Experimental Study and Failure Criterion Analysis of Rubber Fibre Reinforced Concrete under Biaxial Compression-Compression

In order to examine the biaxial compression-compression properties of rubber fibre reinforced concrete(RFRC),an experimental study on RFRC under different lateral compressive stresses was carried out by considering different rubber replacement rates and polypropylene fibre contents.The failure modes and mechanical property parameters of different RFRC working conditions were obtained from the experiment to explore the effects of rubber replacement rate and polypropylene fibre content on the biaxial compression-compression properties of RFRC.The following conclusions were drawn.Under the influence of lateral compressive stress,the biaxial compression-compression failure mode gradually developed from a columnar pattern to a flaky pattern,suggesting that the incorporation of rubber and polypropylene fibres into the concrete resulted in a significant change in the development of cracks.For different rubber replacement rates and polypropylene fibre contents,the vertical compressive stress exhibited the same developing trend under the influence of lateral compressive stress.Specifically,the lateral compressive stress imposed the minimum effect on the vertical compressive stress when the rubber replacement rate and polypropylene fibre content were 20%and 0.4%,respectively,and imposed the maximum effect when the rubber replacement rate and polypropylene fibre content were 20%and 0%,respectively.With the increase of rubber replacement rate,the vertical peak stress was significantly reduced,which implies that an appropriate amount of polypropylene fibres can increase the vertical peak stress to a certain extent.Then,the biaxial compression-compression mechanism of RFRC was analysed from the microscopic level by using scanning electron microscope(SEM).Meanwhile,based on Kupfer’s biaxial compression-compression failure criterion and the octahedral stress space,a biaxial compression-compression failure criterion for RFRC was proposed,which was proven to have good applicability.The research results of this study provide important theoretical basis for the engineering application and development of RFRC.

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.

Finite element failure analysis of continuous prestressed concrete box girders

In order to analyze the load carrying capacity of prestressed concrete box girders, failure behaviors of in-situ deteriorated continuous prestressed concrete box girders under loading are experimentally observed and a finite failure analysis method for predicting behaviors of box girders is developed. A degenerated solid shell element is used to simulate box girders and material nonlinearity is considered. Since pre-stressed concrete box girders usually have a large number of curve prestressed tendons, a type of combined element is presented to simulate the prestressed tendons of box girders, and then the number of elements can be significantly reduced. The analytical results are compared with full-scale failure test results. The comparison shows that the presented method can be effectively applied to the failure analysis of in-situ continuous prestressed concrete box girders, and it also shows that the studied old bridge still has enough load carrying capacity.

Ply-by-Ply Failure Analysis of Laminates under Dynamic Loading

Ply-by-ply failure analysis of symmetric and anti-symmetric laminates under uniform sinusoidal transverse dynamic loading is performed for a specified duration.The study investigates the first ply failure load,followed by the detection of successive ply failures and their failure modes using various failure theories.Some of the well-established failure theories,mostly used by the researchers,are considered for the failure prediction in laminates.The finite element computational model based on higher order shear deformation displacement field is used for the failure analysis and the complete methodology is computer coded using FORTRAN.The ply-discount stiffness reduction scheme is employed to modify the material properties of the failed lamina.The failure theories used in the analysis are compared according to their ability to predict failure load,failed ply,failure mode and progression of failure.The failure analysis is performed for both the cross-ply and angle-ply laminates with all edges simply supported and clamped.The significance of fibre orientation and stacking sequence in terms of the strength of a laminate and failure progression is also highlighted.

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＇.

