Principles of Ultrasonic Testing

How is ultrasound reflected and refracted? 

A sound wave, vertically hitting a plane material interface, is split up into a reflected and transmitted wave. In this connection the amplitudes also change -> reflection and transmission factor. 

R = reflection factor 

T = transmission factor 

Z = acoustic impedance [103 kg/m2 s] 

ρ = density [g/cm3] 

c = sound velocity [m/s] 

Indices: 1 = material, 2 = material 2

What are the basic principles of ultrasound testing and what role do waves play? 

An ultrasonic pulse passes through the work piece at the characteristic sound velocity of that particular material. In the course of this an interaction with the workpiece structure takes place. The corresponding evaluation of the signals received (amplitude and time of flight) allows conclusions to be drawn as to the internal quality of the test object without destroying it. 

With the longitudinal wave the molecules (atoms) oscillate parallel, whereas with the transverse wave they oscillate perpendicular to the propagation axis. The sound velocity c is a material constant mainly depending on the modulus of elasticity and density of the material. With the frequency f of the sound wave this results in the wave length λ. 

How is ultrasound technically implemented?  

 Electric pulses are generated by the transmitter of the instrument and fed to the probe where they trigger ultrasonic pulses. At the same time the electron-beam starts with the initial pulse (IP) in the lower left-hand corner of the screen. Sound waves thus reflected and received back by the probe generate the echoes (reflector echo, backwall echo) displayed on the screen. In this A-scan representation the echo amplitude of a reflector is shown as a vertical trace and the time of flight or distance as a horizontal trace. This allows a clear allocation of the sound path s to each individual echo → location of reflectors, thickness measurement. 

In the pulse echo method the sound portions reflected back to the probe are evaluated. The through-transmission method uses two probes, i.e. one transmitter plus one receiver

What are ultrasonic pulses? 

For this purpose a piezoelectric element (crystal, ceramic, polymer) is used , transforming electrical energy into sound waves and vice versa. Due to mechanical damping of the transducer element this produces a damped oscillation – the ultrasonic pulse and, on receiving a wave signal, the electric RF pulse. The frequency of the pulse is determined by the element thickness, whereas the pulse length or frequency spectrum (bandwidth) is determined by the element damping. 

Weak damping = long pulse duration with distinctive frequency and narrow spectrum (small bandwidth). Especially suited for flaw evaluation according to the DGS method. 

Strong damping = short pulse duration, wide spectrum (large bandwidth): high resolution, good signal-noise ratio especially on coarse microstructure.

What is the sound field in ultrasound?

The form of the sound radiation of a probe depends on its crystal size, its frequency and the sound velocity of the material concerned. The sonogram is received by adding the detectability of circular reflectors to the approximate representation of the sound field. 

A bell-shaped lateral distribution of the sound pressure is observed in the far field. The sound pressure drop to 50% defines the diameter of the sound beam. 

N = near field length [mm] 

Deff = effective crystal diameter [mm] 

Do = mechanical crystal diameter [mm] 

f = frequency [MHz] c = sound velocity [km/s] 

γ6 = angle of divergence 50% limit (-6dB)

How do I evaluate the flow in ultrasound using DGS?  

The sound pressure increases along the acoustic axis up to the near field end, i.e. the focus, reaching its maximum at this point and then dropping again. The reflection characteristic of flaws located completely within the sound field results in an echo amplitude drop proportional to 1/s2 (s= sound path) in the far field. 

This results in the DGS diagram in a log-log system of coordinates. It shows the regularity of echo amplitudes from circular disks of different sizes and from the backwall for a certain probe referred to steel. Differences in the echo amplitudes are outlined as gain differences ΔV. On the other hand, the equivalent reflector size (= diameter of the equivalent circular disk) of the flaw can be determined on the basis of a measured gain difference ΔV between the indication of an unknown reflector (flaw) and the indication of a reference reflector. (DGS method: Distance, Gain, Reflector-Size. 

A flaw evaluation is also possible by way of direct comparison of the flaw echo with the echo of a known reference reflector (reference block method). In field applications the corresponding reference reflectors are scanned at varying distances with their echo amplitudes being traced on the screen as a distance amplitude curve (DAC).

What is phased array probe technology? 

By firing the individual elements of an array transducer at slightly different times, the ultrasonic beam can be focused or steered in a specific direction. By housing a transducer with 16, 32, 64, 128 or 256 individual elements and a corresponding number of pulser preamps, the ultrasonic beam can be „steered“ electronically.