Ultrasonic measurement of stress in steel and concrete

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Advancements in ultrasonic technology may lead to the ability to directly determine stress (strain) in steel and concrete members through measurement of the acoustic birefringence for steel and acoustic wave properties for concrete. The measured birefringence and acoustic wave properties are proportional to the stress in steel and concrete respectively, and thus the magnitude and orientation of the applied stress can be deduced from the ultrasonic velocity measurements. There are currently no technologies that can adequately evaluate the stress of steel and concrete members and determine if steel and concrete members are carrying dangerous levels of load. The significance of the research is to develop a technology that can assess the safety of steel and concrete structural members by directly measuring the stress (i.e. forces) carried in steel and concrete. Such evaluation technologies will be a critical advance toward safer and more reliable infrastructure systems, provide a tool for the condition assessment of infrastructure systems and post-event damage assessments, and aid in innovative new design approaches. The first part in this dissertation looks specifically at the application of direct stress measurement using USM technology to W-flange steel sections commonly used in buildings under axial bending and tension loads. The research found that the ultrasonic stress measurement (USM) shows that the average difference between the predicted and actual stress is only [plus or minus] 8.4 ksi, when using the average natural birefringence measurement (Bo) (0.002208) and the acoustic-stress constant (m1) (7.129x10-8) values for the W6x16 section tested. The average difference between the predicted and actual stress can be decreased to approximately [plus or minus] 1 ksi when location-specific natural birefringence (Bo) values are used. However, there were differences in the acoustic constants in sections from different mills, in the flange vs. the web, and post yielding. The second part of this research aims to provide the essential early-stage research needed to determine feasible techniques to measure stress directly in concrete in compression. The experimental tests evaluated different techniques for stress determination in concrete: Time of Flight (TOF) and Fast Fourier Transform (FFT). The research shows there is a measurable and almost linear increase in TOF with damage in the concrete. The TOF data can be used predict the stress in concrete with an average error of 33%. The development, improvement, and refinement of these methods will lead to accurate direct stress measurement in steel and concrete and a fundamental transformation in the field of structural evaluation.