Non-Destructive Test (Ultrasonic Test)
Ultrasonic testing (UT) comprises a range of non-destructive testing (NDT) techniques that send ultrasonic waves through an object or material. These high frequency sound waves are transmitted into materials to characterise the material or for flaw detecting.
How Does it Work?
Ultrasonic inspection uses a piezoelectrictransducer connected to a flaw detector, which in its most basic form is a pulser-receiver and oscilloscope display. The transducer is passed over the object being inspected, which is typically coupled to the test object by gel, oil or water. This couplant is required to efficiently transmit the sound energy from the transducer into the part, however This couplant is not required when performing tests with non-contact techniques such as electromagnetic acoustic transducer (EMAT) or by laser excitation.
TYPE OF ULTRASONIC TEST
Pulse-echo and Through-transmission
With pulse echo testing, the same transducer emits and receives the sound wave energy. This method uses echo signals at an interface, such as the back of the object or an imperfection, to reflect the waves back to the probe. Results are shown as a line plot, with an amplitude on the y-axis representing the reflection’s intensity and distance or time on the x-axis, showing the depth of the signal through the material.
Through-transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected.
Contact and Immersion Testing
Contact ultrasonic testing is typically used for on-site inspections accessibility or portability. Contact ultrasonic inspection can be performed where only one side of a test specimen as reachable, or where the parts to be tested are large, irregular in shape or difficult to transport.
Immersion ultrasonic testing is a laboratory-based or factory-based non-destructive test that is best suited to curved components, complex geometries and for ultrasonic technique development. In this method, the component or material is submerged in a water, which acts as a couplant in place of the gels used for contact ultrasound. Immersion UT generally uses pulse-echo method, and robotic probe trajectories can be used to inspect complex surfaces which would be hard to cover with contact probes. Immersion UT can be used for a wide range of wall thickness and material types, making it a suitable testing method for a variety of applications and industries.