Strength and failure characteristics of marble spheres subjected to paired point loads

Failure of irregular rock samples may provide implications in the rapid estimation of rock strength,which is imperative in rock engineering practice.In this work,analytical,experimental and numerical investigations were carried out to study the mechanical properties and failure characteristics of rock spheres under paired point loads.Analytical solutions indicted that with the increase in sample size(contact angle)and decrease in Poisson’s ratio,the uneven tensile stress in theta direction decreased.Then laboratory experiments were carried out to investigate the load characteristics and failure mode of spherical marble samples with different sizes subjected to a pair of diametral point loads.The discrete element method(DEM)was adopted to study the failure process of rock spheres.The effect of the sphere diameter on the point load contact angle was examined in terms of peak load,crushed zone distribution and energy dissipation.Experimental and numerical results showed that the samples primarily fail in tension,with crushed zones formed at both loading points.With increase in the sample size,the contact angle,crushed area and total work increase.As the specimen diameter increases from 30 mm to 50 mm,the peak load on the specimen increases from 3.6 kN to 8.8 kN,and the percentage of crushed zone(ratio of crushing zone to sample radius,d/r)increased from 0.191 to 0.262.The results of the study have implications for understanding the failure of irregular rock specimens under point loading conditions and their size effects.

Prediction and Verifcation of Forming Limit Diagrams Based on a Modifed Shear Failure Criterion

The forming limit diagram plays an important role in predicting the forming limit of sheet metals.Previous studies have shown that,the method to construct the forming limit diagram based on instability theory of the original shear failure criterion is efective and simple.The original shear instability criterion can accurately predict the left area of the forming limit diagram but not the right area.In this study,in order to improve the accuracy of the original shear failure criterion,a modifed shear failure criterion was proposed based on in-depth analysis of the original shear failure criterion.The detailed improvement strategies of the shear failure criterion and the complete calculation process are given.Based on the modifed shear failure criterion and diferent constitutive equations,the theoretical forming limit of TRIP780 steel and 5754O aluminum alloy sheet metals are calculated.By comparing the theoretical and experimental results,it is shown that proposed modifed shear failure criterion can predict the right area of forming limit more reasonably than the original shear failure criterion.The efect of the pre-strain and constitutive equation on the forming limits are also analyzed in depth.The modifed shear failure criterion proposed in this study provides an alternative and reliable method to predict forming limit of sheet metals.

Study on Failure Mechanism and Bearing Capacity of Three-Dimensional Rectangular Footing Subjected to Combined Loading

This paper presents two kinematic failure mechanisms of threc-dimensional rectangular footing resting on homogeneous undrained clay foundation under uniaxial vertical loading and uniaxial moment loading. The failure mechanism under vertical loading comprises a plane strain Prandti-type mechanism over the central part of the longer side, and the size of the mechanism gradually reduces at the ends of the longer side and over the shorter side as the corner of rectangular footing is being approached where the direction of soil motion remains normal to each corresponding side respectively. The failure mechanism under moment loading comprises a plane strain scoop sliding mechanism over the central part of the longer side, and the radius of scoop sliding mechanism increases linearly at the ends of the longer side. On the basis of the kinematic failure mechanisms mentioned above, the vertical ultimate bearing capacity and the ultimate bearing capacity against moment or moment ultimate bearing capacity are obtained by use of upper bound limit analysis theory. At the same time, numerical analysis results, Skempton＇ s results and Salgado et al. ＇s results are compared with this upper bound solution. It shows that the presented failure mechanisms and plastic limit analysis predictions are validated. In order to investigate the behaviors of undrained clay foundation beneath the rectangular footing subjected to the combined loadings, numerical analysis is adopted by virtue of the general-purpose FEM software ABAQUS, where the clay is assumed to obey the Mohr-Coulomb yielding criterion. The failure envelope and the ultimate bearing capacity are achieved by the numerical analysis results with the varying aspect ratios from length L to breadth B of the rectangular footing. The failure mechanisms of rectangular footing which are subjected to the combined vertical loading V and horizontal loading H （Vertical loading V and moment loading M, and horizontal loading H and moment loading M respectively are observed in the finite element analysis. ） is explained by use of the upper bound plasticity limit analysis theory. Finally, the reason of eccentricity of failure envelope in H-M loading space is given in this study, which can not be explained by use of the traditional ＇ swipe test＇.

