Quantity Of Microcracks Research Articles

Multi-method comparison and multi-index evaluation of cracking characteristics of asphalt mixture

The micro crack inside asphalt pavement are potential and antecedent factors for macroscopic cracking of the pavement. The position and quantity of micro cracks have a significant impact on the service life and failure mode of asphalt pavement. The purpose of this study was to conduct multi-scale analysis and multi-index evaluation on the whole process of generation, expansion, and transformation of micro-crack inside asphalt pavement (asphalt mixture) based on multiple testing methods. Firstly, synchronous tests were conducted on the cracking process of asphalt mixture by using three-point bending loading method, acoustic emission (AE) monitoring method, and digital image correlation (DIC) analysis method. Secondly, a three-dimensional finite element model of the asphalt mixture beam was established for AE source location simulation. Finally, the advantages and limitations of multi-methods were compared and analyzed. The principal strain points with different sizes were randomly distributed in the tensile area of the asphalt mixture beam bottom at the initial loading stage, and the number of the principal strain points increased with the decrease of the loading rate. The macroscopic main crack of the asphalt mixture beam generated and expanded from the maximum principal strain point (P1) at the later loading stage. The AE dominant-frequency distribution ranges of micro-crack expanding process, macro-crack generating process, and failure process of asphalt mixture beam were 78.125–156.25 kHz, 234.375–312.5 kHz, and 0–78.125 kHz, respectively. Four stages of damage evolution and cracking process inside the asphalt mixture beam were obtained by the AE source location simulation. A comprehensive and systematic evaluation of bending cracking characteristics of asphalt mixture beam was realized by obtaining multiple indexes from multiple scales with multiple methods.

Microwave assistance effect for rock breaking of TBM disc cutter using the coupled method of continuum and grain-based model

Microwave-assisted rock breaking technique can significantly weaken the strength and the stiffness of hard rock, and can consequently enhance TBM efficiency as well as relieve wear of disc cutters. Understanding the mechanism of microwave-assisted rock breaking requires knowledge of the intersection and connectivity properties of freshly formed fractures below the cutter and the fracture network resulting from microwave-induced damage. In this study, a numerical approach, coupled with the grain-based model (GBM) and continuum based microwave electromagnetic analysis, for simulating the microwave-treated granite is proposed firstly. Subsequently, the numerical experiment of TBM cutter penetration is conducted with the consideration of five levels of microwave power irradiation as well as two waveguide arrangements. The temperature field and fracture behavior are analyzed to investigate the thermal damage of the granite after the treatment of the microwave. Furthermore, the reduction of peak normal force and variation of average coordination number are examined to evaluate the impact of waveguide position on the effectiveness of microwave-assisted rock breaking. The simulation results indicate that the suitable power of microwave irradiation (≥ 30 kW) can promote the rise of granite temperature directly below the waveguide, further resulting in an increase in the quantity of microcracks and noticeable damage to the rock block. When the microwave power exceeds 30 kW, the peak normal force and average coordination number progressively start to decline for the case of a waveguide port facing the centerline of a single cutter. In the case of waveguide facing the gap between double cutters, the decreasing trend of the peak normal force is similar to that under the condition of single cutter, but the starting threshold has been raised to 45 kW. When the microwave power is below 60 kW, the average coordination does not change significantly. After comparing the improvement effects of microwave-assisted rock breaking under the two waveguide arrangements, a preferred condition of the microwave for breaking is determined. The waveguide port facing the centerline of a single cutter, and the microwave power of 30 ∼ 60 kW can lead a success in a satisfactory effect in term of the reduction rate of peak normal force and average coordination number.

Mechanical and Microcrack Evolution Characteristics of Roof Rock of Coal Seam with Different Angle of Defects Based on Particle Flow Code

The creation of the natural ceiling rock of the coal seam is rife with fractures, holes, and other flaws. The angle of the defects has a significant influence on the mechanical characteristics and crack evolution of coal seam roof rock. Multi-scale numerical simulation software PFC2D gets adapted to realize the crack propagation and coalescence process in the roof rock of a coal seam with different angles of defects under uniaxial compression. The effect of flaw angles on the micro and macro mechanical characteristics of rock is also discovered. The results show that: (1) the defect angle has influence on the stress-strain, elastic modulus, peak strength, peak strain, acoustic emission (AE) and strain energy of roof rock of coal seam. When the defect angles are different, the starting position of the roof rock in a coal seam fracture is different. The quantity of microcracks firstly reduces with an increase in defect angles before gradually increasing. At the same fault angle, the cracks are mostly tensile ones and only a few shear ones. (2) When the defect angle is less than 90°, tensile and shear fractures are mostly localized at the defect's two tips and propagate along the loading direction. When the defect angle is 90°, the tensile and shear cracks are not concentrated at the tip of the defect. (3) As the defect angles increase, the elastic strain energy rises initially and then falls, and the dissipated energy and total input energy both increase continuously. The elastic strain energy is greatest at the highest strength. The study provides a certain reference for the use of various analysis methods in practical engineering to evaluate the safety and stability of rock samples with pre-existing defects.

