Non- Destructive Testing Lab (CE-606) - Unit 3 Short Notes

3.0 Introduction to Major NDT Methods & Laboratory Framework

Purpose: Transition from theory to practical application of specific discontinuity detection. Focus on method selection based on material properties, defect type (surface, subsurface, volumetric), and component geometry.

Six Primary Laboratory Methods:

1. Ultrasonic Testing (UT)

2. Radiographic Testing (RT)

3. Magnetic Particle Testing (MT)

4. Liquid Penetrant Testing (PT)

5. Eddy Current Testing (ET)

6. Visual Testing (VT)

Laboratory Safety Protocols (Key Hazards):

• UT: High voltage, couplant slip hazards.

• RT: Ionizing radiation (X-ray, gamma) – strict area control, dosimetry, shielding.

• MT: Electrical shock (magnetizing equipment), chemical exposure (particles, cleaners).

• PT: Chemical exposure (penetrants, developers, removers), fire hazard (solvent-based), UV exposure (fluorescent).

• ET: Electrical shock, entanglement in moving parts.

• General: Proper PPE, ventilation, chemical handling, and waste disposal.

3.1 Ultrasonic Testing (UT) - Theory & Thickness Measurement

Fundamental Principles

• Wave Types:

  • Longitudinal (L): Particle motion parallel to wave direction. Fastest, used in thickness gauging.

  • Shear (S): Particle motion perpendicular. Slower, used for flaw detection.

  • Surface (Rayleigh): Travels along surface.

  • Lamb (Plate): Complex modes in thin materials.

• Acoustic Impedance (Z): $$\displaystyle Z = \rho \cdot C $$ (density × sound velocity). Governs reflection/transmission at interfaces.

• Reflection Coefficient (R): $$\displaystyle R = \left( \frac{Z_2 - Z_1}{Z_2 + Z_1} \right)^2 $$ for intensity.

• Techniques:

  • Pulse-Echo: Single transducer transmits/receives. Used for thickness, flaw location.

  • Through-Transmission: Separate Tx/Rx transducers. Used for attenuation measurement.

Equipment & Transducers

• Transducers:

  • Contact: Direct contact, single element.

  • Immersion: For automated scanning.

  • Angle Beam: Uses wedge to introduce refracted S-waves (e.g., 45°, 60°, 70°) for weld inspection.

  • Dual Element: Separate Tx/Rx elements, good for near-surface resolution.

• Flaw Detector Controls:

  • Gain/Attenuation: Amplifies signal.

  • Delay: Shifts baseline for zero offset.

  • Range/Sweep: Sets time base (depth calibration).

  • Repetition Rate (PRF): Pulse frequency.

  • Damping: Pulse width control.

• Couplant: Eliminates air gap (water, oil, gel). Selection based on surface finish, material.

Calibration & Standards

• Calibration Blocks: IIW (International Institute of Welding) US-1, AWS (American Welding Society) block. Contain notches (side-drilled, flat-bottom) for distance/sensitivity calibration.

• Distance-Amplitude Correction (DAC): Curve plotting known reflector amplitude vs. distance. Accounts for beam spread & attenuation.

• Time-Corrected Gain (TCG): DAC implemented electronically in modern instruments.

• Setting Sensitivity: Reference level set using a known reflector (e.g., 80% of screen height for a 3mm SDH at a specific distance).

Laboratory Procedures & Defect Evaluation

• Straight Beam (Normal Incidence): For thickness, laminations parallel to surface.

• Angle Beam (Weld Inspection):

  • Wedge Delay: Time for sound to travel through wedge.

  • Skip Distance: $$\displaystyle S = Y \cdot \tan \theta_r $$ (Y = depth to reflector, $$\displaystyle \theta_r $$ = refracted angle).

  • Beam Path: Total path length calculation for depth location.

• Scan Patterns:

  • Parallel: For welds, scanning parallel to weld axis.

  • Tandem: Tx & Rx on opposite sides, sensitive to planar defects parallel to surface.

  • Raster: Grid pattern for area mapping (C-scan generation).

• Defect Characterization:

  • Location: Range (time base), Depth (using $$\displaystyle \theta_r $$ and trigonometry).

  • Size: Amplitude comparison (6dB drop method for length, 20dB for height) or using DAC/TCG.

  • Shape: Planar (sharp, high amplitude, distinct) vs. Volumetric (rounded, lower amplitude).

• Imaging:

  • A-Scan: Amplitude vs. Time/Depth. Basic signal.

  • B-Scan: Cross-sectional view (2D slice). Shows depth & lateral position.

  • C-Scan: Planar (top-down) view. Shows planarity & location.

