Failure of a bus hoisting platform due to unexpected system behaviour

A bus hoisting platform showed an unexpected behaviour under asymmetric loading. In order to assess the internal restraints and joint forces which had not been considered properly in initial design, a numerical approach was chosen. With a multi-body simulation tool, the loads were calculated for different system configurations and loading situations. The identified loads were used for the stress analysis of individual components (analytical and numerical with FEM). The results revealed joint forces of a higher order than the total external load, which was a consequence of first the underestimated internal loads due to unapparent horizontal force components in the statically indeterminate system, and second, the implemented force control of the hydraulic actuators working in parallel. The correctly identified high internal loads explained the peculiar observations and lead to an insufficient fatigue strength of the welded structure.     

Optimum in situ strength design of laminates under combined mechanical and thermal loads

In this paper, optimum laminate configurations are sought for multidirectional fibre-reinforced composite laminates under combined in-plane mechanical and thermal loads. The design objective is to enhance the value of the loads over and above the first-ply-failure loads which are judged by a transverse failure criterion and the Tsai-Hill criterion, respectively. The in situ strength parameters previously obtained are incorporated in these criteria. It is found that the optimum designs under combined mechanical and thermal loads are not the same as those under pure mechanical loads for three of the four loading cases studied. For all cases the optimum loads are significantly larger than those for a quasi-isotropic design.     

Buckling characteristics and layup optimization of long laminated composite cylindrical shells subjected to combined loads using lamination parameters

The buckling characteristics and layup optimization of long laminated composite cylindrical shells subjected to combined loads of axial compression and torsion are examined on the basis of Flügge’s theory. In the buckling analysis of long laminated composite cylindrical shells, 12 lamination parameters are introduced and used as design variables for layup optimization. Applying a variational approach, the feasible region in the design space of the 12 lamination parameters is numerically obtained. The buckling characteristics are discussed in the design space of the 12 lamination parameters. In the layup optimization, the optimum lamination parameters for maximizing the buckling loads and the laminate configurations for realizing the optimum lamination parameters are determined by mathematical programming methods. It is found that in case of combined loads of axial compression and torsion, the optimum laminate configurations are unsymmetric.     

Static and dynamic analysis of wind turbine blades using the finite element method

Analysis of forces and stresses of horizontal axis propellers and turbines has been the subject of increased interest in recent years because of the need to develop adequate analytical tools for the design and evaluation of-wind turbine rotors. Most of the structural failures of wind turbines occur in the blade root section. Hence, a three-dimensional analytical model to compute the deflection, stresses and eigenvalues in the rotor blades is proposed using bending triangular plate finite elements. Both membrane and bending stiffness are considered in deriving the element stiffness matrix. The consistent mass matrix is used in generating the overall mass matrix. Lift and drag forces created in steady wind conditions are analysed as normal and tangential forces on the blade sections at certain angles of attack. These forces are applied as boundary loads to the computer program to analyse statically and dynamically rotor blades of symmetrical aerofoil NACA 0015 series. Constant chord, tapered and twisted blades were analysed at rated and survival wind speeds. The validity of the computer program used was verified by applying it to a standard cantilever box beam using the beam theory. The results showed that maximum stresses occurred at the root of the blades for all configurations in the spanwise direction and that a tapered blade, in addition to saving material weight, diminished the stresses obtained. The twisting of the blade leads to an increase in stiffness and a decrease in the stresses.     

Dynamic Stability Optimization of Laminated Composite Plates under Combined Boundary Loading

Dynamic stability and design optimization of laminated simply supported plates under planar conservative boundary loads are investigated in current study. Examples can be found in internal connecting elements of spacecraft and aerospace structures subjected to edge axial and shear loads. Designation of such elements is function of layup configuration, plate aspect ratio, loading combinations, and layup thickness. An optimum design aims maximum stability load satisfying a predefined stable vibration frequency. The interaction between compound loading and layup angle parameter affects the order of merging vibration modes and may stabilize the dynamic response. Laminated plates are assumed to be angle-plies symmetric to mid-plane surface. Dynamic equilibrium PDE has been solved using kernel integral transformation for modal frequency values and eigenvalue-based orthogonal functions for critical stability loads. The dictating dynamic stability mode is shown to be controlled by geometric stiffness distributions of composite plates. Solution of presented design optimization problem has been done using analytical approach combined with interior penalty multiplier algorithm. The results are verified by FEA approach and stability zones of original and optimized plates are stated as final data. Presented method can help designers to stabilize the dynamic response of composite plates by selecting an optimized layup orientation and thickness for prescribed design circumstances.     