Pseudo-static stability analysis of rock slopes reinforced by passive bolts using the generalized Hoek-Brown criterion

The stability analysis of passive bolt-reinforced rock slopes under seismic loads is investigated within the framework of the kinematic approach of limit analysis theory.A pseudo-static method is adopted to account for the inertial forces induced in the rock mass by seismic events.The strength properties of the rock material are described by a modified Hoek-Brown strength criterion,whereas the passive bolts are modeled as bar-like inclusions that exhibit only resistance to tensile-compressive forces.Taking advantage of the ability to compute closed-form expressions for the support functions associated with the modified Hoek-Brown strength criterion,a rotational failure mechanism is implemented to derive rigorous lower bound estimates for the amount of reinforcement strength to prevent slope failure.The approach is then applied to investigating the effects of relevant geometry,strength and loading parameters in light of a preliminary parametric study.The accuracy of the approach is assessed by comparison of the lower bound estimates with finite element limit analysis solutions,thus emphasizing the ability of the approach to properly predict the stability conditions and to capture the essential features of deformation localization pattern.Finally,the extension of the approach to account for slipping at the interface between reinforcements and surrounding rock mass is outlined.

Limit Load Analysis of Pipes with Local Wall-thinning

The loss of metal in a pipe due to corrosion usually results in localized thinned areas with various depths and an irregular shape on its surface. In this paper, a number of numerical models of pipes with different size defects are established. The limit loads of these pipes are researched using the nonlinear finite element method. The effect of defect parameters of the local wall-thinning pipes on the limit load is discussed. The results show that limit loads decrease obviously when the depths and lengths of the defect increase. However, when the defect length reaches a certain value, the effect of defect length on limit loads is not significant. These results are compared with the results of the method of API 579. When the defect length is adequately small, the results of FEM are in good agreement with the ones of APl 579, but when the defect depth and length is adequately large, the API 579 is not suitable.

Describing failure in geomaterials using second-order work approach

Geomaterials are known to be non-associated materials. Granular soils therefore exhibit a variety of failure modes, with diffuse or localized kinematical patterns. In fact, the notion of failure itself can be confusing with regard to granular soils, because it is not associated with an obvious phenomenology. In this study, we built a proper framework, using the second-order work theory, to describe some failure modes in geomaterials based on energy conservation. The occurrence of failure is defined by an abrupt increase in kinetic energy. The increase in kinetic energy from an equilibrium state, under incremental loading, is shown to be equal to the difference between the external second-order work,involving the external loading parameters, and the internal second-order work, involving the constitutive properties of the material. When a stress limit state is reached, a certain stress component passes through a maximum value and then may decrease. Under such a condition, if a certain additional external loading is applied, the system fails, sharply increasing the strain rate. The internal stress is no longer able to balance the external stress, leading to a dynamic response of the specimen. As an illustration, the theoretical framework was applied to the well-known undrained triaxial test for loose soils. The influence of the loading control mode was clearly highlighted. It is shown that the plastic limit theory appears to be a particular case of this more general second-order work theory. When the plastic limit condition is met, the internal second-order work is nil. A class of incremental external loadings causes the kinetic energy to increase dramatically, leading to the sudden collapse of the specimen, as observed in laboratory.

