Tuned vibration control of overhead line conductors

Linear and non‐linear theoretical and numerical analysis of ultimate response of overhead line conductors is treated in present paper. Interactive linear and non‐linear conditions in ultimate response are considered. Numerical solution of non‐linear problems appearing is made using the updated Lagrangian formulation of motion. Each step of the iteration approaches the solution of linear problem and the feasibility of the parallel processing FETM technique with adaptive mesh refinement and substructuring for non‐linear ultimate wave propagation and ultimate transient dynamic analysis is established. Some numerical results demonstrating current applicabilities and efficiency of procedures suggested are submitted. Copyright © 2000 John Wiley & Sons, Ltd.     

Fatigue control of thin‐walled structures

The control of the ultimate fatigue behaviour of slender thin‐walled structures is treated in the paper. A macro‐ and micromechanical simulation model adopting the neural network approach is used for the numerical analysis of the problem. The numerical treatment of the non‐linear problems is made using the updated Lagrangian formulation of motion combined with the pseudo‐force technique in the FETM‐approach. Each step of the iteration approaches the solution of the linear problem and the feasibility of the parallel processing and neural network numerical techniques is established. The application on the actual slender bridge is made in order to demonstrate the efficiency of the procedures suggested. Copyright © 2003 John Wiley & Sons, Ltd.     

Experimental characterization of fracture of glass fiber reinforced composites laminates subjected to freeze‐thaw cycles

The simultaneous effect of moisture and freeze‐thaw cycle on the mechanical behavior of glass/epoxy composites laminates is experimentally investigated. The study is planned in order to simulate the detrimental presence of humidity due to rainfall in surface damage of composite structures operating in cold weather. Different mechanisms governing the monotonic response of specimens subjected to freeze‐thaw cycles are pointed out by taking advantages of SEM images. Comparing SEM images taken from dry and wet specimens shows that the failure mechanisms such as matrix cracking and delamination are vastly activated around the notched region when the material is exposed to humidity and freeze‐thaw cycle. The load‐displacement response of examined specimens, namely the linear response, is remarkably altered under these conditions. A reduction of approximately 40% in ultimate load and 30% in slope of tangent line of load‐displacement curves is identified after 100 cycles of freeze‐thaw as well as more than 20% decrease of strain energy release rate.     

Optimization of stress modes by energy compatibility for 4‐node hybrid quadrilaterals

Optimal hybrid stress quadrilaterals can be obtained by adopting appropriate stresses and displacements, and satisfying the energy compatibility condition is shown to be an ultimate key to obtaining optimal stress modes. By using compatible isoparametric bilinear (Q₄) displacements and 5‐parameter energy compatible stresses of the combined hybrid finite element CH(0‐1), a robust 4‐node plane stress element ECQ₄ is derived. Equivalence to another hybrid stress element LQ₆ with 9‐parameter complete linear stresses based on a modified Hellinger–Reissner principle is established. A convergence analysis is given and numerical experiments show that elements ECQ₄/LQ₆ have high performance, i.e. are accurate at coarse meshes, insensitive to mesh distortions and free from locking. Copyright © 2003 John Wiley & Sons, Ltd.     

3D modelling of strong discontinuities in elastoplastic solids: fixed and rotating localization formulations

This paper is concerned with the incorporation of displacement discontinuities into a continuum theory of elastoplasticity for the modelling of localization processes such as cracking in brittle materials. Based on the strong discontinuity approach (SDA) (Computational Mechanics 1993; 12: 277–296) mesh objective 2D and 3D finite element formulations are developed using linear and quadratic 2D elements as well as 8‐noded 3D elements. In the formulation of the finite‐element model proposed in the paper, the analogy with standard formulations is emphasized. The parameter defining the amplitude of the displacement jump within the finite element is condensed out at the material level without employing the standard static condensation technique. This approach results in linearized constitutive equations formally identical to continuum models. Therefore, the standard return mapping algorithm is used to solve the non‐linear equations. In analogy to concepts used in continuum smeared crack models, a rotating formulation of the SDA is proposed in addition to the standard concept of fixed discontinuities. It is shown that the rotating localization approach reduces locking effects observed in analyses based on fixed localization directions. The applicability of the proposed SDA finite‐element model as well as its numerical performance is investigated by means of a three‐dimensional ultimate load analysis of a steel anchor embedded in a concrete block subjected to a shear force. Copyright © 2003 John Wiley & Sons, Ltd.     

