Single Compression
Single element or straight beam transducers are used to measure thickness and to detect flaws on plates, bars, forgings, castings and extrusions. During testing they are applied directly to the flat surfaces of the test material or object. Transducers with smaller diameters can be applied to test slightly curved materials.
Single element contact transducers work by emitting
compression (longitudinal) waves into the test material. Due to the fact that this type of transducer comes into direct contact with test materials when being used, the wear plates are constructed with highly durable material.
Twin Compression
Dual element contact (pitch-catch) transducers measure thickness and detect flaws and corrosion in thin materials, especially where near surface resolution is required. They focus very close to the front surface, making them ideal for pitting and corrosion tests, braze inspection and lamination evaluation. This focusing effect of the dual
transducer makes it ideal for pipes and other curved surfaces.
Dual element transducers utilise separate transmitting and receiving elements, mounted on delay lines that are usually cut at an angle. This configuration improves near surface resolution by eliminating recovery problems. In addition, the “crossed beam” design provides a pseudo-focus that makes duals more sensitive to echoes from
irregular reflectors such as corrosion and pitting.
One consequence of the dual element design is a sharply defined distance-amplitude curve. In general a decrease in the roof angle to an increase in the transducer element size will result in longer pseudo-focal distance and an increase in useful range.
Immersion Transducers
Immersion transducers are usually used for mechanised or automatic systems and, in principle, operate the same way as normal contact compression transducers.
Most applications take place in immersion tanks filled with water, where the test object is placed on a turntable or roller system so that the object is moved at a constant speed past the probe. This technique offers the best coupling conditions to provide reproducible results. Compared to contact transducers where all the parameters are defined “as in steel”, immersion transducer parameters are defined in water. Since the speed of sound in steel is approx. 5920 m per sec and in water is 1480m per sec. This gives a ratio of 4 to 1, which means it takes the same time to travel through 10 mm of water as it does to travel through 40 mm of steel.
Immersion transducers are available as flat faced or focussed. There are two types of focussing spherical and line. Spherical, spot or point focussing gives a reduced but concentrated beam width, which provides the best possible flaw detection capability , but takes longer to scan because of the reduced beam width. Line focussed probes give larger beam width in one axis with a concentrated reduced beam in the other axis. The working range of the focussed probes is much less than the flat-faced probes and in fact the focal length occurs within the near field length.
| | Angle Beam Transducers
Angle beam transducers allow the sound beam to be introduced into the test material at an angle. Plastic wedges of controlled geometry are attached to the transducer active element in order to establish the desired angle. Sonatest wedges are precision engineered to produce a refracted shear wave within the test object at specific angles, as indicated on the wedge or transducer
housing. The refracted beam angle should be selected to ensure that the sound beam angle will be, as much as possible, perpendicular to the plane of expected flaws.
In some cases, the geometry of the test object will dictate the selection of beam angle. With regard to frequency however, the same general rule applies—which is to select the lowest frequency which provides adequate flaw sensitivity. Both material noise and attenuation are minimised at lower frequencies.
Delay Line Transducers
Delay line transducers transmit and receive sound waves with one element, coupled to the surface as with compression transducers. The crystal is held off from the test piece surface by a delay block. This permits inspection very close to the test piece surface.
Wheel Transducers
Wheel transducers operate in a similar fashion to delay line models. They are typically used in applications where a large area must be scanned and/or where the test piece material is sensitive to conventional ultrasonic couplants.
Custom Transducers
Custom transducers are often required for specialist applications. These often contain a number of elements for specific locations and angles. An example of this is the probe used to test railway tracks that incorporates both forward and backward facing twin element arrangements either side of a conventional twin crystal arrangement.
Complex transducers such as this are designed in-house for a variety of specific applications.
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