Roving Hammer Research Articles

On the experimental vibroacoustic modal analysis of a plate-cavity system

This paper proposes a methodology to the experimental vibroacoustic modal analysis on a coupled plate-cavity system. The method is based on the nonlinear curve fitting to the Frequency Response Functions (FRFs) obtained through a roving hammer test. These FRFs are divided into two groups: one is for the acceleration of the plate; the other is for the sound pressure inside the cavity. The modal parameters are estimated independently in each group using both the single/multi-mode all-curve and the all-mode single-curve fitting techniques. Such a way overcomes the identification difficulties when the natural frequencies of the coupled system slightly change over the roving hammer process. Meanwhile, the mode shapes extraction is eased from the different scales between structural and acoustic responses. The two groups of FRFs are fitted by two different types of FRF models, respectively. The results are in agreement with respect to the natural frequencies and modal damping ratios. Particularly, the FRF models are in alternative forms instead of the conventional ones, and they fit the measured FRFs very well with the estimation outcomes applied. Furthermore, the identified modal parameters are verified by the counterpart estimation based on the conventional FRF models formulated by partial fraction expansions. It is shown that the proposed FRF models contain fewer unknown parameters but do not downgrade the estimation accuracy, making the curve fitting process more practical and efficient. Hence, the two-group strategy with the proposed FRF models provides a practical and efficient way for the experimental modal analysis of vibroacoustic systems.

Experimental modal analysis of glass and laminated glass large panels with EVA or PVB interlayer at room temperature

Laminated glass is used for fail-safe structures in many fields of engineering. It contains a polymeric interlayer that improves the brittle glass behavior after fracture, provides damping, increases impact resistance, and at the same time maintains transparency. This paper examines the effect of the type of the interlayer on laminated glass oscillation using a roving hammer test. It also compares the modal response of the laminated glass before and after the low-velocity impact. The experimental modal analysis is validated by numerical modal analysis and using a different experimental methodology. This contribution discusses natural frequencies and damping ratios affecting the impact response and the reduction of noise and vibrations. It is found that the inserted polymer foils increased the damping and the impact resistance of the transparent plate. The interlayer with a higher ratio between loss and storage modulus showed higher damping and thus energy dissipation potential. No significant difference was measured in the natural frequencies of laminated glass before and after low-velocity impact. Inferences drawn from this study can be used by engineers, architects, and contractors when selecting suitable panels for applications such as windshields or curtain walls.

Model updating of uncertain parameters of carbon/epoxy composite plates using digital image correlation for full-field vibration measurement

Model updating is usually based on the contrast between the modal characteristics predicted by the models and those experimentally identified. Traditional experimental methods are based on the use of contacting sensors, but more recently other techniques as 3D Digital Image Correlation (DIC) have also been used successfully. In this paper the results obtained by applying these alternative techniques are compared, to obtain physically-sound models of carbon/epoxy composite plates. Primarily a roving hammer exciting the plates at evenly distributed degrees of freedom (DoF), and a mono-axial accelerometer attached to a single DoF reference point, have been used for modal identification. Alternatively, high speed cameras were applied to measure full-field vibrations of the plates. 3D DIC allowed obtaining a lower number of natural frequencies but much smoother mode shapes and similar results for model updating. The experimental setup has been benchmarked using two different sets of plates varying thickness and ply stacking.

Calculation of the Modulus of Elasticity with Using Natural Frequency Formula and Updated Finite Element Analysis Of Automotive Rear Lamp Lens

In this study; the weight of the modal accelerometer used in the Roving Hammer Impact Test Method was added in the finite element analysis (FEA) and the undamped modal analysis of the automotive rear lamp lenses were performed. Also, calculated new Elastic Modulus for Polymethylmethacrylate (PMMA) material with using natural frequency formula and this value was used in the ANSYS WB program and the undamped modal analysis was repeated. After that, frequency response functions (FRFs) of an automotive rear lamp lens were obtained by using Roving Hammer Impact Test Method. At the same time; dynamic characteristics of the structure, natural frequencies, damping ratios and mode shapes were obtained. Damping ratios were calculated from the FRF’s by using the Half Power Method. Finally, experimental modal analysis (EMA) and FEA results were compared.

