Abstract

In this paper, a simplified finite element model to represent a riveted lap joint in structural dynamic analysis field is proposed. The rivet is modeled by spring‐damper combination elements. Several numerical models are studied with different quantities of rivets (1, 2, 3 and 5) and spring‐damper combination elements (4, 6, 8, 12, 16 and 20) per rivet. Experimental samples of two aluminum material plates connected by different quantities of rivets (1, 2, 3 and 5) are built and tested in order to be known its modal characteristics ‐ natural frequencies and mode shapes. A finite element model updating methodology is used in order to get the best numerical riveted lap joint representation relatively to the experimental samples. An evaluation of the best numerical riveted lap joint is carried out based on the comparison between the experimental and numerical resonance frequencies after updating. It is shown that the riveted lap joints composed by eight and twelve spring‐damper combination elements per rivet have the best representation. A stiffness constant value is obtained for the riveted lap joints in study

Abstract

Specific computer programs for identification in modal analysis of structures are not economical, and they may not always be available. This paper presents a technique for simple optimization, getting fast and quality results, in the estimation of the damping ratios for each separate mode, and that can be resolved with a demo version (downloadable for free from the internet) of commercial optimization software. It is suitable for low levels of structural damping (or non-frequency-dependent damping), which are those that appear in large structures. Likewise, it is not necessary for the modes to be separate, or even that the magnitude of the near modes be less than that of those with which we are working; within the algorithm movable residual complements optimize their position according to the frequency and vertical axes (the response magnitude), to take into account the influence of the modes outside the considered range. In addition, it is not very sensitive to the presence of noise in the signal.

Abstract

Fiber-reinforced laminated composite materials are widely used in aircraft, modern vehicles and light-weight structures. With their high elastic modulus, high strength, and capability of being tailored for a several applications, these materials offer definite advantages compared to more traditional structural materials. However, their behavior under impact is of concern since those events may occur during manufacturing, normal operation or maintenance. The situation is critical for impacts that induce significant internal damage undetectable by visual inspection, which causes large reductions on the strength and stability of the structure. New standardized tests to assess the impact tolerance (ASTM D7136 and D7137) lose some of its appeal by its destructive nature as well as costs involved. Non-destructive techniques such as ultrasonic phased array or experimental modal analysis can be complementary in the evaluation of damage and structural integrity. The results of both destructive and non-destructive experiments carried out on forty-eight composite specimens are presented. A good correlation between incident impact energy and delaminated area as well as variation of eigenfrequencies is noticed. This study does not only provide a better understanding of the impact phenomenon but can also help in design and implementation of new test procedures for structural assessment.

Abstract

A new alternative is presented for the estimation of modal mass, in order to scale the vibration modes obtained from operational modal analysis tests, using optimization techniques, in the frequency-independent proportional damping situation (structural or hysteretic), achieving optimal results even with the presence of a high level of noise in the signal.

After a brief exposition of the theoretical foundations, and the limitations in the current state of the art, the determination of modal parameters is analyzed in a coupled dynamic system, using optimization techniques.

To illustrate the procedure, two examples of application are presented: first, for the case of an elementary beam model of small dimensions; and then, to confirm the capacity of the proposed method to be applied to real civil structures, said capacity is illustrated by a numerical simulation, on a very slender bridge currently in service, and with a very complex internal geometry of its main beam.

Abstract

To determine dynamic response of mechanical system is always a complex issue. One of the core inputs for such a task is a coefficient of logarithmic damping decrement. There is no way how to determine this coefficient analytically; it needs to be measured on the final product. There has been a large amount of dynamic tests carried out on common structures and the results have been incorporated in design codes. Nevertheless, power plant boiler structure differs significantly from common structures. To provide a large number of dynamic tests on real facility is very difficult for many reasons. Using a scaled model is a way how to achieve relevant data based on repeatable experiment.

Abstract

The lack of standardization in vibration mode results has constituted the main shortcoming of operational modal analysis, subsequently resolved by means of the mass modification method. This is a major drawback for civil works, as it interrupts traffic on site, significantly increasing testing time and costs. The purpose of this study is to achieve a balance between traditional modal analysis (accompanied by numerical simulations of mass modification) and operational modal analysis (in conjunction with the mass modification method), thereby obtaining more optimal results with a narrower range of error, even when the signal has a high level of noise. It will thus be possible to reliably determine the modal parameters of the structure and their alteration due to mass modification. For this purpose, the proposed methodology is applied to a large span metal bridge, unique for its slenderness, with a span/depth ratio of 56.6, the lowest recorded on a bridge, as figured in the Guinness Book of Records since 2007.

Abstract

'''High-frequency vibration can result in failure of the working blades. So suppressing vibration failure of blades becomes more and more important. Damping performance of viscoelastic block on blades is researched based on principle of finite element method, contact theory of elastic body and the solving equations of natural frequency. Modal and harmonic response is analyzed to obtain natural frequency, vibration displacement and stress, in which different finite element models of blade are established. Effects of different adding position and thickness of viscoelastic block on damping performance are analyzed. Elastic modulus and loss factor of viscoelastic material is also changed to study their influence on damping performance. The results show that viscoelastic block applied to blades has excellent performance in reducing blade vibration. When viscoelastic block is added to mortise side, the damping performance of blade is the best. In addition, thickness of viscoelastic block and parameters of viscoelastic material have a significant effect on vibration reduction of blade. '''Key words''': Viscoelastic damping material, blades, vibration reduction, modal analysis, harmonic response Corresponding author. E-mail: [mailto:fswang@nwpu.edu.cn fswang@nwpu.edu.cn] (Fusheng Wang)

Abstract

The consideration of the assumption of viscous damping as a starting hypothesis in the dynamic analysis of structures can introduce a substantial error in the calculations. This is due to the energy dissipated by the structure in its oscillation being frequency dependent under that premise, while experimentally the opposite becomes apparent, when structural or hysteretic damping is taken into account. The modes of vibration obtained in the operational modal analysis are not scaled, since the magnitude of the excitation on the structure is unknown. For this reason it is necessary to resort to additional calculation procedures, such as the mass change method, to determine the scale factors of each mode. However, all variants of this method, in the current state of the art, have been deduced in the undamped situation. In this paper we analyze the influence of the deviation of the results for the scale factors, obtained by means of a viscous damping analysis, compared to that same deviation determined under the consideration of hysteretic or structural damping, independent of the frequency, whose results align much better with the experimental behavior of the structure.

Abstract