Ultrasonic pulse-echo devices are built with a transducer which produces a wide-band pulse that is subjected to the test material during testing. The pulse travels through the material until it meets with either discontinuity within the material (such as flaws or defects) or when it reaches a new medium. The wave is then scattered, or reflected back to the transducer. These ultrasonic waves generally occur between approximately 20 kHz and 200 MHz. Conventional ultrasonic transducer operate around 200 kHz and 5 MHz. Since ultrasonic testing equipment generate and receive ultrasonic waves at certain sound velocities in particular materials, ultrasonic testing equipment can be calibrated to the material’s known sound velocity. Once the calibration is complete, the sound velocity is multiplied by the elapsed time of the wave upon which the ultrasonic testing equipment calculates the distance at which the sound wave traveled before reaching a new medium. In most applications, this time interval is a few microseconds or less. The two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material. The result is expressed in the well-known relationship
d = vt/2
Where d is the distance from the surface to the discontinuity in the test piece, v is the velocity of sound waves in the material, and t is the measured round-trip transit time.
The diagram below allows you to see return echoes as they would appear on an oscilloscope. The transducer employed is a 5 MHz broadband transducer 0.25 inches in diameter.
Precision ultrasonic thickness gages usually operate at frequencies between 20 kHz and 200 MHz, by means of piezoelectric transducers that generate bursts of sound waves when excited by electrical pulses. A wide variety of transducers with various acoustic characteristics have been developed to meet the needs of industrial applications. Typically, lower frequencies are used to optimize penetration when measuring thick, highly attenuating or highly scattering materials, while higher frequencies will be recommended to optimize resolution in thinner, non-attenuating, non-scattering materials.
In thickness gauging, ultrasonic techniques permit quick and reliable measurement of thickness without requiring access to both sides of a part. Accuracy’s as high as ±1 micron or ±0.0001 inch can be achieved in some applications. It is possible to measure most engineering materials ultrasonically, including metals, plastic, ceramics, composites, epoxies, and glass as well as liquid levels and the thickness of certain biological specimens. On-line or in-process measurement of extruded plastics or rolled metal often is possible, as is measurements of single layers or coatings in multilayer materials. Modern handheld gages are simple to use and very reliable.