The influence of post‐heat treatments on the tensile strength and surface hardness of selectively laser‐melted AlSi10Mg

To improve the mechanical properties of cast aluminium alloys several post‐heat treatments are known. However, these treatments cannot directly be transposed to additively via selective laser melting manufactured aluminium alloys, e. g., aluminium‐silicon‐magnesium (AlSi10Mg). Therefore, this study aims to determine suitable post‐heat treatments to optimise the mechanical properties of SLM‐built AlSi10Mg specimen. The influence of various post‐heat treatment conditions on the material characteristics was examined through hardness and tensile tests. The findings indicate that the Vickers hardness and ultimate tensile strength could not be improved via secondary precipitation hardening, whereas the fracture elongation shows a value which is distinctly higher than the values of a comparable cast alloy. Solution annealing at 525 °C reduces the hardness and the ultimate tensile strength by about 40 % and increases the fracture elongation three times. A subsequent precipitation hardening allows recovery of 80 % of the as‐built hardness, and 90 % of the previous ultimate tensile strength combined with maintaining an improved fracture elongation of about 35 % compared to the respective as‐fabricated condition.     

Determining the ultimate load of historic masonry arches by using eccentricity charts

The eccentricity charts presented in this paper have been developed on the basis of experimental investigations in order to enable a realistic calculation method of the ultimate load of flat brickwork vaulted floors with standard structural software. The vault is modelled as a three‐hinged arch with eccentric hinges in order to thus represent the non‐linear behaviour of the load‐bearing structure. Furthermore the hinge configuration, which is adapted with the eccentricity charts, takes into account the degree of plastification of historic masonry, existing load‐induced damage, any displacement of the abutments and the location of the thrust line. Two examples are described to explain the applicability of this method, and the results are compared with results from other modelling approaches. This makes clear that the eccentricity charts enable realistic structural analysis of flat brickwork vaults with various geometries and with highly efficient use of time.     

Optimization method for the determination of the most unfavorable imperfection of structures

The paper presents an optimization method for direct determination of the most unfavorable imperfection of structures by means of ultimate limit states. When analyzing imperfection sensitive structures it turns out that the choice of the shape and size of initial imperfections has a major influence on the response of the structure and its ultimate state. Within the optimization algorithm the objective function is constructed by means of a fully nonlinear direct and first order sensitivity analysis. The method is not limited to small imperfections and also allows the imposition of “technological” constraints on the shape of the imperfection, thus making it possible to avoid unrealistically low ultimate loads. When carefully constructed, the objective function and constraints remain linear enabling the use of numerically efficient and readily available linear programming algorithms. Imperfection analyses are shown for thin-walled girders and a cylinder to demonstrate the applicability and efficiency of the proposed method.     

An estimation of true Ramberg‐Osgood curve parameters for materials with and without Luder's strain using yield and ultimate strengths

Engineering tensile stress–strain curves for metallic materials typically show two different behaviours, namely, with Luder's strain and without Luder's strain. Luder's strain is more common for ductile materials, whereas high‐strength steels deform without Luder's strain. Usually, the stress–strain curves of ductile steels exhibit ultimate load where necking starts to develop. On the other hand, steels with low ductility exhibit monotonic increase of the applied load till failure without necking. Recently, Kamaya proposed a method to estimate the Ramberg‐Osgood relationship parameters for true stress–strain curves on the basis of conventional yield and ultimate strengths. This method can be not accurate enough for ductile materials exhibiting Luder's strain. Hence, a more general procedure for the materials exhibiting Luder's strain is proposed. In addition, an inverse method for assessing an ‘apparent ultimate tensile stress’ (akin to the ultimate stress of ductile materials at point of zero slope) for materials with low ductility (due to quenching or carburizing) is suggested.     

预制破片对厚壁圆管的横向高速冲击作用研究

为了获得厚壁圆管在横向高速冲击下的响应规律,进行了预制破片冲击圆管试验,得到不同冲击速度作用下圆管的响应模态及侵彻深度,并采用LS-DYNA对整个动态变化过程进行了仿真研究,获得了侵彻过程中预制破片的速度变化规律及圆管壁厚和预制破片长径比对极限穿透速度的影响规律。结果表明侵彻深度与冲击速度线性相关;圆管壁厚在7mm-8mm之间时对圆管极限穿透速度影响最大;预制破片长径比低于1.5时,对圆管极限穿透速度有显著影响,但其影响效果随自身的增大而逐渐削弱,当达到3.5左右时极限穿透速度不再变化。     

Coupling recognition of the structure and parameters of non‐linear constitutive material models using hybrid evolutionary algorithms

