Ward Heylen

Damage Detection with Parallel Genetic Algorithms and Operational Modes

The detection of damage with model-based methods is a constrained nonlinear optimization problem. Conventional optimization approaches usually lead to local minima. Furthermore, they are highly sensitive to experimental noise or numerical errors. Genetic algorithms (GAs) provide an attractive alternative since they can potentially explore the entire solution space and reach the global optimum. However, GAs are inherently slow when they work with complicated or time consuming objective functions. To overcome this problem parallel GAs are proposed, and they are particularly easy to implement and provide a superior numerical performance. In this study, a real-coded parallel GA is implemented to detect structural damage. The objective function is based on operational modal data; it considers the initial errors in the numerical model. False damage detection is avoided by using damage penalization. The algorithm is verified with two experimental cases. First, a test structure of an airplane subjected to three increasing levels of damage. Second, a multiple cracked reinforced concrete beam that is subjected to a nonsymmetrical increasing static load to introduce cracks. In both cases, the detected damage has a good correspondence with the experimental damage.

Mixed numerical-experimental identification of elastic properties of orthotropic metal plates

Abstract This paper compares results of three different methods to determine the in-plane elastic properties of sheet materials. Results obtained with standard resonant beam and tensile tests are used to assess a mixed numerical–experimental technique developed to determine the in-plane elastic properties of orthotropic plates from the resonance frequencies of rectangular plate samples (the so-called ‘Resonalyser’ technique). Test materials were selected on the basis of an expected low degree of elastic anisotropy in order to put the accuracy and sensitivity of the different techniques to assess anisotropic materials to a test. Therefore, aluminium alloy and stainless steel samples were prepared from hot-rolled plates, deliberately avoiding pronounced cold-rolling textures. The differences between the results obtained with the three experimental approaches are critically evaluated. In the case of very thin plates, the existing mixed numerical–experimental Resonalyser procedure succeeded in accurately identifying the elastic material properties. A slightly adapted procedure is proposed to extend the applicability of the Resonalyser procedure to thicker plates.

Comparison of induction machine stator vibration spectra induced by reluctance forces and magnetostriction

For rotating electric machines, the reluctance forces (Maxwell stresses) acting on the stator teeth are a major cause of noise emission. Next to the reluctance forces, magnetostriction is a potential cause of additional noise from electric machines. First, a thermal stress analogy is used to introduce magnetostriction in the finite-element framework. Next, we present the computation and comparison of the stator vibration spectra caused by these two effects separately, by example of a 45 kW induction machine. Moreover, two kinds of magnetostriction characteristics of the stator yoke material are compared: a quadratic /spl lambda/(B) curve and a /spl lambda/(B) curve with zero-crossing around 1.5 Tesla.

Local magnetostriction forces for finite element analysis

The magnetic materials deformation caused by magnetostriction is represented by an equivalent set of mechanical forces, giving the same deformation to the material as magnetostriction does. This is done in a way similar to how thermal stresses are usually incorporated. The resulting magnetostriction force distribution is summed to other force distributions (external mechanical forces, magnetic forces) before starting the mechanical deformation or vibration analysis. This procedure is incorporated into a weakly coupled cascade solving of the magnetomechanical problem.

Structural damage assessment under varying temperature conditions

Modal parameters such as natural frequencies and mode shapes are sensitive indicators of structural damage. However, they are not only sensitive to damage, but also to the environmental conditions such as, humidity, wind and most important, temperature. For civil engineering structures, modal changes produced by environmental conditions can be equivalent or greater than the ones produced by damage. This article proposes a damage detection method which is able to deal with temperature variations. The objective function correlates mode shapes and natural frequencies, and a parallel genetic algorithm handles the inverse problem. The numerical model of the structure assumes that the elasticity modulus of the materials is temperature-dependent. The algorithm updates the temperature and damage parameters together. Therefore, it is possible to distinguish between temperature effects and real damage events. Simulated data of a three-span bridge and experimental one of the I-40 Bridge validate the proposed methodology. Results show that the proposed algorithm is able to assess the experimental damage despite of temperature variations.