Stability analysis of planar equilibrium configurations of elastic rods subjected to end loads

A semi-analytical method is proposed for investigating the stability of planar equilibrium configurations of an inextensible elastic rod under end-loading conditions. The method is based on representing the second variation of the constrained strain energy of the rod as a diagonal quadratic form using the eigensolutions of an auxiliary Sturm–Liouville problem. The coefficients of the resulting form which determine the sign of the second variation are analyzed by numerically solving an initial-value problem. Examples of curvilinear configurations of rods and a circular ring under point loads are considered and their stability is analyzed using the proposed method.     

Bond characteristics of carbon and stainless steel flat rebars with an alternate rib pattern

The good performance of reinforced concrete depends on the appropriate transfer of forces between reinforcing rebars and concrete, which relies on the bond interaction between the two materials. At an uncracked section, both materials work together by means of the bond forces; however, if the tensile strength of the concrete is reached at a certain part of the structure, a crack will appear and the steel will be the only active element at the cracked section. At increasing loads, the crack will continue opening and large crack openings may lead to a failure of the rebar and to the collapse of the whole structure. A new idea to positively influence the cracking behaviour has emerged, which is based on the combination of smooth and rib zones within the same reinforcing rebar. Furthermore, use of stainless steel flat reinforcement has been considered as an option to optimize the reinforcing of shallow slabs. This paper presents bond tests performed on carbon and stainless steel flat reinforcements embedded in concrete and with different alternate rib configurations. Test results are presented in terms of bond strength and force transfer stiffness, as well as in terms of bond stress–slip relationship. Results show no differences between the bond capacity of carbon and stainless steel rebars if other parameters are kept constant. The use of an alternate surface configuration combining smooth and ribbed zones within the bond length, does affect the bond capacity of the rebar, and the position of the smooth zone within the bond length plays an important role.     

Tests of deck-to-hull adhesive joints in GFRP boats

A test program has been designed for the testing of several deck-to-hull adhesive joints in order to assess the strength and performance while in-service conditions, and the dynamic behavior under cyclic loads of the adhesive bonding in the presence of a debonded area. The tests have also been used for benchmarking of several different adhesive systems as potential candidates to be used in these joints. The fundamental aim has been to reproduce the in-service dynamic loads acting on the deck-to-hull adhesive joint in a suitable test specimen. In the presence of a pre-existing debonding, loads on the adhesive correspond to a mixed modes (modes I and II) fracture situation. A specific load fixture has been developed and used for the testing of the specimens in the mixed mode. Material and manufacturing process are representative of the actual procedure in the yacht industry. The joint design is not assumed to be the optimum for this application, but it has been chosen for the sake of simplicity during testing. Quasi-static tests for the determination of the initial and residual strengths have shown that we cannot point out only one adhesive system as the ideal candidate in all the aspects. All of them have strong and weak features, and the final selection has to be done within the scope of the final performance needed for a certain application. Fatigue tests have not shown specimen failure during the number of cycles selected. Strengths of the damaged specimens have shown in most cases a negligible reduction respect to the undamaged initial situation.     

Deformation and tearing of circular plates with varying support conditions under uniform impulsive loads

Transient dynamic finite element analysis of circular plates with varying support configurations under uniform single square wave form impulsive load has been carried out in FEA package ANSYS. Experimental results of Teeling-Smith and Nurick [The deformation and tearing of thin circular plates subjected to impulsive loads. Int J Impact Eng 1991;11(1):77–91] and Nurick et al. [Tearing of blast loaded plates with clamped boundary conditions. Int J Impact Eng 1996;18(7–8):803–27] for the onset of thinning and tearing at the boundary of clamped circular plates subjected to uniformly loaded air blasts have been used to compare and validate the numerical simulation and procedure. The Mode II failure with respect to clamped circular plates has been simulated using a rupture strain criteria. Mode III failure or plastic shear sliding, has been considered using a shear strain failure criteria as proposed by Wen and Jones for plates. A stiffness reduction scheme has been proposed to decide on the initiation and progression of tearing in conjunction with suitable failure model under Modes II and III. The evolution of deflections, plastic zones, rupture zones and failure modes under the blast loading conditions are found to match well with the experimental results. The validated numerical model has further been used to study the effect of plate thickness on the deformation and tearing response of the circular plates subjected to impulsive loads. The deformation, tearing and shock absorption response of clamped circular plates under uniform impulsive loads with ring support of varying edge configurations at the boundary have also been numerically studied. Further, the response of circular plate–tube combination with varying boundary support configurations has been studied. The plate has been considered at the mid-span of the tube of length equal to the plate diameter with the ends of the tube modelled as clamped. The numerical model has been used to study the effect of tube thickness variations on the deformation and tearing response of the circular plate under shock loads. The response of tube–plate combinations under uniform impulsive loads with ring support at the plate–tube junction have also been numerically studied.     