Macro Meso Response and Stress Wave Propagation Characteristics of MCT High-Voltage Switch Under Shock load

In order to study the dynamic and electrical coupling response characteristics of Metal Oxide Semiconductor Controlled Thyristor(MCT)high-voltage switch under the synergic action of mechanical load and high voltage,the separated Hopkinson pressure bar(SHPB)test system was used to simulate different impact load environments,and combined with the multi-layer high-voltage ceramic capacitor charging and discharging system,the instantaneous electrical signals of MCT high-voltage switch were collected.Combined with numerical simulation and theoretical analysis,the failure mode and stress wave propagation characteristics of MCT high voltage switch were determined.The mechanical and electrical coupling response characteristics and failure mechanism of MCT high voltage switch under dynamic load were revealed from macroscopic and microscopic levels.The results show that the damage modes of MCT high-voltage switches can be divided into non-functional damage,recoverable functional damage,non-recoverable damage and structural damage.Due to the gap between the metal gate and the oxide layer,the insulating oxide layer was charged.After placing for a period of time,the elastic deformation of the metal gate partially recovered and the accumulated charge disappeared,which induced the recoverable functional damage failure of the device.In addition,obvious cracks appeared on both sides of the monocrystalline silicon inside the MCT high-voltage switch,leading to unrecoverable damage of the device.

FAILURE SURFACE AND CAPACITY OF REINFORCED CONCRETE MEMBERS SUBJECTED TO BIAXIAL TENSION

A computer program is developed for the nonlincar analysis of prestressedconcrete and nonprestressed concrete members subjected to combined biaxial bending andaxial load.The strength-interaction diagrams and failure surface are obtained.Thestrength design formulae are proposed.

Experimental and Numerical Studies of Failure Modes and Load-Carrying Capacity of Through-Diaphragm Connections

Shear failure in panel zones and plastic hinges in steel beams are the two major failure modes of connections between concrete-filled steel tubular(CFST) columns and steel beams. To investigate the behavior of this type of connection in both modes,two through-diaphragm connections were tested under cyclic and monotonic loadings and the load-carrying capacity,ductility,and strength of degradation of connections were discussed. Using ABAQUS software,we developed nonlinear finite-element models(FEMs) to simulate the load-carrying capacity and failure modes of the connections under monotonic loading. The finite-element(FE) analysis and test results showed reasonable agreement for the through-diaphragm connections,which confirms the accuracy of FEMs in predicting the load-carrying capacity and failure modes of connections. Based on the validated FEM,a parametric study was then conducted to investigate the infl uence of the thicknesses of the tube and diaphragm on the load-carrying capacity and failure modes of these connections. The results indicate that the strength,stiff ness,and load-carrying capacity are infl uenced less by the thickness of the diaphragm,and more by the thickness of the steel tube. According to the FE analysis results,it can be found that the critical condition between the two failure modes is determined by the shear resistance and bending resistance.

Stress analysis of clamped-free grid composite cylindrical shell under various end loadings

Stress analysis of cylindrical grid-stiffened composite shells was conducted under transverse loading,pure bending,torsion and axial compression under clamped-free boundary condition.Electrical strain gauges were employed to measure the strains in transverse loading case to validate the finite element analysis which was conducted using ANSYS software.Good agreement was obtained between the two methods.It was observed that stiffening the composite shell with helical ribs decreased the average equivalent Von Mises stress on the shell.The reduction of the stress seemed to be higher in the intersection of two ribs.It was also seen that the stress reduction ratio was higher when the structure was under bending compared to torsion and axial compression.The reduction ratio was approximately 75% in pure bending in the intersection point of the ribs,while it was approximately 25% in torsion.Therefore,it is concluded that the presence of the ribs is more effective under bending.Failure analysis was done using Tsai-Wu criterion.The ribs were observed to result in maximum and minimum increase in the failure load of the structure under transverse bending and torsional loading,respectively.

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.

Structural analysis and design of frost resistance function for subgrade of high-speed railway ballasted track in cold regions

