Non Destructive Testing

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Method of finding defects in an object without harming the object.

In the aircraft industry, NDT is used to look for internal changes or signs of wear on airplanes. Discovering defects will increase the safety of the passengers.

The railroad industry also uses nondestructive testing.

If an airplane or a rail had to be cut into pieces to be examined, it would destroy their usefulness.

With NDT, defects may be found before they become dangerous.

What is NDT?

The use of sound waves to determine defects is a very old method. if a piece of metal is struck by hammer, it will radiate certain audible notes, of which the pitch and damping may be influenced by the presence of internal flaws. However, this technique of hammering and listening is useful only for the determination of large defects.

Ultrasonic is a fast, reliable and nondestructive testing method which employs electronically produced high-frequency sound waves that will penetrate metals, liquids, and many other materials at speed of several thousand feet per second.

Ultrasonic Inspection:

A more refined method consists of utilizing sound waves above audible range with a frequency of 1 to 5 million Hz, hence they term ultrasonic.

Quartz is a widely used ultrasonic transducer. A transducer is a device for converting one form of

energy to another.

Ultrasonic Inspection continued:

Ultrasonic waves are introduced to a material The receiving transducer on the opposite side

receives the vibrations and converts them into an electrical signal that can be observed on the cathode ray oscilloscope.

If the ultrasonic wave travels through the specimen without encountering any flaw, the signal received is relatively large.

If there is a flaw in the path of the ultrasonic wave, part of the energy will be reflected and the signal received by the receiving transducer will be reduced.

The pulse-echo method uses only one transducer which serves as both transmitter and receiver.

When the sound wave enters the material being tested, part of it is reflected back to the transducer where it is converted back to an electric impulse. this impulse is amplified and shown on the screen of oscilloscope.

When the sound waves reaches the other side of the material, it is reflected back and shown as another signal on the screen besides the first signal.

Pulse-echo method:

Eddy-current testing uses to detect flaws, irregularities in structure, and variations in composition in conductive materials.

There are several limitations, among them:◦ only conductive materials can be tested, ◦ The material may cause bad readings, ◦ The depth of penetration into the material is

limited

Eddy Current Inspection:

In Eddy current testing, a varying magnetic field is produced if a source of alternating current is connected to a coil.

When this field is placed near a test specimen capable of conducting an electric current, eddy currents will be induced in the specimen.

The Eddy currents, in turn, will produce a magnetic field of their own.

The detection will measure this new magnetic field and convert the signal into a voltage that can be read on a meter.

The testing devices are portable, provide immediate feedback

Eddy Current Inspection:

The scale consists of 10 different standard minerals arranged in order of increasing hardness. For example, TALC is on no. 1 , GYPSUM is on No. 2, etc up to CORUNDUM is on No. 9 and DIAMOND on the last 10th place.

If an unknown material is scratched noticeably by no. 6 and not by no. 5, the hardness value is between 5 and 6.

This test has never been used to any great extent in metallurgy but is still used mineralogy.

Scratch Test:

The primary disadvantage is that the hardness scale is non uniform.

When the hardness of the minerals is checked by another hardness-test method, it is found that the values are compressed between 1 and 9 there is a large gap in hardness values

Scratch Test continued: