In-situ monitoring of remote specimens using ultrasonic guided waves.

摘要

There are a wide variety of engineering applications where it can be advantageous to perform real time ultrasonic measurements. In structural health monitoring (SHM) applications (aircraft, bridges, etc.), real time data can be used to improve safety and reduce maintenance costs. In other applications (such as carbon-carbon production and ceramic sintering) in-situ ultrasonic measurements can be used to study how a material is affected by different manufacturing conditions. Ultrasonic measurements can also be used in place of convention temperature, pressure, fluid flow, or level sensors.;The work presented in this dissertation focused on studying an ultrasonic guided waves based system for performing in-situ measurements on specimens which are commonly used in the accelerated life tests conducted in materials and test reactors (MTRs). Using this system, ultrasonic waves could be sent down a thirty foot long waveguide to a specimen while locating the transducer outside of the hostile environment. Both the theoretical and experimental results demonstrated how individual components of the system influenced the monitoring capabilities.;As one example, the in-situ monitoring system was studied for use in measuring the change in length of creep specimens. Successful measurements of creep elongation were demonstrated using a pulse-echo configuration of the in-situ monitoring system. In addition, the temperature dependence of the in-situ creep measurement was also studied. Both theoretical calculations and experiments showed that the temperature dependence of group velocity was approximately linear for the creep specimen geometry.;'Blister' specimens having a rectangular cross-section were also considered in this work. Theoretically driven semi-analytical finite element (SAFE) calculations, time-domain finite element calculations, and experimental measurements were performed to investigate guided wave propagation in this specimen. The SAFE calculations, finite element predictions, and experimental results all demonstrated good agreement. In general, the guided wave mode with the fastest group velocity appeared to be the most prominent in both the simulated and experimentally measured waveforms. Several types of defects were considered. Defects can sometimes produce temperature anomalies in the specimen. The models indicated that detection of a defect induced temperature gradient in the rectangular 'blister' specimen was possible.;In the design and operation of an in-situ monitoring system based on guided ultrasonic waves, the transducer design plays an important roll in the performance of the system. In this work, a magnetostrictive sleeve design was tested as an alternative to a wire-wire joint configuration. Results showed that the sleeve design was effective for generating ultrasonic guided waves in the wire waveguide. In addition, the sleeve design simplifies specimen fabrication, minimizes the amount of magnetostrictive material required, and eliminates the artifacts associated with the wire-wire joint.