Reliability assessment of post-buckling compressive strength of laminated composite plates and stiffened panels under axial compression

A method for reliability assessment of the post-buckling compressive strength of laminated composite plates and stiffened panels under axial compression is presented in the paper. The prediction of the post-buckling compressive strength is performed by a progressive failure analysis which was developed based on a progressive stiffness degradation model and a nonlinear finite element analysis with a new explicit through-thickness integration scheme. A method coupled with the finite element analysis is proposed for reliability assessment where a finite difference method combined with an improved first-order reliability algorithm that omits the non-important random variables but retains sufficient accuracy was developed for reliability estimation. Two numerical examples are described demonstrating the capabilities of the method developed.     

Finite element analysis of plane soil cutting

This study develops the finite element method (FEM) of solution to provide a theoretical means for determination of soil performance under the actions of a cutting blade—and the forces required to promote cutting. The developed FEM takes into account the effect of progressive and continuous cutting of the clay soil at the tip of the blade, with possible development of failure zones in the soil whenever the shear strength of the soil is exceeded. The solution provides detailed information on stress and deformation fields in the soil, together with tangential and normal pressures developed at the blade soil interface Correspondence between theoretically computed displacement fields and measured values has been obtained. In addition, the theoretically computed and experimentally measured values for forces developed in blade thrust are seen to be in close agreement.     

Constitutive model for high speed tillage using narrow tools

Dynamic effects on soil–tool cutting forces are important when operating at elevated speeds. The rate-dependent behavior of narrow tillage tools was investigated in this study. A hypoelastic soil constitutive relationship with variable Young's modulus and Poisson's ratio was developed to describe the dynamic soil-tool cutting problem. An initial finite element formulation with viscous components incorporated in the stiffness matrix resulted in severe numerical oscillations. A modified model that incorporated lumped viscous components in the equation of motion (independent of the stiffness matrix) was proposed. Numerical oscillations still occurred, but at sufficiently high tool displacements (1–10 mm) to enable the determination of peak draft forces. Experimental data for flat and triangular edged narrow tools were obtained using a 9-m long linear monorail system designed to accelerate narrow tools through a linear soil bin to high speeds. Steady-state speeds from 0.5 to 10.0 m/s over a distance of 1 to 3 m were attained using this system. A reference-tool inverse procedure was used to estimate the dynamic soil parameter in the soil model using draft data obtained for the flat tool. Predictions of triangular tool draft produced correct trends but overestimated experimental data. Draft was overpredicted by less than 1% at a tool speed of 2.8 m/s and by 25% at 8.4 m/s.     

损伤因素对单层平面索网静力性能影响研究：有限元分析与试验验证

运用通用有限元软件ANSYS建立三维有限元模型,对横索与竖索连接节点失效、索预应力损失和锚固端失效这三种损伤因素的不同损伤工况下,单层平面索网结构的受力性能进行了非线性有限元分析,并与相应的试验结果进行了全面对比分析。结果表明：本文的有限元模型能够准确地分析计算上述三种损伤因素对单层平面索网结构受力性能的影响,包括结构...     

3D Dynamic analysis of soil–tool interaction using the finite element method

Previous experimental and finite element studies have shown the influence of both soil initial conditions and blade operating conditions on cutting forces. However, most of these finite element analyses (FEA) are limited to small blade displacements to reduce element distortion which can cause solution convergence problems. In this study a dynamic three-dimensional FEA of soil–tool interaction was carried out based on predefined failure surfaces to investigate the effect of cutting speed and angle on cutting forces over large blade displacements. Sandy soil was considered in this study and modeled using the hypoplastic constitutive model implemented in the commercial FEA package, ABAQUS. Results reveal the validity of the concept of predefined failure surfaces in simulating soil–tool interaction and the significant effect of cutting acceleration on cutting forces.     

A dynamic model for soil cutting by blade and tine

A dynamic model for soil cutting resistance prediction by blade and tine was developed, taking account of shear rate effects both on soil shear strength and soil-metal friction, besides the conventional soil slice inertia, for both brittle and flow failure of soil. The model was verified with a series of tests in a soil bin with a blade and a tine, and the results were acceptable.     

