Structural Dynamic Testing Research Articles

On real-time pseudo-dynamic sub-structure testing: algorithm, numerical and experimental results

The real-time pseudo-dynamic sub-structure (SubPSD) test is a new procedure for dynamic testing of structures. Here, the actual test specimen is a part, i.e. sub-structure, of a larger, complex system. The main structure is simulated on a computer which is in interaction with the test rig. One possible application is the qualification test of a payload (sub-structure) mounted in a carrier rocket (computer model). Consideration of the interaction between main- and sub-structure will yield more realistic results than conventional shaking table tests. This can be a good basis for a more effective, light-weight design of the payload. An algorithm is presented, which is based on the Newmark time domain solution of the equation of motion. It uses sub-stepping instead of iteration to reach equilibrium within each time step. This algorithm and a suitable hardware allow for true real-time performance of the SubPSD test, even with oscillatory sub-structures. Numerical studies are presented which demonstrate the accuracy but also the limits of the procedure. Experimental results of a 4 d.o.f. test model are compared to the numerical results.

Modified predictor–corrector numerical scheme for real‐time pseudo dynamic tests using state‐space formulation

AbstractThis paper deals with an explicit numerical integration method for real‐time pseudo dynamic tests. The proposed method, termed the MPC‐SSP method, is suited to use in real‐time pseudo dynamic tests as no iteration steps are involved in each step of computation. A procedure for implementing the proposed method in real‐time pseudo dynamic tests is described in the paper. A state‐space approach is employed in this study to formulate the equations of motion of the system, which is advantageous in real‐time pseudo dynamic testing of structures with active control devices since most structural control problems are formulated in state space. A stability and accuracy analysis of the proposed method was performed based on linear elastic systems. Owing to an extrapolation scheme employed to predict the system's future response, the MPC‐SSP method is conditionally stable. To demonstrate the effectiveness of the MPC‐SSP method, a series of numerical simulations were performed and the performance of the MPC‐SSP method was compared with other pseudo dynamic testing methods including Explicit Newmark, Central Difference, Operator Splitting, and OS‐SSP methods based on both linear and non‐linear single‐degree‐of‐freedom systems. Copyright © 2004 John Wiley & Sons, Ltd.

Dynamic characteristics of a flexible rotor system supported by a viscoelastic foil bearing (VEFB)

Foil bearings have been considered as an alternative to traditional bearings with the increasing need for high-speed, high-temperature turbomachinery. However, the lack of adequate load capacity and sufficient damping capacity is a key technical hurdle to super-bending-critical operation as well as widespread use of foil bearings in turbomachinery such as turbopumps, turbocompressors and turbochargers. A new foil bearing, ViscoElastic Foil Bearing (VEFB) is suggested in this paper. The super-bending-critical operation of the conventional bump foil bearing and the VEFB is examined, as well as the structural dynamic characteristics. The structural dynamic test results show that the equivalent viscous damping of the VEFB is much larger than that of the bump bearing and that the structural dynamic stiffness of the VEFB is comparable or larger than that of the bump bearing. The results of super-bending-critical operation of the VEFB indicate that the enhanced structural damping of the viscoelastic foil dramatically reduces the vibration near the bending critical speed. With the help of increased damping resulting from the viscoelasticity, suppression of the nonsynchronous orbit is possible beyond the bending critical speed.

High-speed CW step-frequency coherent radar for dynamic monitoring of civil engineering structures

A high-speed coherent radar for dynamic testing of civil engineering structures is proposed. The radar operates a continuous-wave (CW) step-frequency in Ku-band, and the base-band signal is generated by direct digital synthesis. Vibration measurements carried out on a 200 m-long bridge forced by vehicular traffic are reported.

A microwave radar technique for dynamic testing of large structures

In this paper, the authors propose an innovative survey radar technique based on microwave holographic images for dynamic testing of large structures providing both vibration amplitude pattern and frequency. Theoretical background is provided and experimental results obtained during a dynamic test on a concrete and masonry building are reported.

