EDISON WELDING INSTITUTE COOPERATIVE RESEARCH PROGRAM (CRP)

Project Title: Remote Sensor Development for Ultrasonic NDE

Area: NDE

Project Number: J1105/MR8816/9011

Center: Edison Welding Institute

Contact: John C. Lippold

Phone: (614) 486-9400

Principal Investigator: Laszlo Adler

Institution: The Ohio State University - Welding Engineering

Initiation Date: 06/86

Completion Date: 07/88

Funding: N/A

Key Words: NDE, Ultrasonic, Electromagnetic, EMAT

Abstract:

The need for noncontact ultrasonic nondestructive evaluation (NDE) methods in demanding applications has prompted the evaluation of two promising remote sensing techniques: electromagnetic and laser. Currently, electromagnetic transducers (EMATs) are preferred over laser techniques since they are readily available for immediate use at relatively low cost. In addition, EMATs are small, portable devices with sufficient sensitivity for a variety of applications. The results of this study demonstrated the utility of such transducers in applications, including adhesive bond evaluation and surface crack detection. Major problems identified in this study were the severely limited frequency bandwidths of these devices and the necessity of placing the electromagnetic transducers very close to the workpiece under study. Thus, although the EMAT technique is noncontacting, it is well suited as a remote sensing method.

In many practical situations, laser techniques are better suited and more flexible than EMATs. The main advantage of the laser method lies in its ability to generate and detect ultrasonic vibrations through large distances without mechanical contact between the sensors and the sample. It can therefore be readily used in high-temperature or otherwise hostile environments, and is also amenable to rough surfaces, awkward shapes and rapidly moving test pieces.

Generation of ultrasonic waves by pulsed infrared laser irradiation has become a fairly simple technique achieved through the development of inexpensive, rugged and portable Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) solid-state lasers. This technique works especially well at a higher laser intensity where the amplitude and frequency bandwidth of the generated ultrasonic signals compare very favorably to those of the more conventional piezoelectric transducers. It was found that the minor surface damage caused by ablation is usually less than 1 deep and is probably acceptable in most applications.

The main purpose of this project was to provide the groundwork for further technical development by addressing the most challenging inherent problems of laser detections, specifically (1) achieving high ultrasonic sensitivity while maintaining complete mechanical stability and (2) assuring sensitive detection on weakly reflecting surfaces, such as those that are rough or darkened. The first problem was solved by adopting a novel time-delay interferometric method that is inherently insensitive to any mechanical vibration below a carefully chosen ultrasonic cut off (somewhere between 100 kHz and 1 MHz). The effectiveness of this simple technique in completely eliminating adverse vibrations assures full ultrasonic sensitivity even on hand-held or rapidly moving test pieces. The sensitivity of the interferometric detector is better than 1 nm for the conveniently wide bandwidth between 1 and 20 MHz. The ability of this device to detect different bulk and surface modes generated by both laser and conventional sources was demonstrated using various sample types