An Introduction to Ultrasonic Method for Non-Destructive Testing (NDT)

• Non-Destructive Testing (NDT)

Non-destructive tests (NDT) are inspection methods which are usually used to search for the presence of defects in components, without causing any effects on the properties of the components. The type of defects detectable are cracks, porosity, voids, inclusions, etc.

Modern NDT is used by manufacturers to: ensure product integrity and reliability; prevent failure, accidents and saving lives; make profit for users; ensure customer satisfaction; aid in better product design; control manufacturing process; lower manufacturing costs; maintain uniform quality level; ensure operational readiness.

 The table below shows the types of NDT methods used:

Commonly Used Methods	Other Methods
Ultrasonics	Visual Methods
Radiography	Acoustic Emission
Dye Penetrant	Thermography
Magnetic Particle Inspection	Holography
Eddy Current	Potential - drop

• Basic Principles of Ultrasonic Testing

Mechanical vibrations can be propagated in solids, liquids and gases. The actual particles of matter vibrate, and if the mechanical movements of the particles have a regular motion, the vibration can be assigned a frequency in cycles per second, measured in hertz (Hz), where 1 Hz =  1 cycle per second. If this frequency is within the approximate range 10 to 20,000 Hz, the sound is audible; above about 20 kHz, "the sound" waves are referred to as ultrasound or ultrasonics.

The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves.  The waves are not only reflected at the interfaces but also by internal flaws (material separations, inclusions,etc.). 

As an example of a practical application, if a disc of piezoelectric materials is attached to a block of steel (Figure 1a), either by cement or by a film of oil, and a high- voltage electrical pulse is applied to the piezoelectric disc, a pulse of ultrasonic energy is generated in the disc and is propagated into the steel. This pulse of waves travels through the metal with some spreading and some attenuation and will be reflected or scattered at any surface or internal discontinuity such as an internal flaw in the specimen. This reflected or scattered energy can be detected by a suitably-placed second piezoelectric disc on the metal surface and will generate a pulse of electrical energy in that disc. The time- interval between the transmitted and reflected pulse is a measure of the distance of the discontinuity from the surface, and the size of the return pulse can be a measure of the size of the flaw. This is the simple principle of the ultrasonic flaw detector and the ultrasonic thickness gauge. The piezoelectric discs are the "probes" or "transducers"; sometimes it is convenient to use one transducer as both transmitter and receiver. In a typical ultrasonic flaw detector the transmitted and received pulses are displayed in a scan on a timebase on an oscilloscope as shown in Figure 1b.

Figure 1  Basic principle of ultrasonic testing with a compressional probe (a) set-up (b) standard A-scan display