(SEM VI) THEORY EXAMINATION 2023-24 NONDESTRUCTIVE TESTING

NONDESTRUCTIVE TESTING – KME061

Section-wise Important Questions & Ready Answers

SECTION A

(Attempt all – 2 marks each)

(a) Toughness and Hardness

Toughness is the ability of a material to absorb energy and plastically deform before fracture. It represents resistance to impact and shock loading. Hardness, on the other hand, is the resistance of a material to indentation, scratching, or wear. A material may be hard but not tough, and vice versa.

(b) Material Properties Checked in Destructive Testing

Destructive testing is used to evaluate properties such as tensile strength, yield strength, ductility, hardness, impact strength, fatigue strength, and creep behavior. These tests permanently damage the specimen.

(c) Penetrant

A penetrant is a low-viscosity liquid used in liquid penetrant testing that enters surface-breaking defects through capillary action. It helps in revealing cracks and surface flaws invisible to the naked eye.

(d) Developers and Their Types

Developers draw penetrant out of surface defects to make them visible. The main types are dry powder developers, wet water-soluble developers, wet water-suspendable developers, and non-aqueous developers.

(e) Radiographic Testing

Radiographic testing is a non-destructive method that uses X-rays or gamma rays to detect internal defects in materials. Variations in material density cause differences in radiation absorption, forming an image on film or detector.

(f) Decay in Radiographic Testing

Decay refers to the reduction in radioactive intensity of gamma ray sources with time. As radioactive materials decay, their activity decreases, affecting exposure time and image quality.

(g) Immersion Testing

Immersion testing is an ultrasonic testing technique where both the test object and probe are submerged in water. Water acts as a couplant, providing uniform sound transmission and improved inspection accuracy.

(h) Characteristics of an Ultrasonic Beam

An ultrasonic beam has properties such as near field, far field, beam divergence, and intensity variation. These characteristics influence flaw detectability and resolution in ultrasonic testing.

(i) Eddy Current Phenomenon

Eddy currents are circulating electrical currents induced in a conductor when exposed to a changing magnetic field. These currents oppose the magnetic field and are used for defect detection and material property evaluation.

(j) Applications of Eddy Current Testing

Eddy current testing is used for crack detection, thickness measurement, conductivity testing, coating thickness evaluation, and inspection of aircraft components and heat exchanger tubes.

SECTION B

(Attempt any three – 10 marks each)

1. Visual Inspection Technique

Visual inspection is the simplest NDT method used to detect surface defects such as cracks, corrosion, misalignment, and wear. It can be direct (unaided eye), indirect (mirrors, borescopes), or enhanced (magnifiers, cameras). Proper illumination and surface cleanliness are essential for reliable inspection.

2. Zyglo Test (Fluorescent Penetrant Testing)

The Zyglo test uses fluorescent penetrants that glow under ultraviolet light. The steps include surface cleaning, penetrant application, dwell time, excess penetrant removal, developer application, and inspection under UV light. It is highly sensitive to fine surface cracks.

3. Interatomic Spacing Using X-Ray Diffraction

Using Bragg’s law, interatomic spacing is calculated by relating X-ray wavelength, diffraction angle, and order of reflection. This method is widely used in crystallography and material characterization.

4. CRO Technique and Ultrasonic Testing Principle

The CRO (Cathode Ray Oscilloscope) displays ultrasonic echoes for flaw detection. Ultrasonic testing works on the principle of reflection of high-frequency sound waves from material discontinuities. It offers deep penetration and high sensitivity.

5. Eddy Current vs Magnetic Particle Testing Sensitivity

Eddy current testing is less sensitive to very small surface flaws compared to magnetic particle testing because it relies on electromagnetic induction. Magnetic particle testing is more effective for detecting fine cracks in ferromagnetic materials.

SECTION C

Q3(a) Destructive vs Non-Destructive Testing & Ringing Test

Destructive testing permanently damages the specimen to measure mechanical properties, while non-destructive testing evaluates material integrity without damage. The ringing test detects surface cracks by listening to sound produced when a component is struck; cracked parts produce a dull sound.

Q3(b) Oil Whitening and Defects Detectable by NDT

Oil whitening enhances surface defect visibility by applying oil followed by a whitening agent. NDT can detect defects such as cracks, porosity, inclusions, lack of fusion, corrosion, and dimensional variations.

Q4(a) Liquid Penetrant Inspection (LPI)

LPI detects surface-breaking defects by allowing penetrant to seep into flaws. After removal of excess penetrant, a developer highlights defects. Advantages include simplicity and low cost, while limitations include inability to detect subsurface defects.

Q4(b) Magnetic Hysteresis and Magnetization Techniques

Magnetic hysteresis describes the lag between magnetic flux and magnetizing force. Magnetization techniques in magnetic particle testing include longitudinal, circular, and multidirectional magnetization, each offering different flaw detection sensitivity.

Q5(a) Bragg’s Law and Attenuation

Bragg’s law explains X-ray diffraction in crystals. Attenuation refers to the reduction in intensity of electromagnetic radiation as it passes through material due to absorption and scattering.

Q5(b) X-Rays vs Gamma Rays and Safety

X-rays are machine-generated, while gamma rays originate from radioactive sources. Selection depends on material thickness and portability. Radiation safety includes shielding, controlled exposure, and personal dosimeters.

Q6(a) Ultrasonic Transducers and Couplants

Transducers convert electrical energy into ultrasonic waves. Couplants such as water or gel eliminate air gaps and ensure efficient sound transmission during inspection.

Q6(b) Piezoelectricity and Its Use in UT

Piezoelectricity is the property of certain crystals to generate electric charge under mechanical stress. In ultrasonic testing, piezoelectric crystals generate and receive ultrasonic waves.

Q7(a) Eddy Current Probes and Fill Factor

Eddy current probes include absolute, differential, and reflection probes. Probe selection depends on 

material, defect type, and geometry. Fill factor is the ratio of probe area to test area, affecting sensitivity.

Q7(b) Thermography and Acoustic Emission Testing

Thermography detects temperature variations caused by defects using infrared imaging. Acoustic emission testing monitors stress-generated sound waves to identify crack initiation and growth.