Seismic methods applied to ultrasonic testing in civil engineering
Niederleithinger, Ernst; Clauser, Christoph (Thesis advisor); Popovic, John S. (Thesis advisor); Große, Christian (Thesis advisor)
Aachen (2017, 2019)
Habil / Postdoctoral Thesis (Non-german Habil)
Habilitationsschrift, Rheinisch-Westfälische Technische Hochschule Aachen, 2017
The application of ultrasonic testing in civil engineering started several decades ago, mainly limited to pulse velocity measurements in transmission mode for concrete quality control. Starting in the late 1990s, more and more emphasis was put on echo techniques for structural investigations of concrete constructions. Starting with the arrival of point contact transducers without the need of coupling agents these techniques have raised the capabilities of nondestructive testing (NDT) to a new level. Ultrasonic testing (UT) in civil engineering so far mainly benefitted from methods developed for traditional NDT of metals.However, there are still gaps in the currently applied technologies due to their inherent limitations. These gaps might be at least partially closed by applying geophysical techniques, which have been developed for a totally different scale in terms of penetration depths and wavelengths but aiming at similar physical/mathematical challenges (e.g. complex geometries, multiple scattering). This thesis deals with two examples of geophysical methods, which have been adapted to concrete testing.One of the major drawbacks of the imaging techniques currently applied in ultrasonic echo testing for concrete is the limitation to simple geometries. SAFT (synthetic aperture focusing technique), which is widely and successfully used in many variants in NDT, is closely related to geophysical Kirchhoff or Stolt migration. All these techniques aim to focus the reflected energy recorded at the surface back to the reflector, but only direct single reflections are correctly dealt with. Vertical reflectors or the backside of internal features in a construction can`t be imaged. In this thesis, Reverse Time Migration (RTM), another geophysical imaging technique based on the correlation of forward and backward propagating wavefields, is proposed as an alternative for imaging complex geometries. It is shown, that vertical boundaries as those at thickness changes of foundations slabs can be imaged correctly as well as the full geometry of inclusions (voids, tendon ducts). Examples are given based on simulated and measured data. RTM has some drawbacks, as the significant computation time required and artifacts mainly close to the surface, which require additional research and development before widespread practical application in NDT.The second development documented in this thesis is the adaption of algorithms borrowed from seismology to detect subtle changes in concrete due to various loads or degradation in ultrasonic transmission measurements. Since about 10 years several research groups are working on the application of Coda Wave Interferometry (CWI) to evaluate very small changes in the elastic wave velocity (e. g. by stress) or changes in the scattering pattern (e.g. by cracks) inside concrete. Various lab investigations, but also a few first field experiments have shown, that this technology is in fact able to contribute to monitoring of concrete constructions, revealing changes in the material properties (here e. g. by stress or temperature) and to localize the affected area. New instrumental developments as robust and reliable ultrasonic transducers help to path the way for long term implementation in concrete infrastructure (e.g. bridges). Methods to separate different influence factors or to simplify imaging are currently under development.