Mechanisms of destruction of monocore tungsten wire and tungsten harness plastically deformed under uniaxial loading

In the paper the results of studying of mechanical behaviors of tungsten based composites under uniaxial tension deformation are presented. As samples were used wires with different square of cross section and tungsten yarns which were produced by screwing tungsten wires between each other. It is showed that for tungsten wires with 150 μm diameter with increasing of temperature occur evolution of plastic deformation. Thus, for a room temperature happened brittle type of ruining of sample. This are indicated by high quantity of micro cracks which are chaotically placed on a flat surface of rupture. At 300 oC degree microcracks are gradually united to a radial cracks and occur bigger plastic deformation. And at 500 oC of testing digressing mechanical properties of sample to range which are lower that same at room temperature testing. Those results are proved that with increasing of temperature of testing occurs thermal activation of internal processes of deformation and at temperature closed to 500 oC types of mechanisms of deformation are changed. Samples of tungsten yarns are showed the same trends. At room temperature samples are breaking with high value of micro cracks in a surface of rupture. At 300 oC sample are deformed with increased plasticity and larger necking at rupture as for 150 μm wires. But at temperature which is very closed to 500 oC morphology of surface of rupture are changed. And it evidences about changing type of mechanisms of deformation of sample. Information which was got by mechanical testing are proved that in tungsten with increasing of temperature occurs activation of edge dislocations and at temperature which is very close to 500 oC (487 oC for tungsten is 0.206 of melting point temperature) and above proceed changing to mechanisms of destruction based on screw dislocations.

Do pre-existing microcracks play a role in the fracture resistance of roots in a laboratory setting?

To investigate a potential cause-effect relationship between dentinal microcracks and fracture resistance of mandibular incisors that had not been endodontically treated. Sixty mandibular incisors with circular-shaped canals were selected based on micro-computed tomographic scans to create a homogeneous sample. The cross-sectional images of the specimens were screened to identify and quantify the presence of dentinal microcracks. Then, teeth were embedded in polystyrene resin and subjected to axial compressive loading using a universal testing machine. After fracture, the roots were re-scanned and fractography analysis was performed by inspection of 3D models to verify crack propagation. Spearman's rank correlation was used to assess the correlation between the number of microcracks and force required to fracture. Dentinal microcracks were detected in 79% of the specimens (n=44). The incidence of microcracks varied between teeth from 6% to 42% of the total slices per sample (average of 14±17%). The number of microcracks per sample varied from 0 to 1605, with an average of 412±484 (median=221 and IQR 25%=15/75%=658). The load at failure values varied from 227 to 924N, with an average of 560.3±168.1N (median=561 and IQR 25%=458/75%=694). The Spearman correlation coefficient (rho) equalled 0.065. There was no cause-effect relationship between the number of dentinal microcracks and the fracture resistance of nonendodontically treated mandibular incisors. The presence and quantity of microcracks did not make these roots more prone to fracture.

Experimental investigation on the volume stability of magnesium phosphate cement with different types of mineral admixtures

Magnesium phosphate cement (MPC) composited by dead burned magnesia (MgO) and potassium dihydrogen phosphate (KH2PO4, KDP) is a type of excellent repairing material, and its volume stability has great influence on its applications. This paper studied the effects of different types of mineral admixtures on the volume stability of MPC mortar by testing its volume deformation at different curing ages. The main mechanisms to explain how the mineral admixtures affect the volume stability were studied by XRD and ESEM analyses. The experimental results indicate that the influence of ultra-fine fly ash on the volume deformation is different from that of fly ash and silica fume. The volume expansion increases as the content of fly ash in mortar increases while the volume expansion decreases as the content of ultra-fine fly ash increases. For MPC-fly ash system, the use of ultra-fine fly ash can decrease the volume expansion significantly. Moreover, the addition of dipotassium hydrogen phosphate also has significant influence on the volume stability of MPC mortar. The XRD analysis indicates that the amount of sylvite in MPC mortar is the reason for the changes in the volume stability after adding mineral admixtures. The ESEM imaging confirms that the use of ultra-fine fly ash can decrease the quantity of micro cracks in the MPC mortar.

Influence of layer orientation and interlayer bonding force on the mechanical behavior of shale under Brazilian test conditions

The mechanical behavior of inherently anisotropic shale rocks under Brazilian test conditions are investigated in this study based on experimental studies and numerical simulations. The effects of the weak lamination planes and interlayer bonding force of these layers on the failure strength and fracture patterns are studied systematically. Numerical simulations using particle flow code in two dimensions based on the discrete element method showed a good agreement with the experimental results in the failure strength and fracture patterns. The shale revealed strong anisotropic behavior with the failure strength perpendicular to the lamination plane greater than failure strength parallel to lamination plane. The failure strength of the different interlayer bonding force against the layer orientations changed significantly. Four types of fracture patterns were observed: curved fracture, broken-linear fracture, layer-activated fracture, and central-linear fracture. The observed fracture patterns are either or a combination of tensile and/or shear fractures. Increase in interlayer bonding strength decreased the quantity of micro cracks and this directly led to reduction in the anisotropic behavior. Overall the layer orientation and interlayer bonding force of the shale thus play a very important role in the anisotropic behavior of the shale.

