and the Puck criterion was conducted. [ABSTRACT FROM AUTHOR]

In this paper, a brake cylinder coating comprising composite material of Fe3Al and Cr3C2 mixed powder was prepared by laser cladding onto carbon structural steel. The microstructure of the cladding materials was investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The influence mechanism of the Cr3C2 content and process parameters on crack generation of Fe3Al/Cr3C2 composites were studied. The crack is mainly caused by the mismatch between the plastic phase of Fe2AlCr, the brittle phase of Fe3Al, and Cr7C3 in the cladding layer. With the increase in Cr3C2 content, Fe2AlCr and Cr7C3 in the cladding layer increased, while Fe3Al decreased. When the content of Cr7C3 is low, the cladding layer mainly reflects the plastic increase brought by Fe2AlCr; when the content of Cr7C3 is too much, the property of the cladding layer is reflected as the brittleness of Cr7C3. The number of cracks reached the lowest value at 15 wt.%. The process parameters mainly affect the dilution rate of the cladding layer and thus affect the content of Fe2AlCr. [ABSTRACT FROM AUTHOR]

Control of rock failure under stress wave loading is challenging but important to mitigate the negative impacts of underground excavation on the environment (e.g., rock overbreak and ground vibration). This study reports that dynamically amplified expansion pressure from expansive mortar is an effective and efficient solution to promote the dynamic failure of brittle rock. Our experimental and numerical results reveal that the expansion pressure amplified by stress wave loading facilitates the dynamic failure surrounding the expansive mortar, accompanied by the generation of tangential and radial cracks and the attenuation of stress wave. Our data also show that the dynamically amplified expansion pressure is larger than the radial inertia stress and dominates crack generation in the unconfined rock, but the pressurization capacity of expansive mortar should be improved for the rock fracturing application in confined rock. The application of expansive mortar fundamentally modifies the brittleness of surrounding rock under stress wave loading, resulting in changes in failure mode. However, the application of expansive mortar to control rock dynamic failure should be further investigated in field studies. [ABSTRACT FROM AUTHOR]

Crack resistance performance by oxidation must be considered in rubber product development because cracks are the starting point for fracture. When rubber is stretched extensively, mechanical stresses can break apart its molecular chains, generating radicals and promoting mechanochemical oxidation. This reaction is one of the causes of cracking. However, it is not fully understood how cracks are formed in vulcanized rubber with an inhomogeneous crosslinking structure. Atomic force microscopy (AFM) nanomechanics has been utilized to observe vulcanized butadiene rubber in an elongated state. It was shown that crack generation is related to mechanochemical oxidation, and that the cracks could be clearly visualized as stress-concentrated regions at the nanoscale. Additionally, crack formation is accelerated with increasing elongation of the rubber. This demonstrates that AFM nanomechanics is an effective tool for observation of the generation of cracks associated with mechanochemical oxidation in rubber materials. [Display omitted] • Crack generation was investigated using atomic force microscopy (AFM) nanomechanics. • Cracks were generated from stress-concentrated regions in the nanoscale. • Elongation and oxygen in the air cause crack generation. • Crack generation was accelerated with an increasing elongation ratio. [ABSTRACT FROM AUTHOR]

