Introductory Overview

As previously mentioned, the ARPA program is divided into three parts. The major part devoted to defect characterization will be discussed tomorrow. The other parts are the subject of today\u27s program and involve some major NOT problems that are not directly associated with defect characterization in a solid. They have to do with the problem of adhesive bonds, the problem of measuring residual stress and some new techniques that show great ·promise for failure prediction. The solution of these problems not only requires improvements in our understanding of the physical phenomena involved but they also require translation into a device for use in the field. During today\u27s program, we will cover the four distinct areas of: 1) adhesives and composites, 2) new measurements and techniques, 3) internal stress, and 4) acoustic emission. I will begin by going over the program, pointing out the connections between each of the talks and introducing some of the ideas that tie the subjects together

Measurement of Strength of Adhesive Bonds

In order to predict the strength of an adhesive bond between two metal sheets, it is necessary to measure the physical state of the adhesive layer that mechanically joins the two pieces of metal. This requires rapidly performing a detailed analysis of the ultrasonic echoes reflected from the entire structure when it is immersed in a water bath for a normal ultrasonic pulse-echo inspection. To achieve this result, computer-operated ultrasonic inspection systems have been assembled and equipped with special signal processing routines so that particular features of the ultrasonic echo in both the time domain and the frequency domain can be extracted in a time short enough to meet the requirements of a production inspection system. Such features as the relative amplitude of the signals reflected from the top and bottom of the adhesive layer and the frequencies for which standing waves are excited in the adhesive and in the metal adherends are of particular interest for making the strength predictions. It is also important that the interrogating ultrasonic pulse be of very short time duration so that the echoes from the various interfaces in the sandwich-like joint can be resolved in the time domain display. This requires the use of special high frequency pulse generators coupled to broad band transducers and amplifiers. Special procedures are also needed to insure the accuracy of the analog-to-digital conversion at the input to the computer and the subsequent transformations to and from the frequency domain

Trapped Acoustic Modes for Adhesive Strength Determination

The most important aspect of measuring the strength of adhesion at an adhesive to metal interface is to understand the chemistry of the interface and to know what makes a good interface. The existence of a strong interface will make the joint less susceptible to fabrication defects that are going to be there no matter how carefully the joint is prepared

Ultrasonic Inspection of Rubber Sonar Dome Windows

By using a rubber window acoustically matched to sea water, the Sonar system on a destroyer can be made considerably more sensitive. However, if the layered construction of the window develops delaminations while in service, the hydrodynamic characteristics of the structure may become modified and acoustic noise can be generated during high-speed operations. In order to inspect for these delaminations while the ship is tied up to its dock, a pulse-echo ultrasonic scan performed by a diver using a sea-water coupled transducer would appear to be ideal. However, the choice of acoustic parameters suitable for inspecting rubber were unknown. Laboratory studies sho1~ed that by utilizing very short time duration pulses whose center frequencies lie between 0.5 and 1.0 Mhz, it was possible to detect water-filled pockets within 0.2 inches of the outer surface of the window. A prototype instrument suitable for shipboard and dry dock operation has now been constructed. This instrument features optimized pulse excitation of low-frequency broad band transducers when attached to 50 to 100 feet of coaxial cable. A signal light is also incorporated to provide the diver with information on the condition of the rubber window under his transducer

Detection of Bending Stresses in Buried Pipelines

When gas or oil pipelines must be laid through sand or permafrost or other unstable soils, the pipe walls can he subjected to large bending loads if the soil shifts. In order to detect this condition and correct it, it would be useful to monitor the state of stress along the pipeline\u27at regular time intervals using a vehicle that is moved through the line by the fluid or gas in it. The experiments described here demonstrate that such a vehicle, using EMATs to excite and detect ultrasonic waves in the pipe wall, would be feasible because the transduction efficiency of the EMATs and thus the insertion loss of such a system can be related quantitatively to the stress level in the pipe wall

Application of EMATs to In-Place Inspection of Railroad Rails

With the aging of the U.S. railroad system and the increased tonnage being moved, it is more important than ever to monitor the installed railroad track for defects whose growth could lead to track failure and derailments. Current ultrasonic inspection techniques utilize a liquid filled wheel to couple acoustic energy from several piezoelectric transducers into the rail at a variety of angles relative to the head of the rail. This approach limits the speed of inspection to approximately 10 mph, is very sensitive to the surface condition and orientation of the railhead and requires frequent maintenance stops. The feasibility of using EMATs to replace the water filled wheel transducers has been the purpose of this research effort at the Albuquerque Development Laboratory and was sponsored by the Department of Transportation with the cooperation of the Sperry Rail Service Division of Automation Industries, Inc