Trends in experimental modal analysis

Abstract The scope of this paper is to comment on current trends and new developments in the field of experimental modal analysis. The first section covers modal measurement and estimation procedures, with special emphasis on the use and limitations of recent techniques such as multiple input processing, total least square, global time- and frequency domain parameter estimation. In the second section reference is made to applications and use of modal parameters in techniques such as structural modification, fatigue and acoustic analysis. Emphasis is put on applications used at the Katholieke Universiteit, Leuven.

Identification and Prediction of Frame Structure Dynamics by Spatial Matrix Identification Method

This paper presents the results of using an experimental spatial matrix identification method to predict the dynamics of a frame structure under various boundary conditions. The single-input-multiple-output frequency response functions (FRFs) of the test structure under the free-free boundary condition are measured by hammer testing. Using the FRFs, a set of spatial matrices is constructed in order to represent the structural dynamic characteristics of the system by the new method. Using the spatial matrices, the dynamic characteristics of the test structure under the boundary condition of clamping 4 points is predicted. The prediction is adequately accurate for practical application. The results of the prediction demonstrate that the spatial matrices identified by this method can be used for structural modification and substructure synthesis in the field of computer-aided mechanical engineering.

Finite element based expressions for Lorentz, reluctance and magnetostriction forces

The magnetic and mechanical finite element systems are combined into one magnetomechanical system. Investigating the coupling terms results in a finite element expression for the magnetic forces (Lorentz force and reluctance force) for both the linear and nonlinear case. The material deformation caused by magnetostriction is represented by an equivalent set of mechanical forces, giving the same strain to the material as magnetostriction does. The resulting magnetostriction force distribution is superposed onto other force distributions (external mechanical forces, magnetic forces) before starting the mechanical deformation or vibration analysis. This procedure is incorporated into a weakly‐coupled cascade solving of the magnetomechanical problem.

Statistical energy analysis of acoustic noise and vibration for electric motors: transmission from air gap field to motor frame

A large number of vibration measurements are performed on the stator of a standard 5.2 kW electric motor. The rotor and the end-caps are removed. First, vibration measurements are performed on the stator without coils, i.e. consisting of the stator yoke (ferromagnetic iron) and the motor frame (cast iron) only. Second, vibration measurements are carried out on an identical stator with a standard coil system in the stator slots. Subsequently, a statistical energy analysis (SEA) is performed using these experimental data, in order to quantify the internal losses in the stator yoke and in the coils. The SEA also allows us to quantify the transmission of vibrations from stator yoke to motor frame (coupling loss factors), and to consider the influence of the presence of the coil system.

Determining the Elastic Moduli of the Individual Component Layers of Cylindrical Thermal Barrier Coatings by Means of a Mixed Numerical - Experimental Technique

Cylindrical specimens made of the Ni-based super-alloy Inconel 625 (IN 625) were coated with (a) NiCoCrAlY, or (b) NiCoCrAlY and yttria-stabilised zirconia (YSZ: in this case, zirconia with 7-8 wt% yttria), using the electron beam - physical vapor deposition (EB-PVD) technique. In the bi-layer coatings, the YSZ layer is the thermal barrier coating (TBC) and the NiCoCrAlY layer is the metallic bond coat (BC). The BC improves the bonding between the substrate and the ceramic TBC, while the low thermal conductivity of the TBC oers high-temperature protection to the substrate. This paper focuses on the determination of the elastic moduli of the substrate and the coating layers of the test samples. The elastic moduli of the three dierent materials (IN 625, NiCoCrAlY and YSZ) were determined by means of a mixed numerical - experimental technique (MNET). The employed MNET was based on the comparison of the experimentally measured resonant frequencies of the rst bending mode of the test samples to the numerically calculated ones. The unknown elastic properties were determined by ne-tuning the elastic material parameters of the numerical models so as to enable the reproduction of the experimentally measured resonant frequencies.