Advanced UT Concepts (Introduction)

• Phased Array UT (PAUT): Multiple crystal elements with controlled time delays → electronic beam steering & focusing. No mechanical movement needed.

• Time of Flight Diffraction (TOFD): Uses diffracted signals from defect tips (not reflection). Highly accurate for sizing, especially height. Setup: Tx & Rx on opposite sides of weld.

[!TIP] Exam Focus: Be able to calculate depth for an angle beam inspection using skip distance and trigonometry. Know the difference between DAC (curve) and TCG (electronic implementation). Understand why 6dB drop estimates length but not necessarily true size.

3.2 Radiographic Testing (RT) - Theory & Image Interpretation

Fundamental Principles

• Sources:

  • X-ray: Bremsstrahlung & characteristic lines from tube. Voltage (kVp) controls energy/penetration.

  • Gamma: Isotopes (Ir-192, Co-60, Se-75). Fixed energy, no power needed.

• Interactions:

  • Photoelectric: Dominant at low energy, high Z materials. Good contrast.

  • Compton Scatter: Dominant at medium energy. Reduces contrast.

  • Pair Production: >1.02 MeV, high Z.

• Radiographic Contrast:

  • Subject Contrast: $$\displaystyle C_s = \frac{I_1 - I_2}{I_2} = e^{-\mu_1 t_1} - e^{-\mu_2 t_2} $$ (depends on thickness & attenuation difference).

  • Image Contrast: Affected by subject contrast, scatter, and film contrast.

  • Film Contrast (γ): Slope of the straight-line portion of the H&D curve.

• Image Quality Factors:

  • Geometric Unsharpness (Ug):

$$U_g = \frac{F \cdot OID}{SID}$$

(F = focal spot size, OID = object-to-film distance, SID = source-to-image distance). Minimize by maximizing SID and minimizing OID.

*   **Scatter Radiation:** Reduces contrast. Controlled by **filtration** (removes low-energy photons) and **collimation** (limits beam size).

*   **Film Density:** Optical density, $$\displaystyle D = \log_{10} (1/T) $$, where T = transmission. Must be within film's useful range.

Equipment & Setup

• Techniques:

  • Single-Wall, Single-Image (SWSI): Object between source & film. Best for pipes.

  • Single-Wall, Double-Image (SWDI): Source & film on same side, object in middle. Common for welds.

  • Double-Wall, Single-Image (DWSI): Source & film on opposite sides, object in middle. For pipes.

• Distances: SID = SFD + OID (SFD = source-to-film distance). Larger SID → smaller Ug.

• Screens: Lead screens absorb scatter, improve contrast. Fluorescent screens increase film speed (less exposure).

Image Recording Media

• Film: Silver halide emulsion. Requires processing (developer, fixer, wash). Density measured with densitometer.

• Digital Radiography (DR): Flat panel detectors (FPD). Immediate image.

• Computed Radiography (CR): Imaging plates (photostimulable phosphor). Scanned later.

Laboratory Procedures & Interpretation

• Exposure Chart: Plots mAs (or exposure time) vs. thickness for a given material/energy. Guides technique selection.

• Interpretation:

  • Weld Defects: Porosity (dark round spots), Slag (irregular dark with tail), Lack of Fusion (dark linear), Cracks (dark, sharp, often with "tail").

  • Casting Defects: Shrinkage (irregular, often at junctions), Gas holes (round, uniform).

  • Service Damage: Erosion (thinning), Corrosion (loss of material).

• IQI/Penetrameter: Wire-type (e.g., 10-16% sensitivity) or step/hole type. Placed on source side. Must be visible on radiograph to indicate required sensitivity.

• "Radiographic Truth": 2D projection of 3D object. Superimposed features can mask defects. Depth perception is limited.

[!TIP] Exam Focus: Memorize $$\displaystyle U_g = \frac{F \cdot OID}{SID} $$. Know that to improve contrast, you reduce scatter (collimation, filtration) and use high film contrast (γ). Understand the difference between SWDI and DWSI setups.

3.3 Magnetic Particle Testing (MT) - Surface & Near-Surface Flaws

Fundamental Principles

• Magnetism: Ferromagnetic materials (Fe, Ni, Co, alloys). Magnetic Field (H) induces Magnetic Flux (B). $$\displaystyle B = \mu H $$ ($\mu$ = permeability).

• Magnetization:

  • Direct Current (DC): Current passed through part (prod, cable wrap). Deep penetration.

  • Alternating Current (AC): Used with yoke. Skin effect limits penetration to ~0.25mm. Good for surface.

  • Half-Wave DC (HWDC): Pulsating DC. Better surface mobility than AC, some penetration.

• Leakage Field: Discontinuity (crack, void) disrupts flux lines, causing field to "leak" out of surface. Attracts particles.

