(SEM VI) THEORY EXAMINATION 2021-22 NON DESTRUCTIVE TESTING

NON-DESTRUCTIVE TESTING (KME061)

Section-wise Detailed Answers – B.Tech Semester VI

SECTION A

(Attempt all questions – descriptive explanations)

Q1(a) Define Non-Destructive Testing (NDT)

Non-Destructive Testing is a method of evaluating materials, components, or structures for defects, discontinuities, or variations in properties without causing any damage to the object being tested. The primary objective of NDT is to ensure quality, safety, and reliability while allowing the component to remain in service after inspection. NDT is widely used in industries such as aerospace, power plants, construction, and manufacturing.

Q1(b) Explain the meaning of visual inspection

Visual inspection is the simplest and most commonly used NDT method in which surface defects are detected by direct observation with the naked eye or with optical aids such as magnifying glasses, mirrors, or borescopes. It helps identify cracks, corrosion, misalignment, surface irregularities, and welding defects. Visual inspection is usually the first step before applying advanced NDT methods.

Q1(c) Explain the function of penetrant in liquid penetrant testing

In liquid penetrant testing, the penetrant is a low-viscosity liquid that enters surface-breaking defects through capillary action. The penetrant fills cracks, pores, and discontinuities present on the surface. After excess penetrant is removed, a developer draws the trapped penetrant back to the surface, forming visible indications that reveal the presence and shape of defects.

Q1(d) Describe the applications of magnetic particle testing

Magnetic particle testing is used to detect surface and near-surface defects in ferromagnetic materials such as steel and iron. It is widely applied in inspection of welded joints, castings, forgings, railway tracks, pressure vessels, and automotive components. This method is effective in detecting cracks, laps, seams, and inclusions.

Q1(e) Illustrate Thomson scattering

Thomson scattering refers to the elastic scattering of electromagnetic radiation by free or weakly bound electrons. In radiographic testing, it explains the interaction of X-rays or gamma rays with electrons in the material without loss of energy. This scattering affects image contrast and contributes to background radiation.

Q1(f) Explain Compton’s effect

Compton effect is an inelastic scattering phenomenon in which X-rays or gamma rays collide with electrons and lose part of their energy. As a result, the scattered radiation has a longer wavelength. In radiographic testing, Compton scattering reduces image clarity and contributes to radiation hazards, making shielding and collimation essential.

Q1(g) Describe Snell’s law

Snell’s law governs the refraction of waves when they pass from one medium to another. It states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media. In ultrasonic testing, Snell’s law helps determine wave direction, mode conversion, and probe angle selection.

Q1(h) Discuss the advantages of ultrasonic testing

Ultrasonic testing offers high penetration depth, allowing detection of internal defects in thick components. It provides accurate defect location and sizing, produces immediate results, and does not involve radiation hazards. The method is highly sensitive and can be applied to a wide range of materials.

Q1(i) Discuss the scope of eddy current inspection

Eddy current inspection is mainly used for detecting surface and near-surface defects in conductive materials. It is effective for crack detection, material sorting, thickness measurement, and conductivity assessment. The method is widely used in aircraft inspection, heat exchanger tubes, and power plant components.

Q1(j) Describe electromotive force (EMF)

Electromotive force is the potential difference that drives electric current in a circuit. In NDT methods such as eddy current testing, EMF is induced in the test material due to changing magnetic fields, resulting in circulating currents that help identify defects and material properties.

SECTION B

(Attempt any three – detailed explanations)

Q2(a) Difference between destructive and non-destructive testing and visual inspection devices

Destructive testing involves testing materials until failure to determine mechanical properties such as strength and toughness, making the component unusable. Non-destructive testing evaluates material integrity without damaging the component. Visual inspection devices include magnifying lenses, mirrors, endoscopes, borescopes, and cameras that enhance defect visibility and accessibility.

Q2(b) Steps involved in liquid penetrant testing

Liquid penetrant testing begins with surface cleaning to remove contaminants. Penetrant is applied and allowed to dwell so it can enter surface defects. Excess penetrant is removed, followed by application of a developer that draws penetrant out of defects. Finally, inspection is carried out under visible or ultraviolet light, and results are evaluated.

Q2(c) Advantages of gamma-ray radiography over X-ray radiography and radiation safety

Gamma-ray radiography offers greater penetration for thick materials and does not require electrical power at the source. It is portable and suitable for field inspections. However, radiation hazards require strict safety precautions such as shielding, controlled access, warning signs, and monitoring to protect personnel.

Q2(d) Types of probes used in ultrasonic testing

Ultrasonic probes include straight beam probes for normal incidence testing, angle beam probes for weld inspection, dual-element probes for near-surface defects, and immersion probes for automated scanning. Each probe type is selected based on defect orientation and material geometry.

Q2(e) Holography and thermography

Holography uses laser light to record and reconstruct three-dimensional images of objects, allowing detection of minute deformations caused by defects. Thermography detects temperature variations on the surface of a component using infrared cameras. Subsurface defects alter heat flow, producing detectable thermal patterns.

SECTION C

Q3(a) Classification, scope, and limitations of NDT methods

NDT methods are classified into visual testing, liquid penetrant testing, magnetic particle testing, radiography, ultrasonic testing, and eddy current testing. The scope of NDT includes quality control, in-service inspection, failure prevention, and life assessment of components. Limitations include high equipment cost, need for skilled personnel, and restricted applicability depending on material type and defect location.

Q3(b) Visual inspection in welding and its advantages and limitations

Visual inspection in welding detects surface defects such as cracks, undercut, porosity, and misalignment. Advantages include simplicity, low cost, and immediate results. Limitations include inability to detect internal defects and dependence on inspector skill and lighting conditions.