고속 회전 터보 기기용 포일 베어링의 불안정 진동 제진에 관한 연구

A new foil bearing, ViscoElastic Foil Bearing(VEFB) is suggested with the need for a high damping foil bearing. Sufficient damping capacity is a key technical hurdle to super-bending-critical operation as well as widespread use of foil bearings into turbomachinery. The super-bending-critical operation of the conventional bump foil bearing and the VEFB is examined, as well as the structural dynamic characteristics. The structural dynamic test results show that the equivalent viscous damping of the VEFB is much larger than that of the bump bearing, and that the structural dynamic stiffness of the VEFB is comparable or larger than that of the bump bearing. The results of super-bending-critical operation of the VEFB indicate that the enhanced structural damping of the viscoelastic foil dramatically reduces the vibration near the bending critical speed With the help of increased damping resulting from the viscoelasticity, the suppression of the asynchronous orbit is possible beyond the bending critical speed.

Dynamic testing of inflatable structures using smart materials

In this paper we present experimental investigations of the vibration testing of an inflated, thin-film torus using smart materials. Lightweight, inflatable structures are very attractive in satellite applications. However, the lightweight, flexible and highly damped nature of inflated structures poses difficulties in ground vibration testing. In this study, we show that polyvinylidene fluoride (PVDF) patches and recently developed macro-fiber composite actuators may be used as sensors and actuators in identifying modal parameters. Both smart materials can be integrated unobtrusively into the skin of a torus or space device forming an attractive testing arrangement. The addition of actuators and PVDF sensors to the torus does not significantly interfere with the suspension modes of a free–free boundary condition, and can be considered an integral part of the inflated structure. The results indicate the potential of using smart materials to measure and control the dynamic response of inflated structures.

The development of real–time substructure testing

Full-scale dynamic testing of civil engineering structures is extremely costly and difficult to perform. Most test methods therefore involve either a reduction in the physical scale or an extension of the time-scale. Both of these approaches can cause significant difficulties in extrapolating to the full-scale dynamic behaviour, particularly when the structure responds nonlinearly or includes highly rate-dependent components such as dampers. Real-time substructure testing is a relatively new method which seeks to avoid these problems by performing tests on key elements of the structure at full or large scale, with the physical test coupled in real time to a numerical model of the surrounding structure. The method requires a high performance of both the physical test equipment and the numerical algorithms. This paper first reviews the development of structural test methods and the emergence of real-time substructure testing. This is followed by a brief description of the equipment that is needed to implement a substructure test. Several novel developments in the numerical algorithms used in real-time substructure testing are presented, including a new, fast algorithm which allows nonlinear response of the surrounding structure to be computed in real time. Results are presented from a variety of tests which demonstrate the performance of the system at small and large scale, with either linear or nonlinear test specimens, and with varying numbers of degrees of freedom passed between the physical and numerical substructures. Finally, the usefulness and possible applications of the test method are discussed.

VALID STRAIN MEASUREMENTS FOR STRUCTURAL DYNAMICS TESTING. Part 3: Design Rules for Spectral and Waveshape Reproduction

VALID STRAIN MEASUREMENTS FOR STRUCTURAL DYNAMICS TESTING. Part 3: Design Rules for Spectral and Waveshape Reproduction

VALID STRAIN MEASUREMENTS FOR STRUCTURAL DYNAMICS TESTING. Part 2: Dynamic Strain Measurement System Transfer Functions and Linearity

VALID STRAIN MEASUREMENTS FOR STRUCTURAL DYNAMICS TESTING. Part 2: Dynamic Strain Measurement System Transfer Functions and Linearity

Identification of damage parameters of a full-scale steel structuredamaged by seismic loading

Within the European research cooperation COST a two-floor steelframe was defined as a benchmark directed towards investigating damagedetection methods based on structural dynamic test data. In the presentpaper we describe an approach to identify the location and the extent ofthe damage introduced into the steel frame by pseudo dynamic seismic loads.In the first step the measured dynamic response of the original undamagedstructure was used to generate a reference finite element model of thestructure. In the next step the experimental modal data of the damagedstructure were used to identify the location and the extent of the damage.This was based on comparing the changes of stiffness parameters obtainedfrom the undamaged and the damaged structure. After careful selection ofthe parameters representing best the possible stiffness degradations thenumerical values of the parameters were identified by minimizing thetest/analysis differences of eigenfrequencies and mode shapes whilesimultaneously keeping the parameters within physically meaningful limits.With the identified parameters the finite element model was able toreproduce the experimental data as close as possible.