Granite microcracks: Structure and connectivity at different depths

Granite is one rock type used to host high-level radioactive waste repositories, and the structure of microcracks in the rock can influence its hydraulic characteristics. Thus, a quantitative analysis of granite microcracks is relevant for understanding the hydrogeological characteristics of the rocks surrounding geological repositories. The analysis can also contribute scientific data to a seepage model for low permeability rocks and materials with microscopic pores. In this study, seven granite core samples were drilled from different depths up to 600m in Alxa, Inner Mongolia, China. Using a grid survey method and image processing technology, micrographs were converted into binary images of microcracks. The geometric parameters of the microcracks, including their quantity, width, cranny ratio, crack intersections and dimensional parameters of the fracture network, were analyzed in order to fully describe their spatial distribution. In addition, the morphological characteristics and elemental compositions of the microcracks were analyzed by scanning electron microscopy energy dispersive X-ray analysis (SEM-EDS), and the natural moisture content was also determined through heated. Finally, two-dimensional microcrack network seepage models of the granite samples were simulated using the Lattice Boltzmann method (LBM), which revealed the influence of the microcrack structure on their connectivity. The results show that the growth and development of microcracks in the granite samples generally decreases as sampling depth increases in this study area. Connectivity is positively correlated with a number of the geometric parameters: the quantity of microcracks, the cranny ratio, the number of crack intersections and dimensional parameters of the fracture network, which is revealed in the two-dimensional microcrack network seepage models for these granite samples.

EOR (Oil Recovery Enhancement) Technology Using Shock Wave in the Fluid

In this research the alternative method is presented increases of oil recovery of an oil collector. Shock wave impact on layer. The technology is developed, advantage of this method is shown in comparison with layer hydraulic fracturing. For increase oil recovery and restoration of wells the traditional is widely applied technology of hydraulic fracturing of layer (flyuding), allowing on the short period to recover a well. Sense of a flyuding that hydrostatic pressure is created is considerable exceeding mountain and bringing to a rupture of a collector on one or several cracks. In this work the alternative technique of increase in oil recovery is offered by creation of a shock wave small intensity in the bottom fluid. With a high frequency of repeatability set of blows increases quantity of micro cracks in collector rock, increasing coefficient of permeability of layer without breaking its connectivity.

Study on Subsurface Damage Model of the Ground Monocrystallinge Silicon Wafers

In order to better understand the grinding mechanism, the rough, semi-fine and fine ground silicon wafer subsurface damage models are experimentally investigated with the aid of advanced measurement methods. The results show that the rough ground wafer subsurface damage model is composed of large quantity of microcracks with complicated configurations, high density dislocations, stalk faults and elastic deformation layer. Among them microcracks, dislocations and stalk faults are dominant. Apart from the above damage, the amorphous layer and polycrystalline layer (Si-I, Si-III, Si-IV and Si-XII) exist in the semi-fine ground and fine ground wafer subsurface damage models. The amorphous layer depth firstly increases from rough grinding to semi-fine grinding and then decreases from semi-fine grinding to fine grinding. The damage model can be divided in severe damage part and elastic deformation part with high stress. When the material is removed by ductile mode two parts are all small and the ratio of second part is relatively great.

Tensile creep behavior of notched two-dimensional-C/SiC composite

Tensile creep tests of two-dimensional-C/SiC specimens with double-edge arc notches have been carried out at 1100, 1300 and 1500 °C in vacuum. The matrix cracks on the surface and resonance frequency were examined at different creeping times. At 1100 °C, the creep strains of both smooth and notched specimens were concentrated at the transient stage and the steady creep rates were nearly zero, whereas steady creep rates of notched specimens and smooth specimens were similar at 1500 °C. It has been observed that the creep damage mainly concentrated at the area near the notches. Micro-cracks appeared in the area near the notches and on the cross-points of the woven fiber bundles, and the longitudinal fibers near the notches fractured easily. Both types of curves, namely quantity of micro-cracks vs. time and micro-crack width vs. time, were extremely similar as for the creep curves. In general, micro-cracks developed fast during the first 10 h. It has been noticed that within the first 2 h, the micro-cracks near the notches grow faster than those far from the notches, whereas the growth rate of micro-cracks far from notches was faster than those near the notches after 2 h. This phenomenon indicates the stress redistribution during creep. Damage curves at 1300 and 1500 °C have similar trend, though the damage and the quantity of micro-cracks at 1500 °C are higher than those at 1300 °C.