ALBERT

Description:    A general model is presented to unify the explanation of different meta-heuristic algorithms. This model is based on the concept of fields of forces from physics and covers many meta-heuristic algorithms consisting of Genetic Algorithms, Ant Colony Optimization, Particle Swarm Optimization, Big Bang–Big Crunch algorithm and Harmony Search. The properties of these algorithms can be explained using the presented general model that is called the fields of forces (FOF) model. This extension provides efficient means to improve, expand, modify and hybridize the meta-heuristic algorithms. An improved and hybridized algorithm is then developed using the FOF model. Content Type Journal Article Pages 1-20 DOI 10.1007/s00707-011-0494-y Authors A. Kaveh, Centre of Excellence for Fundamental Studies in Structural Engineering, Iran University of Science and Technology, Narmak, Tehran-16, Iran S. Talatahari, Department of Civil Engineering, University of Tabriz, Tabriz, Iran Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    An analytical approach for evaluating the forced vibration response of uniform beams with an arbitrary number of open edge cracks excited by a concentrated moving load is developed in this research. For this purpose, the cracked beam is modeled using beam segments connected by rotational massless linear elastic springs with sectional flexibility, and each segment of the continuous beam is assumed to satisfy Timoshenko beam theory. In this method, the equivalent spring stiffness does not depend on the frequency of vibration and is obtained from fracture mechanics. Considering suitable compatibility requirements at cracked sections and corresponding boundary conditions, characteristic equations of free vibration response are derived. Then, forced vibration response is treated under a moving load with a constant velocity. Using the determined eigenfunctions, the forced vibration response may be obtained by the modal superposition method. Finally, some parametric studies are presented to show the effects of crack parameters and moving load velocity. Content Type Journal Article Pages 1-19 DOI 10.1007/s00707-011-0495-x Authors M. Shafiei, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, P.O. Box 14115–397, Tehran, Iran N. Khaji, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, P.O. Box 14115–397, Tehran, Iran Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    A 3-D, dynamic, non-linear Finite Element Model (FEM) involving contact is used to determine apparent coefficients of friction,  μ , for the interaction between the bristles of a flicking brush for street sweeping and concrete. Also, brush contact arcs, as well as suitable brush penetrations as functions of the debris height, are determined for cutting and F128 brushes. Available experimental results are used to determine the coefficients of friction. The bristles are modelled through beam elements, which are subjected to inertia forces. The results suggest that the model is valid and that the coefficient of friction affects the brush vertical force in the flicking brush, but it does not significantly affect its counterpart in the cutting brush. The results also indicate that the coefficient of friction for the flicking brush depends on the sliding velocity, v , and these were fitted to an exponential  μ − v curve. Content Type Journal Article Pages 1-14 DOI 10.1007/s00707-011-0490-2 Authors L. V. Vanegas-Useche, Facultad de Ingeniería Mecánica, Universidad Tecnológica de Pereira, La Julita, Pereira, Colombia M. M. Abdel-Wahab, Department of Mechanical Construction and Production, Faculty of Engineering, Ghent University, 9000 Ghent, Belgium G. A. Parker, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey, GU2 7XH UK Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    In this article, we use the small strain micromorphic plasticity (MP) to study the wedge indentation of a thin film on a substrate and find qualitative agreement with experiments. A two-dimensional plane strain finite element formulation of the entire MP theory framework is outlined. The generalization of the radial return method for modeling the elasto-plastic deformation is presented. The numerical results show that the MP theory is capable of describing the initial fall in hardness at small depth of indentation and then the rise at larger depth for a soft film on a hard substrate. The indentation force and hardness increase with decreasing film thickness for a given depth. It is also shown that the hardness falls monotonically as the indentation depth increases and never approaches a constant for a hard film on a soft substrate. Contrary to the soft film/hard substrate system, the force