Mechanical properties and fracture surface roughness of thermally damaged granite under dynamic splitting

In order to understand the mechanical properties and the fracture surface roughness characteristics of thermally damaged granite under dynamic splitting,dynamic Brazilian splitting tests were conducted on granite samples after thermal treatment at 25,200,400,and 600℃.Results show that the dynamic peak splitting strength of thermally damaged granite samples increases with increasing strain rate,showing obvious strain‐rate sensitivity.With increasing temperature,thermally induced cracks in granite transform from intergranular cracks to intragranular cracks,and to a transgranular crack network.Thermally induced damages reduce the dynamic peak splitting strength and the maximum absorbed energy while increasing the peak radial strain.The fracture mode of the thermally damaged granite under dynamic loads is mode Ⅱ splitting failure.By using the axial roughness index Z2 a,the distribution ranges of the wedge‐shaped failure zones and the tensile failure zones in the fracture surfaces under dynamic Brazilian splitting can be effectively identified.The radial roughness index Z_(2)^(r)is sensitive to the strain rate and temperature.It shows a linear correlation with the peak splitting strength and the maximum absorbed energy and a linear negative correlation with the peak radial strain.Z_(2)^(r)can be used to quantitatively estimate the dynamic parameters based on the models proposed.

Effect of dynamic loading orientation on fracture properties of surrounding rocks in twin tunnels

For expedited transportation,vehicular tunnels are often designed as two adjacent tunnels,which frequently experience dynamic stress waves from various orientations during blasting excavation.To analyze the impact of dynamic loading orientation on the stability of the twin-tunnel,a split Hopkinson pressure bar(SHPB)apparatus was used to conduct a dynamic test on the twin-tunnel specimens.The two tunnels were rotated around the specimen’s center to consider the effect of dynamic loading orientation.LS-DYNA software was used for numerical simulation to reveal the failure properties and stress wave propagation law of the twin-tunnel specimens.The findings indicate that,for a twin-tunnel exposed to a dynamic load from different orientations,the crack initiation position appears most often at the tunnel corner,tunnel spandrel,and tunnel floor.As the impact direction is created by a certain angle(30°,45°,60°,120°,135°,and 150°),the fractures are produced in the middle of the line between the left tunnel corner and the right tunnel spandrel.As the impact loading angle(a)is 90°,the tunnel sustains minimal damage,and only tensile fractures form in the surrounding rocks.The orientation of the impact load could change the stress distribution in the twin-tunnel,and major fractures are more likely to form in areas where the tensile stress is concentrated.

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient(JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel(V_(S1))and perpendicular(V_(S2)) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased.Simple models for the stress-and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength(JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.

Application of split Hopkinson tension bar technique to the study of dynamic fracture properties of materials

A novel approach is proposed in determining dy- namic fracture toughness （DFT） of high strength steel, using the split Hopkinson tension bar （SHTB） apparatus, com- bined with a hybrid experimental-numerical method. The center-cracked tension specimen is connected between the bars with a specially designed fixture device. The fracture initiation time is measured by the strain gage method, and dynamic stress intensity factors （DSIF） are obtained with the aid of 3D finite element analysis （FEA）. In this approach, the dimensions of the specimen are not restricted by the connec- tion strength or the stress-state equilibrium conditions, and hence plane strain state can be attained conveniently at the crack tip. Through comparison between the obtained results and those in open publication, it is concluded that the ex- perimental data are valid, and the method proposed here is reliable. The validity of the obtained DFT is checked with the ASTM criteria, and fracture surfaces are examined at the end of paper.