不同宽厚比H形截面钢梁抗冲击性能

基于经实验校核的非线性有限元模型,对受横向冲击作用的H形钢梁进行了有限元分析。设计不同宽厚比组配的H形钢梁,分析H形钢梁跨中受横向冲击的动态响应和应力发展过程,并研究宽厚比对H形钢梁抗冲击性能的影响,重点讨论了腹板厚度、翼缘厚度对冲击力平台值和峰值以及耗能的影响。分析结果表明,两端铰接H形钢梁在跨中受冲击载荷作用下的变形模式主要为弯曲变形。相同冲击能量下,冲击力平台值主要受翼缘厚度的影响,冲击力峰值主要受腹板厚度的影响。翼缘厚度对钢梁抗冲击性能的影响要大于腹板厚度。本研究可为不同宽厚比H形钢梁的抗冲击设计提供依据和参考。     

A model of stress distribution and cracking in cohesive soils produced by simple tillage implements

The objective of this research was to describe the mechanical behavior of soil under the action of a tillage tool, with the purpose of finding a relation between the tool geometry and the resultant soil structure. The problem was solved using fundamental principles of soil mechanics and force equilibrium analysis. As a result, a mathematical model was developed which describes three stress zones within the cut soil volume: shear failure, tension failure, and no internal failure. The model was programmed into a computer to generate maps of normal and shear stresses to visualize the three failure zones. The model was tested by cutting soil with flat tillage tools in a laboratory soil bin, and it proved to provide reliable predictions of the pattern of soil shear and tension failure.     

含材料非线性的复合材料单钉接头累积损伤分析

发展了静拉伸复合材料接头层合板三维逐渐损伤模型,考虑了单层复合材料在材料1-2面及3-1面上具有明显非线性剪切应力-应变关系的叠层非线性效应,结合有限元技术即应力分析、失效判定准则及损伤过程中材料性能退化等,对接头层合板损伤扩展进行了模拟,结果表明考虑材料非线性的影响与实验结果吻合更好.     

Theoretical analysis of the spatial variability in tillage forces for fatigue analysis of tillage machines

This paper presents a new theoretical model to describe the spatial variability in tillage forces for the purpose of fatigue analysis of tillage machines. The proposed model took into account both the variability in tillage system parameters (soil engineering properties, tool design parameters and operational conditions) and the cyclic effects of mechanical behavior of the soil during failure ahead of tillage tools on the spatial variability in tillage forces. The stress-based fatigue life approach was used to determine the life time of tillage machines, based on the fact that the applied stress on tillage machines is primarily within the elastic range of the material. Stress cycles with their mean values and amplitudes were determined by the rainflow algorithm. The damage friction caused by each cycle of stress was computed according to the Soderberg criterion and the total damage was calculated by the Miner’s law. The proposed model was applied to determine the spatial variability in tillage forces on the shank of a chisel plough. The equivalent stress history resulted from these forces were calculated by means of a finite element model and the Von misses criterion. The histograms of mean stress and stress amplitude obtained by the rainflow algorithm showed significant dispersions. Although the equivalent stress is smaller than the yield stress of the material, the failure by fatigue will occur after a certain travel distance. The expected distance to failure was found to be d_f = 0.825 × 10⁶ km. It is concluded that the spatial variability in tillage forces has significant effect on the life time of tillage machines and should be considered in the design analysis of tillage machines to predict the life time. Further investigations are required to correlate the results achieved by the proposed model with field tests and to validate the proposed assumptions to model the spatial variability in tillage forces.     

A large-scale finite element point-connector model for self-piercing rivet connections

A new resultant-based point-connector model has been developed for use with large-scale finite element crash simulations for non-linear explicit finite element analysis. The nature of the model has been based on the observed physical failure behaviour of a self-piercing rivet connecting two aluminium sheets. The model is able to describe the complete force-deformation behaviour of the SPR connection from initial loading until failure. The paper gives an overview of the analytical background of the model and shows how the model parameters can be identified from experiments using a reverse engineering approach.     