and hardness diminish with decreasing film thickness for a given depth. Besides, the influences of internal length scale and hardening modulus of the film on hardness predictions are investigated. Content Type Journal Article Pages 1-13 DOI 10.1007/s00707-011-0483-1 Authors Z. H. Zhang, Applied Mechanics Lab., School of Aerospace, Tsinghua University, Beijing, 100084 China Z. L. Liu, Applied Mechanics Lab., School of Aerospace, Tsinghua University, Beijing, 100084 China X. M. Liu, Applied Mechanics Lab., School of Aerospace, Tsinghua University, Beijing, 100084 China Y. Gao, Applied Mechanics Lab., School of Aerospace, Tsinghua University, Beijing, 100084 China Z. Zhuang, Applied Mechanics Lab., School of Aerospace, Tsinghua University, Beijing, 100084 China Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    We study the initiation and propagation of a vertical crack in an elastic semi-infinite plate, reinforced on its boundary by an infinite discontinuous stringer within the limits of the theory of brittle failure. The plate is subjected to uniform distributed tensile forces at infinity, as well as to contact stresses due to application of forces to the stringer. We find the appropriate loading of the coherent stringer, and consequently we consider a problem where the stringer is cracked and a vertical crack has developed within the plate. We deduce the exact analytical solution for the principal singular integral equation for this case; hence the stringer is perfectly rigid and we calculate characteristic parameters of the problem. The results show that the crack tip has a logarithmic singularity, and the tangential contact stresses under the stringer at that end point are finite and generally differ from zero. Content Type Journal Article Pages 1-28 DOI 10.1007/s00707-011-0473-3 Authors Dimosthenis I. Bardzokas, National Technical University of Athens, Athens, Greece S. M. Mkhitaryan, Institute of Mechanics, National Academy of Sciences of Republic of Armenia, 24b Marshal Baghramianave., Erevan, 0019 Republic of Armenia Georgios I. Sfyris, National Technical University of Athens, Athens, Greece Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    This paper discusses the effect of shear deformation of the cross-section on the contact behavior of a buckled Timoshenko beam in frictionless contact with a rigid plane wall. The deformation is assumed to be small, and only planar deformation is considered. The results are compared with those from the Euler beam model. In the case when a buckled Euler beam is in line contact with a plane wall, the plane wall exerts concentrated forces at the boundary points of the line-contact segment but no distributed force. In the case of a Timoshenko beam, on the other hand, the concentrated forces are replaced by a distributed contact force. The distribution of the contact force approaches the two concentrated forces at the boundary points when the shear effect is negligible. The line-contact deformation may become unstable when the edge thrust reaches a secondary buckling load. It is found that the normalized critical length of the contact segment is always smaller when the shear effect is more significant. On the other hand, the normalized secondary buckling load reaches a maximum when the slenderness ratio (beam length over thickness) is in the order of 45. Content Type Journal Article Pages 1-8 DOI 10.1007/s00707-011-0529-4 Authors Jen-San Chen, Department of Mechanical Engineering, National Taiwan University, Taipei, 10617 Taiwan Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    The first passage failure of multi-degree-of-freedom (MDOF) quasi-integrable Hamiltonian systems with damping described by a fractional derivative is studied. The stochastic averaging procedure is applied to derive the averaged equations for first integrals. The conditional reliability function and the conditional mean of first passage failure time are obtained by solving the associated backward Kolmogorv equation and Pontryagin equation together with suitable boundary conditions and initial condition, respectively. One example of two coupled nonlinear oscillators with fractional derivative damping is given to illustrate the proposed procedure. The accuracy of the method is substantiated by comparing the analytical results with those from Monte Carlo simulation. Effects of some parameters of fractional order, damping coefficients and nonlinear strength on the system’s reliability are examined. Content Type Journal Article Pages 1-16 DOI 10.1007/s00707-011-0533-8 Authors Lincong Chen, College of Civil Engineering, Huaqiao University, Xiamen, 361021 China Qingqu Zhuang, College of Mathematical Science, Huaqiao University, Quanzhou, 