Noise and structural dynamics test facilities at Boeing

The noise and structural dynamics laboratories at Boeing provide a wide range of test and measurement services to the Boeing Company. Test data from these laboratories support all phases of the product life cycle across a diverse line of products and applications. The noise laboratory facilities include a low-speed free-jet acoustic wind tunnel, several anechoic and reverberation test chambers, a critical listening facility, and a materials test center. These facilities are supported with a network of data systems for in-lab testing and a variety of transportable data systems for field- and airplane-based testing. Structural dynamics laboratory facilities include large strongbacks and structural floors for component vibration testing, sonic fatigue test facilities, and vibration test facilities. These facilities are supported by a network of dedicated data systems for a wide range of modal, shock, vibration, and fatigue testing. Field tests are supported by a wide range of portable data systems and instrumentation trailers capable of large channel count measurements. This work will provide an overview of the test facilities and measurement capabilities of these laboratories.

Arthroscopy of the wrist for trauma

Over the last decade wrist arthroscopy has proven itself to be a very useful procedure in the management of both bony and soft tissue injuries of the region. Access is achieved through the potential spaces between the dorsal extensor tendons, overlying the joint. Distraction of the wrist joint and instillation of fluid distends the cavity to allow inspection and therapeutic procedures. The radio-carpal and mid-carpal, as well as distal radio-ulna articular surfaces and associated intrinsic and possibly some extrinsic carpal ligaments, are well visualized and dynamic testing of structures is possible. In terms of diagnosis of carpal ligament and triangular fibrocartilage complex (TFCC) injuries it consistently outperforms imaging modalities available. It has also lead to a greater appreciation of the high incidence of occult chondral and ligamentous injury that may co-exist with intraarticular fractures, although the significance of these injuries is debated. Therapeutically it has proven useful in the reconstruction of the articular surface and in the treatment of chondral, intrinsic carpal and TFCC injuries. Results here have been generally comparable to established managements.

MEASURING THE SIX dof DRIVING POINT IMPEDANCE FUNCTION AND AN APPLICATION TO RB INERTIA PROPERTY ESTIMATION

An accurate driving point measurement is imperative in structural dynamic testing. For example, it is used to derive modal scaling, for experimental correlation of finite element models, impedance modelling and extracting the rigid body (RB) inertia properties of an object. A typical driving point measurement gives the linear force/displacement relationship at a single degree of freedom (dof), but any point on an object actually has rotational dofs as well. For example these rotational dofs must be measured in an impedance model where moments are transmitted at the connection point of two substructures. By ignoring the rotations, an inaccurate model will result. In the past, dynamic sensing technology has been limited to the accurate measurement of translational dofs. While rotational sensors do exist, their accuracy is called into question for certain applications. Rotational dofs have tended to be ignored in the measurement process. Applications, which require their use, such as impedance modelling and RB inertia property estimation, have suffered as a result. A process/sensor is being developed to accurately measure the driving point impedance function in all six dofs. The sensor as well as a calibration procedure will be presented here. In order to verify the validity of the calibration and measurement procedure, a new method for measuring the RB inertia properties of an object will be presented. This new method requires an accurate six dof driving point impedance measurement to provide accurate results. The inertia properties of an automotive brake rotor will be measured and compared with the results of a traditional pendulous swing test.

EXACT ADJUSTMENT OF DYNAMIC FORCES IN PRESENCE OF NON-LINEAR FEEDBACK AND SINGULARITY—THEORY AND ALGORITHM

This paper describes the theory and algorithm allowing one to tune a multi-exciter system in order to obtain specified temporal and spatial structural response properties. Considerable effort is being put upon the desire to overcome practical difficulties and limitations as found in real-world systems. The main application that was envisaged for this algorithm is the creation of travelling vibration waves in structures. Such waves may be useful in testing and diagnostic applications or in ultrasonic motors for generating motion. The proposed method adaptively modifies a set of perturbations applied to the model so that an increasing amount of information is extracted from the system. The algorithm strives to overcome the following difficulties: (a) singular model inversion, (b) poor signal to noise ratio, (c) feedback, and (d) certain types of non-linear behaviour. High response levels, exciter–structure coupling and the inherent feedback existing in electro-mechanical systems are demonstrated to cause singularity, poor signal to noise levels and, to some extent, non linear behaviour. These phenomena pose some difficulties under operating conditions commonly encountered during dynamic testing of structures. The tuning of the multi-shaker system is approached in this work, as a non-linear optimisation problem where insight into the physical behaviour is emphasised in choosing the algorithmic strategy. The system's unknown model is inverted in an implicit manner using an automatic orthogonal and adaptive search direction. This adaptation uses the measured responses and forces at each step in order to determine the direction of progression during the tuning process. The non-linear behaviour of the exciters is compensated, in this work, by identification of the high-order (Volterra-like) transfer functions. This high-order model is than inverted allowing one to create a signal that cancels the unwanted harmonics. The proposed approach is analytically shown to converge and the necessary magnitude of perturbations to assure this convergence is derived. A simple case study is used to illustrate the proposed method, while an experimental verification and more examples can be found in a companion paper.