A retrospective study of unfavorable fractures during sagittal split osteotomy:A single surgeon’s experience

Background:Unfavorable fractures are among the most common complications of bilateral sagittal split ramus osteotomy(BSSRO).This study aimed to evaluate unfavorable fracture patterns during BSSRO and assess the related risk factors and treatment measures.Methods:The clinical records and radiographs of 679 patients(1358 sides)who underwent BSSRO at Shanghai Ninth People’s Hospital between September 2013 and December 2021 were examined.Results:Patients with unfavorable fractures who underwent surgical restoration were studied.The incidence of unfavorable fractures was 0.8%(n¼11),with the highest rate in the third year.The unfavorable fractures were divided into three types by location and clinical treatment:(1)SSRO could still be completed after buccal/lingual plate unfavorable fracture(0.6%,n=8);(2)condylar/coronoid process fractures/intermaxillary fixation needed(0.1%,n=2);(3)additional osteotomy required(0.07%,n=1).Conclusion:These results suggest that as a surgeon’s experience increases,the rate of unfavorable fractures may decrease.The novel classification of unfavorable fractures for SSRO might be useful for surgeons to make appropriate treatment choices during orthognathic surgery.

Dynamic notched semi-circle bend(NSCB) method for measuring fracture properties of rocks:Fundamentals and applications

Rocks are increasingly used in extreme environments characterised by high loading rates and high confining pressures.Thus the fracture properties of rocks under dynamic loading and confinements are critical in various rock mechanics and rock engineering problems.Due to the transient nature of dynamic loading,the dynamic fracture tests of rocks are much more challenging than their static counterparts.Understanding the dynamic fracture behaviour of geomaterials relies significantly on suitable and reliable dynamic fracture testing methods.One of such methods is the notched semi-circle bend(NSCB)test combined with the advanced split Hopkinson pressure bar(SHPB)system,which has been recommended by the International Society for Rock Mechanics and Rock Engineering(ISRM)as the standard method for the determination of dynamic fracture toughness.The dynamic NSCB-SHPB method can provide detailed insights into dynamic fracture properties including initiation fracture toughness,fracture energy,propagation fracture toughness and fracture velocity.This review aims to fully describe the detailed principles and state-of-the-art applications of dynamic NSCB-SHPB techniques.The history and principles of dynamic NSCB-SHPB tests for rocks are outlined,and then the applications of dynamic NSCB-SHPB method(including the measurements of initiation and propagation fracture toughnesses and the limiting fracture velocity,the size effect and the digital image correlation(DIC)experiments)are discussed.Further,other applications of dynamic NSCB-SHPB techniques(i.e.the thermal,moisture and anisotropy effects on the dynamic fracture properties of geomaterials,and dynamic fracture toughness of geomaterials under pre-loading and hydrostatic pressures)are presented.

Fracture and Tensile Properties of Polyvinyl Alcohol Fiber Reinforced Cementitous Composites

Experiments were carried out to design polyvinyl alcohol （PVA） fiber reinforced cementitous composites （PVA-FRCCs） holding high ductility and energy consumption ability. Besides, the properties of each ingredients in composites, mixing method and technology for fresh mixture were described in detail. Then, the pseudo-strain-hardening （PSH） behavior was investigated in uniaxial tension test. As a result, the maximum ultimate tensile strain can reach 0.7 percent. On the other hand, the single edge notch （SEN） thin sheet specimens were employed to gain the normal tensile load via crack mouth opening displacement （CMOD） curves, which can show obvious PSH behavior. In addition, the curves can be divided into four zones whose fracture toughness calculation methods were discussed. The wedge splitting （WS） test method can be applied to discuss the fracture toughness. Moreover, fracture energy of SEN and WS specimens were both approximately evaluated.

Dolomite fracture modeling using the Johnson-Holmquist concrete material model:Parameter determination and validation

In this paper,the Johnson-Holmquist concrete(JHC)constitutive model is adopted for modeling and simulating the fracture of dolomite.A detailed step-by-step procedure for determining all required parameters,based on a series of experiments under quasi-static and dynamic regimes,is proposed.Strain rate coefficients,failure surfaces,equations of state and damage/failure constants are acquired based on the experimental data and finite element analyses.The JHC model with the obtained parameters for dolomite is subsequently validated using quasi-static uniaxial and triaxial compression tests as well as dynamic split Hopkinson pressure bar(SHPB)tests.The influence of mesh size is also analyzed.It shows that the simulated fracture behavior and waveform data are in good agreement with the experimental data for all tests under both quasi-static and dynamic loading conditions.Future studies will implement the validated JHC model in small-and large-scale blasting simulations.

