Free vibration of non-uniformly ring stiffened cylindrical shells using analytical, experimental and numerical methods

In this research, the free vibration analysis of cylindrical shells with circumferential stiffeners, i.e. rings with non-uniform stiffeners eccentricity and unequal stiffeners spacing is investigated using analytical, experimental and finite elements (FE) methods. Ritz method is applied in analytical solution while stiffeners treated as discrete elements. The polynomial functions are used for Ritz functions and natural frequency results for simply supported stiffened cylindrical shell with equal rings spacing and constant eccentricity is compared with other's analytical and experimental results, which showed good agreement. Also, a stiffened shell with unequal rings spacing and non-uniform eccentricity with free–free boundary condition is considered using analytical, experimental and FE methods. In experimental method, modal testing is performed to obtain modal parameters, including natural frequencies, mode shapes and damping in each mode. In FE method, two types of modeling, including shell and beam elements and solid element are used, applying ANSYS software. The analytical and the FE results are compared with the experimental one, showing good agreements. Because of insufficient experimental modal data for non-uniformly stiffeners distribution, the results of modal testing obtained in this study could be as useful reference for validating the accuracy of other analytical and numerical methods for free vibration analysis.     

Effect of cutting tool blunting on the performances of various mechanical excavators used in low- and medium-strength rocks

Full-scale laboratory cutting tests that measure the specific energy (SE) are widely used to evaluate rock cuttability by mechanical excavators, and in particular roadheaders fitted with radial or drag-type bits. Radial or drag-type bits are often changed during operation as they wear and become blunt. In this study, full-scale cutting tests were carried out on different rock types using bits with varying degrees of wear in order to assess the impacts of pick bluntness on cutting forces and the SE. The relationships between wear flats and cutting forces, SE, and various rock properties such as uniaxial compressive strength, tensile strength, indentation index, Shore hardness, Schmidt hammer hardness, and density were examined and are discussed in this paper. The mean cutting force increased 2- to 3-fold and the cutting SE rose 4- to 5-fold with a 4-mm wear flat as compared to a sharp pick. Critical wear flats were plotted for different rock property values, and 25 MJ/m³ was considered the threshold SE above which cutting performance was considered to be poor. Best-fit predictive models based on statistical analysis of the laboratory cutting test results are introduced as a means to estimate SE as a function of bit type, wear condition, and various mechanical properties of the rock. These models can be used to predict the performances of mechanical excavators that use radial tools, especially roadheaders, continuous miners, and longwall drum shearers.     

Experimental results of a low-power roadheader driving a gallery with different types of rock at the face

In this paper, the experimental results of a 45-kW and 15-t roadheader excavating a gallery with two different types of rock at the face using two different cutting heads are shown. It is proved that the roadheader works properly with both cutting heads. In comparison with other results in the literature, the principal parameters, i.e. specific energy, cutting rate and tool wear, are at a level that can be considered satisfactory taking into account the low power of the roadheader. On the other hand, the influence of the number of picks, which is the main difference between the two cutting heads, on the operational parameters is shown.     

Assessment of pick wear based on the field performance of two transverse type roadheaders: a case study from Amasra coalfield

A successful excavation of roadheaders depends on the cutting performance and the tool life of conical picks. Tool life is important in terms of wear rate which is affected by different rock parameters such as equivalent quartz content, mineral grain size, as well as cutting parameters on the cutterhead. In this study, analyses among wear rate, specific energy, advance rate, and cutter consumption were carried out. The wear mechanisms of two different models of conical picks were examined from different aspects depending on rock and machine parameters. Their relation with the mechanical and abrasivity properties of rocks and petrographic analyses were investigated. In addition, the metallurgic content and Rockwell hardness of conical picks were determined to describe the metal alloys and their effects on the wear of cutting tool. The results showed that the metallurgic content, pick positions, and other environmental conditions influence the wear mechanism. Finally, two different models were proposed to estimate the pick consumption in sandstone and siltstone rocks based on actual data obtained from coalfield.

