The high-frequency sound waves used for flaw detection and thickness gaging in ultrasonic nondestructive testing applications are generated and received by small probes called ultrasonic transducers. Transducers are the starting point for any ultrasonic test setup, and they come in a wide variety of frequencies, sizes, and case styles to meet inspection needs ranging from flaw detection in enormous multi-ton steel forgings to thickness measurement of paper-thin coatings.
A transducer is generally defined as any device that converts one form of energy into another. The subject of this paper is the ultrasonic transducers used for thickness gaging and conventional flaw detection. Phased array probes that utilize multiple elements to generate steered sound beams are described in detail .
Transducers convert a pulse of electrical energy from the test instrument into mechanical energy in the form of sound waves that travel through the test piece. Sound waves reflecting from the test piece are, in turn, converted by the transducer into a pulse of electrical energy that can be processed and displayed by the test instrument. In effect, the transducer acts as an ultrasonic speaker and microphone, generating and receiving pulses of sound waves at frequencies much higher than the range of human hearing.
Typically, the active element of an NDT transducer is a thin disk, square, or rectangle of piezoelectric ceramic or composite that converts electrical energy into mechanical energy, and vice versa. This element is sometimes informally called the crystal because, in the early days of ultrasonic NDT, elements were made from quartz crystals; however, ceramics such as lead metaniobate and lead zirconium titanate have long been used in most transducers. Recent years have seen increasing use of composite elements in which the traditional solid ceramic disk or plate is replaced by a micro-machined element in which tiny cylinders of piezoelectric ceramic are embedded in an epoxy matrix. Composite elements can provide increased bandwidth and improved sensitivity in many flaw detection applications.
Typical single element and dual element transducer construction.
When it is excited by an electrical pulse, this piezoelectric element generates sound waves, and when it is vibrated by returning echoesit generates a voltage. The active element is protected from damage by a wearplate or acoustic lens and backed by a block of damping material that quiets the transducer after the sound pulse has been generated. This ultrasonic subassembly is mounted in a case with appropriate electrical connections. All common contact, angle beam, delay line, and immersion transducers utilize this basic design. The phased array probes used in imaging applications simply combine a number of individual transducer elements in a single assembly. Dual element transducers, commonly used in corrosion survey applications, differ in that they have separate transmitting and receiving elements separated by a sound barrier, no backing, and an integral delay line to steer and couple the sound energy rather than a wearplate or lens. Figure 1 illustrates typical transducer construction.
While the basic concept is simple, transducers are precision devices that require great care in design, material selection, and manufacturing to help ensure optimum and consistent performance. The transducers commonly used in conventional ultrasonic NDT fall into five general categories based on their design and intended use.