In ultrasonic testing, very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched into materials to detect hidden cracks, voids, porosity, and other internal flaws or to characterize materials. The technique is also commonly used to determine the thickness of the test object.
High frequency sound waves reflect from flaws in predictable ways, producing distinctive echo patterns that can be displayed and recorded by portable instruments. Ultrasonic testing is completely nondestructive and safe, and it is a well-established test method in many basic manufacturing, process, and service industries, especially in applications involving welds and structural metals.
It works on pulse-echo method. The sound pulse coming from the transmitter is radiated in a beam through a given medium at a specific speed or velocity, in a predictable direction, and when they encounter a boundary with a different medium they will be reflected.
All sound waves oscillate at a specific frequency, or number of vibrations or cycles per second, which we experience as pitch in the familiar range of audible sound. Human ears can respond to sound vibrations, which have frequencies in the range of 20Hz to 20kHz. While the majority of ultrasonic flaw detection applications utilize frequencies between 500 KHz to 10 MHz. At frequencies in the megahertz range, sound energy does not travel efficiently through air or other gasses, but it travels freely through most liquids and common engineering materials.
The speed of a sound wave varies depending on the medium through which it is traveling, affected by the medium's density and elastic properties. In steel the velocity of sound wave is 6*1000000 mm/sec.
Any type of wave will have an associated wavelength, which is the distance between any two corresponding points in the wave cycle as it travels through a medium. Wavelength is related to frequency and velocity by the following
λ = v/f
where
λ = wavelength
v = sound velocity
f = frequency
Wavelength is a limiting factor that controls the amount of information that can be derived from the behavior of a wave. In ultrasonic flaw detection, the generally accepted lower limit of detection for a small flaw is one-half wavelength. Anything smaller than that will be invisible. In ultrasonic thickness gauging, the theoretical minimum measurable thickness one wavelength.