Equipment & Materials

• Magnetizing Equipment: Portable yokes (AC/HWDC), prods, coil (for longitudinal magnetization), stationary units.

• Magnetic Particles:

  • Dry: For rough surfaces (castings, forgings). Applied by dusting.

  • Wet: For smooth surfaces (welds). Suspended in liquid (oil/water), applied by spraying/immersion.

  • Fluorescent: Viewed under UV-A (black light). Higher sensitivity.

  • Non-Fluorescent (Visible): Viewed under white light.

• Application: Continuous (during magnetization) vs. Residual (after magnetization).

Laboratory Procedures

1. Pre-cleaning: Remove paint, oil, scale (critical for particle adhesion).

2. Magnetization: Establish adequate field (direction & strength). Check with magnetic field indicator (e.g., pie gauge, shims).

3. Particle Application: Apply while magnetizing (continuous) or after (residual).

4. Inspection: Observe under appropriate lighting. Identify relevant indications (from flaws) vs. non-relevant (from changes in section, permeability).

5. Demagnetization: Often required (especially for aircraft, machinery). Use de-gaussing coil or decreasing AC field.

6. Post-cleaning: Remove particles.

[!TIP] Exam Focus: Know AC has shallow penetration (skin effect), DC has deep. Fluorescent particles require UV-A light and dark adaptation. False indications are common from: abrupt changes in cross-section, sharp corners, residual magnetism, improper magnetization direction.

3.4 Liquid Penetrant Testing (PT) - Surface Breaking Defects

Fundamental Principles

• Capillary Action: Penetrant is drawn into clean, open surface-breaking defects due to surface tension/wetting.

• Key Properties:

  • Penetrant: Low viscosity, good wetting (low contact angle), high fluorescence/color contrast.

  • Developer: Absorbs penetrant from defect (blotting action) and provides contrasting background. Types: dry powder, wet (aqueous/non-aqueous), soluble.

Penetrant Systems & Materials

• Types:

  • Visible (Color): Dye (usually red). Inspected under white light.

  • Fluorescent: Dye fluoresces under UV-A (365 nm). Higher sensitivity, requires dark room.

• Sensitivity Levels (1-4): Level 1 (lowest) to 4 (highest). Higher sensitivity = smaller detectable defect.

• Removers:

  • Water-Washable: Emulsified, removed with water spray.

  • Solvent-Removable: Wiped with solvent-dampened cloth.

  • Post-Emulsifiable: Requires separate emulsifier step before water wash. Most controllable.

Standard Process (6 Steps)

1. Pre-cleaning: MOST CRITICAL STEP. Remove all contaminants (paint, oil, dirt, old penetrant). Use solvent, alkaline cleaner, abrasive.

2. Application of Penetrant: Apply by dipping, spraying, brushing. Dwell Time: 5-60 min (per procedure) for penetration.

3. Removal of Excess Penetrant: Controlled method. Must remove surface penetrant without removing from defect. Follow manufacturer's instructions (e.g., water spray at <40°C, solvent wipe with clean cloths).

4. Application of Developer: Apply dry or wet. Dwell Time: 5-30 min for blotting action.

5. Inspection:

   • Visible: Under white light (100-300 lux).

   • Fluorescent: Under UV-A (365 nm) in darkened area (ambient light < 2 lux). Use UV goggles.

6. Post-cleaning: Remove developer & penetrant after inspection/evaluation.

Interpretation & Limitations

• True Indication: Sharp, defined, continuous outline. Size/shape correlates to defect.

• False Indications: Brush marks, scratches, dirt, porosity in porous materials, contamination.

• Limitations: Only surface-breaking defects. Requires smooth, non-porous, non-absorbent surface. Temperature sensitive (typically 10-50°C). Not for porous ceramics, rubber, unfinished castings.

[!TIP] Exam Focus: Sequence of 6 steps is mandatory. Pre-cleaning is paramount. Know the difference between solvent-removable (wipe) and water-washable (spray). Fluorescent PT requires UV-A light and a dark environment.

3.5 Eddy Current Testing (ET) - Conductivity & Surface/Subsurface Flaws

Fundamental Principles

• Electromagnetic Induction: AC in test coil (primary field) induces eddy currents in conductive test piece.

• Secondary Field: Eddy currents generate opposing magnetic field. Changes in this field alter the impedance (Z) of the test coil.

• Impedance Change (ΔZ): Caused by:

  1. Material Properties: Electrical conductivity (σ), magnetic permeability (μ).

  2. Defects: Cracks, thinning, inclusions.

  3. Test Setup: Lift-off (probe-to-surface distance), coil size, frequency.

• Skin Effect (δ): Penetration depth decreases as frequency increases.

$$\delta \approx \sqrt{\frac{\rho}{\pi f \mu_0 \mu_r}}$$

(ρ = resistivity, f = frequency). Low freq = deep penetration; High freq = surface sensitive.