Transfer of impulsive loading on cladding panels to the fixing assemblies

The fixing assemblies for prefabricated cladding panels on buildings, are often designed on an empirical basis, to support the dead weight of the panel, wind loads and possibly temporary loads arising during construction. If, subsequently, the design loads are accidentally exceeded but the panel is undamaged; it may be necessary to check the condition of the fixings. A physical check would require costly dismantling and reconstruction of the panel but, as an alternative, this paper presents an analysis of the force transferred to cladding fixing assemblies when the panel is subjected to impulsive loading, using the finite element analysis program, DYNA3D. The results were validated with experimental results from impact tests on a steel plate supported on four steel bars. Having obtained this validation, a finite element analysis was then carried out to determine the response of fixings for panels under different levels of blast loading using two different models for the panel-fixing assemblies. The first model considers only the effect of out-of-plane fixing, whilst in the second model both the out-of-plane fixings and in-plane fixings are considered. It was observed that forces transferred to fixings include an axial force, shear forces, and bending moments and these force components can vary along the length of the fixing. Inspections of cladding panels, after being subjected to impulsive loads, often show that the connection between the fixing and the panel and between the fixing and the support structure are more vulnerable to impulsive loading than the panel itself. The FE analysis has shown that forces in the fixings are related to the dynamic response of the cladding panel; hence damage to the fixings could be deduced from damage to the panel.     

Driving Mechanism of a Superconducting Magnetic Bearing System

Driving the rotor of a superconducting magnetic bearing without mechanical contact in the optimum conditions is an important task for high operational speeds. For this reason, an alternative eddy current mechanism is proposed to drive the rotor by means of magnetism with a speed higher than that of the driver. The designed driving system provides strong and stable magnetic coupling between the relatively rotating parts. The drag and lift forces between the rotor and driver disc are discussed by considering various conditions, such as the rotor configurations and the saturation of the magnetic field within the conducting material. The overall results indicate that the designed electromechanical driving system has potential solutions for the various applications for magnetic bearings from the effective driving mechanism point of view.     

Rotating disk containing an internal crack located at an arbitrary position

A rotating disk containing an internal crack at an arbitrary position is treated by using the eigen function expansions of the complex stress potentials, and determining the unknown coefficients from the boundary conditions expressed in terms of the resultant forces. Numerical calculations are done for various configurations and convenient formulae for the stress intensity factors are presented.     

Arbitrary loading problems of doubly symmetric regions containing a central crack

This paper presents a method of analysis of doubly symmetric plates with a central crack subjected to symmetric concentrated forces at points on the outer boundary. The problem is treated by using complex stress functions appropriate for the traction free crack and by means of a kind of boundary collocation method. The method is also valid for arbitrary loading problems, since the applied loads can be replaced by concentrated forces so far as the crack tip stress intensity factors are concerned.Numerical examples are given for elliptical and rectangular plates and plates of other configurations of practical importance. Results of rectangular plates give informations on experimental estimation of crack tip stress intensity factors in complex structures.     

Forces and moments for electrodynamic levitation systems--Large-scale test results and theory

The dynamic circuit theory for the analysis of the suspension characteristics of electrodynamic magnetic levitation schemes for realistic magnet and guideway configurations is reviewed. Electrodynamic forces and moments have been measured on a large-scale stationary superconducting magnet interacting with an aluminum strip mounted on the rim of a 7.6-m diameter rotating test wheel. Good agreement is found between analytical and experimental results over the speed range to 100 km/h for the electrodynamic forces of test configurations which provide a rigorous test for theory. The speed dependence of moments is reported and used to estimate the force distribution on the superconducting coil. The dynamic circuit theory is then used to determine the suspension characteristics of the levitation system for the proposed Canadian Maglev vehicle.     

