Non- Destructive Testing Lab (CE-606) - Unit 4 Short Notes

UNIT 4: NDT Methods - Principles, Equipment, and Practical Application

4.1 Introduction to NDT Methods & Selection Criteria

• Purpose of NDT: To evaluate material properties, detect defects, and assess integrity without compromising the part's serviceability. Used for quality control, failure analysis, and in-service inspection.

• Six Major NDT Methods: VT (Visual), PT (Penetrant), MT (Magnetic Particle), UT (Ultrasonic), RT (Radiographic), ET (Eddy Current).

• Method Selection Criteria: Key factors include:

  • Material type (ferromagnetic vs. non-ferromagnetic, conductive vs. non-conductive).

  • Defect type and orientation (surface-breaking vs. subsurface, planar vs. volumetric).

  • Component geometry and accessibility.

  • Required sensitivity and detection limit.

  • Cost, portability, and speed.

  • Safety and environmental considerations.

[!TIP] Exam Focus: Be prepared to justify method selection for a given scenario (e.g., "Why UT over RT for thick steel welds?").

4.2 Visual Testing (VT) & Optical Methods

• Principle: Direct or enhanced visual observation of surface condition.

• Types:

  • Direct VT: Inspector's naked eye.

  • Remote VT (RVT): Using aids (mirrors, borescopes, fiberscopes, videoscopes).

• Equipment & Lighting: Oblique lighting (raking light) to reveal surface irregularities, diffuse lighting for uniform illumination, magnifying glasses, cameras for documentation.

• Procedure: Ensure adequate surface preparation (cleanliness), optimize viewing conditions (lighting, angle), and maintain documentation (photos, sketches).

• Common Surface Discontinuities to Recognize:

  • Cracks (straight, jagged).

  • Porosity (pinholes, gas bubbles).

  • Undercut (groove along weld toe).

  • Incomplete Penetration/ Fusion (weld root gaps).

  • Misalignment (fit-up issues).

  • Overlap (excess weld metal).

[!TIP] Common Pitfall: VT is not just "looking." Systematic approach, proper lighting, and knowledge of expected defect morphology are critical for reliable detection.

4.3 Liquid Penetrant Testing (PT)

• Principle: Capillary action draws low-viscosity liquid (penetrant) into surface-breaking defects.

• Penetrant Types:

  • Fluorescent (Type 1): Viewed under UV (black) light. Higher sensitivity.

  • Visible (Type 2): Viewed under white light. Used for field/quick checks.

• Removal Methods: Water-washable, Solvent-removable, Post-emulsifiable (with separate remover).

• Standard Process Steps:

  1. Pre-cleaning (remove all contaminants).

  2. Application of penetrant (spray, dip, brush).

  3. Dwell Time (allow capillary action).

  4. Excess Removal (careful, method-specific).

  5. Developer Application (dry powder, wet film, etc. - draws penetrant out).

  6. Inspection (immediate, under appropriate light).

• Interpretation:

  • True Indication: From actual defect. Shape follows defect (linear for cracks, round for porosity).

  • False Indication: From non-defect features (e.g., scratches, stains, porosity in the part itself).

  • Relevant vs. Non-Relevant: Indications within acceptance criteria vs. those outside.

• Critical Factor: Surface roughness/porosity can trap penetrant, causing non-relevant indications.

4.4 Magnetic Particle Testing (MT)

• Principle: Applied magnetic field in a ferromagnetic material. A surface/near-surface defect causes flux leakage, attracting magnetic particles to form an indication.

• Magnetization Methods:

  • Yoke: Portable, induces field between poles.

  • Prod (Point): Direct contact, localized field.

  • Coil (Wrap-around): Longitudinal field along part axis.

  • Central Conductor: For tubular parts, field circles conductor.

• Particle Types:

  • Dry: Powders, used on rough surfaces, dry surfaces.

  • Wet: Suspended in liquid carrier, higher sensitivity, used with UV/fluorescent particles.

• Process: Part must be demagnetized (if previously magnetized), magnetized, particles applied, inspected under white light (non-fluo) or UV light (fluo).

• High-Priority Interpretation:

  • Indication Orientation: Particles form perpendicular to flux lines. Therefore, magnetization direction must be as parallel as possible to the expected defect orientation to maximize leakage.

  • Distinguishing Indications: True indications are sharp, well-defined, and build up. Non-relevant leakage occurs at changes in cross-section (keyways, threads), edges, or material boundaries.

[!TIP] Safety: Ensure no ferromagnetic objects (tools, fixtures) are near the magnetizing area—they can become dangerous projectiles.

4.5 Ultrasonic Testing (UT)

• Principle: High-frequency sound waves (0.5-25 MHz) propagate through material. Reflections from interfaces (defects, back wall) are detected.

• Key Transducers:

  • Straight Beam (Normal Incidence): Longitudinal waves. For thickness, delamination.