     

TBM滚刀破岩动态特性与最优刀间距研究

在对实际工况合理简化的基础上,从岩土细观角度出发,采用颗粒离散元法建立滚刀侵入岩体的二维模拟模型,研究双滚刀作用下岩体的动态响应机制,找出滚刀侵入过程中岩体裂纹、贯入度以及切削力三者的关系。在此基础上,通过数值模拟对常见切深下滚刀最优刀间距问题进行分析,得到不同切深下比能耗与刀间距的规律,并通过试验对双滚刀破岩过程中岩体动态特性以及最优刀间距问题进行验证,最后以工程实例验证研究结论。研究表明：仿真过程中,切削力随贯入度的变化与岩体的跃进破碎特性相一致,岩体破坏服从格里菲斯理论;较小切深下岩体为剪切破坏,较大切深下岩体发生拉应力破坏;切深为10 mm时比能耗有明显拐点,此时刀间距为100 mm;切深为6 mm时,60 mm刀间距下比能耗最小;切深小于2 mm时,实际工况下岩体不能产生贯穿裂纹。     

Dominant rock properties affecting the performance of conical picks and the comparison of some experimental and theoretical results

Conical picks are the essential cutting tools used especially on roadheaders, continuous miners and shearers and their cutting performance affects directly the efficiency and the cost of rock/mineral excavation. In this study, in order to better understand the effects of dominant rock properties on cutter performance, 22 different rock specimens having compressive strength values varying from 10 to 170 MPa are first subjected to a wide range of mechanical tests. Then, laboratory full-scale linear cutting tests with different depth of cut and cutter spacing values are realized on large blocks of rock specimens using one type of conical pick. Specific energy, cutting and normal force values for relieved and unrelieved cutting modes are recorded using a triaxial force dynamometer with capacity of 50 tonnes and a data acquisition system. Cutter force and specific energy values are correlated with rock properties and theoretical force and specific energy values obtained from widely used theoretical approaches.The results indicate that uniaxial compressive strength among the rock properties investigated is best correlated with the measured cutter performance values, which is in good agreement with previous studies. However, it is also emphasized in this study that Brazilian tensile strength, Schmidt hammer rebound values, static and dynamic elasticity modulus are also dominant rock properties affecting cutter performance.Theoretical specific energy defined by different researchers has a meaningful relationship with the experimental specific energy, which is an essential parameter for predicting the instantaneous cutting rates of mechanical excavation systems. It is also demonstrated that the experimental cutter forces obtained for 5 mm depth of cut are in good agreement with theoretical force values, if the friction angle between rock and cutting tool is included in the theoretical formulation. It is emphasized that, to some extend, laboratory tests can help to minimize high cost of a trial–error approach in the field.     

Numerical simulation of rock cutting process induced by a pick cutter considering dynamic properties of rock at intermediate strain rate

Rock excavation is a dynamic process where rock is dynamically loaded by mechanical cutting tools. Previous researches have shown that rock strength in dynamic loading is greater than that in static loading. However, existing prediction models for mechanical excavation of rock do not take into account the dynamic features of the rock. This issue has the potential to deliver misleading outcomes when estimating cutting performance and assigning an equipment for excavation work. The mechanical cutting issue was explored from a dynamic standpoint in this paper. To begin, the dynamic level of mechanical cutting was studied with multiple attempts and was shown to be at intermediate strain rate (ISR). Then, experiments were designed to determine dynamic mechanical properties via a loading device powered by a non-explosive powder reaction capable of creating dynamic loading in the ISR range. Furthermore, numerical simulation of pick cutting using the finite element technique was carried out, incorporating dynamic and quasi-static parameter settings. Also, a laboratory linear cutting test was conducted under various cutting configurations to validate the numerical results. The numerical simulation with quasi-static inputs was shown to be less accurate than the modeling with dynamic inputs. The simulation with dynamic inputs matches the laboratory findings closer by exhibiting a narrower margin. Thus, this study offers a unique view of evaluating mechanical excavation issues from a rock dynamic perspective.