According to the technical characteristics of short fixed wheelbase of a high-speed carriage, a subgrade-track integrated space mechanical response analysis model is proposed for trains under the action ofbiaxial load after the comparison of the stress distribution characteristics of the ballast track subgrade bed structures for high-speed railway under the action of uniaxial load and biaxial load. The loading threshold value （high-cycle long-term dynamic strength） under the circum- stance where the cumulative deformation of subgrade structure gradually develops and finally reaches the convergent state, and its relationship with the foundation coefficient K30 were deduced, based on the characteristics of cumulative defor- mation evolution obtained from the unit structure filling model test under the action of cyclic loading. In view of structure stability and frost resistance requirements of the railway subgrade in cold regions, technical conditions to maintain good service performance of subgrade structure of high-speed railway ballasted track are discussed and analyzed. Study results show that the additive effect manifests itself obviously for railway train bogies under the action of biaxial load than uni- axial load, which has a significant dynamic effect on the subgrade bed bottom and a slight effect on the surface layer. Thus, the adoption of a biaxial load model in the design of a high-speed railway subgrade accurately reflects the vehicle load. Pursuant to the structure design principle, the design method of the subgrade structure of high-speed railway ballasted track is proposed to meet the technical requirements such as structural strength, bearing stiffness and high-cyclic and long-term stability. Technical indicators are obtained for the variation of thickness of the surface layer of reinforced sub- grade bed in the double-layer subgrade mode along with the change of K30 at the subgrade bed bottom. The double-layer structure mode of ＂closure on the upper layer and drainage on the lower layer＂ was proposed in order to meet the water- proofing and drainage requirements of the upper layer of the subgrade bed in cold regions. A dense-framework graded gravel filler with weak water permeability at a coefficient of 10 4 cm/s is used on the upper layer and the void-framework graded gravel filler at the water permeability coefficient of 10 2 cm/s is adopted on the lower layer.

Damage response of conventionally reinforced two-way spanning concrete slab under eccentric impacting drop weight loading

Reinforced concrete(RC)structures are generally designed to carry quasi-static gravity loads through almost indispensable components namely slab,however,it may be subjected to high intense loads induced from the impact of projectiles generated by the tornado,falling construction equipment,and also from accidental explosions during their construction and service lifespan.Impacts due to rock/boulder falls do occur on the structures located especially in hilly areas.Such loadings are not predictable but may cause severe damage to the slab/structure.It stimulates structural engineers and researchers to investigate and understand the dynamic response of RC structures under such impulsive loading.This research work first investigates the performance of 1000×1000×75 mm^(3)conventionally reinforced two-way spanning normal strength concrete slab with only tension reinforcement(0.88%)under the concentric impact load(1035 N)using the finite element method based computer code,ABAQUS/Explicit-v.6.15.The impact load is delivered to the centroid of the slab using a solid-steel cylindroconical impactor(drop weight)with a flat nose of diameter 40 mm,having a total mass of 105 kg released from a fixed height of 2500 mm.Two popular concrete constitutive models in ABAQUS namely;Holmquist-Johnson-Cook(HJC)and Concrete Damage Plasticity(CDP),with strain rate effects as per fib MODEL CODE 2010,are used to model the concrete material behavior to impact loading and to simulate the damage to the slab.The slab response using these two models is analyzed and compared with the impact test results.The strain rate effect on the reinforcing steel bars has been incorporated in the analysis using the Malvar and Crawford(1998)approach.A classical elastoplastic kinematic idealization is considered to model the steel impactor and support system.Results reveal that the HJC model gives a little overestimation of peak displacement,maximum acceleration,and damage of the slab while the predictions given by the CDP model are in reasonable agreement with the experimental test results/observations available in the open literature.Following the validation of the numerical model,analyses have been extended to further investigate the damage response of the slab under eccentric impact loadings.In addition to the concentric location(P1)of the impacting device,five locations on a quarter of the slab i.e.,two along the diagonal(P2&P3),the other two along the mid-span(P4&P5),and the last one(P6)between P3 and P5,covering the entire slab,are considered.Computational results have been discussed and compared,and the evaluation of the most damaging location(s)of the impact is investigated.It has been found that the most critical location of the impact is not the centroid of the slab but the eccentric one with the eccentricity of 1/6th of the span from the centroid along the mid-span section.

Strength Prediction of Cruciform Specimen Under Biaxial Loading

In order to achieve a better understanding of failure behavior of cruciform specimen under different biaxial loading conditions,a three-dimensional finite element model is established with solid and interface elements.Maximum stress criterion,two Hashin-type criteria and the new proposed criteria are used to predict the strength of plain woven textile composites when biaxial loading ratio equals 1.Compared with experimental data,only the new proposed criteria can reach reasonable results.The applicability of the new proposed criteria is also verified by predicting the tensile and compressive strength of cruciform specimen under different biaxial loading ratios.Moreover,the introduction of interface element makes it more intuitive to recognize delamination failure.The shape of the predicted delamination failure region in the interface layer is similar to that of the failure region in neighboring entity layers,but the area of delamination failure region is a little larger.