单层空间网格结构刚性节点优化设计方法研究

单层网格结构中的节点不仅须传递轴力，也须传递两个方向上的弯矩。因此，单层网格结构中的节点为受力复杂的刚性节点，应具有合理的结构形式和良好的力学性能。同时，单层网格结构节点连接复杂、高空施工难度大，节点与杆件应有可靠、方便、通用的连接方式。该文提出了单层网格结构刚性节点优化设计方法:通过构造设计节点端，实现节点与杆件的连接；通过对受力复杂的节点核进行拓扑优化，开发高效的受力节点形式。在优化模型中，通过与外荷载无关的自平衡力系表示节点弯曲刚度，使得优化后的节点在任意方向上均有良好的弯矩传递能力。为避免拓扑优化中将力直接施加在设计域而导致的应力奇异问题及节点形式受限的缺点，通过施加加速度场，以体积惯性力等效集中荷载，降低了优化结果对边界条件的敏感性，获得了稳定、可靠的优化结果，解决了刚性节点拓扑优化中的关键技术难点。并以一个实际结构中的节点为例进行优化设计。通过分析节点端连接性能、力学性能以及与传统节点进行对比，验证了将拓扑优化和构造设计相结合的刚性节点优化设计方法。     

Numerical simulation of gas-jet target in the laser-produced-plasma short-wave-radiation source

Numerical simulation of Xe gas jet, outflowing from a nozzle into the vacuum and considered as a target in the LPP radiation source, has been carried out. Different nozzle configurations and different gas conditions before the nozzle have been considered. A criterion based on the simulation results and describing observable plasma radiation intensity has been proposed. The criterion makes it possible to select an optimum configuration for a further experimentation. Results of the calculation are compared with earlier published experimental data.     

The optimum support of horizontal pressure vessels made from reinforced plastic

The deformation and stress state of horizontal pressure vessels are usually determined by the loads resulting from the support. The most favourable state can be realized only with a flexible support arch since its small flexural rigidity results in even distribution of the support pressure, while its small tensile rigidity provides for equal support forces. These facts have been proved by theoretical tests and experiments carried out on a pressure vessel made from reinforced plastic material and have resulted in the development of a structure providing for optimum support.     

Convergence and stability analysis of the deformable discrete element method

This work investigates numerical properties of the algorithm of the discrete element method (DEM) employing deformable circular disks presented in the authors' earlier publication. The new formulation called the deformable DEM (DDEM) enhances the standard DEM (SDEM) by introducing an additional (global) deformation mode caused by the stresses in the particles induced by the contact forces. An accurate computation of the contact forces would require an iterative solution of the implicit relationship between the contact forces and particle displacements. In order to preserve efficiency of the DEM, the new formulation has been adapted to the explicit time integration. It has been shown that the explicit DDEM algorithm is conditionally stable and there are two restrictions on its stability. Except for the limitation of the time step as in the SDEM, the stability in the DDEM is governed by the convergence criterion of the iterative solution of the contact forces. The convergence and stability limits have been determined analytically and numerically for selected regular and irregular configurations. It has also been found out that the critical time step in DDEM remains unchanged with respect to the SDEM.     

A reduced modal parameter based algorithm to estimate excitation forces from optimally placed accelerometers

This paper presents a time domain technique for estimating dynamic loads acting on a structure from acceleration time response measured experimentally at a finite number of optimally placed accelerometers on the structure. The technique utilizes model reduction to obtain precise load estimates. The structure essentially acts as its own load transducer. The approach is based on the standard equilibrium equations of motion in modal coordinates. The modal parameters of a system – natural frequencies, mode shapes and damping factors – can be estimated experimentally from measured data, analytically for simple problems, or using the finite element method. For measurement of the acceleration response, there can be a large number of locations on the structure where the accelerometers can be mounted, and the precision with which the applied loads are estimated from measured acceleration response may be strongly influenced by the locations selected for accelerometer placements. A solution approach, based on the construction of D-optimal designs, is presented to determine the number and optimum locations of accelerometers that will provide the most precise load estimates. An improvement in the algorithm, based on reduced modal matrix, is further proposed to reconstruct the input forces accurately. Numerical examples that help understand the main characteristics of the proposed approach are also presented. The numerical results illustrate the effectiveness of the proposed technique in accurately recovering the loads imposed on discrete as well as continuous systems.     

A finite element algorithm to study creep cracks based on the creep hardening surface

This work addresses finite element (FE) modelling of creep cracks under reversed and cyclic loads in steels. A constitutive model based on the creep hardening surface developed by Murakami and Ohno has been selected for this purpose. This model is particularly accurate for describing creep under reversed and cyclic loads and requires no additional material constants. An FE algorithm for this model has been derived and implemented into a research code FVP. The algorithm is verified by comparing the numerical predictions with closed form solutions for simple geometries and loading configurations. FE predictions are compared with experimental data for a stationary crack in a compact tension specimen. The stress and strain fields in the vicinity of a crack under a sustained load are compared with those for the intermediate unloading case. Several integral fracture parameters are investigated as to their appropriateness for describing creep cracks under reversed and cyclic loads.