Abstract: The development, evaluation, and comparison of wireless IoT and cyber-physical systems requires testbeds supporting inspection of logical states and accurate observations of physical performance metrics. We present FlockLab 2, a second generation testbed supporting multi-modal, high-accuracy and high-dynamic range measurements of power and logic timing and at the same time in-situ debug and trace infrastructure of modern microcontrollers allowing for reproducible evaluation and benchmarking. We detail the architecture, provide a characterization and demonstrate the interface, the supported services and the tools of the FlockLab 2 testbed. Data Availability Statement. The hardware design and the software for server and observer of the presented testbed architecture and the data for the plots in this paper are openly available at XXX.

Abstract: Experimental modal analysis of electrical transmission tower has been a challenging task for transmission tower researchers and design engineers in industry all over the world. Requirement of large numbers of sensors and accelerometers have been major constrain. In this study an innovative approach has been developed to investigate the dynamic characteristics and behavior of tower structure through analytical and experimental modal analysis. Firstly, a scale down (1:15) prototype model of transmission tower structure has been constructed with mild steel straps, joint together by welding, for modal testing. Modal hammer test has been conducted on the prototype tower model for extracting modal parameters; modal frequency, modal damping and modes, of the tower model which representing the actual tower structure. Secondly, the transmission tower structure has been modeled in standard finite element tools and analyzed analytically for natural frequencies. The first six natural frequencies and corresponding mode shapes have been determined analytically and first six natural frequencies have also been determined experimentally and compared with each other. The first six natural frequencies are determined analytically, the frequency range of 2–9 Hz has been found. The analytical and experimental modal analysis of transmission tower structure has been found to be in correlation with some differences. The maximum natural frequencies percentage difference 11.1% has been found; between the scale down model and the stand software model. Additionally, the tower structure has been modified and optimized to improve the stiffness of the diaphragm as per specification and practical limitations. The first order natural frequency of the modified tower has been reduced to 2.171 Hz from the 2.1773 Hz.

Abstract: Cross-laminated timber (CLT) is gaining popularity worldwide with the development of tall wood buildings A non-destructive testing (NDT) technique employing modal testing for the measurement of natural frequencies and a genetic algorithm for minimizing the difference between calculated and measured natural frequencies has been proposed for the inverse determination of in-plane and out-of-plane elastic constants of orthotropic Mindlin plates Based on the exact solution of free transverse vibration of orthotropic Mindlin plates under a pair of opposite edges simply supported and the other pair free (SFSF), the effective bending and shear stiffness of two three-layer and one five-layer symmetric CLT panels were inversely determined Moreover, the results showed that the inclusion of five sensitive vibration modes was more efficient than having more modes for the inverse determination This study demonstrated the efficiency and potential of the non-destructive method in measuring the effective bending and shear stiffness of mass timber panels

Abstract: Determining the natural frequency distribution is of great importance in studying the vibration of the steering system in a commercial vehicle. A high-speed vibration frequency sweep experiment on ...

Abstract: Composite materials are widely used for their peculiar combination of excellent structural, mechanical, and damping properties. This work presents an experimental study on the dissipation properties of disk-shaped composite specimens exploiting vibration tests. Two different polymer matrix composites with the same number of identical laminae, but characterized by different stacking sequences, namely unidirectional and quasi-isotropic configurations, have been evaluated. An ad-hoc steel structure was designed and developed to reproduce an in-plane torsional excitation on the specimen. The main idea of the proposed approach relies on deriving the damping properties of the disks by focusing on the modal damping of the overall vibrating structure and, in particular, using just the first in-plane torsional deformation mode. Experimental torsional damping evaluations were conducted by performing vibrational hammer excitation on the presented setup. Two methods were proposed and compared, both relying on a single-degree-of-freedom (SDOF) approximation of the measured frequency response function (FRF).

