Ultrasonic Inspection Techniques

Ultrasonic Inspection

This method is used to find internal defects by using ultrasonic inspection sound waves. Very minute defects such as cracks, porosity, blowholes etc. can be accurately detected in castings. Sound waves can pass through solids without any absorption. It can also be reflected from a surface. Hence, ultrasonic waves are used in this test. These ultrasonic waves are produced by a transducer. The transducer can change the high frequency electrical energy into ultrasonic sound waves. It is called transmitter which can also change the ultrasonic sound waves into electrical energy.

Principle : Ultrasonic Inspection Techniques

High frequency sound waves are sent into a material by the use of a transducer. The sound waves travel through the material and they are received by the same transducer or a second transducer. The amount of energy transmitted or received and time to receive the energy is analyzed to determine the presence of flaws. Changes in material thickness and
properties can also be measured.

If the work is defect free, the wave will strike the bottom of the work and return to the receiver. The striking of waves at the bottom surface and top surface are indicated in the form of pip (echo) in CRT as 1 and 3 as 3 shown in image. If there is any defect in between top and bottom surfaces, the wave is reflected back from that spot and it is indicated as a pip in CRT as 2.

Two basic ultrasonic inspection techniques are employed such as pulse-echo and through-transmission.

(a) Pulse-Echo inspection : This process uses a transducer to both transmit and receive the ultrasonic pulse in image. The received ultrasonic pulses are separated by the time. It takes the sound to reach the different surfaces from which it is reflected. The size (amplitude) of a reflection is related to the size of the reflecting surface. The pulse-echo ultrasonic response pattern is analyzed on the basis of signal amplitude and separation.

(b) Through-transmission inspection : This inspection employs two transducers. Among them, one is to generate and the second one is to receive the ultrasound as shown in image. A defect in the sound path between two transducers will interrupt the sound transmission. The magnitude (the change in the sound pulse amplitude) of the interruption is used to evaluate test results. Through- transmission inspection is less sensitive to small defects than pulse-echo inspection. Ultrasonic inspection is used to detect surface and subsurface discontinuities such as cracks, shrinkage cavities, bursts, flakes, pores, delaminations and porosity.

Steps used in the testing

The following steps should be applied during the inspection:
(a) The couplant should be applied on the inspected area.
(b) For the circular test specimen, the prop will be placed in the corresponding space in the supporting fitting tool. Enough couplant should be used between probe and tool.
(c) For the flat specimen, no tool is needed and couplant is only applied between inspected surface and probe.
(d) Special attention should be paid on the location where the possible cracks exist.
(e) A discontinuity like a crack produces a peak on the screen.
(f) Attention should also be given to the movement of the possible peak caused by the cracks on the specimen.

Advantages

It is a fast and reliable process.
Minimum part preparation is required.
This method can be used for much more than just flaw detection.
It is sensitive to both surface and subsurface discontinuities.
The depth of penetration for flaw detection or measurement is superior to other NDT methods.
Only single-sided access is needed when the pulse-echo technique is used.
It is highly accurate in determining reflector position and estimating size and shape.
It provides instantaneous results.
Detailed images can be produced with automated systems.
It is nonhazardous to operators or nearby personnel and does, not affect the material being tested.
Its equipment can be highly portable or highly automated.

Limitations

Surface must be accessible to transducer and couplant so that ultrasound can be transmitted.
Surface finish and roughness can interfere with inspection.
Thin parts may be difficult to inspect.
Skill and training are more extensive than with some other methods.
It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
Linear defects oriented parallel to the sound beam may go undetected.
Reference standards are required for both equipment characterization and the calibration of flaws.