点载荷作用下聚乙烯燃气管道的损伤分析

通过单轴拉伸试验获得了聚乙烯(PE)材料弹性及屈服参数。建立了点载荷下的管-土有限元模型,研究了埋地燃气PE在点载荷下的力学响应,分析了点载荷尺寸、管道内压及管道直径的影响。基于DFDI(ductile failure damage indicator)模型,结合有限元仿真结果,对点载荷下的PE管进行了定量损伤计算。结果表明,与无点载荷管道的损伤结果对比可知,点载荷的存在增大了管道损伤;管道内压变化时管道损伤变化较大;管道直径变化时对PE管损伤影响较小,各管径下的损伤值都比较接近。

粒子群自进化算法求解物流装箱问题

为了解决当今物流行业中装载货物类型为强异构的情况,提高装载填充率和效率,提出了一种求解三维装箱问题的元启发式算法——粒子群自进化算法。算法包含两部分:极限点构造启发式算法和粒子群自进化规则。极限点构造启发式算法引入了极限点的概念,利用新的极值点思想推导出了三维装箱问题的启发式算法。粒子群自进化规则提出了在货物装载序列中表示粒子的方法,推导了粒子间交叉、变异算子,在极限点构造启发式算法的基础上不断迭代进化完成货物的装载。通过不同结果的比对,证明该算法显著提高了物流装载的空间利用率,强异构货物的平均装载率达到了85%,验证了算法在强异构货物下的有效性与优越性,并给出了货物装载的三维模型。由于实际测试集的缺少,分别为机腹仓装载类和集装板类模型提出了实例生成器,通过生成器的测试集验证了算法在实际应用中的紧凑性、实用性和快捷性。

软黏土中吸力锚承载特性离心试验研究

加载倾角和系泊点位置是吸力锚基础最重要的设计要素之一,能够改变锚体的破坏姿态以影响极限承载力。在饱和软黏土地基中选择不同的加载倾角及系泊点位置进行位移控制下的张紧式吸力锚离心模型试验,利用六自由度磁力计装置定量分析了加载倾角和系泊点位置对吸力锚破坏姿态与极限承载力的影响。研究发现,系泊点位置在锚体约2/3高度时,吸力锚发生平动破坏,当加载倾角由35°变化至20°时,在相同的系泊点位置,吸力锚发生后仰破坏,归一化极限承载力稍增大,并且达到极限承载力后仍保持一定的承载力余量;系泊点位置在锚体2/3高度以上时,吸力锚发生前倾破坏,归一化极限承载力降低了25%左右,并且破坏后的承载力余量大大降低。无论破坏模式如何,均未发现锚内土塞与锚有明显分离。

混凝土材料动态破坏过程的近场动力学模拟

采用黏弹性近场动力学方法,根据混凝土单轴强度J准则类比得到近场动力学键伸长率的动态断裂准则。通过对骨料下的填充混凝土板、梁等构件进行模拟,探究了不同的加载速率下,骨料对裂纹形态及裂纹扩展路径的影响。结果表明,加载速率越大,混凝土中填充的骨料断裂度也越大。动态断裂J准则能准确描述混凝土在动态荷载下的破坏形态,该方法为混凝土的损伤防治工程提供了指导依据。

浮式生产储卸油装置单点系泊缆失效识别研究

针对南海某FPSO单点系泊系统,建立数值计算模型,结合百年一遇、五十年一遇、十年一遇环境载荷,对完整系泊系统、单根缆失效系泊系统、同组双缆失效系泊系统进行数值分析。结果表明,该系泊系统在一根缆破断时能够承受百年一遇环境载荷;结合环境载荷及单点位置可以判断是否存在系泊缆失效。

地震荷载作用下非饱和土边坡三维极限承载力研究

研究旨在对地震荷载作用下非饱和土边坡稳定性进行分析。为了考虑地震荷载时间效应、边坡放大效应及土体阻尼特性的影响,采用修正拟动力法对边坡动力响应进行描述。采用考虑基质吸力影响下非饱和土广义黏聚力和饱和土摩擦角定义非饱和土剪切强度参数。基于三维离散破坏模型,采用极限分析上限法构建了地震荷载作用下非饱和土边坡三维极限承载力分析模型,通过已发表文献结果验证了所提出模型的有效性。最后,提供了非饱和土边坡三维极限承载力设计图表。研究结果表明:①与二维破坏模型相比较,所提出三维破坏模型能实现对边坡稳定性更为准确的评估;②忽略非饱和土剪切强度参数影响往往导致边坡稳定性评估结果偏于保守。

