Increasing Impact Times Research Articles

Progressive Damage Behaviours of Triaxially Confined Rocks under Multiple Dynamic Loads

Investigation of rock progressive damage under static confinement and strain rates facilitates the generation mechanism of natural fault damage zones. A triaxial Hopkinson bar apparatus is used to perform dynamic triaxial compression tests to examine the damage and degradation process of rocks subjected to multiple impacts. Dynamic mechanical properties are determined under a static triaxial pre-stress of (30, 20, 10) MPa and multiple dynamic loadings, with the repetitive impact velocity of 27 m/s and strain rates from 50 to 150/s. The acoustic characteristics are identified by ultrasonic measurement to qualify the damage values. The micro-crack parameters, including crack area and volumes are detected using synchrotron X-ray micro-computed tomography (μCT) to characterize the progressive damage. In addition, the microcrack orientation, density and fractal dimension are analysed from thin section. Experimental results show that dynamic stress-strain curves can be divided to elastic, nonlinear deformation and unloading phases. Dynamic peak stress, Young’s modulus and ultrasonic wave velocity decrease with increasing impact times. The high frequency of ultrasonic wave is filtered by the induced microcracks. The progressive damage and evolution of fracture networks are associated highly with microcrack initiation, propagation, branching and coalescence. Shear bands are commonly generated in granite, and tensile cracks are dominant in marble, while sandstone is mainly failed by compaction and deformation band. The absorbed energy of rock increases nonlinearly with increasing crack surface and volume. Besides, microcracks propagate primarily along the maximum principal stress; the density and fractal dimension exhibit an anisotropic distribution controlled by true triaxial confinement and dynamic impacts.

Microstructure evolution and mechanical properties of a lamellar near-α titanium alloy treated by laser shock peening

Laser shock peening (LSP) is a novel surface treatment technique for strengthening metal materials. The microstructural response of lamellar Ti834 alloy was investigated after treated by LSP. Micro-hardness of LSPed specimens on the surface and along the depth direction was measured. Microstructural features of plastic deformation layer induced by LSP were identified by transmission electron microscopy (TEM). Experimental results showed that the surface micro-hardness of Ti834 alloy increases gradually with increasing impact times and the peak of the micro-hardness appear in the subsurface layer of the specimen. The depth of the strengthening layer was 200 μm and 500 μm for the one and two impacted specimens, respectively. The compressive residual stress along the depth of the specimen increases with increasing of impact times, and the maximum compressive residual stress at the depth of 100 μm from the surface of the specimen after two times impact. The microstructural evolution of Ti834 alloy with lamellar structure includes the formation of sub-grain and dislocation cell, twin–matrix (T-M) lamellae divided by the parallel MTs, MT-MT intersecting at an angle with the α-plates, the crossed arrangement of dislocation arrays (DAs-DAs) and the segmentation of DWs/DTs in the transverse direction.

Dynamic mechanical characteristics and failure mode of serpentine under a three-dimensional high static load and frequent dynamic disturbance.

The improved split Hopkinson pressure bar (SHPB) was used to study the dynamic mechanical properties and failure characteristics of surrounding rock in deep rock mass engineering that is under high stress and affected by blasting excavation and other dynamic disturbances. In a three-dimensional high static load and frequent dynamic disturbance test, the preload high axial pressure and confining pressure are used to simulate the high crustal stress of deep rock, and the effect of small disturbances on the rock is simulated by the low impact load. The results show that there are two types of dynamic stress-strain curves of deep rock: an elastic-plastic curve and plastic-elastic-plastic curve. The curves consists of five parts: the compaction stage, micro-crack steady development stage, micro-crack unstable propagation stage, fatigue damage stage, and fatigue failure stage. Reductive phenomena of constringent strain after dynamic peak stress appear because of the different degrees of rock damage. Moreover, these phenomena include two conditions, namely, whether rebound occurs or not. The impact resistance of rock is strongest when the ratio of the confining pressure to axial pressure is optimal, and the dynamic average strength of rock and accumulative impact times decrease with the increase of the preloaded axial compression and increase with the increase of the preloaded confining pressure. Both the dynamic deformation modulus and dynamic peak stress decrease with the increase of the accumulative impact time, while the maximum strain and the dynamic peak strain increase. The corresponding rebound strain as a whole first increases and then decreases with the increasing impact times. For deep rock, tensile failure and single-bevel plane shear failure are the main failure modes, and pull-compression mixed friction failure is the auxiliary failure mode. Additionally, the lumpiness of broken rock decreases with the increase of the preloaded axial compression and increases with the increase of the preloaded confining pressure.

