Rune Brincker

Modal identification of output-only systems using frequency domain decomposition

In this paper a new frequency domain technique is introduced for the modal identification of output-only systems, i.e. in the case where the modal parameters must be estimated without knowing the input exciting the system. By its user friendliness the technique is closely related to the classical approach where the modal parameters are estimated by simple peak picking. However, by introducing a decomposition of the spectral density function matrix, the response spectra can be separated into a set of single degree of freedom systems, each corresponding to an individual mode. By using this decomposition technique close modes can be identified with high accuracy even in the case of strong noise contamination of the signals. Also, the technique clearly indicates harmonic components in the response signals.

Pullout behavior of steel fibers from cement-based composites

Abstract A comprehensive experimental program on pullout tests of steel fibers from cement based matrices is described. A specially designed single fiber pullout apparatus was used to provide a quantitative determination of interfacial properties that are relevant to toughening brittle materials through fiber reinforcement. The parameters investigated included a specially designed high strength cement based matrix called Densified Small Particles system (DSP), a conventional mortar matrix, fiber embeddment length, and the fiber volume fraction. The mediums from which the fiber was pulled included a control mortar mix without fibers, a mortar mix with 3, and 6 percent fibers by volume. The results indicate that: (1) The dense DSP matrix has significantly improved interfacial properties as compared to the conventional mortar matrix. (2) Increasing the fiber embeddment length and the fiber volume fraction in the cement matrix increase the peak pullout load and the pullout work. (3) The major bond mechanism in both systems is frictional sliding.

Size Effects on the Bending Behaviour of Reinforced Concrete Beams

ABSTRACT Load-deformation curves for reinforced concrete beams subjected to bending show size effects due to tensile failure of the concrete at early stages in the failure process and due to compression failure of the concrete when the final failure takes place. In this paper these effects are modelled using fracture mechanical concepts, and size effects of the models are studied and compared with experimental results.

Fracture mechanical Markov chain crack growth model

Abstract On the basis of the B -model developed in [J. L. Bogdanoff and F. Kozin, Probabilistic Models of Cumulative Damage. John Wiley, New York (1985)] a new numerical model incorporating the physical knowledge of fatigue crack propagation is developed. The model is based on the assumption that the crack propagation process can be described by a discrete space Markov theory. The model is applicable to deterministic as well as to random loading. Once the model parameters for a given material have been determined, the results can be used for any structure as soon as the geometrical function is known.

Impact Excitation Processing for Improved Frequency Response Quality

Impact excitation is the most common excitation type for measurements of frequency response functions for modal analysis and other purposes. The method used is almost always based on setting the data acquisition system up with triggering, fixed FFT analysis settings, and then using an accept/reject step where each impact is either accepted if the impact seems good, or rejected if it contained some error such as double impacts or overload. This method has several drawbacks that often lead to non-optimal frequency responses. In this paper, an improved method based on time recording of all signals and subsequent post processing is proposed. The data acquisition part is made easier with the proposed method, while at the same time the importance of a skilled operator is reduced. It is shown on a real test structure that the quality of the resulting frequency responses can be significantly improved (measured by the coherence function) compared to the traditional method, and at the same time the total acquisition time can be shortened. An automatic optimization procedure which allows for fully automated post processing is proposed.

Modal Parameters from a Wind Turbine Wing by Operational Modal Analysis

Operational Modal Analysis also known as Ambient Modal Analysis has an increasing interest in mechanical engineering. Especially on big structures where the excitation and not less important the determination of the forces is most often a problem. In a structure like a wind turbine wing where the modes occur both close in frequency and bidirectional the Ambient excitation has big advantages. In this paper modal parameters are identified from the wing by operational modal analysis. For the parameter identification both parametric and non-parametric techniques are used. Advantages and disadvantages are discussed and results from the different techniques are compared.

A general procedure for estimating dynamic displacements using strain measurements and operational modal analysis

Measurement systems are being installed in more and more civil structures with the purpose of monitoring the general dynamic behavior of the structure. The instrumentation is typically done with accelerometers, where experimental frequencies and mode shapes can be identified using modal analysis and used in health monitoring algorithms. But the use of accelerometers is not suitable for all structures. Structures like wind turbine blades and wings on airplanes can be exposed to lightning, which can cause the measurement systems to fail. Structures like these are often equipped with fiber sensors measuring the in-plane deformation. This paper proposes a method in which the displacement mode shapes and responses can be predicted using only strain measurements. The method relies on the newly discovered principle of local correspondence, which states that each experimental mode can be expressed as a unique subset of finite element modes. In this paper the technique is further developed to predict the mode shapes in different states of the structure. Once an estimate of the modes is found, responses can be predicted using the superposition of the modal coordinates weighted by the mode shapes. The method is validated with experimental tests on a scaled model of a two-span bridge installed with strain gauges. Random load was applied to simulate a civil structure under operating condition, and strain mode shapes were identified using operational modal analysis.

Estimation of Unmeasured DOF’s Using the Local Correspondence Principle

This paper will present a new method to estimate unmeasured Degrees of Freedom (DOF’s) in a structure using only a limited amount of sensors. The method applies the Locale Correspondence Principle (LCP) to estimate a linear transformation between experimentally determined mode shapes, and mode shapes from a Finite Element (FE) Model. Mode shapes from the FE model are then altered to fit the real mode shapes of the structure, creating a set of estimated mode shapes covering all nodes.

Expansion of Mode Shapes and Responses on the Offshore Platform Valdemar

There is a need in the future for maintaining and increasing oil and gas production in the Danish North Sea. Related to this are studies for exploring the potential for extending the lifetime of offshore platforms by implementation of Structural Monitoring Systems (SMS). The project, which this paper is based on, uses an expansion technique as a first step in the sequence of assessing the actual lifetime of a platform. Mode shapes and natural frequencies are estimated using operational modal analysis. The mode shapes are then expanded by expressing each experimental mode shape as an optimal linear combination of selected modes from a finite element model. The offshore platform, Valdemar, which is fully instrumented with accelerometers, GPS, strain gauges and wave radars, is chosen as a case study. Results show that the measured response can be expanded with high precision, which provides valuable information when assessing the actual lifetime of the platform. Also it is shown that the expansion technique can be used for assessment of measurement uncertainties.

Strain Estimation in a Glass Beam Using Operational Modal Analysis

A potential application of operational modal analysis is the prediction of strain or stress time histories which, on the other hand, are one of the most important sources of uncertainty in fatigue design and remaining fatigue life calculations. This is due to the difficulty of estimating the stiffness, mass and damping properties with accuracy, as well as the use of simplified loading models.