The structure of the non‐linear constitutive models is a key to control non‐linear behaviours of materials. Because the non‐linear mechanical mechanism is not clearly understood in most cases, it is very difficult to assume the structure of the model in advance. The recognition of the structure of the model from experimental results can help understanding of the mechanism. This recognition is a dynamic search problem being highly multimodal, multi‐variable with high order, and needing a large parameter space. How to obtain a global optimum solution is a key to this problem. In this paper, a hybrid evolutionary algorithm is proposed for coupling recognition of the structure of the non‐linear constitutive material model and its coefficients in global space using global response information, e.g. load vs deflection data, obtained from the structural test. Genetic programming is used to recognize the structure of the non‐linear stress–strain relationship without any assumption in advance and the genetic algorithm is then used to recognize its coefficients. The non‐linear stress–strain relationship thus found can not only satisfy the dynamic change in its structure but also its variables and coefficients. Non‐linear finite element analysis is used to transfer the load–deflection information to the stress–strain data. The potential of the proposed method is demonstrated by applying it to the macro‐mechanical modelling of the non‐linear behaviour of composite materials. A non‐linear material model for the unidirectional ply is recognized by using experimental data of a lamina plate [(±45)₆]_s. The obtained non‐linear constitutive model gave good predictions in coincidence with the non‐linear behaviours of the [(±30)₆]_s, [(0/±45)₃]_s and [(0/±45)₄]_s plates. The results indicate that the coupling non‐linear constitutive model of the structure and its coefficients can identify the model which the traditional constitutive model theory is unable to recognize. Copyright © 2004 John Wiley & Sons, Ltd.     

Some useful approximations for wrought aluminum alloys based on monotonic tensile properties and hardness

The objective of the present paper is to derive some useful approximations for estimating the strain‐controlled fatigue properties and cyclic deformation of wrought aluminum alloys from hardness and monotonic tensile properties. A variety of relationships and correlations among monotonic tensile properties, Brinell hardness, cyclic deformation and strain‐controlled fatigue properties are developed for wrought aluminum alloys. A simple method is proposed for prediction of the strain‐life curve requiring only ultimate tensile strength and modulus of elasticity. Prediction capability of the proposed method is evaluated for 25 kinds of wrought aluminum alloys with ultimate tensile strength between 120 MPa and 650 MPa. The proposed method provides good approximations of the strain‐life curve.     

潜孔钻机单支腿支撑平台受力分析

在潜孔钻机支撑平台结构设计时,存在工况考虑相对简单、忽视误操作导致极限工况等问题.为研究不同工况下平台的危险区域,详细地对潜孔钻机单支腿支撑的平台进行受力分析,确定正常和极限工况,基于力学平衡理论和变形相容条件,建立其力学模型,利用ANSYS对平台进行静力学有限元分析,得出各个工况下平台的最大应力、变形以及局部应力集中区域.结果表明:极限工况下,平台的最大应力、变形值均超过正常工况,且该结构大部分区域处于低应力状态,具有一定的静强度,符合工况的设计要求.研究结果为平台结构的优化提供参考.     

Evidence on the corrosion-induced hydrogen embrittlement of the 2024 aluminium alloy

The present work aims to provide evidence of corrosion‐induced hydrogen embrittlement of the aircraft aluminium alloy 2024. An extensive experimental investigation involving metallographic and fractographic analyses as well as mechanical testing was performed. The corrosion exposure led to a moderate reduction in yield and ultimate tensile stress and a dramatic reduction in tensile ductility. Metallographic investigation of the specimens revealed a hydrogen‐rich embrittled zone just below the corrosion layer. Furthermore, fractographic analyses showed an intergranular fracture at the specimen surface followed by a zone of quasi‐cleavage fracture and further below an entirely ductile fracture. Mechanical removal of the corroded layers restored the yield and ultimate stress almost to their initial values but not the tensile ductility. The tensile ductility was restored to the level of the uncorroded material only after heat treatment at 495°C. Measurement of hydrogen evolution with temperature showed that by heating the corroded alloy at 495°C, the trapped hydrogen is released.     

Numerical modelling of non‐linear electroelasticity

The numerical modelling of non‐linear electroelasticity is presented in this work. Based on well‐established basic equations of non‐linear electroelasticity a variational formulation is built and the finite element method is employed to solve the non‐linear electro‐mechanical coupling problem. Numerical examples are presented to show the accuracy of the implemented formulation. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of Stress‐Aging Processing on Precipitates and Mechanical Properties of a 7075 Aluminum Alloy

Two‐step stress‐aging tests, as well as pre‐treatment plus stress‐aging experiments, are performed on a 7075 aluminum (Al–Zn–Mg–Cu) alloy. Influences of stress‐aging parameters on mechanical behavior and fracture mechanism are investigated through uniaxial tensile test and fracture morphology analysis. It is revealed that the stress‐aging dramatically influences the mechanical properties and fracture characteristics of the studied alloy, which is contributed to the sensitivity of microstructures to stress‐aging. When the alloy undergoes two‐step stress‐aging, the ultimate tensile strength and yield strength first increase and then decrease with the increased first step stress‐aging temperature, while the elongation first decreases and then increases. For the retrogression pre‐treated plus stress‐aged alloy, the yield strength first increases and then drops with the increased retrogression pre‐treatment time, while the ultimate tensile strength almost remains stable. Furthermore, the elongation continuously increases with the increased retrogression pre‐treatment time. The observation of fracture morphology indicates that the dimple‐type intergranular fracture is the main fracture mechanism for the two‐step stress‐aged and retrogression pre‐treated plus stress‐aged alloys.     

