History of Ultrasonics

Prior to World War II, sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In 1929 and 1935, Sokolov studied the use of ultrasonic waves in detecting metal objects. Mulhauser, in 1931, obtained a patent for using ultrasonic waves, using two transducers to detect flaws in solids. Firestone (1940) and Simons (1945) developed pulsed ultrasonic testing using a pulse-echo technique.

Shortly after the end of World War II, researchers in Japan began to explore the medical diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-mode presentation with blips on an oscilloscope screen. That was followed by a B-mode presentation with a two dimensional, gray scale image.

Japan's work in ultrasound was relatively unknown in the United States and Europe until the 1950s. Researchers then presented their findings on the use of ultrasound to detect gallstones, breast masses, and tumors to the international medical community. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation.

Ultrasound pioneers working in the United States contributed many innovations and important discoveries to the field during the following decades. Researchers learned to use ultrasound to detect potential cancer and to visualize tumors in living subjects and in excised tissue. Real-time imaging, another significant diagnostic tool for physicians, presented ultrasound images directly on the system's CRT screen at the time of scanning. The introduction of spectral Doppler and later color Doppler depicted blood flow in various colors to indicate the speed and direction of the flow.

The United States also produced the earliest hand held "contact" scanner for clinical use, the second generation of B-mode equipment, and the prototype for the first articulated-arm hand held scanner, with 2-D images.

How Is Ultrasound Used in ndt?

Sound with high frequencies, or ultrasound, is one method used in NDT. Basically, ultrasonic waves are emitted from a transducer into an object and the returning waves are analyzed. If an impurity or a crack is present, the sound will bounce off of them and be seen in the returned signal. In order to create ultrasonic waves, a transducer contains a thin disk made of a crystalline material with piezoelectric properties, such as quartz. When electricity is applied to piezoelectric materials, they begin to vibrate, using the electrical energy to create movement. Remember that waves travel in every direction from the source. To keep the waves from going backwards into the transducer and interfering with its reception of returning waves, an absorptive material is layered behind the crystal. Thus, the ultrasound waves only travel outward.

One type of ultrasonic testing places the transducer in contact with the test object. If the transducer is placed flat on a surface to locate defects, the waves will go straight into the material, bounce off a flat back wall and return straight to the transducer. Sound waves propagate into a object being tested and reflected waves return from discontinuities along the sonic path. Some of the energy will be absorbed by the material, but some of it will return to the transducer.

Ultrasonic measurements can be used to determine the thickness of materials and determine the location of a discontinuity within a part or structure by accurately measuring the time required for a ultrasonic pulse to travel through the material and reflect from the backsurface or the discontinuity.

When the mechanical sound energy comes back to the transducer, it is converted into electrical energy. Just as the piezoelectric crystal converted electrical energy into sound energy, it can also do the reverse. The mechanical vibrations in the material couple to the piezoelectric crystal which, in turn, generates electrical current.

A pulse-echo ultrasonic measurement can determine the location of a discontinuity with a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of the material. Then it reflects from the back or surface of a discontinuity and is returned to the transducer.

The ultrasonic tester graphs a peak of energy whenever the transducer receives a reflected wave. Sound is reflected any time a wave changes mediums. Thus, there will be a peak anytime the waves change mediums. Right when the initial pulse of energy is sent from the tester, some is reflected as the ultrasonic waves go from the transducer into the couplant. The first peak is therefore said to record the energy of the initial pulse. The next peak in a material with no defects is the backwall peak. This is the reflection from waves changing between the bottom of the test material and the material behind it, such as air or the table it is on. The backwall peak will not have as much energy as the first pulse, because some of the energy is absorbed by the test object and some into the material behind it.

The amount of distance between peaks on the graph can be used to locate the defects. If the graph has 10 divisions and the test object is 2 inches thick, each division represents 0.2 inches. If a defect peak occurs at the 8th division, we know the defect is located 1.6 (0.2 x 8) inches into the test object.

If the thickness of the object is unknown, it can be calculated using the amount of time it takes for the back wall peak to occur. The thickness of the object is traveled twice in that time, once to the back wall and once returning to the transducer. If we know the speed of our sound, then we can calculate the distance it traveled, which is the thickness of the object times two.

If a defect is present, it will reflect energy sooner also. Another peak would then appear from the defect. Since it reflected energy sooner than the back wall, the defect's energy would be received sooner. This causes the defect peak to appear before the backwall peak. Since some of the energy is absorbed and reflected by the defect, less will reach the backwall. Thus the peak of the backwall will be lower than had there been no defect interrupting the sound wave.

When the wave returns to the transducer, some of its energy bounces back into the test object and heads towards the back wall again. This second reflection will produce peaks similar to the first set of backwall peaks. Some of the energy, however, has been lost, so the height of all the peaks will be lower. These reflections, called multiples, will continue until all the sound energy has been absorbed or lost through transmission across the interfaces.

Often straight beam testing will not find a defect. For example, if the defect is vertical and thin enough, it will not reflect enough sound back to the transducer to let the tester know that it exists. In cases like this, another method of ultrasound testing must be used. The other method of ultrasound testing is angle beam testing. Angle beam testing uses an incidence of other than 90 degrees. In contact testing, an angled plastic block is placed between the transducer and the object to create the desired angle. For angle beam testing in immersion systems, a plastic block is not needed because the transducer can simply be angled in the water.

If the angle of incidence is changed to be anything other than 90 degrees, longitudinal waves and a second type of sound wave are produced. These other waves are called shear waves. Because the wave entered at an angle, it does not all travel directly through the material. Molecules in the test object are attracted to each other because solids have strong molecular bonds. The molecules carrying the sound are attracted to their surrounding molecules. Because of the angle, those sound carrying molecules get pulled by attracting forces in a direction perpendicular to the direction of the wave. This produces shear waves, or waves whose molecules travel perpendicular to the direction of the wave.

Angle beam testing and a change in the angle of incidence also creates further complications. When a wave hits a surface at an angle, it will be refracted, or bent, when it enters the new medium. Thus, the shear waves and the longitudinal waves will be refracted in the test object. The amount of refraction depends on the speed of sound in the two mediums between which the wave is traveling. Since the speed of shear waves is slower than the speed of longitudinal waves, their angles of refraction will be different. By using Snell’s law, we can calculate the angle of refraction if we know the speed of sound in our material.