     

延长盾构机刀具使用寿命的措施

对盾构刀盘和刀具进行了针对性的论述,在盾构刀盘、刀具的设计开发过程中,通过分析刀盘和刀具的受力状况,统计刀盘和刀具的损坏形式,对刀具进行了有针对性的研究改进,从而使产品质量、性能更加可靠完善。     

Study on the influence of joint spacing on rock fragmentation under TBM cutter by linear cutting test

Joint spacing is one of the most important geological factors influencing rock fragmentation by TBM cutters and TBM performance. In order to study the influence of joint spacing, full-scale linear cutting tests have been conducted for the Beishan granite samples with different joint spacing (i.e. one intact sample, two jointed samples with joint spacing of 100 mm and 400 mm). For different joint spacing, the influence of penetration depth on rock fragmentation was also explored by varying the penetration depth with an interval of 0.5 mm. During the test process, the three directional forces acting on the TBM cutter were recorded, and the rock chips formed by each cutting pass were weighed, respectively. By analysing the cutting force, crack initiation/propagation and rock chips, the influences of joint spacing on rock fragmentation process by TBM cutter were investigated. The test results showed that the increase of penetration depth cannot improve the TBM breakage efficiency after reaching a certain value for the intact rock sample, and the normal force for intruding the intact rock is larger than that for intruding the rock jointed samples. It is also found that the sample part below the joint plane is intact, thus joint can restrain the crack propagating cross the joint plane and facilitates the chips formation on the cutting surface. For the rock sample with joint spacing of 100 mm, two rock fragmentation modes were found during the cutting process. One mode is that the cracks initiate from the crushed zone under TBM cutter, and the cracks propagate to the joint plane, consequently form large rock chips. The other one is that the cracks initiate from the joint plane and then propagate to the rock cutting surface, and the cracks initiate before the formation of the crushed zone under the cutter. For the rock sample with joint spacing of 400 mm, there are two rock fragmentation stages, i.e., the normal rock fragmentation stage and the joint-controlled rock fragmentation stage. There is a transitional process between these two stages, and also the median crack can be promoted to propagate vertically to joint plane due to the joint existence. This study can provide useful guidance for operation optimization and performance prediction for TBM operating in jointed rock masses.     

Predictive plots for conical pick performance using mechanical and elastoplastic properties of rocks

Conical picks are by far the most widely used drag type cutting tools employed on partial face rock excavation machines. The cutting force and specific energy are two important design parameters for the conical pick performance, and the rock cutting testing is considered as the promising tool for determining these parameters. In the absence of an instrumented cutting rig, researchers generally rely on empirical predictive plots. For this, this paper suggests predictive plots for estimating the cutting force and specific energy, in consideration of the cutting depth to define the cuttability with conical picks. In this context, rock cutting tests were carried out on six volcanic rock samples with varying cutting depths using the unrelieved and relieved cutting modes. The cutting force and specific energy were correlated with the uniaxial compressive strength, Brazilian tensile strength, elasticity modulus, and plasticity index. Predictive plots were proposed for different cutting depths in the unrelieved and relieved cutting modes and exponential relationships were obtained among the cuttability parameters, and mechanical and elastoplastic properties of rocks.     