  • Angle Beam (Shear Wave): Introduced at an angle (e.g., 45°, 60°, 70°) to detect flaws in welds (mode-converted to shear).

  • Dual-Element (Split): Separate transmitting/receiving crystals, better for near-surface resolution.

  • Phased Array: Multiple elements controlled electronically for beam steering/focusing (advanced).

• Testing Techniques:

  • Pulse-Echo (A-scan): Single transducer. Most common. Displays amplitude vs. time/depth.

  • Through-Transmission: Separate Tx/Rx probes. Measures attenuation.

  • Resonance: For thickness measurement of thin materials.

• Calibration: Use reference standards:

  • IIW (International Institute of Welding) blocks: For angle beam calibration (distance, sensitivity).

  • Distance/Sensitivity Blocks (DSB): For straight beam calibration and DAC/DGS curve generation.

• A-scan Interpretation (Pulse-Echo):

  • Horizontal Baseline: Represents time/depth.

  • Vertical Deflection: Represents amplitude (signal strength).

  • Key Signals: Initial pulse (front surface), flaw echo, back-wall echo.

  • Thickness Measurement: $$\displaystyle T = \frac{V \cdot t}{2} $$, where $V$ = sound velocity in material (m/s), $t$ = time for round trip (s).

  • Mode Conversion: Longitudinal wave hitting boundary at angle can convert to shear/other modes, creating additional echoes.

[!TIP] Critical Skill: Practice reading A-scans. Identify the first back-wall echo and measure its time to calculate thickness. A missing or shifted back-wall can indicate a large reflector.

4.6 Radiographic Testing (RT)

• Principle: Differential absorption of ionizing radiation (X-rays, Gamma rays) by materials of different density/atomic number.

• Radiation Sources:

  • X-ray Generators: Electrical, controllable energy (keV), lower penetration than gamma for same energy.

  • Gamma Isotopes: Ir-192 (0.47 MeV), Co-60 (1.25 MeV). Higher penetration, constant output, no power needed.

• Image Formation:

  • Radiograph: Permanent image on film.

  • Radioscopy/Real-Time: Digital Detector Array (DDA), Computed Radiography (CR - imaging plates), Fluoroscopy (continuous view).

• Film Radiography Basics:

  • Film Types: Single-emulsion (general), double-emulsion (high contrast).

  • Processing: Developer, Stop bath, Fixer, Wash.

  • Film Density ($D$): $$\displaystyle D = \log_{10} \left( \frac{I_0}{I} \right) $$, where $$\displaystyle I_0 $$ = incident light, $I$ = transmitted light. Higher density = darker film.

  • Contrast: Difference in density between adjacent areas.

• Geometry & Exposure:

  • Source-to-Object Distance (SOD)

  • Object-to-Film Distance (OFD)

  • Source-to-Film Distance (SFD = SOD + OFD)

  • Penumbra (Blur): $$\displaystyle P = F \cdot \frac{OFD}{SOD} $$, where $F$ = focal spot size. Minimize OFD/SOD ratio for sharpness.

  • Exposure: $$\displaystyle E \propto \frac{1}{(SOD)^2} $$ (Inverse Square Law).

• Interpretation of Weld Defects (2D Projection):

  • Porosity: Small, round/oval dark spots (gas bubbles).

  • Slag Inclusions: Irregular, elongated dark areas with fuzzy edges.

  • Lack of Fusion: Sharp, straight dark lines at weld toe or root.

  • Cracks: Straight, dark, often with "tails" (separation at ends).

  • Undercut: Dark, crescent-shaped area along weld edge.

  • Beam Angle/Film Orientation: Defect's true shape is distorted based on its angle relative to the beam. A planar defect (crack) parallel to beam may be invisible.

[!TIP] Safety First: ALARA (As Low As Reasonably Achievable) is the core principle. Use shielding, interlocks, area monitors, and personal dosimeters. Never look directly at an exposed source.

4.7 Eddy Current Testing (ET)

• Principle: Electromagnetic induction. AC in primary coil creates changing magnetic field, inducing eddy currents in conductive test piece. Defects alter coil impedance.

• Probes:

  • Surface Probes: For surface cracks, conductivity measurements.

  • Tubing Probes: For in-service inspection of heat exchanger tubes (rotate probe, detect inside/outside defects).

  • Array Probes: Multiple coils for faster scanning or imaging.

• Display:

  • Impedance Plane (X-Y or Lissajous): Shows real (resistance) vs. imaginary (reactance) components. Lift-off signal (probe height change) moves signal along a characteristic curve; defect signal moves perpendicular to it.

  • Time-Sweep (Scan): Signal amplitude vs. position (for scanning).

• Factors Affecting Coil Impedance: Conductivity (σ), Permeability (μ), Geometry (size, shape, distance - lift-off), Frequency, Defect presence.

