分级循环加卸载下砂岩损伤力学特性研究

通过选取陕西省略阳市某滑坡表层的中风化砂岩进行静态单轴压缩试验与三轴循环加卸载试验,研究低围压循环加卸载条件下岩石的损伤力学特性,得到力学参数、损伤参数、滞回环面积等随循环过程的演化规律及循环荷载作用下砂岩的变形破坏机制。结果表明:循环加卸载试验过程中,围压增强砂岩抵抗变形破坏能力,随着围压增大,动弹性模量明显增大而动泊松比逐渐降低,砂岩试样内凹现象逐渐“模糊”;滞回环面积受围压与轴向载荷影响明显,与围压变化呈负相关,与轴向载荷变化呈正相关,轴向载荷增大时,不可逆应变逐渐累积,砂岩损伤加剧;砂岩内部原始裂隙在初始较低循环荷载作用下压密,后过渡到弹性变形与裂纹稳定扩展阶段,随着试验进行,裂纹加速演化不稳定扩张,岩样逐渐变形失稳破坏。

基于声发射和DIC特征的层状复合岩石力学损伤试验及模型研究

为研究层状复合岩石的力学损伤特性,以深部致密储层砂岩及泥岩为研究对象,通过相似材料模型试验制备层状复合类岩石,开展单轴压缩试验并辅以声发射系统及数字图像修正(DIC)系统全过程监测,得到层状复合岩石的强度及弹性模量等物理力学参数。研究声发射振铃计数与岩石损伤演化过程对应规律,将单轴压缩下层状复合岩石损伤过程划分为初始损伤、损伤稳定发展及损伤加速三个阶段。以声发射内部损伤演化特征及DIC表面损伤演化特性为基础,建立了基于双损伤因子表征的层状复合岩石损伤本构模型。结果表明:3类岩石中,A类岩石的强度及弹性模量分别高于B类岩石21%和24%,AB类层状复合岩石的强度及弹性模量则低于B类岩石3%和4%。A类岩石以拉伸破坏为主,B类岩石多呈单斜面剪切破坏,AB类复合岩石主要沿胶结层理面或强度较弱部分产生拉伸-剪切滑移型破坏。三类岩石加载初期声发射活动较弱,在峰值应力时信号值达到最大。文中提出的模型合理地揭示了单轴压缩下层状复合岩石内部结构发展与外部裂纹萌发、扩展与贯通的损伤演化机理。

冻融环境下岩体损伤力学特性多尺度研究及进展

岩体因天然缺陷存在易受到冻融环境影响而产生损伤,冻融损伤过程实质上为反复冻胀荷载作用于缺陷引起的疲劳演化过程。但从冻融损伤存在显著的尺度效应,缺陷的不同空间尺度往往决定了冻融损伤评价尺度,如初始损伤以空洞、孔隙、微裂纹等为主的多孔岩石,其冻融损伤多表现为孔隙或微裂隙萌发、扩展、连通等,反映的是微、细观尺度范畴;而初始损伤以节理、裂隙等为主的岩体,其冻融损伤多体现沿宏观缺陷的局部化冻融损伤,属于宏观损伤尺度范畴。着眼于岩石冻融损伤尺度及对应损伤识别尺度两大关键问题,首先,依据岩石天然缺陷空间尺度进行量化分级,明确不同层级下的损伤特征及对应损伤识别方法;而后系统归纳微、细、宏观尺度下岩石冻融损伤识别及评价方法,明确不同尺度条件下冻融损伤识别手段及损伤力学机制。最后,结合现今岩石冻融损伤研究现状,提出岩体冻融多尺度损伤识别及评价机制发展方向,并初步探讨冻融诱发岩体损伤的微–细–宏观的跨尺度认知思路,有望对冻融诱发岩体损伤特性及对应性识别系统认知提供参考。

含水率对花岗岩残积土动态流变损伤力学特性与损伤度影响试验研究

以厦门地区某建筑工程地基浅层花岗岩残积土为试样,利用SDT-10微机控制电液伺服动态三轴流变仪为主要试验装备,研究了10%、15%、20%和23%等4种不同含水率试样的动态黏弹塑性流变损伤力学特性,分析了其正弦变化动态荷载作用下的动态流变损伤变形及动弹性模量变化特点。选取动弹性模量作为表述损伤过程的重要参数,由应变等效性假设推导出损伤变量,并研究了损伤程度随试样含水率的变化规律。研究结论对揭示软弱岩土动态流变损伤破坏动力学机制等具有重要的理论和现实意义。

Dynamic mechanical properties and instability behavior of layered backfill under intermediate strain rates

To obtain dynamic mechanical properties and failure rule of layered backfill under strain rates from10to80s-1,impactloading test on layered backfill specimens(LBS)was conducted by using split Hopkinson pressure bar system.The results indicatethat positive correlation can be found between dynamic compressive strength and strain rate,as well as between strength increasefactor and strain rate.Dynamic compressive strength of LBS gets higher as the arithmetic average cement-sand ratio increases.Compared with static compressive strength,dynamic compressive strength of LBS is enhanced by11%to163%.In addition,theenergy dissipating rate of LBS lies between that of corresponding single specimens,and it decreases as the average cement contentincreases.Deformation of LBS shows obvious discontinuity,deformation degree of lower strength part of LBS is generally higherthan that of higher strength part.A revised brittle fracture criterion based on the Stenerding-Lehnigk criterion is applied to analyzingthe fracture status of LBS,and the average relevant errors of the3groups between the test results and calculation results are4.80%,3.89%and4.66%,respectively.

