connectors Housing
Transducer
Rev 0 August 2008 Probe Design
10.2 Angle probes
An angle probe is a piezo-electric crystal mounted on a perspex wedge at an angle calculated to generate a shear (transverse) wave in the test material.
The wedge is made of perspex because:
a The compressional speed of sound in perspex (2730m/sec) is lower than the shear velocity of sound in steel (3250m/sec) so refracted angles are greater than incident angles.
b Perspex is very absorptive and attenuates unwanted echoes from the compressional wave as it hits the perspex test material interface.
The piezo-electric crystal generates a compressional wave which it transmits into the perspex wedge. When the compressional wave hits the bottom surface of the wedge most of the energy is reflected away from the interface and back into the perspex. It is dampened by tungsten powder in epoxy resin placed on the perspex wedge as damping.
Angle probe
If there is no couplant on the bottom surface of the perspex wedge, all the energy is reflected back into the probe. If there is couplant and if the probe is placed on test material, sound energy passes into the test material and generates a shear wave. Angle probes utilise compression probes mounted on a wedge of perspex. The wedge of such a probe is cut to a particular angle to enable the beam to refract into the test material at a chosen angle.
Angle probes usually transmit a shear or sometimes a surface wave into test materials and are used largely in weld testing, casting and forging inspection and in aerospace applications.
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Rev 0 August 2008 Probe Design
10.3 Twin crystal probes
A single crystal probe transmits and receives ultrasound with one crystal:
the crystal transmits the pulse and vibrates when the pulse returns from a backwall echo or a flow. However, when a single crystal probe is used, a signal appears on the screen at the beginning of the time base. It is caused by vibrations immediately adjacent to the crystal and is called by several names: initial pulse, transmission signal, crystal strike or main bang.
For a single crystal probe the length of the initial pulse is the dead zone and any signal from a reflector at a shorter distance than this will be concealed in the initial pulse. We deliberately delay the initial pulse beyond the left of the time base, by mounting the transducers of a twin (or double) crystal probe onto plastic wedges. This and the focusing of the crystals reduces the dead zone considerably and it is only where the transmission and receptive beams do not overlap that we cannot assess flaws.
A twin or double crystal probe is designed to minimise the problem of dead zone. A twin crystal probe has two crystals mounted on perspex shoes angled inwards slightly to focus at a set distance in the test material. Were the crystals not angled, the pulse would be reflected straight back into the transmitting crystal.
Twin crystal probe
The perspex shoes hold the crystals away from the test surface so that the initial pulse does not appear on the CRT screen. The dead zone is greatly reduced to the region adjoining the test surface, where the transmission and reception beams do not overlap.
Rev 0 August 2008 Probe Design
There are other advantages
• Double crystal probes can be focused
• Can measure thin plate
• Can detect near-surface flaws
• Has good near-surface resolution Disadvantages
• Good contact is difficult with curved surfaces
• It is difficult to size small defects accurately as the width of a double-crystal probe is usually greater than that of a single-crystal probe
• The amplitude of a signal decreases the further a reflector is situated from the focal distance - a response curve can be made out.
Therefore single and twin crystal probes are complementary.
10.4 Other probe types
Immersion probes are designed for use where the test part is immersed in water. They are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications. Immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and in turn, into the component being inspected. Immersion transducers can be purchased in a flat, cylindrically or spherically focused lens. A focused transducer can improve sensitivity and axial resolution by concentrating the sound energy to a smaller area.
Delay line probes, as the name implies, introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the crystal to complete its transmission function before it begins to receive returning signals. Delay line transducers are recommended for applications that require a contact transducer with good near-surface resolution and are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezo-electric element from the heat.
High frequency broadband probes with frequencies between 20 and 150MHz are commercially available and can improve flaw resolution and thickness measurement capabilities dramatically.
Rev 0 August 2008 Test Techniques