Revealing the interlaminar shear failure behavior of unidirectional laminate under combined compression-shear loads

Due to the coupling effects between stresses in different directions,the mechanical behavior of an ad-vanced composite material under multiaxial loading is extremely complex.In this study,the influence of through-thickness compressive stress on the interlaminar shear performance of a carbon fiber-reinforced composite was experimentally investigated.Hollow cylindrical unidirectional laminate specimens were fabricated to conduct combined compression-shear tests,and the fracture morphologies of the specimens were characterized to reveal their failure behavior.The results indicate that a moderate compression load significantly enhanced the shear properties of the laminate by inhibiting crack propagation and improv-ing the friction effect.The shear strength and modulus of a laminate specimen subjected to combined stresses improved up to a maximum of 76%and 231%,respectively,over those of an equivalent specimen subjected to pure shear.However,as the applied through-thickness load approached the compressive strength of the laminate,the specimen shear capacity began to decline owing to the transition of frac-ture mechanisms.Indeed,the specimens exhibited mixed failure modes corresponding to the different stress states,which were induced by the combined effects of through-thickness compressive and shear stresses.As the applied through-thickness compressive stress increased,the dominant failure mode of the laminate specimen changed from fiber-matrix debonding to fiber shearing and then to fiber break-age,resulting in various shear performances.

Geiger型索穹顶冲击动力响应与失效机理研究

针对当前索穹顶结构缺乏冲击荷载作用下结构动力响应和失效机理的研究现状,该文基于ANSYS/LS-DYNA分析软件,提出了Geiger型索穹顶结构冲击动力分析模型,详细分析了冲击作用下结构的节点位移和杆件内力响应,并进一步结合美国规范UFC4-023-03的倒塌准则,对结构的失效模式进行评价,提出了三种失效模式,即结构轻微损伤、结构局部损伤和结构整体失效。随后,基于冲击破坏杆件数和杆件内力损伤程度,提出了结构整体损伤指数的概念,通过参数分析,探索了索穹顶结构在不同冲击物参数下整体损伤指数的变化特点及不同设计参数下的抗冲击性能。研究发现了整体损伤指数与三种失效模式之间的关系:当失效模式为结构轻微损伤时,其整体损伤指数小于0.3;当失效模式为结构局部损伤时,其整体损伤指数位于0.3~0.8之间;当失效模式为结构整体失效时,其整体损伤指数大于0.8。基于此提出了失效模式控制方程。冲击荷载越靠近结构中心,结构发生整体失效概率越大,反之则局部损伤和轻微损伤概率越大。提高初始预应力水平和杆件截面面积能有效提高结构的抗冲击性能。

海洋纤维增强热塑性立管极限承载拉力预测方法

海洋新型纤维增强热塑性立管因其可盘卷、耐腐蚀、耐疲劳和轻质化等优点,在深水油气开发中应用前景十分广阔。热塑性立管具有复合材料的各向异性、受力耦合效应及复杂的本构关系,且承受浮体运动和复杂海洋环境载荷,其失效模式尚未明确。针对轴对称载荷作用下纤维增强热塑性立管极限承载力问题,进行热塑性管稳态热传导和热应力的理论推导,求解了稳态温度和应力分布,首次给出了在任意温度载荷作用下管体径向位移的解析解,并直接求解其径向、轴向、环向和剪切应力。采用各向同性层Von Mises和各向异性层最大应力(Max Stress)准则或Tsai-Hill准则判定热塑性管的失效,基于应力分布、失效准则和二分法计算了热塑性管的极限载荷。温度载荷、纤维铺设角度和径厚比对管道的应力分布影响显著。不同温度载荷会改变失效指数沿径向的变化趋势,增大轴向拉力将增大热塑性管的失效指数,选用不同的失效准则在管体失效判定上存在一定的差异。热塑性管温度越低、纤维铺设角越小及径厚比越大,管道对轴向拉伸载荷的承载能力越强。

