NDT : Introduction to NDT (Non Destructive Testing)

NDT means Non Destructive Testing,

It is a technique used to find defects present inside any material. 

QA/QC testing process could be classified into Destructive testing and nondestructive testing. Destructive testing process results in damaging the test specimen.

Non Destructive testing: Testing process is conducted without physically damaging the test specimen 

NDT is of four types 

1. Ultrasonic testing [UT]
2. Liquid Penetrant testing [LTP]
3. Magnetic particle testing [MPT]
4. Radiographic testing [RT]

1. Ultrasonic Testing : UT

UT is performed using sound waves/ Ultrasonic waves. 

Sound waves can be classified into three based on frequency:

1. Infrasonic waves (with frequency less than 20 Hz, these waves are produced by the vibration of huge bodies like earth) 
2. Audible/Sonic waves(with frequency range extends from 20 Hz to 20 KHz, only these waves are audible to human ears)
3. Ultrasonic waves(with frequency more than 20 KHz, these waves proved very useful to mankind Eg . communication , testing , Agriculture etc ..
   In UT, ultrasonic waves when forced to propagate through the test material gets reflected back either from the defective region (cracks) or from the backside of the test material. The reflected waves is received using transducers processed to analyzed and understand about the defects in the material. 

2. Liquid Penetrant testing [LTP]

Liquid penetrant testing is used to find defects on the open surfaces (0-3 mm) of the test materials 

3. Magnetic particle testing [MPT]

MPT is used to find defects on the open surfaces of test material to a depth of (0-6 mm)/ subsurface level .Only ferromagnetic metals(iron, cobalt, nickel….) can be inspected using MPT.

4. Radiographic testing [RT]

RT is performed by transiting light rays/ electromagnetic rays  (X-ray , alpha , beta , gama ...). The resultant ray that emerges out of the test material is copied to a photographic film and analyzed to understand details(size , shape , depth..) about the defects 

For more details reference: 

http://techshoreinspections.blogspot.in/2016/09/techshore-inspection-services-explains_6.html

https://en.wikipedia.org/wiki/Ultrasonic_testing