Improvement in the efficiency of diamond tools is inextricably linked with the use of high-strength heat-resistant diamond grains for tooling. Hence, study of the effect exerted by heat treatment on changes in the mechanical characteristics of diamonds produced in various growth systems is a relevant task. The effect of heat treatment (in an inert atmosphere) of diamond powders produced in growth systems using ferroalloys to dissolve carbon and promote the conversion of graphite into diamond on the mechanical characteristics of diamonds and elemental composition of inclusions formed on the crystal surfaces during heat treatment is examined. Following heat treatment in the range 700–1100°C, inclusions are observed on the crystal surfaces in diamonds produced in the Fe–Co–C and Fe–Ni–C growth systems. In our opinion, the effect results from ejection of the liquid metal phase to the surface by capillary forces. In crystals with a higher content of inclusions, the phenomenon of capillary ejection is manifested at lower temperatures than the melting of carbon solvent alloys in the diamond growth process. This is confirmed by a sharp change in the specific magnetic susceptibility shown by the samples with a high content of intracrystalline inclusions in the temperature range 400–800°C. For diamonds with a high content of intracrystalline inclusions, with increase in the heat treatment temperature in the range 700–1100°C, the precipitation of the solvent alloy on the surface of diamond crystals leads to crack generation and decreases the strength of diamond crystals. For diamond samples with a low content of inclusions, when the heat treatment temperature increases to 800°C, the specific magnetic susceptibility and strength hardly change (taking into account the relative error of the data obtained). [ABSTRACT FROM AUTHOR]

Most existing freight railway tunnels have experienced basement cracking, damage, and even mud-pumping after a certain period of operation. For heavy-haul railway tunnels with high traffic densities, large capacities, and large axle loads, this situation will be more serious. This paper establishes a simplified model of a single-track heavy-haul railway tunnel using the two-dimensional particle discrete element method. The meso-evolution process of the concrete structure and surrounding rock void at the bottom of the tunnel is studied in the later stages of repeated loading by heavy-haul trains. The difference in the base void between waterless and water-saturated bases is compared. The cracks in the surrounding rock are first generated at the lowest position of the invert center, and run from the middle to the sides and from the top to the bottom. The terminal cracks return to the free surface, exhibiting a parabolic shape that opens upward. There are more cracks in the middle and fewer at the sides. The asynchronous deformation of the surrounding rock and the inverted arch is positively correlated with the number of cracks. The voiding process under water-saturated conditions is similar to that without water, but the depth and width over which the rock mass is crushed and the range of voiding increase exponentially in the water-saturated case, and the void appears earlier. It is anticipated that these research results will contribute to the structural design and operational safety of heavy-haul railway tunnels. [ABSTRACT FROM AUTHOR]

A molecular dynamics (MD) simulation study was performed to investigate the effects of helium (He) on the mechanical properties of nanocrystalline body-centered cubic iron (BCC Fe). Simulated X-ray diffraction (XRD) was used to explore the relationship between the generation of cracks and the change of the crystal structure in nanocrystalline BCC Fe during tensile deformation. It is observed that the peak stress and the elastic modulus decrease with increasing concentration of He atoms, which are introduced into the grain boundary (GB) region of nanocrystalline Fe. The generation and connection of intergranular cracks are enhanced by He atoms. Significant peak separation, which is associated with the generation of cracks, is found in the simulated XRD patterns of nanocrystalline Fe during the tensile process. The lower diffraction angle of the {211}′ peak suggests a more serious lattice distortion during loading. For all nanocrystalline Fe deformed to 6% strain, the degree and fraction of the lattice distortion increases with the increasing loading stress.

An in-depth study of the failure of granular materials, which is known as a mechanism to generate defects, can reveal the facts regarding the origin of the imperfections, such as cracks in the carbon anodes. The initiation and propagation of the cracks in the carbon anode, especially the horizontal cracks below the stub-holes, reduce the anode efficiency during the electrolysis process. The failure analysis of coke aggregates can be employed to determine the appropriate recipe and operating conditions in order to avoid the formation of cracks in the carbon anodes. In this paper, it will be shown that a particular failure mode can be responsible for the crack generation in the carbon anodes. The second-order work criterion is employed to analyze the failure of the coke aggregate specimens and the relationships between the second-order work, the kinetic energy, and the instability of the granular material are investigated. In addition, the coke aggregates are modeled by exploiting the discrete element method (DEM) to reveal the micro-mechanical behavior of the dry coke aggregates during the compaction process. The optimal number of particles required for the failure analysis in the DEM simulations is determined. The effects of the confining pressure and strain rate as two important compaction process parameters on the failure are studied. The results reveal that increasing the confining pressure enhances the probability of the diffusing mode of the failure in the specimen. On the other hand, the increase of strain rate augments the chance of the strain localization mode of the failure in the specimen.