362021 China Weiqiu Zhu, Department of Mechanics, State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University, Hangzhou, 310027 China Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    This paper presents the stochastic elastic modulus reduction method for system reliability analysis of spatial variance frames based on the perturbation stochastic finite element method (PSFEM) and the local average of a random field. The stochastic responses and reliability index of each element of a structural frame are characterized by the PSFEM and the first-order second-moment method, to properly handle the correlation structures and scale of fluctuation of random fields. A strategy of elastic modulus adjustment for the estimation of system reliability is developed to determine the range and magnitude of elastic modulus reduction, by taking the element reliability index as a governing parameter. The collapse mechanism and system reliability index of a stochastic framed structure are determined through iterative computations of the PSFEM. Compared with the failure mode approaches in traditional system reliability analysis, the proposed method avoids two major difficulties, namely the identification of significant failure modes and estimation of the joint probability of failure modes. The influences of the correlation structure and scale of fluctuation of the random field upon system reliability are investigated to demonstrate the accuracy and computational efficiency of the proposed methodology in system reliability analysis of spatial variance frames. Content Type Journal Article Pages 1-16 DOI 10.1007/s00707-011-0546-3 Authors Lu Feng Yang, Key Laboratory of Disaster Prevention and Structural Safety of Ministry of Education, School of Civil Engineering and Architecture, Guangxi University, Nanning, 530004 China Bo Yu, Key Laboratory of Disaster Prevention and Structural Safety of Ministry of Education, School of Civil Engineering and Architecture, Guangxi University, Nanning, 530004 China J. Woody Ju, Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    The large amplitude free vibration of a laminated composite parabolic plate with parabolically orthotropic plies is investigated for the first time. The effects of out-of-plane shear deformations, rotary inertia, and geometrical nonlinearity are taken into account. The geometry of the plate is described, and the analysis performed in the parabolic coordinate system. The problem is solved numerically using a new parabolic hierarchical finite element. The nonlinear equations of free motion are mapped from the time domain into the frequency domain using the harmonic balance method. The resultant nonlinear equations are solved iteratively using the linearized updated mode method. Results for the fundamental linear and nonlinear frequencies are obtained for symmetric and antisymmetric laminates with clamped and simply supported edges. Comparisons are made with the finite element method for clamped and free isotropic parabolic plates and show excellent agreement. The aspect ratio, thickness ratio, moduli ratio, number of plies, layup sequence, and boundary conditions are shown to affect the hardening behavior. Content Type Journal Article Pages 1-16 DOI 10.1007/s00707-011-0550-7 Authors Abderrahim Houmat, Faculty of Engineering, Department of Mechanical Engineering, University of Tlemcen, B.P. 230, 13000 Tlemcen, Algeria Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970

Description:    This paper deals with the formulation of an analytical model for the dynamic behavior of anisotropic plates, with an arbitrarily located internal line hinge with elastic supports and piecewise smooth boundaries elastically restrained against rotation and translation among other complicating effects. The equations of motion and its associated boundary and transition conditions are derived using Hamilton’s principle. By introducing an adequate change of variables, the energies that correspond to the different elastic restraints are handled in a general framework. The concept of transition conditions and the determination of the analytical expressions are presented. Analytical examples are worked out to illustrate the range of applications of the developed analytical model. One of the essential features of this work is to demonstrate how the commonly formal derivations used in the applications of the calculus of variations can be made rigorous. Content Type Journal Article Pages 1-20 DOI 10.1007/s00707-011-0552-5 Authors Ricardo Oscar Grossi, INIQUI-CONICET, Facultad de Ingeniería, Universidad Nacional de Salta, Avenida Bolivia 5150, Salta, República Argentina Journal Acta Mechanica Online ISSN 1619-6937 Print ISSN 0001-5970