Equipment & Probes

• Probe Types:

  • Surface: For crack detection.

  • Encircling: For tube/rod inspection (whole circumference).

  • Internal Diameter (ID): For inside of tubes.

  • Pancake: Flat, for surface scanning.

  • Differential: Two coils (reference & test). Excellent for crack detection, rejects slow changes.

  • Absolute: Single coil. Measures absolute properties (conductivity, lift-off).

• Display:

  • Impedance Plane (Dot): X = resistance, Y = reactance. Defect signal is vector change.

  • Strip Chart (Time Base): Signal vs. scan position.

Laboratory Procedures & Applications

• Conductivity Measurement: For material sorting, heat treatment verification (e.g., aluminum alloys). Uses absolute probe, calibrated standards.

• Crack Detection: Differential probe, high frequency, scan surface.

• Tubing Inspection (EDDY current Array - ECA): Encircling/ID probes for heat exchangers, aircraft. Detects wall thinning, pitting, ovality.

• Coating Thickness: Non-magnetic coating on ferrous base. Measures lift-off effect.

• Lift-Off: Critical variable. Signal changes with probe distance. Must be controlled or compensated for in flaw detection.

[!TIP] Exam Focus: Skin effect is key: frequency selection is a trade-off between depth and surface resolution. Lift-off is the biggest source of noise in flaw detection. Differential probes are best for finding small cracks as they reject uniform signals.

3.6 Visual Testing (VT) - The Foundation Method

Role in NDT

• Primary method. First and last step in most inspections.

• Prerequisite for other methods (surface must be clean/accessible).

• Often used in conjunction (e.g., verify UT indication, document RT finding).

Laboratory Enhancement Tools

• Lighting:

  • Direct: Bright, specular. Highlights smooth surfaces.

  • Diffuse: Soft, shadowless. For general viewing.

  • Low-Angle (Raking): Light skimmed across surface. Excellent for revealing surface-breaking defects (scratches, laps, tool marks).

• Magnification:

  • Optical Comparator: Projects silhouette, measures dimensions.

  • Borescope/Fiberscope: For internal cavities (engines, pipes).

  • Video Scopes: Digital imaging, documentation.

• Documentation: Still photography, video. Must include scale, orientation, and lighting direction.

Procedural Aspects

• Pre-cleaning: Essential for visibility.

• Adequate Illumination: Minimum 200-300 lux for general, 500+ for critical.

• Systematic Procedure: E.g., for weld: start at one end, inspect root, then face, then toe on both sides. Use consistent viewing angle.

• Recognition of Service Damage:

  • Corrosion: Pitting, general loss, rust staining.

  • Erosion: Smooth, rounded loss (fluid flow).

  • Wear: Abrasive, directional.

  • Deformation: Buckling, bending, denting.

• Remote Visual Testing (RVT): Use of cameras/scopes for inaccessible areas.

[!TIP] Exam Focus: Raking light is the single most important technique for surface defect detection in VT. Always document with a scale and directional arrow. VT is qualitative, not quantitative (except with comparators).

3.7 Comparative Analysis, Reporting & Laboratory Management

Method Selection Matrix (Simplified)

Advantages & Limitations Summary

Laboratory Record Keeping & Reporting

• NDT Report Essentials:

  • Procedure/Standard used (e.g., ASME Sec V, API 1104).

  • Equipment (make/model), probes, settings.

  • Personnel (certification level).

  • Findings: Location (weld ID, station, orientation), size (length, height, depth if known), type (porosity, crack), acceptance (per code).

  • Sketches: Mandatory. Show component, defect location, orientation, dimensions.

  • Photographs/Images: When possible.

• Acceptance Criteria: Defined by applicable construction/engineering code (ASME Boiler & Pressure Vessel Code, API, AWS D1.1). Not decided by NDT technician.

• Calibration & Verification:

  • Daily: Equipment checks (UT gain stability, RT exposure, MT magnetizing force, ET reference standard).

  • Reference Standards: Maintained, traceable, used for setup/calibration only.

  • Procedure Qualification Record (PQR): Demonstrates procedure can find specified defects.

  • Written Practice (WP): Company procedure for NDT personnel training/certification (per ASNT SNT-TC-1A or ISO 9712).

[!TIP] Exam Focus: Know which method is best for a given scenario (e.g., surface crack in steel = MT or PT; internal porosity in weld = RT or UT; tubing wall loss = ET). Always reference a code for acceptance. A report must include a sketch.