Abstract: Structural dynamic modification is a popular approach to obtain desire frequencies and dynamic characteristics. It has been observed that reanalyzing the modified structure usually involves complicated calculations when modifications are concerned with numerous degrees of freedom (DOFs), especially adding substructures to these DOFs. This paper proposed a method to reanalyze the frequency response functions (FRFs) of structures with multiple co-ordinates modifications. Two different cases are taken into consideration in the modifications, including adding (or decreasing) masses, stiffness, and damping, as well as adding spring-mass substructures, which makes the method more practical. This method is developed by employing Sherman–Morrison and Woodbury (SMW) formula based on the FRFs related to the modifications coordinates of the original system. The advantage of this method is that neither a physical model nor a modal model is required; instead, it needs only the FRFs, which can be directly measured by experimental modal testing. Another salient feature of this proposed strategy is that the FRFs of the modified structure can be calculated in only one step. Validation of this proposed method is demonstrated using various numerical examples. It is shown that the method is very effective and can be considered for real applications.

Abstract: Real-time hybrid simulation techniques have developed significantly in the last decades, driven by scientific research and steady progress in simulation and modeling techniques together with powerful automatic code generation tools. In engineering applications, hybrid simulation is mainly used for physical testing of critical components, while the remaining rest of the system is incorporated as computer model and properly coupled to the elements being tested. The focus of this research is to connect virtual nonlinear components with arbitrary characteristics to an existing mechanical model. This approach allows experimental verification of almost any classical nonlinear dynamic phenomena. The methodology can be applied for both, the optimal design of nonlinear components as well as testing of complex elements or materials based on experimental data, respectively. The laboratory setup comprises of standard hardware components used in modal testing including electrodynamic shakers, force, displacement and acceleration sensors and furthermore a real-time processing unit with feedback control for proper system coupling. For periodic forcing, the efficiency of the proposed method is demonstrated in a laboratory setup consisting of a rigid mass attached to a virtual nonlinear spring-damper element. All tests performed confirm the efficiency of the proposed system because nonlinear oscillations are reproduced with a very high level of accuracy.

Abstract: Modal tests are performed to validate analysis models of structures, and it is important to support a test article use fixtures that allow an engineer to focus his time and effort on updating the analysis model instead of the supports. Oftentimes, however, inadequate boundary condition fixtures are used in modal surveys because the design and manufacture of a proper boundary condition may be too expensive for a program. An alternative approach of creating appropriate boundary conditions by using accelerations as references to fix degrees of freedom is presented in this paper and is demonstrated using test results from a tap test on an aluminum beam. Frequency response functions (FRF) are generated directly and indirectly using a partial inversion of the FRF matrix for several different boundary condition cases using the same set of test data. Modes are extracted from the resulting FRF and are compared to an analysis model.

Abstract: The paper presents results of an experimental study related to a non-destructive diagnostic technique used for preliminary determination the location and size of delamination in composite coatings of wind turbine blades. The proposed method of damage detection is based on the analysis of the ten first mode shapes of bending vibrations, which correspond to displacements of rotor blades perpendicular to the rotor plane. Modal parameters depend on the physical properties of the structure. On the other hand, failures can affect these properties (e.g. locally reduce the stiffness of the structure). Monitoring of selected modal parameter can allow determination the technical condition of the structure. The main assumption of the presented method is a comprehensive analysis of the measured data by determination the root mean square value (RMS) for each measurement point from all forms of free vibration obtained from the experiment. As a result, information contained in all modes of vibrations that may indicate damage of the blade will be included in a single characteristic. The investigations were carried out on a scaled-down model of a wind turbine blade of a rotor diameter of 36 m. The modal parameters have been determined only experimentally using a Laser Doppler Scanning Vibrometer. Damage was simulated for three localizations by additional high stiffness elements fixed to the surface of the blade. The results of the research presented in this paper confirm the effectiveness of RMS calculation in detection damage using modes of vibrations.