Blasting-Induced Permeability Enhancement of Ore Deposits Associated with Low-Permeability Weakly Weathered Granites Based on the Split Hopkinson Pressure Bar

By utilizing the improved split Hopkinson pressure bar (SHPB) test device, uniaxial, constant-speed cyclic, and variable-speed cyclic impact compression tests were conducted on weakly weathered granite samples. By combining nuclear magnetic resonance (NMR) and triaxial seepage tests, this study investigated the change laws in the mechanical properties, porosity evolution, and permeability coefficients of the samples under cyclic impacts. The results showed that in constant-speed cyclic impacts with increasing impact times, deformation modulus decreased, whilst porosity firstly decreased and then increased. Furthermore, dynamic peak strength firstly increased and then decreased whereas peak strain constantly increased before failure of the samples. In the variable-speed cyclic impacts, as impact times increased, deformation modulus firstly increased and then declined with damage occurring after four impact times. The compaction process weakened and even disappeared with increasing initial porosity. Three types of pores were found in the samples that changed in multiscale under cyclic loading. In general, small pores extended to medium- and large-sized pores. After three variable-speed cyclic impacts, the porosity of the samples was larger than the initial porosity and the permeability coefficient was greater than its initial value. The results demonstrate that the purpose of enhancing permeability and keeping the ore body stable can be achieved by conducting three variable-speed cyclic impacts on the samples.

Experimental study on shale fracturing assisted by low-temperature freezing

Under the cold impact of liquid nitrogen, the shale suffers from the significant freezing damage, which provides the possibility of liquid nitrogen fracturing. Moreover, shale fracturing assisted by liquid nitrogen can effectively reduce reservoir pollution. In this paper, the hydraulic fracturing experiments of natural shale samples frozen by liquid nitrogen were carried out to investigate the factors affecting the crack propagation of shale after low temperature fracturing. The results show that a large number of cracks or macropores form inside the natural shale sample after freezing treatment by liquid nitrogen. The fracture pressure of the shale decreases with increasing impact time at the beginning of the immersion time, and remains substantially stable after an immersion of 2 hours. When the freezing time increases, the crack initiation time increases accordingly. After low temperature impact, the fracture pressure of shale decreases with the increase of stress difference, but the cracking times vary with the stress with obvious regularity. It is easier to form main fracture with larger displacement on the premise of well-developed shale bedding.

Surface topography evolution of Ni-based single crystal superalloy under laser shock: Formation of the nano-scale surface reliefs

The aim of the study was to investigate the effect of laser shock peening (LSP) on surface topography evolution of metallic targets. Samples manufactured by a Ni-based single crystal superalloy with polished finish were treated by LSP, and the surface topographies before and after LSP were examined by non-contact White-Light Interferometer (WLI). Results showed the following three aspects: (a) By taking advantage of WLI, the shrinkage porosities and the interdendritic structures were observed simultaneously. (b) With the increasing impact times, the round pit induced by laser shock became deeper. (c) The nano-scale surface reliefs were found on the bottom of round pit induced by LSP, and the specific plastic flow of metallic materials under the action of compressive stresses was deemed as the primary contributor to the formation of surface reliefs. It revealed a novel microscale plastic deformation phenomenon of metallic materials in surface strengthening.

Deforming TC6 titanium alloys at ultrahigh strain rates during multiple laser shock peening

The deformation mechanism of TC6 titanium alloys at ultrahigh strain rates (>106s−1) during multiple LSP was investigated. When nanosecond pulse and 1000MW laser irradiated on the materials, the high pressure plasma shock wave was induced and ultrahigh strain rates response was caused in the material. The TEM observation of the surface layer indicates that a layer of nanocrystalline has been formed and is unevenly distributed after a single impact. Increasing impact times could provide longer time and more energy to dislocation movement which refines the grains into smaller size and makes the distribution uniform. The hardness of TC6 titanium alloy has been improved by a single laser shock peening, forming a severe plastic deformted layer with a depth of 200μm. Increasing impact times will improve the hardness and effective depth. The XRD test shows that the position of diffraction peak did not show any change, but the Bragg diffraction peak was broadened. Dislocation activity is a prevalent deformation mechanism of TC6 titanium alloy in the grain refinement study.

Crumb rubber in concrete: Static and dynamic evaluation

In this study, six different concrete mix designs containing various amounts of coarse and fine crumb rubber were tested for properties important to concrete safety barriers. Eighteen samples – three of each mix design – were tested under static compression to determine the compressive strength and elastic modulus values. Another eighteen samples were subjected to dynamic drop tests to assess the effect of rubber on energy dissipation. Test results show that increasing the amount of rubber decreases the compressive strength and elastic modulus of the concrete, while significantly increasing impact time and energy dissipation capacity. It was determined that replacing 20–40% of aggregates with crumb rubber creates concrete mixes that would be useful for concrete safety barriers in locations where strength, fracture resistance and energy dissipation are required. Finally, concrete mixes with greater than 60% of aggregates replaced by crumb rubber would be useful for concrete impact attenuators in locations where low impact severity is of ultimate importance, and fracture or fragmentation upon impact is acceptable.

Analysis the Dynamic Strain of AM60 Magnesium Alloy by Means of a Single Laser Shock Processing

In order to study the dynamic response of metal of laser shock processing, dynamic strain curves of AM60 Magnesium alloy during laser shock processing were measured by resistance strain gauges. Dynamic strain curves of three equiangular rosette near the shock spot and three strain gauges of different distances from the spot center were studied. The results indicated that the strain rate of AM60 Magnesium alloy decreased and plastic deformation increased with increasing impact times. And one dimensional strain hypothesis of laser shock processing was reasonable.

Dynamic damage and stress-strain relations of ultra-high performance cementitious composites subjected to repeated impact

Ultra-high performance cementitious composites (UHPCC) were prepared by replacing 60% of cement with ultra-fine industrial waste powders. The dynamic damage and compressive stress-strain relations of UHPCC were studied using split Hopkinson pressure bar (SHPB). The damage of UHPCC subjected to repeated impact was measured by the ultrasonic pulse velocity method. Results show that the dynamic damage of UHPCC increases linearly with impact times and the abilities of repeated impact resistance of UHPCC are improved with increasing fiber volume fraction. The stress waves on impact were recorded and the average stress, strain and strain rate of UHPCC were calculated based on the wave propagation theory. The effects of strain rate, fibers volume fraction and impact times on the stress-strain relations of UHPCC were studied. Results show that the peak stress and elastic modulus decrease while the strain rate and peak strain increase gradually with increasing impact times.

Analysis of Impact between Racket and Tennis Ball

In this research, the impact between the racket and the ball was studied experimentally. The behavior of the ball during the impact was analyzed using high speed images. The purpose of this research is to clarify the effect of both impact area and strings tension on the rotational speed and the velocity of ball after impact. The deformation of the ball, the impact time, and the movements of the ball during the impact were studied experimentally. According to the experimental results, the following conclusions can be obtained. It was found that the rotational speed and the velocity after the impact increased with increasing deformation of the strings and decreasing deformation of the ball. Moreover, the rotational speed and the velocity after the impact increased with increasing impact time. And, the rotational speed of the ball after the impact increased with increasing displacement of the ball in the parallel direction to the surface of the strings.

ARGON PLASMA COAGULATION FOR THE ENDOSCOPIC TREATMENT OF A NON‐RESECTABLE KLATSKIN TUMOR: IN VITRO AND IN VIVO STUDY

Background: Palliative treatment including stenting is limited in patients with Klatskin tumor. Argon plasma coagulation (APC) is a new local treatment modality for the devitalization and debulking of tumors. Argon plasma coagulation could be a candidate method for relief of biliary strictures in patients with non‐resectable Klatskin tumor in whom biliary stenting has failed. This study provides an evaluation of the technical feasibility, safety, and effect of APC as a palliative strategy in patients with non‐resectable Klatskin tumor.Methods: In vitro studies were performed in order to investigate the dimension of coagulation necrosis in 11 human gallbladders. The currents were applied in normal air conditions and a bowl filled with normal saline in five and six specimens, respectively. Argon plasma coagulation was also performed on three patients with Klatskin tumor who showed no effective drainage via percutaneous transhepatic approach with a cholangioscope.Results: A coagulation current was delivered to the specimen even if in normal saline. The maximum depth and diameter of necrosis was 3 and 6.5 mm under normal air conditions, compared with 2 and 5 mm in water conditions. No perforation of the gallbladder wall occurred in any of the lesions. The dimension of the necrosis increased with increasing impact time and energy settings. Argon plasma coagulation application was possible on tumors of patients without severe complication.Conclusion: Argon plasma coagulation seems to be applicable, effective and relatively safe in palliative treatment for advanced non‐resectable Klatskin tumor via cholangioscopy. Longer follow ups and comparative trials with other treatment modalities are, however, required.

Simulated Traffic on Turfgrass Topdressed with Crumb Rubber

AbstractTopdressing plays a vital role in turfgrass subjected to traffic. Sand is commonly used for topdressing; under suboptimal growing conditions, however, methods to maintain wear tolerance are limited. There has been recent interest in using crumb rubber from recycled tires as an amendment for turfgrass areas. A topdressing study was initiated in July 1993 to determine the effect of crumb rubber on turfgrass systems subjected to traffic. A factorial randomized complete block design with three replications was implemented with two crumb rubber particle sizes (large, 6.0–2.0 mm; small, 2.0–0.05 mm) and five topdressing rates (0.0, 17.1, 34.2, 44.1, and 88.2 t ha−1) on a 1‐yr‐old Kentucky bluegrass‐perennial ryegrass (Poa pratensis L.‐Lolium perenne L.) stand. In 1993 and 1994, 96 passes were made with a Brinkman traffic simulator. Surface hardness characteristics measured were peak deceleration, time to peak deceleration, and impact duration. The small crumb rubber size was more effective in increasing impact time periods than the large crumb rubber, but had no effect on peak deceleration values. Shear resistance values decreased by as much as 40% as crumb rubber volumes increased in 1993, but were increased by 20% in 1994 after rubber particles had settled to the soil surface. There was generally an increase in turf cover under traffic as crumb rubber rates increased above 34.1 t ha−1, and the small crumb rubber was more effective in 1993. Our results suggest that crumb rubber can alter surface characteristics and increase wear tolerance of turfgrass exposed to traffic.