(SEM VIII) THEORY EXAMINATION 2017-18 NON DESTRUCTIVE TESTING

SECTION A (Brief Explanation)

Define NDT
Non-Destructive Testing (NDT) is a method of inspecting materials or components without damaging or destroying them. It is used to detect defects, cracks, or irregularities while keeping the object in usable condition.

Defects
Defects are imperfections or discontinuities in a material that reduce its strength, quality, or performance. They may occur during manufacturing, welding, casting, or service use.

Fields of Application of NDT
NDT is widely used in aerospace, automobile, railway, power plants, oil and gas industries, shipbuilding, and construction to ensure safety and reliability of structures.

Equipment in Visual Inspection
Visual inspection requires basic tools such as magnifying glass, borescopes, mirrors, measuring gauges, proper lighting equipment, and cameras to detect surface defects.

Dwell Time
Dwell time is the time allowed for a penetrant or testing medium to remain on the surface so that it can properly enter surface defects during inspection.

Nature of Radiation
Radiation is energy transmitted in the form of waves or particles. In radiographic testing, electromagnetic radiation such as X-rays or gamma rays is used to detect internal defects.

Film Handling Recommendations
Radiographic films must be handled carefully to avoid scratches, exposure to light, moisture, or dust. Proper storage and safe darkroom conditions are necessary to maintain image quality.

Probe Used in Restricted Spaces
Small or angle probes are used in restricted spaces because they allow inspection at specific angles and can access limited or confined areas.

Eddy Current
Eddy current is a circulating current induced in a conductive material when exposed to a changing magnetic field. It is used to detect surface and near-surface defects.

Optical Inspection
Optical inspection uses visual and optical instruments such as microscopes and cameras to examine surfaces for cracks, corrosion, or defects.

SECTION B (Brief Explanation)

Machine Vision and Human Eye
Machine vision is a technology that uses cameras and image processing software to inspect materials automatically. The human eye detects light reflected from objects and sends signals to the brain for interpretation. Machine vision improves accuracy and reduces human error in inspection.

Radiography (X-ray and Gamma-ray)
Radiography is an NDT method that uses radiation to detect internal defects. X-ray radiography uses electrically generated X-rays and is commonly used in industrial testing. Gamma-ray radiography uses radioactive isotopes and is suitable for field inspections. Both methods are used in welding inspection and casting examination.

Destructive Testing and Difference from NDT
Destructive testing involves breaking or damaging a material to test its properties, such as tensile strength testing. Unlike NDT, destructive testing cannot be performed on finished products intended for use. NDT preserves the material while detecting defects.

Bragg’s Equation
Bragg’s equation explains how X-rays reflect from crystal planes to determine interplanar spacing. It is expressed as:
nλ = 2d sinθ
where λ is wavelength, d is spacing, θ is angle, and n is order of reflection.

Zyglo Fluorescent Penetrant Test
This test uses fluorescent dye to detect surface cracks. The penetrant is applied, allowed to dwell, excess removed, and developer applied. Under UV light, defects glow clearly. It is used in aerospace and automotive parts inspection.

SECTION C (Brief Explanation)

Ultrasonic Testing
Ultrasonic testing uses high-frequency sound waves to detect internal defects. A probe sends sound waves into the material, and reflected waves indicate flaws. It is widely used in weld and thickness inspection. It offers high accuracy but requires skilled operators.

A-scan, B-scan, and C-scan
A-scan shows defect signals as amplitude versus time. B-scan provides a cross-sectional view of the material. C-scan gives a top view image of defects. These scans help in analyzing internal flaws clearly.

EMAT Working Principle
Electromagnetic Acoustic Transducers generate ultrasonic waves without contact using electromagnetic fields. They do not require couplant and work well on rough or hot surfaces.

Ultrasonic Probes and Couplant
Different probes include normal, angle, and dual probes. The pulse depends on material properties and probe frequency. Couplant is important to remove air gap between probe and material for proper sound transmission.

Eddy Current Inspection
Eddy current inspection works by inducing currents in conductive materials and detecting changes caused by defects. It is used in tube inspection and crack detection.

Liquid Penetrant Inspection Steps
The process includes cleaning, applying penetrant, dwell time, removing excess penetrant, applying developer, and inspection. The penetrant must have high sensitivity and good surface wetting properties.

Rail Inspection by Eddy Current
Rail tracks are inspected using eddy current testing to detect surface cracks and wear. Special probes are used to ensure safe railway operation.