Effects of approximations in analyses of beams of open thin‐walled cross‐section—part II: 3‐D non‐linear behaviour

In a companion paper, the effects of approximations in the flexural‐torsional stability analysis of beams was studied, and it was shown that a second‐order rotation matrix was sufficiently accurate for a flexural‐torsional stability analysis. However, the second‐order rotation matrix is not necessarily accurate in formulating finite element model for a 3‐D non‐linear analysis of thin‐walled beams of open cross‐section. The approximations in the second‐order rotation matrix may introduce ‘self‐straining’ due to superimposed rigid‐body motions, which may lead to physically incorrect predictions of the 3‐D non‐linear behaviour of beams. In a 3‐D non‐linear elastic–plastic analysis, numerical integration over the cross‐section is usually used to check the yield criterion and to calculate the stress increments, the stress resultants, the elastic–plastic stress–strain matrix and the tangent modulus matrix. A scheme of the arrangement of sampling points over the cross‐section that is not consistent with the strain distributions may lead to incorrect predictions of the 3‐D non‐linear elastic–plastic behaviour of beams. This paper investigates the effects of approximations on the 3‐D non‐linear analysis of beams. It is found that a finite element model for 3‐D non‐linear analysis based on the second‐order rotation matrix leads to over‐stiff predictions of the flexural‐torsional buckling and postbuckling response and to an overestimate of the maximum load‐carrying capacities of beams in some cases. To perform a correct 3‐D non‐linear analysis of beams, an accurate model of the rotations must be used. A scheme of the arrangement of sampling points over the cross‐section that is consistent with both the longitudinal normal and shear strain distributions is needed to predict the correct 3‐D non‐linear elastic–plastic behaviour of beams. Copyright © 2001 John Wiley & Sons, Ltd.     

A novel non‐linearly explicit second‐order accurate L‐stable methodology for finite deformation: hypoelastic/hypoelasto‐plastic structural dynamics problems

A novel non‐linearly explicit second‐order accurate L‐stable computational methodology for integrating the non‐linear equations of motion without non‐linear iterations during each time step, and the underlying implementation procedure is described. Emphasis is placed on illustrative non‐linear structural dynamics problems employing both total/updated Lagrangian formulations to handle finite deformation hypoelasticity/hypoelasto‐plasticity models in conjunction with a new explicit exact integration procedure for a particular rate form constitutive equation. Illustrative numerical examples are shown to demonstrate the robustness of the overall developments for non‐linear structural dynamics applications. Copyright © 2004 John Wiley & Sons, Ltd.     

A model updating strategy of non‐linear vibrating structures

The objective of this paper is to present a model updating strategy of non‐linear vibrating structures. Because modal analysis is no longer helpful in non‐linear structural dynamics, a special attention is devoted to the features extracted from the proper orthogonal decomposition and one of its non‐linear generalizations based on auto‐associative neural networks. The efficiency of the proposed procedure is illustrated using simulated data from a three‐dimensional portal frame. Copyright © 2004 John Wiley & Sons, Ltd.     

A discontinuous Galerkin formulation of non‐linear Kirchhoff–Love shells

Discontinuous Galerkin (DG) methods provide a means of weakly enforcing the continuity of the unknown‐field derivatives and have particular appeal in problems involving high‐order derivatives. This feature has previously been successfully exploited (Comput. Methods Appl. Mech. Eng. 2008; 197 :2901–2929) to develop a formulation of linear Kirchhoff–Love shells considering only the membrane and bending responses. In this proposed one‐field method—the displacements are the only unknowns, while the displacement field is continuous, the continuity in the displacement derivative between two elements is weakly enforced by recourse to a DG formulation. It is the purpose of the present paper to extend this formulation to finite deformations and non‐linear elastic behaviors. While the initial linear formulation was relying on the direct linear computation of the effective membrane stress and effective bending couple‐stress from the displacement field at the mid‐surface of the shell, the non‐linear formulation considered implies the evaluation of the general stress tensor across the shell thickness, leading to a reformulation of the internal forces of the shell. Nevertheless, since the interface terms resulting from the discontinuous Galerkin method involve only the resultant couple‐stress at the edges of the shells, the extension to non‐linear deformations is straightforward. Copyright © 2008 John Wiley & Sons, Ltd.