Abstract: A fixed base correction method that uses acceleration constraint shapes as references to transform flexible or dynamically active boundary conditions into fixed boundaries has been recently implemented for modal tests. The method uses test data directly to generate constraint shapes associated with accelerometer measurements at the test article and test fixture interface that are then used as references when calculating corrected fixed base frequency response functions (FRFs). The main challenge with the method is that at least one disturbance source, such as a modal shaker, must be applied to the boundary structure for each constraint shape used, so it is advantageous to understand how many constraint shapes may be required to fix a boundary for test planning purposes. This paper outlines a procedure that uses multipoint constraint equations in an analysis model of an integrated test article and its test fixture to determine the number of exciters necessary to apply the fixed base correction method. The method is verified by comparing mode shapes of the fixed base test article to the system model with a number of multipoint degrees of freedom constrained.

Abstract: In many engineering applications, the vibration analysis of a structure requires the set up of a large number of sensors These studies are mostly performed in post processing and based on linear modal analysis However, many studied devices highlight that modal parameters depend on the vibration level non linearities and are performed with sensors as accelerometers that modify the dynamics of the device This work proposes a significant evolution of modal testing based on the real time identification of non linear parameters (natural frequencies and damping) tracked with a linear modal basis This method, called Kinematic-SAMI (for multiSensors Assimilation Modal Identification) is assessed firstly on a numerical case with known non linearities and secondly in the framework of a classical cantilever beam with contactless measurement technique (high speed and high resolution cameras) Finally, the efficiency and the limits of the method are discussed

Abstract: When conducting a vibration test on an electrodynamic shaker or shaker table, the layout of fixtures and test components on the shaker adapter changes the vibration modes and anti-resonances of the system. During a test, these dynamics can be excited by the shaker itself, leading to an invalid test if the desired environment is exceeded at some points, and potentially damaging the shaker or test components. A high-fidelity shaker model would allow for accurate pretest planning, in which test component layout and control accelerometer placement can be optimized to mitigate problem areas. However, shaker systems are notoriously difficult to model analytically due to a multitude of joints with indefinite properties, unknown stiffness and damping of the magnetic field, and the scarcity of available technical drawings for the internal components. This work explores the use of hybrid modal substructuring to create a test-based model of a shaker table with a dynamically complex shaker fixture. The transmission simulator method is used with an experimentally derived modal model of the shaker and a fixture to decouple a finite element model of the fixture and replace it with a finite element model of the fixture with an attached test article. Special care is taken to ensure that an optimized test layout and accurate FEMs are created. The resultant modal model for the total system is shown to accurately retain the fixture dynamics while successfully adding the test component dynamics through the usable frequency range of the shaker.

Abstract: Although the dynamic behavior of rectangular plates has been the subject of much research for many decades, it remains of a crucial importance in various engineering fields and some edge conditions have not yet been treated, especially those involving edges connected to distributed rotational springs and non-linear vibrations. Also, in the practice of Modal Testing, theoretical models are needed for quantitatively estimating the flexibility of the real plate supports. A complementary work is presented here corresponding to plates connected to a distribution of rotational springs at two opposite edges vibrating in the geometrically non-linear regime occurring at large vibration amplitudes. To build the plate trial functions, defined as products of beam functions in the x and y directions, the mode shapes of simply supported beams connected to rotational springs are first calculated. Then, after exposing the general formulation of the non-linear problem, based on Hamilton’s principle and spectral analysis, the plate case is examined. Using the single mode approach, the backbone curves are determined, giving the non-linear frequency-amplitude dependence for plates having different combinations of stiffness and aspect ratios. It is noticed, as may be expected, that the obtained hardening non-linearity effect becomes more accentuated with increasing the rotational spring stiffness.

Abstract: In recent years, smart structures have attracted much interest as morphing structures. One of the simplest types of these structures is bistable composite plate, which has many applications in aero...

