INTRODUCTION

The name Acousto-Ultrasonics (AU) is derived from a combination of Acoustic-emission monitoring and Ultrasonic characterization. “Invented” by Alex Vary at NASA-Lewis, the method consists of monitoring – and analyzing – the acoustic signals received from a controlled and complete insonification of the test material.

Acousto-Ultrasonics is a highly sophisticated and advanced technique using digital signal processing and pattern recognition algorithms. As such it is orders of magnitude superior to any other conventional ultrasonic technique, where the human recognizance capability is much inferior to modern data processing, or where automatic scanning systems are limited by considering only the amplitude in a small portion (gate) of the ultrasonic signal. Acousto-Ultrasoncs considers the entire ultrasonic response, in time as well as in frequency, of the entirely insonified material.

ACOUSTO-ULTRASONIC SYSTEM

Learning Process

One major task of setting up an acousto-ultrasonic system consists of the training or learning process. Several identical standard specimens are needed for each feature that the system will have to be able to extract. A number of measurements have to be repeated several times on each specimen. After many digital feature extraction processes on all the waveform data, templates are built for each feature.

The feature templates are tested by evaluating the same standard specimen and the system is tested for its feature regognition reliability. If the reliability is insufficient, the ultrasonic interrogation system (transducer frequency, number, and location) may have to be altered and the training process will have to be repeated.

Inspection System

The inspection system concists of a cradle with all the transducers and of a mechanical subsystem to move the parts and to place the transducers in contact with the parts. It further contains an ultrasonic pulser/receiver unit, a switch matrix to control all the transducers, DSP boards, and a Computer.

Automated Test Procedure

(1) After the part is in place, all transducers are brought into contact.

(2) Waveform data are being acquired, controlled by the switch matrix.

(3) The acquired waveform data are being processed in the following phases:

Signal Enhancement, e.g. increase SNR via ensemble averaging, correlation, and linear filtering; transducer compensation via Wiener filtering, and Domain Transform.

Feature Extraction, e.g. estimate signal properties that relate to the physics of the propagation problem such as time of arrival of certain wave modes, localized density estimates from frequency content, beat-frequency content or specific joint-time-frequency occurrences.

Signal Classification, where each feature group is sequentially evaluated against feature group templates from the learning set, and decision about “good” or “bad”

EXAMPLES

In the case of a complex composite part, measuring 20 feet in length, and having a diameter varying from 1 to 4 inches, each part was completely tested in 50 seconds.

The method previously used, CATSCAN, took 17 minutes per part

A-Scans and Frequency Spectra

Plates with acceptable bonding

Plates with insuffiencient bonding

Corrosion of Riveted Plates

Ultrasonic energy plotted versus time and frequency

APPLICATIONS

Acousto-Ultrasonics is usefull for the inspection of components that have repeatable shapes, i.e. for

• High Volume Production with 100% Inspection Requirement
• Short Inspection Time
• Complex Material Systems

The method has been used for

• Composites strength prediction
• Wood internal bond strength prediction
• Rocket Motor propellant bond-line adhesion
• Titanium and other adhesive bond strength prediction
• Corrosion Detection in Riveted Aluminum Plates
• Tire Damage Evaluation
• Thermal-Oxidative Damage detection

REFERENCES

Vary, A., The Acousto-Ultrasonic Approach, in Acousto-Ultrasonics, Theory and Applications, J. C. Duke, Jr., ed., Plenum Press, 1988.

Srivastav, V. K., Prakash, R., Acousto-Ultrasonic Evaluation of the Strength of Composite Material Adhesive Joints, Acousto-Ultrasonics, Theory and Applications, J. C. Duke, Jr., ed., Plenum Press, 1988.

Vary, A., Acousto-Ultrasonics, in Vol. 2 of Non-Destructive Testing of Fibre-Reinforced Plastics Composites, J. Summerscales, ed., Elsevier Science Publishers, 1990.

Beall, F. C., Overview of Acousto-Ultrasonics Applied to Wood and Wood-based Materials, Proceedings of the Second International Conference on Acousto-Ultrasonics, Alex Vary, ed., Copyright ASNT, 1993.

Bartos, A.L., J.O. Strycek, R.J Gewalt, H. Loertscher, and T.C. Chang, “Ultrasonic and Acousto-Ultrasonic Inspection and Characterization of Titanium Alloy Structures,” Nondestructive Characterization of Materials VIII, R. E. Green, Jr., et al., eds., Plenum Press, 1998.

Shiloh, K., A.L Bartos, A. Frain, and E. Lindgren, “Ultrasonic Detection of Corrosion between riveted Plates”, Proceedings of SPIE 3994-09, 2000.