Abstract Boron in low carbon steel can improve hardenability and hot ductility remarkably. However, defects including cracks, internal half way cracks, and even breakouts commonly appear during the continuous casting process, due to the segregation and remelting behaviors of boron-containing steel. In the present investigation, morphology characteristics around the crack were observed by optical microscope (OM). The segregation and distribution of boron around grain boundary were observed by secondary ion mass spectroscopy (SIMS), and the remelting phenomenon around the crack was observed by the high temperature confocal laser scanning microscope (CLSM). Results show that BN particles are detected in the crack region, Boron is concentrated at grain boundaries. Due to the boron segregation, liquid phase is observed at 1420–1400 °C and 1340–1330 °C in cooling process with boron content even of 0.0013 wt%. The low and high temperature brittle zones are revealed based on boron segregation and remelting behavior. [ABSTRACT FROM AUTHOR]

Hard and brittle materials like glass and ceramics are highly demanded in modern manufacturing industries. However, their superior physical and mechanical properties lead to high cost of machining. Ultrasonic machining has been regarded as one of the most suitable fabrication techniques for these kinds of materials. A smoothed particle hydrodynamics model was proposed to study the material removal mechanism of the ultrasonic machining in this study. Influences of abrasive materials and the particle shapes on the crack formation of work substrates were investigated using this smoothed particle hydrodynamics model. Experiments were also conducted to verify the simulation model. Both of the simulation and experimental results show that using hard and spherical abrasive particles is helpful to improve the material removal efficiency. This work was the first to demonstrate the crack formation mechanisms during ultrasonic machining with different abrasive particles using smoothed particle hydrodynamics, which is significant for improving the machining performance of the ultrasonic machining process. [ABSTRACT FROM AUTHOR]

In order to study the characteristics of Ti 2 AlC coating using hypersonic flame spraying, Ti 2 AlC powder was synthesized and deposited on low carbon steel. The tribological and mechanical properties of Ti 2 AlC coating at room temperature and 800 °C were investigated by frictional tests under different contact loads and sliding velocities. Stresses and crack generations of Ti 2 AlC coating were analyzed by finite element analysis. The phases at room temperature and 800 °C were shown, and the reasons of low frictional coefficients were explained. Results show that Ti 2 AlC can be used as an excellent self-lubrication material at a wide range of temperature due to the special mix of metallic/covalent bonds and phase change performance. Contact loads and sliding velocities have significant effects on the frictional properties of Ti 2 AlC coating. As the buffer action of Ti 2 AlC coating, stress of substrate is decreased by 41.3%. The deformation of substrate is circular in nature and the crack is not easily generated due to the coating protection. [ABSTRACT FROM AUTHOR]

The crack geometry and associated strain field around Berkovich and Vickers indents on silicon have been studied by X-ray diffraction imaging and micro-Raman spectroscopy scanning. The techniques are complementary; the Raman data come from within a few micrometres of the indentation, whereas the X-ray image probes the strain field at a distance of typically tens of micrometres. For example, Raman data provide an explanation for the central contrast feature in the X-ray images of an indent. Strain relaxation from breakout and high temperature annealing are examined and it is demonstrated that millimetre length cracks, similar to those produced by mechanical damage from misaligned handling tools, can be generated in a controlled fashion by indentation within 75 micrometres of the bevel edge of 200 mm diameter wafers. [ABSTRACT FROM AUTHOR]

The crack geometry and associated strain field around Berkovich and Vickers indents on silicon have been studied by X-ray diffraction imaging and micro-Raman spectroscopy scanning. The techniques are complementary; the Raman data come from within a few micrometres of the indentation, whereas the X-ray image probes the strain field at a distance of typically tens of micrometres. For example, Raman data provide an explanation for the central contrast feature in the X-ray images of an indent. Strain relaxation from breakout and high temperature annealing are examined and it is demonstrated that millimetre length cracks, similar to those produced by mechanical damage from misaligned handling tools, can be generated in a controlled fashion by indentation within 75 micrometres of the bevel edge of 200 mm diameter wafers.

