Modern Practice in Stress and Vibration Analysis VI

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Authors: J. Gong, C. Liu, P.P. Conway, Vadim V. Silberschmidt
Abstract: SnAgCu solder is a promising lead-free material for interconnections in electronic packages. However, its melting temperature (490°K) is considerably higher than that of the traditional SnPb solder (456°K). At the same time, SnAgCu has much better creep resistance at high temperature. These properties may cause large residual stresses during manufacturing processes due to the mismatch of thermal properties of electronic components that can influence the reliability of solder joints in electronic packages. This paper studies the residual stresses in solder joints in a flip chip package under different cooling conditions and their influence on the subsequent cyclic test by means of a finite element approach. The results show that the initial temperature of 453°K is high enough to induce residual stresses due to manufacturing procedures. Simulations, based on traditional creep-fatigue models, demonstrate that the residual stresses affect the mechanical behaviour of solder joints in several initial thermal cycles but have little effect on their reliability.
Authors: C. G. Dumitraş
Abstract: Due to robotic deburring development, the research gains a new orientation and focused on the cutting forces and the chip control. The present paper will emphasize the main difference which occurs between the normal cutting process and the deburring process, the way it develops and the parameters which characterize this process. Also the dynamics of the process are considered. Based on a central composite design one determine a relation between the geometry of the tool, workpiece hardness and cutting force.
Authors: E.M. Anawa, Abdul Ghani Olabi
Abstract: Welding dissimilar materials become inevitable in engineering industries. There are many issues/problems associated with the welding of dissimilar materials, related to the welding process and its parameters. The current work investigates the effect of laser welding conditions on the toughness of dissimilar welded components. In this study, CO2 laser welding has been successfully applied for joining 316 stainless steel with low carbon steel (F/A). Design of experiment techniques has been used for different effective welding parameters (laser power, welding speed, and focus position) to optimize the dissimilar F/A joints in terms of its mechanical properties. Taguchi approach was applied to optimize the welding parameters. Three factors with five levels each (L-25) were employed in these models. Impact strength was measured at room temperature by using the universal pendulum impact tester. The results were compared with the impact strength of the base material. The results were analysed using ANOVA and S/N ratio for optimal parameters combination. It is evident that Taguchi approach has decreased the number of experiments without negative effects on the result.
Authors: D.T.G. Katerelos, C. Galiotis
Abstract: Composite laminates are used for the construction of a wide range of products; from civil infrastructure to aeronautical or space structures. Since all these structures cannot be built at once, but they are composed by several structural elements connected in various patterns, the problem of stress concentrations strikes importantly. Additionally, the structures are often designed in a “fail safe” manner, which bespeaks the damage tolerance of the material. This means that the structural element and the structure are called to operate in the presence of various forms of damage. Damage locations within a structural element or a structure cause stress rising. In the present paper experimental investigation of the stress concentration arising in composite laminates, (a) around a circular notch and (b) due to damage onset and growth during their lifetime, is presented. The experimental results are compared to analytical model predictions.
Authors: S.J.I. Walker, G.S. Aglietti, P.R. Cunningham
Abstract: In the current world of engineering, structural vibration problems continue impact the design and construction of a wide range of products. Amid the parameters that determine the dynamic behaviour of a structure the one that takes into account the dissipation of energy resulting in the decay of the vibration is the least understood and the most difficult to quantify [1]. The estimation of damping factors is of interest in most branches of engineering sciences. In the field of aircraft structures the damping directly affects the fatigue life, a parameter which is applied conservatively due to the inherent complexity in modelling the damping of built up structures and the potentially catastrophic consequences of a fatigue failure. One of the most important problems is the limited knowledge of how joints affect the damping of the complete structure. This work therefore addresses this issue and focuses on the damping of joints in metal plates as part of a larger project to investigate the damping of built up structures. Various plate configurations are experimentally investigated using two different approaches. The results from the configurations are compared and discussed along with the advantages and disadvantages of each experimental approach. This enables a link to be identified between the damping magnitudes and the mode shapes and joint stiffnesses.
