sonal gupta 
Ultrasonic testing (UT) uses sound waves to detect flaws and determine wall thickness. It is non-destructive and can be used on a variety of materials and structures.
UT is typically performed by using a coupling medium between the transducer and the test object. This may be water, oil, or gel depending on the application.
Transducer
Ultrasonic testing uses sound waves to detect internal flaws and determine thickness of materials. The size of the signal waves, seen on an electronic screen, can indicate the location and type of defect.
The key component of any ultrasound system is the transducer, which is usually made of a dual crystal. It transmits longitudinal waves into the test object using a gel, oil or water as a coupling medium. This allows the waves to penetrate the material, and a probe is moved over the surface of the object to produce an image.
A variety of transducers are available, but dynamic range is the most important attribute. The higher the dynamic range, the more sensitive and precise the transducer is. The dynamic range is determined by the ratio of the largest amplitude to the smallest amplitude signal the transducer can translate.
UT can be used on many different kinds of materials, including metals, ceramics and plastics. It is also useful on denser materials, such as concrete and metal-based composites, although the resolution of these methods may be less defined.
The reflection method of UT is one of the most common, and relies on sound waves being reflected at interfaces in the test material. This can be the back wall of the material, or it could be an imperfection within the material such as a crack or void. The diagnostic machine displays this information as a signal, with the amplitude representing the strength of the reflection and the distance represented by its arrival time.
Transmitter
Unlike magnetic particle inspection and radiography, which use ionizing radiation and magnetism to evaluate objects, ultrasonic testing uses sound waves at frequencies higher than humans can hear. This means it can safely be used on many types of equipment, from metal ingots and welded components to concrete structures and aircraft frames.
The ultrasonic transmitter is the device that creates and sends the high-frequency sound wave to be tested. It must be designed to match the size of the object and type of defect you are looking for. The transmission frequency must also be chosen to ensure the sound reaches the defect, without being attenuated by it.
Ultrasonic testing uses ultrasound to detect and assess a wide range of structural defects in materials like metal, composite, plastic and ceramic. These include voids, disbonds, inclusions, cracks, and other discontinuities that could compromise the integrity of an item.
The most common type of ultrasonic flaw detector is a pulse-echo device that transmits a short burst of ultrasound to the test material. If the test material has a different acoustical impedance than the surrounding environment, some of the ultrasound will reflect back to the source device. The diagnostic machine then displays this reflected signal on-screen, along with its amplitude and the time it took to reach the transducer. This information helps technicians identify and locate the location of a flaw.
Receiver
A receiver converts ultrasonic energy into electrical signals, which are routed to a display unit. The amplitude of reflected waves is recorded, and the size of defects in the material can be determined by analyzing the results on an electronic screen. The receiver is powered by a pulser, which emits short bursts of high energy to drive the transducer and introduce ultrasonic vibrations into the test object. A piezoelectric disk is used to transform the electrical signal into ultrasonic vibrations which are transmitted through the test object and reflected back from imperfections in the surface or structure. The transmitter and receiver are separated by a coupling medium, which allows for the maximum transmission of sound energy.
Ultrasonic testing is able to detect flaws, discontinuities and measure the thickness of materials. It is commonly used on metals, ceramics, plastics and composites. It is less effective on dense materials like concrete, and highly porous biological materials such as wood and paper.
Various ultrasonic techniques are utilized depending on the desired inspection. For example, oblique defect detection is often achieved by tilting the transducer. Other methods include angle beam scanning, which involves moving the transducer linearly over a surface to collect A-scan data for inspection. This method is also suitable for welds and other components with oblique cracks. It’s important that technicians are trained to use UT equipment correctly in order to get the best and most accurate results.
Calibration
Calibration is an essential element of non-destructive testing that ensures the accuracy and precision of results. This process is usually done by the equipment manufacturer, but it can also be completed by a third party that specializes in ultrasonic testing. In addition to reducing costs, these companies can typically provide access to training that will allow in-house employees to complete calibrations in the future.
Regardless of which method is used, the main component of ultrasonic flaw detection is the pulser-receiver. This device emits and receives ultrasonic vibrations and converts them into an electrical signal that is then routed to a display unit. The pulser-receiver is equipped with a piezoelectric disk that provides the energy required to transmit and detect ultrasonic vibrations in an item. It also contains a pulse generator that produces short bursts of electrical energy and sends them to the transducer. These pulses are reflected off of the test object, and the receiver-transmitter system converts these reflections into an electronic signal that can be measured using the standard dB (decibel) measurement.
The reference standards and calibration blocks used in ultrasonic testing come in a variety of shapes and sizes, depending on the NDE application. The material of the standard must be the same as the test subject, and it should feature an artificially induced defect that closely resembles the type of defect you are attempting to find in your test specimen. A typical reference block will contain a series of drilled holes or notches that are smaller than the actual flaws you are trying to locate in your test specimen.