Disc cutting tests in Colorado Red Granite: Implications for TBM performance prediction

A series of full-scale laboratory disc cutting tests was conducted with a single disc cutter (432 mm diameter and a constant cross-section profile) and a single rock type (a coarse-grained red granite). Normal, rolling, and side forces were measured for a series of spacings and penetrations, from which other cutting parameters also were calculated. Although the increases of normal and rolling forces with increased spacing and penetration are as expected, the results illuminate additional aspects of performance prediction. Specific energy (SE) considerations indicate that a spacing of 76 mm is close to optimum in this hard, brittle crystalline rock. At this spacing, penetration has very little effect on SE. These results show why spacings near 76 mm are commonly found on tunnel boring machines operating in hard rock. The relationship of rolling force to normal force was close and consistent: A nearly linear rise of the ratio of rolling force to normal force with increased penetration, and, conversely, a nearly unchanged ratio with increases in spacing. The results tend to validate performance prediction methodologies based on normal force-penetration models.     

Behavior of foundations using driven vertical and inclined piles under horizontal loading

Conclusions 1. It was found that for equal spacing of the piles in the clusters, the bearing capacity of the foundations consisting of inclined piles with a low grating in clay soils of semihard and hard consistency is greater by a factor of 1.7–2.1 than the capacity of foundations consisting of vertical piles. 2. The rigid embedment of the heads of the inclined piles in the slab of the low grating increases their strength under horizontal loading for operation in a cluster by a factor of 3.9, whereas for vertical piles it is increased by factor of only 1.9, in comparison with the individual piles with free heads of equal rigidity. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 11–13, March–April, 1982.     

内含圆柱域热源的非饱和土介质水热耦合作用的SPH数值模拟

光滑粒子流体动力学(SPH)作为一种拉格朗日型无网格粒子方法,已经成功地应用于多种类型耦合问题的求解。基于SPH方法探索求解二维求解域内含有圆形局部热源的水热耦合问题,其中揭示了程序前处理阶段已有粒子布置方案存在的不足。针对所要求解的具体问题,使用一种新的粒子布置方案以弥补数值计算过程中的精度损失,该粒子布置方案中径向粒子间距严格相等,环向间距大致等于径向粒子间距。通过对水热耦合方程数学特性的分析,表明了SPH算法可以用于此类耦合问题的数值模拟。最后,基于SPH算法自行编制的程序对一种非饱和土介质内含有局部热源的水热耦合问题进行数值模拟,对非饱和土介质中热能传输以及水分迁移的规律进行了分析。     

实验室测定岩石两种裁割性能指标的方法

岩石的截割性能指标是反映岩石抵抗截割能力的重要参数 ,是设计、选择截割刀具和工作机构以及整个机器的依据。文章介绍了实验室测定岩石两种截割性能指标的装置与方法 ,并分析了其各自的特点 ,这对于正确地测定岩石的截割性能有一定的指导作用。     

252kV 四级污秽棒形支柱瓷绝缘子花釉缺陷原因分析及解决方法探讨

对比分析了干法生产的252kV四级和三级污秽棒形支柱瓷绝缘子的工艺特点,认为四级污秽棒形支柱瓷绝缘子产生花釉缺陷的原因是:伞间距小、伞伸出大,施釉操作时坯体旋转,伞根部位暴露不完全,釉浆很难喷到伞根部位。在保证产品机械性能、电气性能和冷热性能的前提下,将大伞根厚度由22.67 mm减小到17.61 mm,小伞根厚度由20.07 mm减小到15.66mm,大伞与小伞之间杆径直线距离增加2.64mm、小伞与大伞之间杆径直线距离增加2.69mm,两伞之间距离由67.76mm增加为70.11mm;爬电距离增加(用一组伞作为对比)5.61mm;杆径未变化;伞间最小距离70.11mm;伞间距和伞伸出之比0.926;大小伞伸出之差17.5 mm;伞倾角17°。实验证明伞形结构优化后,为施釉操作创造了条件,瓷检合格率明显提高,较好地解决了花釉缺陷问题。     

整体式太阳热水器的评价

阐述了等截面与不等截面整体式太阳热水器的特点及机理,通过对比试验,对其进行分析和评价。经测试及计算,不等截面太阳热水器的平均热效率、吸热板与水之间的换热系数均高于等截面太阳热水器,而其热损系数则低于等截面太阳热水器。因此,不等截面太阳热水器的热性能优于等截面太阳热水的热性能。     