Authors: M. Fouinneteau, A.K. Pickett
Abstract: An extensive test campaign has been conducted to characterise the different failure modes observed in heavy tow (24k) carbon and glass braided composites. The Digital Image Correlation (DIC) technique was used to obtain complete strain field measurement of large strains to failure. Failure in the fibre direction, under tension and compression loading were investigated; in the tension direction relatively large strains to failure were measured due to tow straightening damage mechanisms. Another important test is tensile shear loading which can undergo very large strains to failure due to fibre re-orientations in the loading direction. This latter, so-called ‘scissoring’ mechanism, has been quantified through changes of fibre angle during the complete loading history. Data gathered from coupon testing was used to characterise a material damage, with failure, model for the heavy tow braids, using a general composite damage model available in the explicit Finite Element (FE) code PAM-CRASHTM. Finally, four-point bending tests on braided carbon and glass composite beams with full field strain measurements to failure was carried out and used to validate the numerical model. A good agreement between simulation and experimental results was obtained.
Authors: Mohammad Mohammadi Aghdam, M.R.N. Farahani, M. Dashty, S.M. Rezaei Niya
Abstract: Bending analysis of thick laminated rectangular plates with various boundary conditions is presented using Generalized Differential Quadrature (GDQ) method. Based on the Reissner first order shear deformation theory, the governing equations include a system of eight first order partial differential equations in terms of unknown displacements, forces and moments. Presence of all plate variables in the governing equations provide a simple procedure to satisfy different boundary condition during application of GDQ method to obtain accurate results with relatively small number of grid points even for plates with free edges .Illustrative examples including various combinations of clamped, simply supported and free boundary condition are given to demonstrate the accuracy and convergence of the presented GDQ technique. Results are compared with other analytical and finite element predictions and show reasonably good agreement.
Authors: Jan Sieber, B. Krauskopf
Abstract: We demonstrate a method for tracking the onset of oscillations (Hopf bifurcation) in nonlinear dynamical systems. Our method does not require a mathematical model of the dynamical system but instead relies on feedback controllability. This makes the approach potentially applicable in an experiment. The main advantage of our method is that it allows one to vary parameters directly along the stability boundary. In other words, there is no need to observe the transient oscillations of the dynamical system for a long time to determine their decay or growth. Moreover, the procedure automatically tracks the change of the critical frequency along the boundary and is able to continue the Hopf bifurcation curve into parameter regions where other modes are unstable.We illustrate the basic ideas with a numerical realization of the classical autonomous dry friction oscillator.
Authors: C. Massow, Veronica Vidal, Alan R. Champneys, John H.G. Macdonald
Abstract: Cable-stayed bridges frequently experience vibrations due to a variety of mechanisms, exacerbated by their very low inherent damping. A research group of the University of Bristol has focused lately on the study of cable-stayed bridges, some advances have led to the identification of vortex-induced deck vibrations occurring at the Second Severn Crossing (SSC) and improved methods of analysis of field vibration data. Based on such experience, it aims to study the autoparametric excitation which, due to very great amplitudes, can seriously damage the structure. It has been suggested that this may have been the mechanism of excitation of some large amplitude cable vibrations on real bridges, but the details of the behaviour are not very well understood and several cases of large cable vibrations on full scale bridges have not been fully explained. In this paper we examine a previously established cable-deck model and compare it to a new, more exact model in a different coordinate basis.
Authors: Reyolando Manoel Lopes Rabelo da Fonseca Brasil, L.C.S. Feitosa, José Manoel Balthazar
Abstract: We present a simple mathematical model of a wind turbine supporting tower. Here, the wind excitation is considered to be a non-ideal power source. In such a consideration, there is interaction between the energy supply and the motion of the supporting structure. If power is not enough, the rotation of the generator may get stuck at a resonance frequency of the structure. This is a manifestation of the so-called Sommerfeld Effect. In this model, at first, only two degrees of freedom are considered, the horizontal motion of the upper tip of the tower, in the transverse direction to the wind, and the generator rotation. Next, we add another degree of freedom, the motion of a free rolling mass inside a chamber. Its impact with the walls of the chamber provides control of both the amplitude of the tower vibration and the width of the band of frequencies in which the Sommerfeld effect occur. Some numerical simulations are performed using the equations of motion of the models obtained via a Lagrangian approach.

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