钢筋混凝土双肢剪力墙静力弹塑性分析

建立了由墙肢单元模型、连粱单元模型和连接单元模型组成的钢筋混凝土双肢剪力墙的静力弹塑性分析计算模型。墙肢单元采用以有限元为基础的宏模型;按是否出现对角线剪切破坏,分别建立短连粱计算模型和长连粱计算模型;为计及连粱与墙肢连接界面的相对位移,建立用复合弹簧模拟的连接单元计算模型;给出了确定模型参数的方法。对有关文献的短连粱和长连粱双肢剪力墙试件进行了静力弹塑性分析,分析结果与试验结果符合较好。     

Materially and geometrically non-linear woven composite micro-mechanical model with failure for finite element simulations

A computational micro-mechanical material model of woven fabric composite material is developed to simulate failure. The material model is based on repeated unit cell approach. The fiber reorientation is accounted for in the effective stiffness calculation. Material non-linearity due to the shear stresses in the impregnated yarns and the matrix material is included in the model. Micro-mechanical failure criteria determine the stiffness degradation for the constituent materials. The developed material model with failure is programmed as user-defined sub-routine in the LS-DYNA finite element code with explicit time integration. The code is used to simulate the failure behavior of woven composite structures. The results of finite element simulations are compared with available test results. The model shows good agreement with the experimental results and good computational efficiency required for finite element simulations of woven composite structures.     

纤维增强复合材料风机叶片宏细观一体化有限元分析An unified FEM for macroscopic and mesoscopic analysis of fiber reinforced composite fan blades

为了建立纤维增强复合材料风机叶片宏观性能和细观组分的直接关联,得到一般有限元分析时无法获得的细观参量值,利用FORTRAN程序把细观力学的失效／损伤分析模块,嵌入到有限元软件ABAQUS中的USD‐FLD 用户子程序中,建立了风机叶片宏细观一体化模型。该模型能够实现基于细观组分级损伤／失效判据的宏细观渐进损伤分析和强度预报功能。该模型计算结果与文献中的试验结果有较好的一致性。     

局部有限元模型边界条件的直接传递方法

提出一种将整体分析得到的节点力或节点位移直接传递到精细化局部有限元模型的方法,即部分混合单元法。沿精细化局部有限元模型周边建立一组过渡单元,该组过渡单元采用与整体模型一致的单元类型和模拟方式,其外侧边界上的节点与整体模型节点的相对坐标对应,内侧边界与精细化局部有限元模型采用基于面约束的方式连接。在外侧边界上根据节点坐标对应施加整体分析获得的节点力或节点位移,过渡单元就可直接将边界条件传递到精细化局部有限元模型。通过贵州红水河特大桥钢-混结合段的精细化有限元分析,验证了本文方法的实用性和有效性。     

Finite element analysis of the piezocone test in cohesive soils using an elastoplastic–viscoplastic model and updated Lagrangian formulation

In this research, the finite element analysis of piezocone penetration has been conducted using the elastoplastic–viscoplastic bounding surface model in the updated Lagrangian reference frame. A finite element formulation has been performed considering the viscoplastic contribution of the model and the theory of mixtures has been incorporated to explain the behavior of the soil. The formulated model has been implemented into a finite element program, EPVPCS-S (elastoplastic–viscoplastic coupled system-soil), to analyze the mechanism of piezocone penetration. The results of the finite element analysis have been compared and investigated with the experimental results from the piezocone penetration and dissipation tests conducted using LSU/CALCHAS (Louisiana State University Calibration Chamber System).     