Design and experimental validation of a metamaterial solution for improved noise and vibration behavior of pipes

This paper investigates a metamaterial solution for efficient vibration attenuation and acoustic radiation reduction of an aluminum pipe. To this end, using unit cell predictions, locally resonant structures are designed to have a pronounced flexural resonance frequency at the vicinity of a dominant vibration mode of the pipe. A direct approach of the Bloch-Floquet theorem is adopted to provide the dispersion relation representing wave motion in an infinite metamaterial pipe. Using these wave dispersion relations, the frequency range of the stopband zone created by the metamaterial solution is predicted. The dynamic behavior of the finite counterpart is predicted using the Finite Element Method (FEM). The resonant structures are produced from polymethyl methacrylate (PMMA) panels and are added to the host structure. In order to properly characterize both the vibrational behavior of the metamaterial pipe and the acoustic radiation from its wall, impact tests using roving hammer technique is performed on the pipe and both accelerations and acoustic pressures are measured at different locations. The experimental results show a pronounced stopband zone created by the addition of a few rows of resonant structures. Moreover, comparisons between the measurements and numerical predictions show a good agreement.

Model updating of uncertain parameters of carbon/epoxy composite plates from experimental modal data

This work presents a methodology to obtain physically-sound models of composite structure laminates using a combination of modal analysis, numerical modelling and parameter updating, avoiding the common uncertainties on the constructions of similar numerical models. Moreover this model establishes the baseline (pristine situation) of the dynamic behaviour of the set of composite plates. Therefore it could be applied for condition assessment or quality manufacturing control of existing structures through a non-destructive Structural Health Monitoring (SHM), and hence it could help to detect degradation or defects of the composite components. The driven data of the methodology were the modal frequencies and shapes of composite plates. To obtain these values an extensive experimental campaign of modal analysis has been performed on a set of carbon/epoxy laminates. A multiple input single output technique has been applied, using a roving hammer exciting the plates at evenly distributed Degrees of Freedom (DoF), and a mono-axial accelerometer attached to a single DoF reference point. The use of a high dense grid of points has allowed to identify a number of natural frequencies greater than usual in similar works, as well as improving the smoothness of the mode shape. Modal characteristics numerically obtained from a Finite Element Method (FEM) model based on manufacturer reference data were compared with experimental results. This baseline model was updated through a gradient based optimization algorithm. Before the process of model updating, a sensitivity analysis has been performed to identify the driven uncertain parameters using a Montecarlo approach. This technique reduces the number of parameters to be optimized to a small set increasing the efficiency of the methodology. As a result of the whole process, a physically more accurate model is obtained on which discrepancies with the corresponding experimentally measured modal parameters are drastically reduced. Analysis of the consistency of the adjusted numerical parameters has been done with alternative experimental tests (Quasi Static Loading (QSL) and Ultrasonic inspection).

Effects of elastic supports and flexural cracking on low and high order modal properties of a reinforced concrete girder

A number of nondestructive evaluation (NDE) methods are available to detect cracks and damage in reinforced concrete structures. Methods such as ultrasonic, impact echo, or X-ray testing have high resolution but are time consuming to perform and therefore only used locally. Since the early 1970s, researchers have also studied the modal properties of structures for damage detection and identification. In this paper, we discuss the effects of elastic supports and flexural cracking on the dynamic response of a large-scale laboratory reinforced concrete girder. For this purpose, an instrumented hammer used for impulse response testing was employed to initiate vibrations and to subsequently estimate the natural frequencies and modes of a large-scale laboratory concrete girder. Natural frequencies up to the 13th mode were successfully extracted from the frequency response function (FRF). In addition, the mode shapes were extracted up to the 6th mode due to the limitation of the linearity of the FRF response. These modal properties were determined from the measured accelerations of two points on the girder using a roving hammer. The test procedure was performed on the uncracked beam and repeated after the beam had been cracked. The experimental measurements were verified with a finite element (FE) model consisting of one-dimensional beam elements that consider both flexural and shear deformations. Support flexibility was modeled as linear-elastic springs, and was found critical in order to explain experimentally-obtained measurements. The effects of cracking were found to be most obvious in the higher modes. Additionally, utilizing the modal flexibility method on the first mode (natural vibration frequency and mode shape) was an accurate predictor of flexural crack locations.