Biomechanical characteristics of an anterior cruciate ligament injury in javelin throwing

Purpose： The purpose of this study was to understand the mechanism of an anterior cruciate ligament （ACL） injury in javelin throwing and javelin throwing techniques relevant to this ACL injury. Methods： The patient in this study was an elite female javelin thrower who completed the first three trials and sustained a non-contact ACL injury on her left knee in the fourth trial of javelin throwing during a recent track and field meet. Three-dimensional kinematic data were collected in the injury and non-injury trials. The kinematic data of 52 male and 54 female elite javelin throwers were obtained from a javelin throwing biomechanical database. Results： The patient had greater forward center of mass velocity and less vertical center of mass velocity after the first 25% of the delivery phase in the injury trial compared to non-injury trials. The patient had less left knee flexion angle and angular velocity but similar left knee valgus and internal rotation angles during the first 21% of the delivery phase in the injury trial compared to non-injury trials. The video images showed an obvious tibia anterior translation at the 30% of the delivery phase in the injury trial. The left knee flexion angle and angular velocity at the time of the left foot landing and the maximal left knee flexion angle during the delivery phase were not significantly correlated to the official distance for 52 male and 54 female elite javelin throwers. Conclusion： The ACL injury in this study occurred during the first 30% of the delivery phase, most likely during the first 25% of the delivery phase. A stiff landing of the left leg with a small knee flexion angle was the primary contributor to this injury. Javelin throwers may have a soft left leg landing with a flexed knee, which may help them prevent ACL injuries without compromising performance.

CT IDENTIFICATION OF THE MECHANIC CHARACTERISTICS OF DAMAGE PROPAGATION OF ROCK

The computerized tomography technique is applied to study the damage propagation of rock for the first time in this paper. CT values and their distribution regularity of damage propaga-tion of rock are analyzed in detail. The relation between CT values and stresses (strains) of the damage propagation of rock is then discussed. This provides the foundation for establishing the constitutive relation of damage propagation of rock.

Mechanical properties of deep sandstones under loading rate effect

The advance speed of the working face in coal mines can significantly affect the fluctuation frequency of abutment pressure in front of the coal body.Moreover,it has a certain correlation with the change of axial loading rate in coal and rock mechanics test.Therefore,uniaxial compression tests under various loading rates of 0.05,0.1,0.15,0.25,0.5 MPa/s were conducted using 2000 kN triaxial testing machine and PCI-2 acoustic emission test system to study the loading rate effect on the mechanical properties of deep sandstones.The results show that 1)the peak strength and elastic modulus of the deep sandstone increase with the loading rate increasing;2)with the loading rate increasing,the deep sandstone transforms from plastic-elastic-plastic to plastic-elastic and moreover,the failure mode gradually transfers from type I to type III;3)With the loading rate increasing,the total input strain energy,elastic strain energy,and dissipated strain energy generally increase;4)the damage variable presents the evolution characteristics of inverted“S”shape with time,and with the loading rate increasing,the damage degree of the deep sandstone is aggravated.The conclusion obtained can provide the theoretical basis for the stability control of the surrounding rock in deep engineering.

STUDY ON THE DAMAGE CONSTITUTIVE RELATION OF ROCK UNDER UNIAXIAL STRESS CONDITION

The damage propagation constitutive relation of rock under uniaxial stress condition is studied by CT scanning in this paper. Damage variables of rock are calculated using CT scanning method and Lemaitre’s equivalent strain hypothesis respecively, the damage propagation constitutive relation of rock is discussed.

Biomechanical behavior of brain injury caused by sticks using finite element model and Hybrid-III testing

Objective： To study the biomechanical mechanism of head injuries beaten with sticks, which is common in the battery or assaultive cases.Methods： In this study, the Hybrid-III anthropomorphic test device and finite element model （FEM） of the total human model for safety （THUMS） head were used to determine the biomechanical response of head while being beaten with different sticks. Total eight Hybrid-III tests and four finite element simulations were conducted. The contact force, resultant acceleration of head center of gravity, intracranial pressure and von Mises stress were calculated to determine the different biomechanical behavior of head with beaten by different sticks.Results： In Hybrid-III tests, the stick in each group demonstrated the similar kinematic behavior under the same loading condition. The peak values of the resultant acceleration for thick iron stick group, thin iron stick group, thick wooden stick group and thin wooden stick group were 203.4 g, 221.1 g, 170.5 g and 122.2 g respectively. In finite element simulations, positive intracranial pressure was initially observed in the frontal comparing with negative intracranial pressure in the contra-coup site. Subsequently the intracranial pressure in the coup site was decreasing toward negative value while the contra-coup intracranial pressure increasing toward positive values.Conclusions： The results illustrated that the stiffer and larger the stick was, the higher the von Mises stress, contact force and intracranial pressure were. We believed that the results in the Hybrid-III tests and THUMS head simulations for brain injury beaten with sticks could be reliable and useful for better understanding the injury mechanism.