横向载荷与预紧力对螺栓断裂寿命的影响

螺栓连接依靠自身简单可靠的特点被广泛应用于各个行业,然而螺栓的疲劳断裂却是一个非常突出的问题.因此,为准确预测螺栓断裂失效寿命,进行了螺栓材料的疲劳裂纹扩展实验,求得了螺栓材料的断裂参数,从而建立了Paris裂纹扩展模型.然后运用有限元方法与疲劳裂纹扩展实验进行对标,并取得较好的效果,为螺栓断裂失效寿命的计算奠定了基础.最后,考虑到螺栓预紧力与横向载荷对螺栓断裂的影响,建立了带裂纹的螺栓有限元模型,并采用有限元方法进行螺栓失效寿命计算的定量分析.结果表明:横向载荷与螺栓预紧力对应力强度因子和螺栓使用寿命具有较大的影响.

不同起爆条件下双钢壁LNG储罐抗爆性能分析

由于双钢壁储罐具有建造周期短、投资小等优点,在LNG储存中应用越来越广泛,但其易在外罐受到外部冲击时引发内罐的破坏。为了分析储罐在不同外爆载荷条件下的破坏行为,以双钢壁LNG储罐为研究对象,考虑内、外罐及保冷材料之间的载荷传递与相互作用,建立了外罐-保冷材料-内罐多介质的多相耦合有限元模型,并对多影响因素下储罐在外爆载荷作用下的破坏进行了分析,探究其抗爆性能。结果表明:在外爆载荷作用下,内、外罐从迎爆面中心向内凹陷直至发生破坏,最大应力均在迎爆面中心,且越靠近中心应力值越大;内、外罐材料均先达到屈服强度后发生塑性变形,随后达到失效应变值并发生破坏;与外罐相比,内罐发生破坏的时间晚42.9%,变形范围和破坏范围分别小12.3%、6.7%;储罐的抗爆性能随着储存液位的升高和罐体容积的增大而增强,随着起爆位置从罐壁中心向上、下两侧移动而提高,在罐壁中心起爆时储罐的破坏程度最大。该研究成果可为双钢壁LNG储罐的设计和储存液位的控制等方面提供理论参考依据。

强对流天气下输电塔-线体系倒塌破坏与风雨组合系数研究

极端气象灾害给输电线路的安全运行带来了严重威胁,相对低电压等级的角钢输电塔出现了较高频次的倒塌破坏事故。基于现象学非线性滞回模型,开展了输电塔-线体系在强对流天气下的倒塌破坏研究,总结了雨荷载对输电塔响应的影响规律;基于风雨气象记录,进行了风雨组合分析并提出了风雨组合系数。结果表明,在极端强对流天气中,所研究输电塔的薄弱部位主要为第3、4节段,该部位的大量斜材和少量主材陆续屈曲是造成输电塔倒塌的主要原因,极端风雨气象条件下有必要考虑雨荷载对输电塔-线体系响应的影响。不同重现期下风速和雨量的组合值均小于其相应的标准值,该地区50 a重现期下风速和雨量的组合系数建议值分别为1.0和0.7。

多向荷载耦合作用下复合基础承载特性分析

针对水下石油生产系统复合基础在不同方向荷载耦合作用下的承载特性,分析复合加载模式下的破坏包络线。采用非线性有限元数值模拟方法,通过固定位移比法绘制竖向—倾覆、水平—扭转、竖向—水平—倾覆、竖向—水平—扭转荷载空间内的地基承载力破坏包络线,分析不同长径比和桩间距对不同方向荷载耦合作用下地基承载力包络线的影响,并分别将归一化处理后的竖向—倾覆、水平—扭转荷载包络线拟合为关于长径比和桩间距的包络线数学表达式。结果表明:随竖向荷载分量的增大,基础在二维荷载空间内包络线尺寸有所减小。结合实际工程应用,通过绘制地基承载力破坏包络线,对基础设计方案校核和评估,能很好指导工程实践与设计。