碳纤维复合材料纵梁承载与吸能特性研究

为验证某轨道交通车辆碳纤维复合材料纵梁的设计值是否满足实际工况要求,采用有限元分析,依据某轨道交通车辆纵梁运营工况的要求,设定了试件连接区域、加载工况及加载点,进行了100%和130%限制载荷试验以及两种工况下的极限载荷试验,并在130%限制载荷试验基础上继续做破坏试验。试验结果显示:在限制载荷试验中,试件未发出明显的响声,试验结束后试件没有发生破坏、纤维分层、开裂等损伤现象,同时加载连接处未发生破坏,表明该试件满足轨道交通车辆实际运营工况要求;破坏试验加载至300%限制载荷,试件整体结构完好,表面未出现明显损伤,加载至500%限制载荷时,夹具出现破坏,试件目测完好无损,整体结构完好,表面未出现明显的损伤,表明该结构的碳纤维复合材料纵梁设计安全裕度较大。

Addressing Economic Dispatch Problem with Multiple Fuels Using Oscillatory Particle Swarm Optimization

Economic dispatch has a significant effect on optimal economical operation in the power systems in industrial revolution 4.0 in terms of considerable savings in revenue.Various non-linearity are added to make the fossil fuel-based power systems more practical.In order to achieve an accurate economical schedule,valve point loading effect,ramp rate constraints,and prohibited operating zones are being considered for realistic scenarios.In this paper,an improved,and modified version of conventional particle swarm optimization(PSO),called Oscillatory PSO(OPSO),is devised to provide a cheaper schedule with optimum cost.The conventional PSO is improved by deriving a mechanism enabling the particle towards the trajectories of oscillatory motion to acquire the entire search space.A set of differential equations is implemented to expose the condition for trajectory motion in oscillation.Using adaptive inertia weights,this OPSO method provides an optimized cost of generation as compared to the conventional particle swarm optimization and other new meta-heuristic approaches.

悬链线钢管混凝土单管拱破坏试验研究

为探究悬链线钢管混凝土拱的力学性能,依托净跨径95 m的单管混凝土拱桥—瓦石窝大桥,开展1︰16的缩尺模型试验。开展2根钢管混凝土悬链线拱面内单点加载试验,试验拱跨度为5.938 m,矢跨比为1/3.5,拱轴系数为1.5,加载工况分别为拱顶加载和四分点加载。试验结果表明:拱顶和1/4加载工况下,拱的破坏模式均为四铰破坏;2种加载工况下,拱加载点处外部变形较大,但未发生局部屈曲现象。在达到极限荷载的55%后,拱肋钢管开始对核心混凝土产生约束作用,钢管对核心混凝土的约束效应比抛物线拱要提前发挥作用。L/4加载工况下拱的变形和应变均大于拱顶加载工况,说明L/4加载工况更不利。另外,利用ANSYS梁单元法对悬链线钢管混凝土拱进行了参数扩展分析,分析了不同加载方式、拱轴系数、矢跨比,以及含钢率对拱极限承载力的影响。结果表明:不同加载方式、矢跨比、含钢率对拱的承载能力影响较大,拱轴系数影响较小。随着含钢率和矢跨比的增大拱的承载力相应增加。研究成果可以增补钢管混凝土拱承载力试验库和为工程设计提供借鉴,同时也为悬链线钢管混凝土拱的理论计算铺垫基础。