Abstract: Model updating methods can be adopted to improve the correlation level between the finite element model of a spot welded structure and its test model. However, in the presence of contact interfaces in the vicinity of the welded areas, improving the correlation level is problematic and challenging. An approach for correlating the finite element model of a welded structure with contact interfaces using finite element model updating and modal testing is proposed. The proposed approach was tested on a car front-end module structure that consisted of nine components and 76 resistance spot-welded joints used to assemble the components. CWELD and CELAS1 element connectors were used to represent the spot-welded joints and contact interfaces in the finite element modelling and updating. This approach was applied successfully to predict the modal parameters of the car spot-welded front-end module. The total error of the initial finite element model of the structure was reduced from 27.13% to 5.75%. The findings of this work suggest that the proposed approach has a great potential for use in investigating the dynamic behaviour of various spot-welded structures without a significant decline in accuracy.

Abstract: High cycle failure of blades and vanes caused by the vibration is one of the major causes reducing the lifetime of turbomachines. For multiblade packets, the failure may occur at vibrations with high frequencies that can reach up to tens of kHz. The experimental modal testing of blades is crucial for the validation of numerical models and for the optimization of turbomachine design. In this paper, the test rig and procedure for measurements of dynamic characteristics of lightweight multiblade packets in wide and high frequency ranges are developed. The measurements are based on a noncontact excitation and noncontact measurement method, which allows the determination of the modal characteristics of the packets with high accuracy in wide frequency ranges. The responses of the multiblade packets are measured using a Scanning Laser Doppler Vibrometry (SLDV), while vibrations are excited by the acoustic excitation technique. Modal tests of the blade packet comprising 18 vane blades connected by shrouds are performed. The measurements are performed within the high frequency range of 0-30 kHz, and the natural frequencies and mode shapes are obtained for first 97 modes. To capture the complex high frequency blade mode shapes, each blade in the packet is scanned over 25 reference points uniformly distributed over the blade concave surface. In order to obtain the high frequency resolution, the frequency range used for the measurements is split into several frequency intervals accordingly to the number of spectral lines available in the used data acquisition system, and for each such interval, the test is performed separately. The finite model of the packet is created, and the numerical modal analysis is performed to compare the calculated natural frequencies and mode shapes with the experimental measurements. The comparison shows the satisfactory with those from finite element analysis. It illustrates the measurement method described in this work is effective and reliable.

Abstract: The development of predictive models requires several assumptions along with known system properties and boundary conditions to generate a correlated model. When a prototype product is available, modal analysis can be used to benchmark the current product and extract modal properties. The extracted values are often cross referenced with FEA solutions and utilized to feed forward into CAE models for data replication and future prediction. This study was used to perform modal testing on a full sized pickup truck driveline to build a one-dimensional lumped parameter model. The successful extraction of modal parameters was able to provide benchmark stiffness and damping estimates for use in CAE model updating to achieve better correlation with experimental vehicle data. The resulting lumped parameter changes reduce the number of model assumptions and allow for modification of stiffness design targets for new prototype driveshafts and/or additional driveline components.

Abstract: In modal testing and finite element model correlation, analysts desire modal results using free-free or rigid boundary conditions to ease comparisons of test versus analytical data. It is often expensive both in cost and schedule to build and test with boundary conditions that replicate the free-free or rigid boundaries. Static test fixtures for load testing are often large, heavy, and unyielding, but do not provide adequate boundaries for modal tests because they are dynamically too flexible and often contain natural frequencies within the frequency range of interest of the test article. Dynamic coupling between the test article and test fixture complicates the model updating process because significant effort is required to model the test fixture and boundary conditions in addition to the test article. If there were a way to correct the modal results for fixture coupling, then setups used for other structural testing could be adequate for modal testing. In the case described in this paper, a partial static loads testing setup was used, which allowed significant schedule and cost savings by eliminating a unique setup for a modal test. A fixed base correction technique was investigated during modal testing of a flexible wing cantilevered from part of a static test fixture. The technique was successfully used to measure the wing modes de-coupled from the dynamically active test fixture. The technique is promising for future aircraft applications, but more research is needed.