The change in resistance upon bending in metal films as thick as 1 mm used for underpanel force touch applications is limited by the low sensitivity, thus requiring high-performance readout circuitry. In this paper, we report inkjet-printed silver thin films having crack-inducing underlayers, which further increases the sensitivity of their resistance changes under deformation. This allows for detecting weak vertical forces even through the plates (force-receiving layer), such as 0.4 or 1.2 mm thick polyethylene terephthalate or 0.4 mm thick glass. The underplate sensors will detect a force level as low as 10 gf, which corresponds to the amount of force required for fingerprint recognition. Furthermore, such highly sensitive strain sensors can potentially solve the inaccuracy issue of wearable devices, which can occur when misplaced sensors detect relatively weak biosignals, such as heart rate and blood pressure. The sensor detects the accurate pulse patterns of the wrist artery even though it is off-centered from the artery by 6 mm or larger. The crack-based strain sensor and its usage as a hidden underplate force sensing device will create various wearable and user-machine interface applications.

The characteristics of cyclic bending deformation and fatigue fracture are studied on polycrystalline aluminum and Al-1.0mass%Mg alloy from the metallurgical point of view. It is found that the fatigue life mainly depends on grain size and the kind of materials. Cracks are preferentially formed at grain boundaries inclined 40 – 60°to the tension-compression direction, suggesting that shear stress affects the crack formation. EBSD measurements reveal the inhomogeneity of deformation. Intense development of sub-grain is seen in the grain interior close to a crack. It indicates that the work hardening close to grain boundaries triggers the crack formation. [ABSTRACT FROM AUTHOR]

Abstract: Ordered mesoporous TiO2 films with increased pore connectivity are introduced in this work. Crack generation was accomplished by thermal shock using rapid thermal annealing to obtain a connected pore structure. Due to the increased pore connectivity caused by crack generation, the specific surface area of mesoporous TiO2 films increased by 2.6 times. As specific surface area increased, gas sensing properties of the ordered mesoporous TiO2 film were enhanced, thus, ordered mesoporous TiO2 films with increased pore connectivity are ideal candidates for gas sensor applications. [Copyright &y& Elsevier]

Abstract: This paper presents a study on the fracture of NiTi SMA wire undergoing thermal cycling, activated through resistive heating, under a constant load referred to as thermo-mechanical cycling (TMC). In general, TMC results in initiation of a large number of surface cracks on the SMA wire perpendicular to the wire axis. After initiation, these cracks propagate simultaneously inside the material under cyclic strain. But, the important observation in this study is the generation of cracks in the bulk of the material during TMC, preferentially at the core region of the wire. It has been shown that generation of cracks at the core region is due to strain inhomogeneity during TMC as a consequence of variation in the volume fraction of austenite and martensite phases across the wire cross section. This strain inhomogeneity is attributed to temperature and stress gradients that can exist across the cross section of the wire during thermal cycling. Study shows that nucleation of cracks and their growth at the core region of the wire plays an important role in the fracture of thermally activated SMA wire. [Copyright &y& Elsevier]