越顶破坏模式下沉埋桩受荷段及沉埋段推力算法

滑坡推力的确定对于抗滑桩设计极其重要,沉埋桩作为对传统抗滑桩的优化,其受荷段推力的研究目前主要借助于模型试验和数值模拟,缺乏深入的理论分析。为了建立沉埋桩后侧受荷段及桩顶沉埋段滑坡推力计算方法,针对沉埋桩加固的基岩-覆盖层式滑坡,基于潜在越顶破坏模式,由桩顶位置将越顶滑面分为顶部、底部两段,其中,顶部滑面上水平方向合力即为沉埋段推力,可由积分求得,底部滑面上各个方向的力求解方法与此类似;在此基础上,利用刚体极限平衡理论对底部滑面与桩受荷段所围滑体进行受力分析,进而可得受荷段推力计算公式。实例分析表明:理论算法所得沉埋段与受荷段推力值与FLAC3D结果非常接近,其中,受荷段推力、沉埋段推力、设桩位置处总推力随沉埋比的增大而分别非线性减小、增大、减小;沉埋比位于0~0.67范围内时,沉埋段与受荷段推力之比由0缓慢增大到0.30~0.50,随着沉埋比增大到0.8,该比值急剧增大到1.47~2.12;一般沉埋深度下,沉埋段推力小于受荷段推力。沉埋桩推力的理论研究对于桩体内力优化、沉埋深度确定具有重要的现实意义,将有助于该桩型的进一步推广应用。

球形颗粒接触破碎试验

目前对于粗粒土颗粒接触破碎性质的研究,多集中在单点接触的情形,少有对多点接触情形下颗粒破碎特性的探讨。而实际粗粒土堆体中颗粒间大多是以多点接触的形式存在。本文利用石膏试样模拟粗粒土颗粒进行单点和三点接触情形下的破碎模型试验,研究球形石膏颗粒在单点和多点接触情形下的破碎特性,比较单点和多点接触破碎特性的异同,并总结不同侧向接触力条件下颗粒的破碎规律。

工程结构整体屈曲的临界荷载分析

很多工程结构的承载力取决于整体屈曲分析所确定的稳定承载能力．一般认为特征值屈曲计算值是非线性屈曲计算值的上限,而且按照第一阶屈曲模态施加初始缺陷必将减小临界荷载,通过数值计算表明实际工程中会出现相反的结果,并根据屈曲理论研究了上述现象的机理．比较了不同的屈曲类型屈曲后平衡路径的发展特征及其缺陷影响,采用若干典型结构作为算例对特征值屈曲分析进行探讨,发现其适用范围为直柱、框架及平板结构,并以实际结构为例进行了验证．通过分析得知:不同类型的屈曲形式有着不同的缺陷敏感性;屈曲后平衡路径的发展特征决定了特征值屈曲分析的偏差情况;对极值点屈曲的结构进行临界荷载分析,需要施加正负缺陷进行比较以得到最低临界荷载值．

覆冰荷载下输电铁塔体系可靠度研究

研究在覆冰荷载作用下,输电铁塔主要失效模式及其体系可靠度。建立覆冰荷载作用下寻找输电铁塔主要失效模式的方法,该方法以覆冰厚度为控制量,可以方便地获得铁塔的主要失效模式及相应的极限覆冰厚度。利用基于响应面的随机有限元法来获得各失效模式上各个单元的极限状态方程,然后用逐步等效线性化法得到各失效模式的等效线性化极限状态方程。所有的失效模式组成一个串联系统,计算各失效模式间的相关系数,最后由Ditlevsen界限法计算输电铁塔体系可靠度。分析在2008年初的冰雪天气里发生倒塌的我国南方某220kV输电铁塔的主要失效模式及其体系可靠度。通过计算指出由于不平衡张力的存在,使输电铁塔的极限覆冰厚度接近于它的设计覆冰厚度;同时指出:如果借鉴构件的目标可靠度指标的话,既有输电铁塔在设计覆冰条件下,体系可靠度指标不能满足要求。

某输电线路铁塔覆冰条件下的失效模式分析

建立了覆冰荷载作用下寻找输电铁塔主要失效模式的方法,该方法以覆冰厚度为控制量,可以方便地获得铁塔的主要失效模式及其相应的极限覆冰厚度。分析了南方某既有输电线路铁塔在两种工况下的主要失效模式,指出该类铁塔的薄弱处位于挂导线处、上曲臂以及塔身上部。同时指出如果存在20%不平衡纵向张力,挂导线处及上曲臂的极限覆冰厚度接近于该铁塔的设计覆冰厚度,塔的安全储备不足。铁塔的失效模式分析的结果与实际倒塔情况相吻合。