![]() ![]() The ones on the left are for manual testing, the ones on the right are water-proof probes, used for immersion technique, while the probes in the middle are angle beam probes.ģ0K, Fig. 1b,c,d: Basic construction of ultrasonic probes.ĭepending on the application, probes also differ with respect to the size of the active piezoelectric elements, their frequency, bandwidth and the basic design. Use of delay-line probes is a simple measure to have an excellent near field resolution.įig. The time delay until the ultrasonic signal enters the workpiece avoids, that echoes from flaws close to the surface appear within the dead zones of the ultrasonic flaw detector, which are caused by the high voltage excitation pulses. The delay line can be either rigidly fixed to the front face of the vertical probe or can be interchangeable. However here, the sound is transmitted into the workpiece via an additional delay-line made of plastics with low ultrasonic absorption. ![]() 1d) complies basically with that of the vertical beam probe. The construction of the "Delay line" probe ( Fig. Therefore, angle beam probes build up like this do not need a separate backing material (damping block) on the rear face of the piezoelectric element, as far as not the extremely short pulses of thickness probes are expected from the probe. These facts lead to a high sensitivity, rather short pulses and a high bandwidth. It also acts as a medium mechanical damping of the piezoelectric element. This ensures good energy transmission from the piezo element into the wedge. Usually an acoustic matching layer is placed between the piezoelectric element and the plastic wedge. The piezo-elements are mounted on plastic wedges, which are generally made of plexiglass, polystyrene or other plastic materials of low acoustic absorption. Their sound field characteristics overlap in the workpiece. 1c) consists of one separate transmitter and receiver element each. 1b), the ultrasound is transfered into the workpiece under at a specified angle. 1a: Basic construction of ultrasonic probes. It also protects the probe against mechanical damage, while it is moved over a workpiece, which may have a rough surface, or against chemical damage, when chemically agressive fluids are used as couplants.įig. A protective and/or matching layer in front ensures that as much of the acoustic energy as possible is transmitted into the workpiece. The second task of the damping block is to absorb that part of ultrasonic energy, generated by the piezoelectric element, which is going backward. The acoustic impedance of the damping block must be close to that of the piezoelectric material in order to suppress ringing resp. 1a), the piezoelectric element, which converts electrical energy into mechanical energy and vice versa, is mechanically attached to a backing material, most often called the damping block. In the vertical or straight beam probe ( Fig. 1 shows the basic construction of four fundamental probe types. Today, ultrasonic probes work almost exclusively according to the piezoelectric effect. In any case is the choice of the correct probe is decisive for the quality and the reliability of inspection results. In numerous cases, especially if a workpiece has a complicated geometry or the inspection has to be done under unusual conditions, ultrasonic inspection becomes only feasible by use of probes, which have appropriate acoustic properties. Probes form the actual core in all non-destructive ultrasonic inspection procedures: The fact whether a workpiece can be inspected or not depends upon them. The choice of the correct type of probe is often made easier through this knowledge.ģ. if a probe is coupled to a specimen, which it is not designed for. Knowing about these details however does not only contribute towards a better technical understanding of the basic action of the probe, but it does also help towards understanding the behaviour of the probe under certain operation conditions, e.g. Although the overall characteristics of probes, such as pulse shape, frequency and sound field are given in data sheets and are basically well understood, the knowledge about constructional details with regard to the utilised piezoelectric materials is less common. Depending on the application and the required probe characteristics, the one or the other material is more advantageous, sometimes for acoustic or technical reasons, sometimes even for economic reasons. These materials differ in their physical properties. RiesĪ series of powerful piezoelectric materials are available to generate ultrasound in ultrasonic probes. Piezoelectric materials for ultrasonic probes NDTnet - September 1996, Vol.1 No.09 Piezoelectric materials for ultrasonic probes Authors: M. ![]()
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