The subject of this thesis is the development of new ultrasound inspection techniques for detection of cracks that are smaller than the wavelength of the inspecting wave and the characterization of cracks in fluid-filled pipes as either surface-breaking or subsurface. The spectrum of the scattered field of a partially closed crack comprises harmonic components not expected to be found in the case of linear scatterers such as pores or inclusions. Paper A presents an experimental investigation into the linear reflection and generation of the 2nd harmonic component following the incidence of an ultrasonic wave onto a dry or water-confining interface formed by elasto-plastic steel-steel surfaces in contact. The results indicate that water has an unexpected effect on the reflection, at low interfacial pressures, suggesting that fluid mediated forces play a role not accounted for in current models. The level of the generation of the 2nd harmonic measured provides support for further development of the technique for detection of dry, partially closed cracks or fluid-filled, nearly open cracks. A theoretical model describing the nonlinear scattering of acoustic waves by surface-breaking cracks with faces in partial contact is presented in Paper B. Both linear and nonlinear response of the crack are shown to be the largest for a SV wave incident on the surface containing the crack at an angle just above the critical angle for longitudinal waves. A method which provides information on whether a fracture is surface-breaking or subsurface has been modelled and its optimal experimental set-up examined in Paper C. The main assumption of the model is that water carried by pressurized pipes infiltrates and fills a surface-breaking crack, while a subsurface crack is dry. The model simulates an inspection in which the modulation technique is employed and the surface hosting the crack is not accessible. A parameter, constructed with signals recorded in backscattering configuration during a modulation cycle, is examined and shown to provide a clear criterion to distinguish subsurface from surface-breaking cracks when a SV wave at 45 degree incidence is employed as a probe. Finally, in Paper D the modulation technique is experimentally tested on steel beams that host surface-breaking fatigue cracks. The method is shown to be a successful tool to distinguish a dry from a fluid-filled crack. Furthermore, it is revealed that the dynamics of the fluid needs to be accounted for in a more accurate simulation tool. / QC 20100906

The purpose of this study is to develop a non-contact air-coupled NDT method to identify and characterize surface-breaking cracks in concrete structures using surface wave transmission measurements. It has been found that the surface wave transmission (SWT) across a surface-breaking crack is related to the crack depth. However, inconsistence was noticed in surface wave transmission measurements. In this dissertation, the author first summarized limitations of the current SWT method for application to concrete structures, which include inconsistent sensor coupling, near-field effect of sensors, effects of crack width, external loading effect on surface wave transmission coefficient, and lack of a repeatable source. In this dissertation, the author attempts to find solutions to the aforementioned problems. First, non-contact air-coupled sensors were applied to the SWT method to reduce experimental errors caused by inconsistent coupling condition of conventional contact sensors. Air-coupled sensing enables reliable and consistent results, and significantly improves test-speed. Results from laboratory and field tests demonstrate effectiveness of air-coupled sensors. Second, appropriate sensor-to-source configurations are proposed to reduce undesirable effects: (i) the near-field effect of sensors around a crack, and (ii) contribution of multiple modes in a plate-like structure with a finite thickness. Near-scattering of surface waves interacting with a surface-breaking crack was investigated using numerical simulations (finite element method) and experimental studies over a wide range of the normalized crack depth (h/λ: crack depth normalized by wavelength of surface waves) and the normalized frequency-thickness ratio (f-H/CR: frequency-thick normalized by Rayleigh wave speed). Third, effects of external loadings on transmission coefficient of surface waves in concrete were investigated through a series of experimental studies. In the research, variation of the transmission coefficient is presented as a function of crack mouth opening displacement (CMOD). This provides a guideline on minimum CMOD to which the SWT method can be reasonably applied. In addition, the author experimentally demonstrates that using low-cost piezoceramic sensors is effective in generating consistent stress waves in concrete. Finally, the author demonstrates that the air-coupled SWT method developed in this study is effective for in-situ estimates of a surface-breaking crack in large concrete structures. / text

In this thesis, two ultrasonic timing methods are used in order to investigate the accuracy and reliability of measurements for surface breaking cracks having different orientations and heights. Also the best applicable measurement technique is searched by comparing the received test results. These methods are the Time of Flight Diffraction (TOFD) Method using diffraction of longitudinal waves and another method using the reflection of shear waves from the crack tips. In order to simulate and measure the height of surface breaking cracks three sets of test blocks from steel, and two sets of wedges from plexiglas material are manufactured. Also several probes having frequencies of 2Mhz, 4Mhz, 5Mhz and angles of 45o and 70o are used. Some test procedures are created to make realistic comparisons between the test results and the ones found by previous studies in literature. The results are compared according to the standard deviations of errors in crack height measurements and it is found that the depth, orientation of defects and the frequency of probes have considerable affect on the results. With wider probe angles and higher frequencies of probes to some extent the errors are observed to be running low and the height of cracks could be measured closer to the original size. The amount of the errors is increased in measurements with the increasing angle of cracks. The results of both methods are found to be very satisfactory. A range of &plusmn / 0.5 mm for means of error from the original vertical crack heights is determined. The results agree with the previous studies.

This research investigates the modeling of randomly distributed surface-breaking microcracks and their effects on higher harmonic generation in Rayleigh surface waves. The modeling is based on micromechanical considerations of rough surface contact. The nonlinear behavior of a single microcrack is described by a hyperelastic effective stress-strain relationship. Finite element simulations of nonlinear wave propagation in a solid with distributed microcracks are performed. The evolution of fundamental and second harmonic amplitudes along the propagation distance is studied and the acoustic nonlinearity parameter is calculated. The results show that the nonlinearity parameter increases with crack density and root mean square roughness of the crack faces. While, for a dilute concentration of microcracks, the increase in acoustic nonlinearity is proportional to the crack density, this is not valid for higher crack densities, as the microcracks start to interact. Finally, it is shown that odd higher harmonic generation in Rayleigh surface waves due to sliding crack faces introduces a friction nonlinearity.