Abstract: When the particle simulation method is used to simulate large length-scale problems, such as crustal length-scale problems in the field of geology, it is necessary to include gravity effect into the particle model. To consider gravity effect appropriately, there is a minimum value requirement for the contact stiffness of a particle so that unexpected particle overlap can be avoided. This minimum value is called the critical contact stiffness of a particle due to gravity effect. Using the vertical force equilibrium condition of a particle at the bottom of a two-dimensional particle model, a theoretical formula has been successfully derived for determining the critical contact stiffness of the particle in this paper. For the two-dimensional particle model consisting of circular particles of unit thickness, there is a relationship between the critical contact stiffness of a particle and the critical elastic modulus of the particle material when a linear-elastic-contact model between any two particles is used. Since the elastic moduli of most crustal rocks are greater than the corresponding critical elastic modulus of the particle material, it has been theoretically demonstrated that the particle simulation method, which is based on the linear-elastic-contact model between any two particles, is suitable for simulating gravity effect within the particle model of a large length-scale. The related simulation results have demonstrated the usefulness of the particle simulation method for simulating spontaneous crack generation patterns within brittle crustal rocks. [Copyright &y& Elsevier]

Abstract: The particle simulation method is used in this paper to explicitly simulate spontaneous crack generation in brittle crustal rocks. Although the method was previously used to simulate crack generation in small-scale laboratory specimens and mining sites, it has seldom been used to simulate spontaneous crack generation in large-scale geological systems. If a geological system can be treated as a quasi-static one, the mechanical response of the system should be independent of the loading rate. Based on this understanding, we have compared the commonly-used continuous loading procedure with a newly-proposed discontinuous loading procedure, which is independent of the loading rate within the elastic range of a particle system. The use of the discontinuous loading procedure enables the macroscopic elastic modulus of a two-dimensional particle system to be directly evaluated from the particle stiffness. However, the particle-size sensitivity analysis of at least two different models needs to be conducted for dealing with the particle size-dependent issue when the particle simulation method is applied to solve crack generation problems in geological systems. Through the phenomenological modeling of spontaneous crack generation problems, it has been demonstrated that the particle simulation method is useful for simulating spontaneous crack generation phenomena at geological length scales. [Copyright &y& Elsevier]

An in-depth study of the failure of granular materials, which is known as a mechanism to generate defects, can reveal the facts regarding the origin of the imperfections, such as cracks in the carbon anodes. The initiation and propagation of the cracks in the carbon anode, especially the horizontal cracks below the stub-holes, reduce the anode efficiency during the electrolysis process. The failure analysis of coke aggregates can be employed to determine the appropriate recipe and operating conditions in order to avoid the formation of cracks in the carbon anodes. In this paper, it will be shown that a particular failure mode can be responsible for the crack generation in the carbon anodes. The second-order work criterion is employed to analyze the failure of the coke aggregate specimens and the relationships between the second-order work, the kinetic energy, and the instability of the granular material are investigated. In addition, the coke aggregates are modeled by exploiting the discrete element method (DEM) to reveal the micro-mechanical behavior of the dry coke aggregates during the compaction process. The optimal number of particles required for the failure analysis in the DEM simulations is determined. The effects of the confining pressure and strain rate as two important compaction process parameters on the failure are studied. The results reveal that increasing the confining pressure enhances the probability of the diffusing mode of the failure in the specimen. On the other hand, the increase of strain rate augments the chance of the strain localization mode of the failure in the specimen. [ABSTRACT FROM AUTHOR]

A molecular dynamics (MD) simulation study was performed to investigate the effects of helium (He) on the mechanical properties of nanocrystalline body-centered cubic iron (BCC Fe). Simulated X-ray diffraction (XRD) was used to explore the relationship between the generation of cracks and the change of the crystal structure in nanocrystalline BCC Fe during tensile deformation. It is observed that the peak stress and the elastic modulus decrease with increasing concentration of He atoms, which are introduced into the grain boundary (GB) region of nanocrystalline Fe. The generation and connection of intergranular cracks are enhanced by He atoms. Significant peak separation, which is associated with the generation of cracks, is found in the simulated XRD patterns of nanocrystalline Fe during the tensile process. The lower diffraction angle of the {211}′ peak suggests a more serious lattice distortion during loading. For all nanocrystalline Fe deformed to 6% strain, the degree and fraction of the lattice distortion increases with the increasing loading stress. [ABSTRACT FROM AUTHOR]