Ultrasonic Testing - Probe Systems (Theory)

The operator's wish to accurately know the actual discontinuity size is understandable, therefore it is expected that a nondestructive testing method, such as ultrasonic testing, gives this information.  However, due to the fact that on the screen of the Ultrasonic Flaw Detector only the reflected sound (echo) coming from the discontinuity can be interpreted, it is therefore very difficult to reliably assert the size of the discontinuity.  In fact, the echo height plays the decisive part when evaluating discontinuities during manual ultrasonic testing.

It is also important to know the signal characteristic of different types of defect so that upon detection of the defect, the type of defect can be identified.  The signal characteristics can be found in the section below "Defect Characterization".


There are generally 3 types of probe systems.  Regardless of which probe system is used in the detection of discontinuity, if the reflected portion of the sound wave is not received by the probe, then it is unlikely that the discontinuity will be detected.  The possibilities of detection only increase when the plane discontinuity is hit vertically by the sound beam.  The 3 types of probe systems are:

• Straight-Beam Probe
• Angle-Beam Probe
• Immersion Probe

Java simulations of the probe systems and the defect characterization can be found in their respective section below.  The type of defect in the Java simulations of the probe systems is the fine linear slag inclusion, the symbols in the applets are as follow:


IP  = Initial Pulse
BE = Backwall Echo
R   = Reflector(flaw) Echo
(In through-transmission technique, R simply symbolizes the signal detected by the receiver)
F   = Front Echo in Immersion Probe System
B   = Backwall Echo in Immersion Probe System

1) Straight-Beam Probe

Figure 1  A single crystal longitudinal wave probe

Probes whose beams are normal to the surface are called straight-beam probes.  Most standard straight-beam probes transmit and receive longitudinal waves.

The typical design of a single crystal longitudinal wave probe is shown in Figure 1. Such probe is called a 0⁰ longitudinal wave probe.  If a metal wedge is placed between the piezoelectric element and the specimen surface, of the same material as the specimen, longitudinal waves can be propagated into the specimen at an angle.

The piezoelectric element (the crystal, or ceramic plate), of a suitable thickness to produce the resonant frequency required, is usually circular in shape, and typical diameters are 6 to 30 mm (1/4 to 1 inch), with frequencies in the range 1-15 MHz. The crystal faces are metallized, either by coating them with electro conductive ink which gives a deposit of silver or copper after baking.

The piezoelectric crystal is backed with a damping backing as shown in Figure 1. This material must have a similar acoustic impedance to that of the crystal, so the back wave travels into it without reflection. It should be highly absorbent, and obviously well bonded to the piezoelectric element. Nowadays, the acoustic backing is one of the two kinds, (1) a scattering, diffusing backing, made of tungsten powder in epoxy resin or some form of sintered metal; (2) a quarter- wavelength layer.

There are a number of basic straight-beam probe configurations which are applicable to a range of testing problems.  3 configurations will be discussed as follow:

• Basic Longitudinal Wave Pulse Echo System

Figure 2  Basic longitudinal wave pulse echo system

Figure 2 shows the basic longitudinal wave pulse echo system using a normal (0⁰) longitudinal wave combined transmitter and receiver (T/R) probe.

A Java simulation of the probe configuration can be found at this link>Basic Pulse Echo System. 

• Through-Transmission Technique

Figure 3  Through-transmission technique

Figure 3 shows the through-transmission technique with the transmitter and receiver separately on opposite sides of the specimen.  If a flaw is detected, the signal R is lost or reduced as shown in cases B and C of Figure 3.

A Java simulation of the probe configuration can be found at this link>Through-Transmission Technique. 

• Double-Probe Longitudinal Wave System

Figure 4  Double-probe system

For a double-probe longitudinal wave system, normally there is no input signal shown on the display.

A Java simulation of the probe configuration can be found at this link> Double-Probe System. 

2) Angle-Beam Probe

Figure 5  A single crystal transverse wave probe

Probes whose beams enter at an angle are called angle-beam probes because they transmit and receive the sound waves at an angle to the surface of the test specimen.  Most standard angle-beam probes transmit and receive, due to technical reasons, transverse waves.

The transverse wave probe shown in Figure 5 is the most-widely-used for weld inspection. The piezoelectric element is cemented to the sloping face of a Perspex block, the angle of this face to the base being chosen so that when the Perspex flat face is placed on a metal specimen, the longitudinal wave in the Perspex is mode converted into a transverse wave in the specimen, at a chosen angle. The angle of the transverse wave beam, for any particular probe, will of course depend on the velocity of ultrasound in the specimen material. Thus a 70⁰ probe for use on steel is not a 70⁰ probe when used in aluminum.  Such probes are commonly sold in terms of the nominal angle of the transverse wave beam in steel.

Materials other than Perspex have been proposed for the probe wedge in a transverse wave probe. For example, for use on copper and cast iron specimens, Nylon wedges have been used.

The most important points about the transverse wave probe design are to make sure that the probe angle (nominal value in steel) of the transverse waves and the point of entry of the centerline of the beam into the specimen are known. These are two very important characteristics of any probe, which the ultrasonic equipment user needs to determine in the calibration procedure for each individual probe.

The usable range for the nominal probe angle is determined from the 2 critical angles as illustrated in Figure 6.  For more information on critical angle, pls refer to this link> Critical Angle.

Figure 6  Usable range for nominal probe angle

There are a number of basic angle-beam probe configurations which are applicable to a range of testing problems.  3 configurations will be discussed as follow:

• Basic Transverse Wave Pulse Echo System

Figure 7  Basic transverse wave pulse echo system

Figure 7 shows the basic transverse wave pulse echo system using the half-skip technique.

A Java simulation of the probe configuration can be found at this link> Half-Skip Technique.

• Full-Skip Technique

Figure 8  Full-skip technique

Figure 8 shows the full-skip technique where the transverse wave can be reflected off the lower surface.

A Java simulation of the probe configuration can be found at this link>Full-Skip Technique. 

• Tandem Probe

Figure 9  Tandem probe

Often in thick-walled test specimens, in which there are vertical discontinuities, a T/R probe cannot be used since the reflected sound waves from the discontinuity and the surface of the test specimen do not return to the T/R probe.  In this case, a second probe is used for receiving the reflected portions of the sound wave, thus enabling detection of the discontinuity.  In the Tandem technique, one probe is used as a transmitter and the other probe is used as the receiver.  Both the probes are mechanically-linked at a fixed distance apart.  Scanning is made for vertical discontinuities at different depths of the test specimen, depending on the probe spacing.

A Java simulation of the probe configuration can be found at this link> Tandem Probe.

3) Immersion Probe

1 configuration of the immersion probe will be discussed as follows:

• Water-Immersion Technique

Figure 10  Water-immersion technique

Figure 10 shows the water-immersion technique.  The path length in water is large, so the distance between the front and backwall echoes in the specimen may be rather small, unless "beam expansion" is used on this part of the display. "R" in Figure 10 represents the 3 flaw echoes resulting from the reverberations between the front of the specimen and the flaw.

A Java simulation of the probe configuration can be found at this link> Water-Immersion Technique.

4) Defect Characterization

The common defects in welds and their signal characteristics are listed in Table 1.  For the following echo patterns shown in Table 1, the sensitivity is adjustable to produce a full screen height echo from a 1.5mm horizontal hole at the same range as the defects discussed.

A Java simulation of the defect characterization can be found at this link> Characteristics Simulation.

Table 1 Flaw Characterization from Welded Defects

Defects	Pulsed Shape
Gas Pore Echo amplitude depends on pore size but usually between 1/5 and 3/5 screen height	Single range reflector with a narrow pulse display at time base
Group Porosity Echo amplitude usually less than 1/5 screen height	Multi-range reflector with a wide pulse display at time base
Isolated Slag Inclusion Echo amplitude generally about 2/3 screen height with reflection from more than one range	Forked type display with some pulse width at time base
Fine Linear Slag Inclusion Echo amplitude usually about 1/2 to 3/4 screen height	Usually narrow pulse width at time base
Cracks Echo amplitude tends to be high with numerous peaks	Due to the multi-faceted nature and range of cracks, the pulse will have multiple peaks and usually wide at time base