Ultrasonic Testing
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Oscillations and waves are found everywhere. A simplified form of an oscillation is a pendulum. If you hang a ball on a string and move it slightly, the pendulum will swing back and forth continuously. This movement across a resting point is called an oscillation.

If the oscillation energy is consumed, for example by air resistance, it is called a damped oscillation. If it is heavily damped, it is referred to as an impulse. If the consumed energy is restored, it is called an undamped oscillation. An undamped oscillation has a constant amplitude.

The deflection indicates the distance from the resting position at any given time. The maximum deflection from the resting position is called the amplitude.

The oscillation period T(s) is the time span in which one full oscillation cycle occurs. The frequency is derived from the oscillation period and represents the number of oscillation cycles per unit time (e.g., 4 cycles per second = 4 Hertz).

In ultrasonic material testing, frequencies in the megahertz range are commonly used. This means several million oscillations per second. In contact technique testing between 1 to 5 MHz, and in immersion technique testing up to 25 MHz.

The wave is the spatial propagation of an oscillation. During an oscillation period T, the oscillation travels one wavelength in space. This distance is called wavelength λ. One wavelength per oscillation period T is therefore the sound velocity c.

λ = Wavelength lambda (mm)
c = Sound velocity (m/s)
f = Frequency

Formula: c = λ : T or c = λ * f

Ultrasound waves are mechanical oscillations and cannot travel in a vacuum. In metals, atoms are tightly bound. Due to these attractive forces, it is referred to as elastic coupling. Different metals have typical, material-specific sound velocities. The sound velocity is a material constant. Choosing the correct frequency can be crucial for successful ultrasonic testing. The wavelength can be used to estimate the detection limit for the smallest detectable reflector size. This detection limit is approximately half a wavelength.