岩石不等长裂纹应力强度因子及起裂规律研究

天然岩体中含有数量众多的裂隙且绝大多数为不等长裂隙,而那些具有一定尺度和规模的主控裂隙对岩体稳定性起决定作用。为研究不等长裂纹相互作用规律及主控裂纹起裂规律,基于Kachanov法,推导了远场受力的两条拉剪不等长共线裂纹的尖端应力强度因子表达式并从理论上分析了裂纹间距对裂纹相互作用的影响;通过最大周向应力准则,计算了单轴受拉时不同裂纹倾角的主控裂纹起裂角,绘出理论断裂准则曲线;推算滑动裂纹的应力强度因子,对含不等长裂纹的类岩试件进行单轴压缩试验。结果表明:当裂纹间距大于等于小裂纹长度时,小裂纹对主控裂纹的起裂几乎无影响;主控裂纹起裂角只与裂纹角有关,单轴受拉时,起裂角随裂纹角增大而减小,单轴受压时,起裂角随裂纹角增大而增大;当裂纹倾角小于30°时,起裂临界荷载急剧增大。     

Evaluation of cutting efficiency during TBM disc cutter excavation within a Korean granitic rock using linear-cutting-machine testing and photogrammetric measurement

In TBM excavation, estimation of cutting performance is of great importance in design stage as well as during construction. The performance is highly dependent on the geological conditions, i.e. characteristics of rock and discontinuities, and operational conditions, i.e. selection of cutter, cutting forces, cutter spacing, etc. For performance estimation, full scale test is most reliable and accurate since it takes full advantage of using real cutter and real size specimen. Linear cutting machine (LCM) is usually used for a full size test to evaluate the cutting performance. This paper presents the results of LCM tests carried out under various cutting conditions to assess the cutting performance of a TBM disc cutter for granitic rock in Korea. In LCM test, the excavated rock volume was determined by ShapeMetrix3D photogrammetric measurement system. This system was employed to ensure the accurate determination of cutting volume and subsequently calculated specific energy (SE). The optimum cutting condition for the Korean granitic rock was obtained at the minimum value of SE. In addition, three-dimensional numerical analysis was performed to simulate the rock cutting behavior in the LCM test. The results of the numerical simulation were closely comparable with the results of the LCM test. This study presents the cutting performance of a disc cutter by LCM test for a Korean granitic rock and demonstrates the applicability of numerical analysis as an alternative for the prediction of the cutting performance.     

Drag pick cutting tests: A comparison between experimental and theoretical results

This paper aims at reporting the results of a number of drag pick cutting tests on selected igneous rock samples to compare the experimentally determined maximum cutting force (FC^′) values with theoretically estimated ones. First, a review on theoretical rock cutting models proposed for both chisel and conical picks was presented in detail. Experimental study consists of both chisel and conical pick cutting tests in unrelieved (single-pick) cutting mode with varying cutting depths. FC^′ values were determined from experimental results, and theoretical models were utilized to compute FC^′ for all cutting conditions. Computed and experimentally determined FC^′ data were then compared for a referenced cutting depth. It is shown that the theoretical models might overestimate or underestimate FC^′ and cannot give reliable results. Finally, explanations for these mismatches were presented.     

变电站等高差短导线受力性能分析

为研究等高差导线的受力性能,提出了一种简化导线模型,并验证了该简化模型的精度可以满足研究需要。使用该简化模型对等高差导线在不同间距下的水平作用力和端部弯矩进行分析。结果表明抛物线公式具有局限性,当间距较小时其计算结果可能存在较大的误差;水平作用力和端部弯矩均随间距增大先增大后减小,水平作用力为0时对应的间距与弯矩最小时对应的间距基本一致。