Theoretical analysis of soil reaction on a lug of the movable lug cage wheel

The measurement of soil reaction forces on a lug of a movable lug cage wheel was carried out in a soil bin. To elucidate the experimental results, a theoretical analysis of soil reaction forces on the lug of the movable lug cage wheel was made by using an analysis of the lug trajectory and a modified theory in soil–vehicle mechanics. The existing theory was modified and adjusted by considering the actual lug trajectory and the soil trench made by the preceding lug. The results showed that the theoretical analysis gave a good representation of the reaction forces measured experimentally. The higher pull and lift forces of the movable lug cage wheel compared with those of the fixed lug wheel was supported by the theoretical analysis. Although the theoretical representation of soil reaction forces should be improved by further works, it is sufficiently accurate to estimate the performance of the movable lug cage wheels by the proposed theory.     

Tyre flexibility and mobility on soft soils

Five model tyres were tested in the soil bin to investigate the effects of wheel flexibility on the tyre-soil performances. Two different soil types were used together with various inflation pressures which governed the tyre flexibility. The results confirm that tyre flexibility contributes significantly to the development of all the energy components [equation (1)] in the tyre-soil system. As can be seen from the contrasting performances shown, increasing the inflation pressure may allow for a favourable increase in the drawbar pull in one soil (frictional soil) so long as the input energy available can be increased, whilst the reverse may be true in the case of the other (clay) soil. The finite element model used satisfactorily confirms the measered values obtained and is seen to be able to account for tyre flexibility as shown in Figs. 11–14.     

Numerical simulation and testing verification of the interaction between track and sandy ground based on discrete element method

Tractive effort of tracked vehicles plays an important role in military and agricultural fields. In order to solve the problem of low precision in numerical simulation of the interaction between track and sandy ground, a systematic and accurate discrete element modeling method for sandy road was proposed. The sandy ground was modeled according to the mechanical parameters measured by soil mechanics tests. The interaction coefficients of sandy soil were measured by the repose angle test and triaxial compression test combined with the corresponding simulation. On this basis, a discrete element interaction model of track-sandy ground was established, which can be used to test the tractive effort of track. Numerical simulation calculation of track model at different speeds was carried out, and the simulation results were compared with the results of indoor soil bin test for verification. The verification results show that the interaction between track and sandy ground based on DEM simulation is consistent with the actual soil bin test. The discrete element modeling method in this paper can be used to model the track and sandy ground accurately, and the simulation model can be used to test the tractive effort of tracked vehicle.     

Non-linear finite element analysis of cone penetration in layered sandy loam soil – Considering precompression stress state

Axisymmetric finite element (FE) method was developed to simulate cone penetration process in layered granular soil. The FE was modeled using ABAQUS/Explicit, a commercially available package. Soil was considered as a non-linear elastic plastic material which was modeled using variable elastic parameters of Young’s Modulus and Poisson’s ratio and Drucker–Prager criterion with yield stress dependent material hardening property. The material hardening parameters of the model were estimated from the USDA-ARS National Soil Dynamics Laboratory – Auburn University (NSDL-AU) soil compaction model. The stress–strain relationship in the NSDLAU compaction model was modified to account for the different soil moisture conditions and the influence of precompression stress states of the soil layers. A surface contact pair (‘slave-master’) algorithm in ABAQUS/Explicit was used to simulate the insertion of a rigid cone (RAX2 ABAQUS element) into deformable and layered soil medium (CAX4R ABAQUS element). The FE formulation was verified using cone penetration data collected on a soil chamber of Norfolk sandy loam soil which was prepared in two compaction treatments that varied in bulk density in the hardpan layer of (1) 1.64 Mg m⁻³ and (2) 1.71 Mg m⁻³. The FE model successfully simulated the trend of cone penetration in layered soils indicating the location of the sub-soil compacted (hardpan) layer and peak cone penetration resistance. Modification of the NSDL-AU model to account for the actual soil moisture content and inclusion of the influence of precompression stress into the strain behavior of the NSDL-AU model improved the performance of FE in predicting the peak cone penetration resistance. Modification of the NSDL-AU model resulted in an improvement of about 42% in the finite element-predicted soil cone penetration forces compared with the FE results that used the NSDL-AU ‘virgin’ model.