Structural uncertainty identification using mode shape information

This thesis is concerned with efficient uncertainty identification (UI) – namely the nonlinear inverse problem of establishing specific statistical properties of an uncertain structure from a practically-limited supply of low-frequency dynamic response information. An established UI approach (published in 2005) which uses Maximum Likelihood Estimation (MLE) and the Perturbation Method of uncertainty propagation is adopted for the study using (for the first time) mode shape information rather than just natural or resonant frequencies. The thesis develops a method based on the use of selected coefficients in a generalized displacement model i.e. a weighted series of spatially-continuous multiply-differentiable base functions to approximate the structural free-vibration response of an uncertain structure. The focus is placed on the estimation (from relatively small data sets) of the statistical properties of the location of an attached point-mass with normally-distributed position. Simulated data for uncertain point-mass-loaded linear beam and plate structures is initially used to test the method making use of as much exact or closed-form differentiable information as possible to obtain frequencies and mode shapes. In the case of plate structures, extensive use is made of the Rayleigh Ritz method to generate the required response coefficients. This is shown to have significant advantages over alternatives such as the Finite Element method. The approach developed for use with free vibration information is then tested on measured experimental data obtained from an acoustically-forced clamped plate. Structural displacement measurements are taken from the plate using Vibromap 1000, a commercially-available ESPI-based holomodal measurement system capable of wide-field vibration response observation in real-time, or quantitative displacement response measurement. The thesis shows that the developed uncertainty identification method works well for beams and plates using simulated free-vibration dat

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Applications on Ultrasonic Wave

This book presents applications on the ultrasonic wave for material characterization and nondestructive evaluations. It could be of interest to the researchers and students who are studying on the fields of ultrasonic waves

Advanced experimental methods for the characterization of welded structures

Welding is one of the most prevalent techniques for mechanical fastening of metals. Recent developments in welding technology have led to welding techniques being more readily employed in safety-critical engineering structures. Since the existence of residual stresses and material property variation around welds affects the mechanical performance and thereby structural integrity, it is essential to improve our knowledge in understanding and modelling the mechanical response of the welded structures. The present work focuses on mechanical characterizations of such structures. This work can be broadly classified into two parts; the first part investigates the residual stress distribution in welded specimens of different metals and the second part presents investigations of mechanical properties in welded specimen using full field optical techniques. A newly invented destructive technique for residual stress measurement, the contour method, was used for the investigations of the residual stress in welded joints in this study. The principle of the contour method is based on a variation of Bueckner's superposition theory. By means of a single straight cut, the 2D residual stress component normal to the region of interest can be determined. In this work, first the numerical simulations of the contour method using two and three dimensional bodies have been demonstrated. The contour method was then applied to one-pass and three-pass groove weld specimens and the results obtained from the contour method were compared to those obtained by the neutron diffraction strain measurement technique. The capability of the contour method to measure multiple residual stress components was also investigated in this project. A recent development of the contour method of stress measurement, the multi-axial contour method, permits measurement of the 3D residual stress distribution in a body, based on the assumption that the residual stresses are due to an inelastic misfit strain (eigenstrain) that does not change when a sample containing residual stresses is sectioned. The eigenstrain is derived from measured displacements due to residual stress relaxation when the specimen is sectioned. By carrying out multiple cuts, the full residual stress tensor in a continuously processed body can then be determined. In this study, finite element simulations of the technique were carried out to verify the method numerically. The method was then used to determine the residual stresses in a VPP A-welded sample, and the results were validated by neutron diffraction measurements. As part of the characterization of the welded structures, a study was undertaken to develop a method of extracting local mechanical properties from weld metal by strain mapping using the digital image correlation (DIC) technique. The feasibility of determining local stress-strain behaviour in the weld zone of a 316H stainless steel pipe with a girth weld was investigated by tensile tests on miniature and standard tensile test specimens. In addition, electron speckle pattern interferometry (ESPI) was utilized to obtain the full-field strain maps of a standard tensile specimen during loading and compared to those obtained in the same specimen by digital image correlation in order to verify the DIC measurements

Analysis of error functions in speckle shearing interferometry

Electronic Speckle Pattern Shearing Interferometry (ESPSI) or shearography has successfully been used in NDT for slope (δw/δx and/or δw/δy) measurement while strain measurement (δu/δx, δv/δy, δu/δy and δv/δx) is still under investigation This method is well accepted in industrial applications especially in the aerospace industry. Demand of this method is increasing due to complexity of the test materials and objects. ESPSI has successfully performed in NOT only for qualitative measurement whilst quantitative measurement is the current aim of many manufacturers. Industrial use of such equipment is being completed without considering the errors arising from numerous sources, including wavefront divergence. The majority of commercial systems are operated with diverging object illumination wavefronts without considering the curvature of the object illumination wavefront or the object geometry, when calculating the interferometer fringe function and quantifying data. This thesis reports the novel approach in quantified maximum phase change difference analysis for derivative out-of-plane (OOP) and in-plane (IP) cases that propagate from the divergent illumination wavefront compared to collimated illumination. [Continues.

Vibration-based structural health monitoring of composite structures

Composite materials are in use in several applications, for example, aircraft structural components, because of their light weight and high strength. However the delamination which is one of the serious defects often develops and propagates due to vibration during the service of the structure. The presence of this defect warrants the design life of the structure and the safety. Hence the presence of such defect has to be detected in time to plan the remedial action well in advance. There are a number of methods in the literature for damage detection. They are either 'baseline free/reference free method' or using the data from the healthy structure for damage detection. However very limited vibration-based methods are available in the literature for delamination detection in composite structures. Many of these methods are just simulated studies without experimental validation. Grossly 2 kinds of the approaches have been suggested in the literature, one related to low frequency methods and other high frequency methods. In low frequency approaches, the change in the modal parameters, curvatures, etc. is compared with the healthy structure as the reference, however in the high frequency approaches, excitation of structures at higher modes of the order of few kHz or more needed with distributed sensors to map the deflection for identification of delamination. Use of high frequency methods imposes the limitations on the use of the conventional electromagnetic shaker and vibration sensors, whereas the low frequency methods may not be feasible for practical purpose because it often requires data from the healthy state which may not be available for old structures. Hence the objective of this research is to develop a novel reference-free method which can just use the vibration responses at a few lower modes using a conventional shaker and vibration sensors (accelerometers/laser vibrometers). It is believed that the delaminated layers will interact nonlinearly when excited externally. Hence this mechanism has been utilised in the numerical simulations and the experiments on the healthy and delaminated composite plates. Two methods have been developed here - first method can quickly identify the presence of the delamination when excited at just few lower modes and other method identify the location once the presence of the delamination is confirmed. In the first approach an averaged normalised RMS has been suggested and experimentally validated for this purpose. Latter the vibration data have then been analysed further to identify the location of delamination and its size. Initially, the measured acceleration responses from the composite plates have been differentiated twice to amplify the nonlinear interaction clearly in case of delaminated plate and then kurtosis was calculated at each measured location to identify the delamination location. The method has further been simplified by just using the harmonics in the measured responses to identify the location. The thesis presents the process of the development of the novel methods, details of analysis, observations and results.EThOS - Electronic Theses Online ServiceHigher Education Commision of PakistanNESCOM, PakistanGBUnited Kingdo