Statistical analysis of fracture properties based on particle swarm optimization and Pearson correlation coefficient method

Prediction of reservoir fracture is the key to explore fracture-type reservoir. When a shear-wave propagates in anisotropic media containing fracture,it splits into two polarized shear waves: fast shear wave and slow shear wave. The polarization and time delay of the fast and slow shear wave can be used to predict the azimuth and density of fracture. The current identification method of fracture azimuth and fracture density is cross-correlation method. It is assumed that fast and slow shear waves were symmetrical wavelets after completely separating,and use the most similar characteristics of wavelets to identify fracture azimuth and density,but in the experiment the identification is poor in accuracy. Pearson correlation coefficient method is one of the methods for separating the fast wave and slow wave. This method is faster in calculating speed and better in noise immunity and resolution compared with the traditional cross-correlation method. Pearson correlation coefficient method is a non-linear problem,particle swarm optimization( PSO) is a good nonlinear global optimization method which converges fast and is easy to implement. In this study,PSO is combined with the Pearson correlation coefficient method to achieve identifying fracture property and improve the computational efficiency.

Fractographic and Fracture Mechanical Investigations of Refractories under Different Loading Conditions

Crack initiation and propagation have been investigated under tensile and shear loading in ceramically and carbon bonded refractories.A wedge splitting test procedure and a modified shear test have been applied.Test results have been used for material characterization especially with respect to brittleness.Furthermore a microscopic fractographic test procedure was developed and applied on fractured test specimens.In order to explain brittleness dependence on structure properties correlation of fractographic and fracture mechanical results has been evaluated.Frequently brittleness reduction is achieved by a lower amount of transgranular crack propagation associated with a strength decrease while maintaining specific fracture energy unchanged.Deviations from pure linear fracture mechanics increase with decreasing brittleness and contribute to specific fracture energy.Shear specimens may show two generations of cracks,a first one initiated by tensile loads （stable propagation） and a second one by shear loads （unstable propagation）.

Relation between Loading Rate and Fracture Velocity on Limestone

The relation between loading rate and fracture velocity is the key to determining the fracture toughness of rock mass under dynamic loading.While designing an optimal blast design for any limestone mines,understanding the relationship between blast detonation pressure and rock fragmentation can increase the energy utilisation in any limestone mine blast.The detonation pressure is directly related to dynamic loading rate and fracture velocity is directly related to stress wave propagation speed during blasting.This paper discusses the relationship between dynamic loading rate and fracture velocity for limestone samples.It was observed that crack propagation velocity increases with fracture toughness of rock samples.It may be concluded that as the dynamic loading increases,the fracture velocity increases.

Experimental study on tensile failure process of double-K fracture parameters in roller compacted concrete layer

Study on failure of soft stratum of roller compacted concrete (RCC) is an important aspect of stability of high RCC dam. Six kinds of specimens with different interfaces were investigated by wedge splitting method. Double-K fracture parameters (initial fracture parameter and unsteady fracture parameter) were calculated by the concrete double-K fracture theory. It is indicated that the approach of construction joint or old joint after RCC final set is the most efficient among the six cases, and its fracture parameter is the largest among them. Its propagation path is sinuous. Its failure surface is scraggly. Bedding plane crack fails at the underside of the concrete surface, bond course or the surface between them for each approach. So disturbance on the underside of the concrete surface should be avoided or decreased at best during RCC construction.

