Notes on Unit : Ultrasonics

Flaw detector:
Principle :

When there is change in the medium, ultrasonic waves get reflected. This property of ultrasonics  is used as principle in flaw detection technique

Basic ultrasonic testing systems :

There are two important techniques involved in ultrasonics scanning of materials :

1. Pulse echo system
2. Pulse transmission system

Pulse echo system :

Principle :

It is based on the principle of echo reflection of ultrasound at the interfaces. This is the most commonly used method in ultrasonic testing.  The pulse is either fully or partially reflected by any defect in the material.  The reflected signal is collected by the single probe which acts both as a transmitter and receiver. 

Working :

The block diagram for ultrasonic flaw detector is shown in the diagram.  It consists of a high frequency generator and a cathode ray oscilloscope (CR0).

It uses two separate probes, one for transmitting the ultrasonic waves and other to receive them after passing through the specimen. Ultrasonic waves are sent from transmitter probe into the specimen.  Waves  strike the surface of the specimen and produce an echo which is given to CRO.

If there is no defect in the specimen then a peak is produced in the right side of CRO while a peak is seen in left side due to the standard reference echo sent direct from transmitter to CRO.

If there is a flaw or defect in the specimen then waves would get reflected from that defect and a corresponding peak is produced in middle of left and right peaks in CRO.

                     

The time interval 't'

between the transmission and reception of the sound may be recorded in CRO and distance of the defect from the surface of specimen can be easily found.

                          

Advantages : 

1. It can reveal internal defects
2. This method is highly sensitive to most of the cracks and flaws.
3. It gives immediate results at low costs.
4. It gives the size and location of the flaws exactly.
5. Since there is no radiation in this process, it is a safe process

Limitations:

1. It is difficult to find flaws with complex sizes and shapes
2. Trained technicians alone can handle this method.