Key Engineering Materials Vols. 293-294

Abstract: Life consumption evaluation presently represents a high priority topic for Industry and University. In the specific work of this paper, we performed an analysis about the effects of thermal transients on life consumption of steam turbine rotors and an optimization of the thermal transient during runups. This kind of operation, more and more recurrent in power generation, produces a stress in the mechanical elements that could compromise their integrity. The work was developed in its larger part in the MGMV laboratory of the University of Genova, using the equipment (hardware and software) present in this structure, in cooperation with industry. The study moves from a classification of the damage mechanism and from a description of the most important manufacturing technologies of turbine rotors. Then, after a glance at the most important commercial software for estimating life consumption, a classification is given referring to the checks that Ansaldo, the industrial collaborator, performs during its service interventions. Finally, the paper reports the calculation module developed according to the specifications given by the industrial collaborator; the software was developed in Matlab and needs as input the characteristic constants of the rotor and the temperature measurements (with high precision transducers) at the inlet of the HP and IP sections of the turbine. This study confirms that thermal transients are very important to define the rotor’s residual life; the results could help the manufacturer, who designed the machines, and the operators, who run the power plant, to ensure a correct use of the installation, without unexpected failures that could create an economical damage connected with a missed production.

Abstract: Thermoelastic Stress Analysis (TSA) has been used to detect and evaluate the severity of damage on a flat metallic plate. The damage takes the form of a semi-circular notch that represents a surface flaw. Thermoelastic data was gathered from the undamaged side of the plate. The experimental results show that shallow surface flaws can be detected by using phase information from thermoelastic data. This information can then be used to indicate the flaw severity in terms of the notch depth. It is shown that the phase data is dependent on the heat conduction effects around the notch, which enable an assessment of the damage. This is modelled using a simple finite element simulation of the effects of heat conduction on the thermoelastic response. A discussion on the potential of using phase variation across damaged regions to analyse damage severity is provided.

Abstract: The paper describes an on-going research effort aimed at detecting the presence of delamination damage in composite panels based upon their higher-frequency structural response. Two alternative damage indexes are examined that facilitate the identification of the location and extent of delaminations. The damage indexes do not require vibration measurements to be performed on the undamaged structure. Use is made of the bending and twisting curvatures corresponding to the higher-frequency mode shapes that are post-processed via two different smoothing techniques. The modal data are obtained via finite element models based on Mindlin theory and including delaminations. These are introduced using a sub-laminate strategy that permits multiple damages to be modelled through the thickness. Various delamination sizes and locations are examined with a random noise superposed on the data in order to ascertain the degree of sensitivity of the damage index to the noise in the experimental data.

Abstract: In this contribution we present a validation of an identification procedure and a modeling method with regard to detection, localisation and quantification of damage in a structure. Vibration measurements of an excited experimental structure are used as input for a stochastic subspace system identification algorithm. The identified experimental modal data (eigenvalues and mode shapes) serve to update the underlying finite element model. The experimental setup consists of a cantilever beam and an additional equipment to damage the structure locally and progressively. In contrast to earlier contributions the evolution of damage is quantified in order to estimate the lifetime of the structure.

Abstract: This paper presents a comparative study of three enhanced signal processing methods to locate damage on mode shape data. The first method called curvature mode shape is used as a reference. The second tool uses wavelet transform and singularity detection theory to locate damage. Finally we introduce the windowed fractal dimension of a signal as a tool to easily measure the local complexity of a signal. Our benchmark aims at comparing the crack detection using optimal spatial sampling under different severity, beam boundary conditions (BCs) and added noise measurements.

Abstract: In this paper a method for estimating the damage location in beam and plate structures is presented. A plexiglas cantilever beam and a plexiglas plate with one fixed boundary condition are tested experimentally. The dynamic response of the structures is analyzed using one and twodimensional continuous wavelet transforms of the structural mode shapes. The location of the crack is indicated by a peak in the spatial variation of the transformed response. The proposed wavelet analysis can effectively identify the crack position without knowledge of either the structure characteristics or its mathematical model. The practical application of the technique for civil engineering structures requires only the response of the structure.

Abstract: This work investigates the use of two different vibration-based methods for health monitoring of aircraft wings. A finite element model of a simplified wing is used to model and predict the vibration response of an aircraft wing in an intact condition and in the presence of different types and levels of damage. Two main types of damage are considered- cracks and distributed damage. This study first explores the sensitivity of the lower modal frequencies to different damage levels of the studied types. Then the employment of the frequency response functions subjected to principal components analysis is discussed. This is an early model-based study which is intended to establish if the considered procedures can be used as damage detection tools.

Abstract: Structural properties of machinery elements and sub-assemblies are often used for diagnostic purpose. Occasionally in engineering practice for some technical objects there is no description of distribution of structural parameter values available. Results of experimental evaluation of this distribution are difficult to be applied for diagnostic purpose as a result of lack of a reference distribution for comparison purposes. Nevertheless, for some technical objects, the inherent properties of the structural parameter values distribution may be used in such a case. In the paper an example of a rotating shaft is considered for which the axial symmetry of structural parameter values distribution is expected. Application of impact testing for the purpose of identification of the modal model is an easy way to check this symmetry and to use the results for diagnostic purpose. The technique applied to an example shaft indicated its failure but proved to be inefficient in localization of the failure.

Abstract: The paper presents the course and results of crack propagation simulation research. The object taken into account is a large power turbo-set rotor. The computer code system NLDW is presented. It uses a non-linear model of journal bearings, and well known crack model. Crack depth is marked by a crack coefficient. It is shown the crack generates a coupled forms of lateral, axial and torsional vibrations in multi-support rotor. Their intensity depends on the axial and circumferential crack location on the shaft. The attempt at pointing a proper diagnostic indicator for crack detection in large rotating machine is made according to obtained results.

Abstract: Minor and random slip between rolling elements and races in rolling element bearings makes vibration signals have periodically time-varying ensemble statistics, which is known as cyclostationarity. Two second-order cyclostationary methods, the spectral correlation density (SCD) and the degree of cyclostationarity (DCS), are talked about in this paper based on a statistical model of rolling element bearings. The SCD provides redundant information in bi-frequency plane and cyclic frequency domain embodies the majority of it, which is a series of non-zero discrete cyclic frequencies completely reflecting the fault characters of rolling element bearings. The DCS has virtues of less computation and clearer representation, at the same time keeps the same characters with SCD in cyclic frequency domain. And the DCS is also proved to be resistant to the additive and multiplicative stationary noise. Simulation and experiential results from three rolling element bearing faults: outer race defect, inner race defect and rolling element defect, indicate practicability of the DCS analysis in rolling element bearing condition monitoring and fault diagnosis.