Finite element model calibration of a nonlinear perforated plate

This paper presents a case study in which the finite element model for a curved circular plate is calibrated to reproduce both the linear and nonlinear dynamic response measured from two nominally identical samples. The linear dynamic response is described with the linear natural frequencies and mode shapes identified with a roving hammer test. Due to the uncertainty in the stiffness characteristics from the manufactured perforations, the linear natural frequencies are used to update the effective modulus of elasticity of the full order finite element model (FEM). The nonlinear dynamic response is described with nonlinear normal modes (NNMs) measured using force appropriation and high speed 3D digital image correlation (3D-DIC). The measured NNMs are used to update the boundary conditions of the full order FEM through comparison with NNMs calculated from a nonlinear reduced order model (NLROM). This comparison revealed that the nonlinear behavior could not be captured without accounting for the small curvature of the plate from manufacturing as confirmed in literature. So, 3D-DIC was also used to identify the initial static curvature of each plate and the resulting curvature was included in the full order FEM. The updated models are then used to understand how the stress distribution changes at large response amplitudes providing a possible explanation of failures observed during testing.

Vibrational characteristics of wood, aluminum, and composite hockey sticks

The stick used by ice hockey players consists of a long straight shaft attached to a curved blade. Composite materials have replaced wood and aluminum shafts for the vast majority of current professional and amateur players. The stiffness of the shaft plays a crucial role in the amount of potential energy that can be stored and released during a slap shot. Shafts are available in a wide range of stiffness and flex ratings in order to match player preference. Several wood, aluminum, and composite shafts were tested using a roving hammer experimental modal analysis with and without their blades. Mode shapes of the shaft tested alone were found to be those of a free-free beam. For shafts of similar lengths, aluminum shafts had the highest resonance frequencies and the lowest damping coefficients, while wood shafts had the lowest frequencies and the highest damping. Vibrational characteristics of whole sticks, including one-piece, two-piece sticks (shaft and blade from same material), and hybrid (shaft and blade of different materials) show that the presence of the blade significantly lowers the frequencies of the torsional modes of vibration. These results, especially damping coefficients, suggest reasons for the anecdotally preferred “feel” provided by composite sticks.

Investigation into the use of low cost MEMS accelerometers for vibration based damage detection

When a structure is damaged there is often a concomitant localized change in structural stiffness, which may affect the dynamic characteristics of the structure. One characteristic that has proved suitable for damage identification and location is the operational curvature shape which exhibits changing features at locations of stiffness change. A method that finds theses features, SIDER, employs an algorithm that operates on broadband operational curvature shapes. To obtain data for SIDER, a small number of accelerometers, typically about four, is installed on the structure and the structure is then excited at a large array of test points using a ‘roving hammer’ technique. Frequency response functions are individually measured between each excitation point and the reference accelerometers, and the required operating curvature shapes are determined from the resulting array of frequency response functions. While setup of the experiment is reasonably quick, data acquisition can be time consuming and it is not easily amenable to automation. This paper investigates an alternative approach, which relies on the reciprocity theorem. An array of response transducers is installed on the structure, and then only a few locations are excited. There are significant benefits to this method, including the possibility of automation and remote sensing. The cost of hundreds of conventional accelerometers can be prohibitive, and therefore this project investigated using an array of low cost MEMS accelerometers. MEMS are also attractive in that they may be embedded into a composite structure during manufacture. In the study reported here the roving hammer and MEMS array methods are compared by testing a composite vertical stabilizer (tail plane) from an Airbus A320 aircraft. Despite the different test procedures, and the lower quality of the MEMS transducers, it is shown that an array of low cost MEMS transducers can determine results comparable with those obtained using high performance transducers.