Abstract: NASA is developing an expendable heavy lift launch vehicle capability, the Space Launch System, to support lunar and deep space exploration. To support this capability, an updated ground infrastructure is required including modifying an existing Mobile Launcher system. The Mobile Launcher is a very large heavy beam/truss steel structure designed to support the Space Launch System during its buildup and integration in the Vehicle Assembly Building, transportation from the Vehicle Assembly Building out to the launch pad, and provides the launch platform at the launch pad. The previous Saturn/Apollo and Space Shuttle programs had integrated vehicle ground vibration tests of their integrated launch vehicles performed with simulated free-free boundary conditions to experimentally anchor and validate structural and flight controls analysis models. For the Space Launch System program, the Mobile Launcher will be used as the modal test fixture for the ground vibration test of the first Space Launch System flight vehicle, Exploration Mission ? 1( now referred to as Artemis 1), programmatically referred to as the Integrated vehicle modal test. The Integrated vehicle modal test of the Exploration Mission - 1 integrated launch vehicle will have its core and second stages unfueled while mounted to the ML while inside the Vehicle Assembly Building, which is currently scheduled for the late spring or early summer of 2020. The Space Launch System program has implemented a building block approach for dynamic model validation. The modal test of the Mobile Launcher is an important part of this building block approach in supporting the integrated vehicle modal test since the Mobile Launcher will serve as a structurally dynamic test fixture whose modes will couple with the modes of the Exploration Mission ? 1 test vehicle. The Mobile Launcher modal test will further support understanding the structural dynamics of the Mobile Launcher and SLS during rollout to the launch pad, which will play a key role in better understanding and prediction of the rollout forces acting on the launch vehicle. The Mobile Launcher modal test is currently scheduled for the summer of 2019. Due to a very tight modal testing schedule, this Mobile Launcher modal pretest analysis has been performed to ensure there is a high likelihood of being able to successfully complete the modal test (i.e. identify the primary target modes) using the planned instrumentation, shakers, and excitation types. This paper will discuss this Mobile Launcher modal pretest analysis and the unique challenges faced due to the Mobile Launcher's size and weight, which are typically not faced when modal testing aerospace structures.

Abstract: − In this paper, a method is proposed for establishing an approximate prediction model of rotordynamics through modal testing. In particular, the proposed method is applicable to systems that cannot be established according to conventional methods owing to the absence of information regarding the dimensions and material of the rotor–bearing system. The proposed method is demonstrated by employing a motor dynamometer driven by a 1 MW class induction motor without dimension and material information. The proposed method comprises a total of seven steps, wherein an initial model is established by incorporating approximate dimensions and material information, and the model is improved on the basis of the natural frequency characteristics of the system. During model improvement, the modification factor is introduced for adjusting the elastic modulus and shear modulus of the system. Analysis of critical speed and imbalance response indicates that the separation margin is 67% and the maximum vibration amplitude is less than the amplitude limit of 0.032 mm under the API 611 standard, which means that the motor dynamometer can stably operate at a rated speed of 1800 rpm. Hence, the obtained results validate the feasibility of the proposed method. Furthermore, for broad usage, it is necessary to accordingly apply and validate the proposed method for various rotor–bearing systems.

Abstract: In large civil engineering structures, the output-only modal identification is the most applicable technique for estimating the modal parameters such as damping. However, due to no measurement and control of excitation force, the identified parameters obtained by output-only technique have more uncertainty than those derived from the input-output technique. Given the different nature and uncertainties of the two modal identification techniques, in the present study, the damping related to the first 12 modes of a double-layer grid developed from the ball joint system were identified via the two techniques and compared with each other. For this purpose, a double-layer grid was constructed by pipes and balls with free-free boundary conditions provided for both input-output and output-only experiments. Exciting the grid, its acceleration response was measured at appropriate degrees of freedom. Then, by using these data and performing modal analysis, involving four different methods of input-output and five different methods of output-only, the natural frequencies and damping ratios of the desired modes were extracted. The results indicated that despite the good agreement between the modal damping of the grid, as identified by different methods of input-output together and by different methods of output-only together, the results of input-output and output-only methods were different with each other. The damping values through the input-output modal identification methods were on average 65% higher than the corresponding values of the output-only modal identification methods.