• Interpretation Basics:

  • Lift-off Signal: Large, smooth signal as probe moves away from surface.

  • Defect Signal: Sharp, distinct signal superimposed on lift-off baseline. Depth affects signal amplitude (shallower = larger).

  • Conductivity Changes: Signal shift due to material variations (heat treatment, alloy).

[!TIP] Key Concept: ET is extremely sensitive to surface conditions and lift-off. Consistent probe positioning and speed are vital for reliable inspection.

4.8 Emerging & Specialized NDT Methods (Brief Overview)

• Acoustic Emission (AE): Listens for transient elastic waves generated by active defects (crack growth, friction). Used for continuous monitoring of structures (pressure vessels, pipelines).

• Thermography:

  • Passive: Detects natural thermal patterns (e.g., delaminations in composites).

  • Active: Applies heat (flash, lamp) and monitors cooling. Used for disbonds, water ingress.

• Guided Wave UT (Long-range): Uses low-frequency UT (10-100 kHz) that travels long distances along structures (pipes, rails). Detects volumetric defects over tens of meters from a single probe location.

• Leak Testing: Detects fluid/gas leakage. Methods: Pressure decay, Vacuum decay, Tracer gas (Helium mass spectrometer).

4.9 NDT Procedure Development & Qualification

• Role of Written Procedures: Mandatory for consistency, repeatability, and regulatory compliance. Based on standards: ASNT SNT-TC-1A (personnel cert), ISO 9712 (personnel cert), API 1104 (welding), ASME Section V (codes).

• Key Procedure Elements:

  1. Scope & References.

  2. Method & Technique (e.g., UT pulse-echo, straight beam).

  3. Equipment (makes, models, settings).

  4. Personnel Requirements (certification level).

  5. Calibration & Reference Standards (type, frequency).

  6. Step-by-Step Process (pre-clean, apply, scan, evaluate).

  7. Acceptance Criteria (defect size, type, location limits).

  8. Documentation & Reporting.

• Procedure Qualification vs. Personnel Certification:

  • Procedure Qualification: Demonstrates the method/technique can reliably detect specified defects in a representative sample (often using reference standards with known flaws).

  • Personnel Certification: Demonstrates an individual can competently perform the method to a specified standard (via exam and practical).

[!TIP] Exam Distinction: A certified technician must follow an approved, qualified procedure. The procedure defines how to test; certification proves the person can do it.

4.10 NDT Reporting & Documentation

• Essential Report Elements:

  • Client, component identification, procedure used.

  • Equipment (make, model, serial), calibration status.

  • Personnel (names, certification levels).

  • Results: Clear description of all indications with sketches/photos, location (coordinates), size (length, height), and classification.

  • Interpretation & Conclusion: Accept/Reject based on acceptance criteria. Disposition recommendation.

• Indication Classification:

  • Relevant: Indication that exceeds acceptance criteria → Reject.

  • Non-relevant: Indication within criteria or from non-critical feature → Accept.

  • False: Not from a defect (e.g., dirt, scratch).

• Traceability: All records (reports, calibration certificates, procedure revisions) must be linked to the specific component/test.

• Communication: Reports must be clear, objective, and unambiguous. Separate observations (e.g., "linear indication at 12 o'clock, 50mm long") from interpretations (e.g., "likely a crack").

4.11 Laboratory Safety in NDT

• General Principle: Understand hazards specific to each method. Use SDS (Safety Data Sheets) for all chemicals.

• Method-Specific Hazards & Protocols:

  • RT (Ionizing Radiation):

    • Hazard: Acute/chronic radiation exposure, genetic damage.

    • Protocols: ALARA, controlled areas (posting, barriers), interlocks, area monitors, personal dosimeters (TLD/film badge), strict time/distance/shielding rules.

  • PT (Chemicals):

    • Hazard: Toxicity (inhalation/skin), flammability, skin/eye irritation.

    • Protocols: Ventilation (fume hoods), gloves/goggles, proper storage (flammables cabinet), no smoking, proper waste disposal.

  • MT (Electrical & Projectiles):

    • Hazard: Electrical shock, magnetically attracted flying objects (tools, equipment).

    • Protocols: Inspect cables/grounding, keep work area clear of ferromagnetic objects, wear safety glasses, demagnetize part after test.

  • UT (Electrical & Couplant):

    • Hazard: Electrical shock from equipment, slip hazards from couplant (water/gel), skin irritation.

    • Protocols: Equipment grounding, avoid water near electrical outlets, clean spills immediately, use non-irritating couplants.

  • General: Laser safety (for videoscopes with laser diodes - avoid eye exposure), noise (UT pulse), ergonomics (repetitive motion, lifting).

[!TIP] Mandatory: Never bypass safety interlocks on RT equipment. Always wear appropriate PPE (gloves, goggles) when handling PT chemicals. Conduct a pre-job safety briefing for every new setup.