NON DESTRUCTIVE TESTING

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X-rays
In 1895, Roentgen found that a highly penetrating radiation of unknown nature is produced when fast moving electrons strikes on matter. Because of their unknown nature, Roentgen called these radiations as X-rays. Actually X-rays are electromagnetic waves of very short wavelength.
Properties of X-rays
1. They are electromagnetic waves of very short wavelength.
2. They travel in straight line with the velocity of light.
3. They can affect a photographic film.
4. They cause fluorescence in many materials.
5. They can ionize the gas through which they pass.
6. They can penetrate through matter.
7. They can cause photoelectric effect.
8. They are not deflected by electric and magnetic fields.
9. Under suitable conditions, X-rays are reflected and refracted like ordinary light.
Production of X-rays
A Coolidge tube (discharge tube) is used for the production of X-rays. The cathode is a tungsten filament F heated by a 5-10 V battery. By thermionic emission, electrons are emitted from the cathode. These electrons are accelerated to high velocities by applying a very high potential difference between the cathode and the anode. When accelerated electrons strike the target (anode), X-rays are produced.

X-ray radiography

Radiography means producing an image on a film using electromagnetic radiation. X-ray radiographic methods are used for non destructive testing. Cracks, voids, discontinuities in welded joints etc. can be detected by this method. X-ray radiography operates on the principle of dissimilar transmission of X-rays through different materials. The ability of the material to block X-rays increases with the density of the material.
X-rays can penetrate easily through soft body tissues and are largely blocked by bone structure of other solid materials. So if a photographic plate is placed behind the patient, a photo can be taken showing bones as white shadows.
There are 3 main disadvantages:
X-ray image is basically a shadow and there is no scope for 3-dimensional informations.
X-rays are not easy to focus; hence images are of low resolution.
It is difficult to distinguish between various types of tissues in the body because they all have similar X-ray absorption properties.
To overcome these problems, stereo radiography and computed tomography (CT) are used.
Stereo radiography

Stereo radiography consists of superimposing radiographs taken from two positions of the X-ray tube which provides a 3-dimensional image.
Computed tomography (CT Scan)
Here X-rays from a finely collimated source are made to pass through a slice of the object or patient from a variety of directions. Thus X-ray beam moves all around the patient, scanning from hundreds of different angles. The computer collects the ‘slices’ of data and puts them together to form a 3-dimensional image of the body.
Ultrasonics
The human ear cannot hear sound vibrations of frequencies less than 20 Hz and greater than 2o KHz. This frequency range (20-20Khz) is audible range for human ear. Sound waves of frequencies above 20 KHz are called ultrasonic waves or ultrasonics. Though human beings cannot hear ultrasonic sound, some birds and animals are found to respond to ultrasonic sound. Bats produce and detect ultrasonic waves and are able to avoid obstacles on their way. Dogs can hear ultrasonic sound of low frequency.
Properties of ultrasonics
1. Attenuation: The amplitude of the ultrasonic waves diminishes as it propagates through a medium.
2. Ultrasonic waves undergo diffraction just like light and sound waves.
3. Ultrasonic waves require a material medium for propagation.
4. Ultrasonic waves obey law of reflection and law of refraction just like light waves.
5. Ultrasonic waves carry much more energy than audible sound waves.
6. Ultrasonic waves can penetrate large distances through matter.
7. Ultrasonic waves produce heat while passing through a medium.
Nondestructive testing using ultrasonics

Ultrasonic waves can be used to detect like cracks, cavities and the presence of foreign particles in a material. Strong pulses of ultrasonics are sent through the object to be tested. If object is defect free, then pulses will reflect from the other side. The incident pulse A and reflected pulse C are seen on the screen of CRO. If any defect is present on the object, ultrasonics gets reflected from the defect, and the reflected wave, called echo pulse B can be seen on the CRO. The position of the pulse B and its distance from A gives the nature of the defect and its location.
Ultrasound scanning
Internal organs of human body can be examined via the images produced by reflection and absorption of ultrasonic waves. Here a beam of high frequency ultrasonic energy is directed into the body. Waves reflected by the body tissues are detected and transformed into an image. A computer constructs the image that reveals the shape and density of internal structures.
Through ultrasound images are much safer than x-ray images, they do not contain much defect. They also contain large amount of noises. But contain organs such as liver is impossible to scan by x-rays, but can be imaged by ultrasonic waves.
MRI Scan
80% of our body’s atoms are hydrogen atoms. Hence most parts of the body can serve as sources for hydrogen NMR signals. In MRI scan, a varying magnetic field is applied across the tissue. A radio signal can be used to flip by spies of nuclei. The signals emitted during the relaxation of protons are detected by a sensitive detector. Since the proton density varies with type of the tissue, this helps in reconstructing the image. MRI scan is better for tissues, but less effective for detecting breaks in bones. MRI scan is expensive compared to x-ray images.
Electrical method of non-destructive testing
The electrical method is based on the principle of variations in the electrical resistivity due to the presence of defect in the specimen. When a coil carrying current is held near a conductor (specimen), eddy current are induced in the conductor. This is associated with a magnetic field. The defects in the specimen will be affect the resistivity and eddy currents. As a result, induced magnetic field changes. These variations in the induced magnetic field can be detected by a search coil and hence defects can be detected.
Magnetic method of non destructive testing
In this method the specimen is magnetized by spraying a fine powder of ferromagnetic substance on the specimen. Defects produce irregularities and discontinuities in the magnetic flux lines which help to locate the size and dimension of the defects. This method can be used for magnetic materials.