预裂缝宽度对爆破效果影响研究

采用LS-DYNA软件建立数值模型,模拟分析了预裂缝宽度分别为3 cm和5 cm时对爆破效果的影响。结果表明,预裂缝越宽,减振效果越好,相差比例最高可达46%。选择不同孔径预裂爆破进行现场爆破振动监测试验,验证了数值模拟所得规律的正确性。预裂爆破参数的选取需结合现场实际工况和周围建(构)筑物的保护进行综合分析。

应力波作用下红砂岩复合动态断裂特征研究

为探究应力波作用下红砂岩的复合动态断裂特性,采用分离式霍普金森压杆和数字图像技术对带预制裂缝的半圆盘三点弯曲试样开展冲击加载实验,分析加载率、波长对红砂岩动态拉伸、断裂特征的影响。结果表明:①红砂岩试样加载率与动态抗拉强度、断裂韧度及破坏率均呈一次函数关系;断裂能随加载率增大而增长415.27%。②随着波长增加,动态抗拉强度增长742.14%,其中断裂能增长54.49%,但能量吸收率呈下降趋势;裂纹扩展平均速度增长4.09%,且首尾裂纹应变增长时间出现滞后现象;破坏率增长效应得到强化。③冲击速度8 m/s时试件主裂纹首尾监测点位的应变平均增加84.31%。

预制裂隙砂岩巴西劈裂特性试验研究

为研究裂隙对岩石宏观破裂形态的影响,开展了单裂隙、共线双裂隙、平行双裂隙3种裂隙类型下砂岩的巴西劈裂试验,分析3种裂隙类型不同裂隙倾角下的破裂形态并对其分类,同时分析不同裂隙倾角下3种裂隙类型抗拉强度的变化。试验结果表明:裂隙会削弱岩石的抗拉强度,裂隙数量、裂隙倾角及裂隙间几何排布对试样的破裂形态及抗拉强度造成影响;单裂隙、共线双裂隙与平行双裂隙砂岩试样分别存在5种、4种与3种类型的破裂形式,岩石非均质性导致同一裂隙倾角下试样的破裂形态有所不同;单裂隙与共线双裂隙砂岩的抗拉强度总体在0°裂隙倾角时最大,随着裂隙倾角的增大,单裂隙砂岩抗拉强度呈先下降后上升再下降的变化规律,共线双裂隙砂岩抗拉强度呈现下降趋势,而平行双裂隙砂岩的抗拉强度在45°裂隙倾角时最大。

裂隙发育岩体水电工程地下洞室预裂爆破试验研究

硬梁包水电站地下厂房地质构造复杂,围岩裂隙发育,岩性杂乱,岩体具有“硬、碎、杂”特点,厂房开挖成型难度较大、预裂爆破效果差,为解决此问题,结合地下厂房第Ⅲ层中部拉槽施工预裂,开展了系统的爆破试验。初期试验中,预裂爆破线装药密度低于规范计算值近100 g/m的情况下,两侧边墙岩体在爆破后十分破碎,无法明显看到预裂孔的痕迹。开展的岩体声波测试和岩体完整性分析表明:厂房岩体全孔平均纵波速度为4.03 km/s,整体完整性差;岩体沿预裂孔轴向0~1.5 m、1.5~3.9 m、3.9~7.4 m深度的分段波速平均值分别为2.59 km/s、3.58 km/s、4.70 km/s,表现出在深度方向上的分段完整性差异。根据上述特点,在厂房开挖施工过程中,预裂爆破的平均单孔线装药密度取为0.123~0.284 kg/m,并针对不同深度选取不同的线装药密度,各分段采用小药卷均匀间隔装药,试验和应用效果良好,保证了壁面基本成型,半孔率显著提高。