Three-dimensional experimental spatial dynamics modelling of a reciprocating Freon compressor housing

Experimental spatial dynamics modelling encompasses the modelling, postprocessing, and visualization of 3D spatial dynamic response of structures as statistically-qualified complex-valued continuous vector fields derived from experimental structural dynamics testing. A weighted least-squares discrete finite element formulation has been developed to reconstruct the continuous 3D velocity response fields from experimental dynamics data. Dynamic 3D rotations, displacements, and accelerations were naturally derived from the full field velocity response model. Also, dynamic strain and stress fields across the shell elements were extracted by directly postprocessing the dynamic response. A Scanning Laser Doppler Vibrometer (SLDV) was used to measure a single frequency of the 3D dynamic response in an in situ test of an operating reciprocating Freon compression. The SLDV measured the velocity response at thousands of locations on the compressor housing from multiple, positionally-registered locations. The geometry from a preexisting finite element model provided the shape model of the compressor. An experimentally derived spatial dynamics model of the compressor housing was solved and postprocessed results were obtained.

Structural Dynamics Test Simulation and Optimization for Aerospace Components

Structural Dynamics Test Simulation and Optimization for Aerospace Components

Review of full-scale dynamic testing of bridge structures

Full-scale dynamic testing of structures can provide valuable information on the service behaviour and performance of structures. With the growing interest in the structural condition of highway bridges, dynamic testing can be used as a tool for assessing the integrity of bridges. From the measured dynamic response, induced by ambient or forced excitation, modal parameters (natural frequencies, mode shapes and modal damping values) and system parameters (stiffness, mass and damping matrices) can be obtained. These identified parameters can then be used to characterize and monitor the performance of the structure. Analytical models of the structure can also be validated using these parameters. Reasons for conducting full-scale tests are given, and a discussion of the types of excitation devices used in forced vibration testing of large civil engineering structures is also included in the paper. A detailed review of published full-scale dynamic tests conducted on bridge structures is also given.

An Excitation System for Dynamic Testing of Large Structures

Abstract The service behavior and real performance of engineering materials and systems are best evaluated using data obtained from prototype or full-scale testing, or both. For large engineering structures composed of materials such as concrete, with variable properties, data from such tests are invaluable. Full-scale forced vibration testing is a useful tool for obtaining information on the condition of structural systems. As compared to ambient vibration testing, forced vibration testing can provide more accurate estimates of damping capacity and yield data from which better system identification of structural parameters is possible. However, full-scale forced vibration field testing is much more difficult to achieve when compared with laboratory tests on prototypes. The (large) size of the structure usually means special excitation systems are required. The development and performance characteristics of such an excitation system is described in the article. The system consists of an hydraulic actuator, mounted within a purpose built frame, two pumps, an electronic control unit, and a personal computer. It can be used to induce vertical excitation of highway bridges, long-span floor slabs, and similar structures.

Dynamic Scaling of Flexible Structures With Viscoelastic Components

Laboratory dynamic testing of large and complex space structures can be difficult and expensive, if practical at all. The use of dynamically scaled test articles is a standard approach to laboratory testing which helps to avoid these problems. The theory and practice of dynamic scaling for elastic structures are well established and validated; however, the issue of scaling structures with viscoelastic components has not been thoroughly investigated. In a structure that is “replica scaled” by a factor 1/λ, the natural frequencies are each scaled by a factor λ. For proper dynamic scaling, each corresponding damping ratio should be unchanged from that of the original full scale structure. The difficulty that arises with structures incorporating viscoelastic components is that the viscoelastic material properties vary with frequency in a nonlinear fashion. Thus, for a particular mode of a scaled model, the viscoelastic material’s damping properties are evaluated at a frequency which is modified by a factor 1/λ and as a consequence the damping ratio does not scale properly. To solve this problem, the authors propose taking advantage of the temperature dependence of viscoelastic material properties. By appropriately controlling the experimental temperature of the scaled structure, it is possible to obtain accurate dynamic scaling results. In this paper, the proposed frequency-temperature compensation method of dynamic scaling for viscoelastically damped structures is developed and experimentally validated.