Ultrasonic measurement techniques in NDT

The three basic ultrasonic measurement techniques most widely used today are:

1. Pulse echo

2. Transmission

3. Resonance

Pulse-Echo Technique

The pulse-echo technique is the most popular of the three basic ultrasonic, nondestructive testing techniques. The pulse-echo technique is very useful in detecting flaws and for thickness measurement. The initial pulse of ultrasonic energy from a transducer is introduced into the test specimen through the couplant. This sound wave travels through the thickness of the specimen until a reflecting surface is encountered, at which time the sound wave reflects back to the transducer. This is called the back-wall echo. If the wave encounters a fl aw in its path, the fl aw acts as a reflecting surface and the wave is reflected back to the transducer. The echo in this case is referred to as a flaw echo. In both cases, the reflected wave travels back to the transducer, causing the transducer element to vibrate and induce an electrical energy that is normally amplified and displayed onto a CRT or other such device. The echo wave coming from the back wall of the specimen is marked by its transit time from the transducer to the back wall and return. Similarly, the transit time for the fl aw echo can also be determined by this technique. Since transit time corresponds to the thickness of the specimen, it is quite possible to calculate the thickness of the specimen using simple computer logic. One other technique, known as the immersion test technique, has generated tremendous interest among the manufactures that are in favor of automated inspection techniques. In the immersion technique, the specimen is completely immersed in the liquid.

Transmission Technique

In this technique, the intensity of ultrasound is measured after it has passed through the specimen. The transmission technique requires two transducers, one to transmit the sound waves and one to receive them. The transmission testing can be done either by direct beams or reflected beams. In either case, the flaws are detected by comparing the intensity of ultrasound transmitted through the test specimen with the intensity transmitted through a reference standard made of the same material. The best results are achieved by using the immersion technique since this technique provides uniform and efficient coupling between transducers and test specimen. The main application of the transmission technique is in detecting flaws in laminated plastic sheets.

Resonance Technique

This method is primarily useful for measuring the thickness of the specimen. This is accomplished by determining the resonant frequencies of a test specimen.

APPLICATION OF ULTRASONIC NDT IN PLASTICS

Ultrasonic nondestructive testing (NDT) has gained popularity in the past decade along with the growth of the plastic industry and along with an increasing emphasis placed on automation and material saving. Two major areas in which ultrasonic testing concepts are applied extensively are fl aw detection and thickness measurement. The pulse-echo technique is used to detect a flaw such as voids and bubbles in an extruded rod of rather expensive materials such as Teflon and nylon. The flaw detection unit and other auxiliary equipment can be programmed so that the specific portion of the rod with a flaw is automatically cut off and discarded without disturbing the continuous extrusion process. The transmission technique is commonly used to detect flaws in laminates. Thickness measurement by ultrasonic equipment is simple, reliable, and fast. This NDT technique simplifies the wall thickness measurement of parts with hard-to-reach areas and complex part geometry. Automated wall thickness measurement and control of large diameter extruded pipe is accomplished by using the immersion technique. An ultrasonic sensing unit is placed in a cooling tank to continuously monitor wall thickness. In the event of an out of control condition, a closed-loop feedback control system is activated and corrections are made to bring the wall thickness closer to the set point. Many such systems are commercially available. The ultrasonic NDT technique is used extensively by gas companies to examine the integrity of plastic pipe socket joints after they have been solvent cemented together. Ultrasonic measurements can also be used for determining the moisture content of plastic. In materials like nylons, the attenuation and the acoustic velocity change with the change in moisture content. The use of ultrasonics in testing reinforced plastics and missiles and rockets has been discussed.