Transient displacement analysis using double-pulsed ESPI and fringe processing methods

This thesis deals with techniques for the displacement measurement of fast transient phenomena using ESPI. Four main contributions are presented. First, a computer model for speckle noise and ESPI fringe generation is proposed. An assessment methodology for speckle noise reduction algorithms is then derived using the computer model. Then the noise in the ESPI fringe patterns is analysed using computer generated speckle and several solutions for its reduction are proposed and assessed. Finally, a fast electro-optical system is presented as a solution to the unambiguous phase extraction problem from a single interferogram. With this novel system, whole field transient displacements occurring in time intervals as short as 20ns can be successfully registered and retrieved. [Continues.

Dynamic digital shearography for on-board robotic non-destructive testing of wind turbine blades

Structural integrity plays a critical role in development of infrastructural construction and support facilities. During the lifespan of most large-scale equipment, condition monitoring and periodic inspection is indispensable for ensuring structural health and evaluation of service condition. Wind turbine blades are the most important component of wind turbines and demands regular inspection to detect defects, which often occur underneath a blade surface. Current methods used to inspect wind turbine blades include to send NDT operators to climb the tower for on-site inspection of the blades’ surface or to dismantle the blades for inspection on the ground. These approaches are time-consuming, costly and pose risks of injury to human inspectors. Thus, it is necessary to develop a technological method for wind turbine blade on-site inspection of wind turbine blades. Digital shearography based on laser interferometry has demonstrated its prominent capability for inspecting composite material which is the main material used in the construction of wind turbine blades. Shearography is a ramification of holography interferometry and is more efficient to be used as a non-destructive testing (NDT) technique owing to its improved robustness and sensitivity to surface displacement. Robotic climbers, on the other hand, have recently drawn significant interest in NDT applications to replace human inspectors in extreme conditions. Thus, this thesis presents investigations into the development of a robotic NDT method using digital shearography for on-site inspection of wind turbine blades. The development of the shearography unit with correlation fringe pattern acquisition and the integration of this unit with the robotic climber adhering to wind turbine blades using vacuum generators are described in this thesis. The successful conduction of the indoor and outdoor trails for the integrated system verifies that shearography holds the ability to be used as an NDT tool for on-site wind turbine blade inspection, and that the climbing robot is able to access most areas of a wind turbine blade and stabilise itself to remove the impact on the shearography of the high frequencies from the climber’s vacuum motor and the low frequencies from the blade swing. Temporal phase shift shearography, and the fast phase map acquisition methods with less steps are evaluated in the thesis. Experiments are performed in lab with phase maps obtained using different algorithms. Apart from the conventional 4 steps and 3 steps phase shift algorithms, the modified 4+1 and 3+1 temporal phase shifting algorithms are developed for more suitability of semi-dynamic inspection by firstly calculating the correlation fringes and followed by the phase map calculations. The results of these modified methods are compared with the conventional 4 steps and 3 steps methods and are shown with equal qualities. Moreover, the reduced steps of phase shifting, i.e., 2+1 phase shifting methods are conducted for semi-dynamic phase map acquisition. It is found that the temporal phase shifting methods are not suitable for dynamic wind turbine blade inspection, however, the fast semi-dynamic temporal phase shift algorithms are able to produce phase maps with lower clarity. Pixelated spatial phase shift shearography is developed to remedy the limitation of temporal phase shift techniques. It adopts a micro-polarization sensor in the complementary metal oxide semiconductor (CMOS) camera, two linear polarizers, and a quarter waveplate as a new arrangement of optical path to replace the piezoelectric transducer stepper as the phase stepper. Three algorithms are introduced based on this novel developed system. Additionally, the site of view is enlarged for upgrading of the system. The development of the pixelated spatial phase shift shearography has mitigated the static processing limitation on temporal phase shift shearography, which caters for the demands of on-site NDT operation. At the same time, it remedies the current real-time shearography system which is not able to produce phase distributions for further quantitative analysis. The new developed pixelated spatial phase shift shearography system is thus more suitable for WTB on board inspection than both conventional and less-steps temporal phase shift shearography system. The field of view enlargement optimisation in the new developed spatial phase shift system indirectly reduces the distance for the inspection process and meanwhile enlarges the site of view, which consequently reduces the weight and structural complexity of the robotic-shearography integration system. The research addresses and resolves the difficulty of on-board wind turbine blade inspection with a novel robotic NDT approach using digital shearography. The approach is significant for real world industrial applications. Moreover, through the temporal and spatial phase shift evaluation, the research proves the feasibility of dynamically obtaining phase maps by the shearography system for further quantitative analysis without using temporal phase shift devices

Some comments on particle image displacement velocimetry

Laser speckle velocimetry (LSV) or particle image displacement velocimetry, is introduced. This technique provides the simultaneous visualization of the two-dimensional streamline pattern in unsteady flows as well as the quantification of the velocity field over an entire plane. The advantage of this technique is that the velocity field can be measured over an entire plane of the flow field simultaneously, with accuracy and spatial resolution. From this the instantaneous vorticity field can be easily obtained. This constitutes a great asset for the study of a variety of flows that evolve stochastically in both space and time. The basic concept of LSV; methods of data acquisition and reduction, examples of its use, and parameters that affect its utilization are described

Some applications of holographic interferometry and Speckle correlation techniques to the study of plant growth and physiology

Imperial Users onl

Development of optical phase evaluation techniques: Application to fringe projection and digital speckle measurement

Ph.DDOCTOR OF PHILOSOPH

On the integration of deformation and relief measurement using ESPI

The combination of relief and deformation measurement is investigated for improving the accuracy of Electronic Speckle-Pattern Interferometry (ESPI) data. The nature of sensitivity variations within different types of interferometers and with different shapes of objects is analysed, revealing significant variations for some common interferometers. Novel techniques are developed for real-time measurement of dynamic events by means of carrier fringes. This allows quantification of deformation and relief, where the latter is used in the correction of the sensitivity variations of the former

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.

Optical Techniques for Defect Evaluation in Vehicles

The optical techniques are a powerful tool on situations where either the physical contact or invasive techniques for evaluation are not suitable. Vehicle environments constitute an application field for the optical techniques and are the focus of this chapter. In order to reinforce this kind of techniques, it must be clarified that the idea to manipulate the light backs to the second century before our age, when Archimedes planned to destroy enemy ships using a solar heat ray with an array of actuators to change the shape of a mirror (Bifano T., 2011). Therefore, the field of photonics is the one that offers the possibility to achieve one of the greatest realizations and applications because the light is present in all aspects of the human life and our way of living is impossible without light (Carmo J. P. et al., 2012a). Optical measurement systems are also suitable for harsh monitorization because they are non-contact and full-field techniques. This is the case of Moiré Interferometry, which is used for many optoelectronic applications as displacement measurements (Wronkowski L., 1995), evaluation of microelectronics devices deformation (Xie H. et al., 2004), optical communications (Chen L. et al., 2000), strain measurements with Fiber Bragg Grattings, FBGs, (Silva A. F. et al., 2011) and spectrography (Kong S. H. et al., 2001). In this context, it must be noted that the recent nuclear disaster in Fukushima, Japan, confirms the need of tighter security measures be done within harsh environments (which includes the automobiles) in order to increase both the safety of people and the reliability of vehicles’ parts