横波分裂和纵波方位各向异性在油气勘探中的应用

本文首先回顾了横波分裂和纵波方位各向异性应用于油气勘探的发展历程;在此基础上重点讨论了二者在上地壳中的分布情况、相互关系及应用条件.横波分裂是地震波传播经过裂缝时所独有的横波双折射现象,40年的科学研究及生产应用表明这一物理现象在上地壳中广泛存在,并主要存在于近地表1200 m以浅.纵波方位各向异性从浅到深在上地壳中分布差异较大;应用于检测裂缝时仅对含气裂缝发育区敏感,对含其他流体的裂缝区不敏感.针对裂缝发育区的预测,本文利用理论和实验数据分析论证了横波分裂与纵波方位各向异性的关系;阐明在油气勘探中横波分裂分析与纵波方位各向异性预测裂缝储层须首先消除近地表的影响,这在一定程度上限制了二者的应用.同时阐明二者的应用还需要满足近地表相对简单、裂缝储层相对较厚等条件;且需采集非零偏或环形垂直地震剖面(VSP)资料进行标定.即多分量地震数据品质、近地表地震地质条件及储层方位各向异性发育程度是影响横波分裂及纵波方位各向异性应用效果的3个关键因素.

基于平台巴西劈裂试验下WC-Co硬质合金的动态响应与失效特性

为获取WC-Co硬质合金在动态拉伸加载下的力学性能和失效机制,设计了动态平台巴西劈裂试验。结果发现,WC-Co硬质合金具有典型的弹脆性特征,且断裂应变随着加载应变率的增加而略有增加。在一维应力波加载下,WC-Co硬质合金的动态抗拉强度随应变率的增加而增加,表明其应变率效应具有明显的正相关性,该效应的产生机制与典型陶瓷类材料是一致的,即由I型裂纹的亚临界扩展决定。对回收破碎试样进行微观形貌观察,发现平台圆盘中间位置处微观断裂模式主要为沿晶断裂,且在拉应力作用下形成韧窝断裂形貌;在靠近加载点位置区域,受多向应力作用,材料不仅存在韧窝断裂,在单个晶粒的局部劈裂表面还存在河流花样的解理断裂。宏观角度上,WC-Co硬质合金表现出明显的脆性特征,但微观角度却发现有局部塑性变形特征。

多分量裂缝预测技术研究及其在川南页岩气工区的应用

与常规油气相比,页岩中普遍含气,但是单井的产量变化大,通过改造形成裂缝网络的页岩气井才具有工业开发价值。目前,页岩裂缝预测的方法较多,大多数是基于纵波地震资料的,而利用多分量地震资料预测裂缝发育区仍是研究人员探索和努力的方向。这里首次运用转换横波地震资料对页岩气的裂缝发育情况进行预测研究。首先,利用转换波模型正演,结合多波地震资料对比识别页岩的小断裂、小断层。其次,通过基于横波分裂特性的裂缝预测技术对高角度的裂缝走向和密度进行预测。通过四川盆地实际资料应用表明,多分量裂缝预测技术能够有效识别页岩气的裂缝发育情况,多分量裂缝预测效果明显优于纵波叠前裂缝预测,成果更加合理。

基于SHPB的切缝药包防护特性研究

为探究定向断裂控制爆破中切缝药包对围岩的防护特性,利用分离式霍普金森压杆(SHPB)试验对用药包材料制成的垫片防护下的花岗岩试块进行冲击,分别研究垫片与岩石间距、垫片材料和厚度因素对围岩防护效果的影响,分析在高速冲击下岩石的破碎情况和应力峰值,并建立多个切缝药包单孔爆破数值模型,通过对比不同情况下围岩损伤而进一步揭示切缝药包对围岩的防护特性机理。研究结果表明:岩石在强冲击荷载作用下严重破碎,当采用外壳垫片防护后,岩石的破碎程度有所降低。相同垫片厚度和垫片与岩石间距离固定的情况下,PVC外壳垫片防护效果要优于ABS外壳垫片。随着垫片厚度的增加和岩石与垫片间距的增加,岩石受损程度逐渐减弱,且增加垫片厚度对岩石的防护效果要优于增加岩石与垫片外壳间距的防护效果,研究成果可为控制爆破提供技术参考。