Session: 10-01: Nonlinear Ultrasonic Techniques for NDE I

Paper Number: 134599

134599 - Harmonic Imaging of Local Plastic Regions In Metals By Mode Converted Transverse Wave and Longitudinal Thickness-Mode Local Resonance Techniques 

Abstract: 

Ultrasonic imaging techniques using mechanical scanning and focused immersion transducers are routinely used in many inspection applications. These established techniques are linear, i.e., increasing the amplitude of the excitation results in a corresponding increase in the amplitude of a signal of interest, such as an echo from a defect. In the laboratory, measurements based on nonlinear ultrasonic phenomena, e.g. harmonic generation, have shown sensitivity to damage such areas of plastic deformation. However, numerous difficulties have kept nonlinear ultrasonic techniques from being incorporated into practical inspection and monitoring applications. In this work we present and compare two scanning imaging techniques based on the nonlinear phenomenon of harmonic generation: harmonic imaging using 1) mode converted transverse waves (MCTW) and 2) longitudinal thickness-mode local resonances excited.

Both methods presented make use of harmonic generation, which refers to the harmonic distortion developed in a sinusoidal ultrasonic wave as it propagates through a material with a nonlinear response. The MCTW technique presented measures the relative harmonic content of wave packets that have travelled through the sample thickness with at least 1 trip as a transverse wave generated at a boundary reflection. When the longitudinal wave generated by a focused transducer encounters a planar sample (i.e., a flat plate) the following wave packets are received: the front surface reflection, the back surface reflection, subsequent echo signals as the longitudinal wave reflects multiple times at the sample interfaces, and additional signals due to mode conversions that occur at the sample interfaces. When an off-centered longitudinal wave is reflected at the back surface, both longitudinal and transverse waves (i.e., MCTW) are generated at the reflecting interface. The position of the focused transducer is chosen such that the signal from the MCTW arrives at the front surface within the focus beam diameter, and the MCTW is detected by the transducer. In the local resonance technique the frequency of the excitation applied to the transducer is chosen to correspond to one of the longitudinal thickness modes of the sample, and the duration is chosen such that a strong reverberation is developed in the sample. One advantage of this technique is that the longitudinal wave nonlinearity is enhanced by transmitting large-amplitude long burst wave of the through-thickness resonance frequency of a plate sample.

For both techniques, a focused transducer is excited with a tone burst wave of appropriate frequency and duration, and the feature of interest (i.e., the MCTW signal or the thickness mode resonance signal) is gated out of the received signal. The fundamental and harmonic amplitudes are acquired from the digitized waveforms and displayed as C-scan images. Results are shown for several samples, including 1) an aluminum alloy plate subjected to a maximum tensile plastic strain of 0.3%, and 2) a steel (SM460) plate subjected to tension-tension low-cycle fatigue. We find that harmonic images obtained by the local resonance technique show multiple reflections as well as the plastic region, and the technique is very sensitive to sample thickness variation. Images using the MCTW technique show the plastic regions more clearly, and the technique has the additional advantage of being less sensitive to sample thickness variations.

Presenting Author: Thomas Grimsley RITEC Inc.

Presenting Author Biography: Thomas Grimsley received his Ph.D. in Physics from Brown University in 2012 and joined Ritec in 2011. Tom is RITEC’s President and Senior Applications Scientist. Tom assists customers with instrumentation and ultrasonic measurements, which often includes feasibility and proof of concept measurements. He is also involved in all aspects of hardware design.

Authors:

Thomas J. Grimsley II RITEC, Inc.
Kazuhiro Kyotani Insight K,K.
Takeru Shirota Insight K.K.
Koichiro Kawashima Ultrasonic Materials Diagnosis Laboratory