Date of Award

August 2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Al AG Ghorbanpoor

Committee Members

Habib HT Tabatabai, Konstantin KS Sobolev, Roshan RD D'Souza, Ghoncheh GM Mashayekhi

Abstract

Magnetic-based methods to evaluate structural defects have gained interest over recent decades. Magnetic flux leakage (MFL) has been used as an important nondestructive evaluation (NDE) method for detection of structural flaws such as surface cracks or corroded regions. MFL has been studied through experimental works, numerical simulations, and field testing for flaw detection. This work utilizes experimental results and 3-D finite element simulations to develop a methodology for flaw detection in pre-stressing steel in concrete members. MFL occurs after magnetic saturation of the defect region through the application of a strong DC magnetic field. Hall-effect sensors are located in the MFL apparatus, which also contains two blocks of strong DC magnets, to measure the vertical magnetic field variation. The superposition of the individual effects of stirrups and flaws on the MFL signal is validated by comparing signals given from experiments, numerical simulation, and an analytical method. A 3-D analysis of MFL signals is developed to determine the position and orientation angle of stirrups in addition to initial estimate of a flaw’s location along the sample. The 2-D analysis of the MFL signals characterizes the flaw more accurately and identifies geometric properties of flaws. From experiments and numerical simulations, the MFL-related properties of flaws are tabulated and visualized for several section loss percentages and lengths of flaws as well as the various distances between the magnet blocks and embedded strands within concrete. The methodology suggested by this work determines the stirrups’ position and orientation with good accuracy and detects and estimates the level of corrosion in embedded prestressing steel in pre-stressed concrete girders.

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