7.6 Non-Destructive Testing

7.6 Non-Destructive Testing (NDT)

1. Introduction to NDT

1.1 Definition and Purpose

1. Definition: Inspection methods that evaluate materials, components, or systems without causing damage or altering their serviceability.

2. Primary Objectives:

   • Detect internal and surface flaws.

   • Measure dimensions and thickness.

   • Determine material properties.

   • Evaluate structural integrity.

   • Monitor degradation over time.

3. Key Advantage: Allows testing of components that remain in service.

1.2 Comparison with Destructive Testing

Aspect	Non-Destructive Testing (NDT)	Destructive Testing
Specimen	Remains usable	Destroyed or damaged
Cost	Lower per test	Higher due to specimen loss
Sampling	100% testing possible	Statistical sampling
Location	In-service testing possible	Laboratory mostly
Purpose	Quality control, maintenance	Material qualification

1.3 Common Applications

1. Aerospace: Aircraft components, engine parts.

2. Civil Engineering: Bridges, pipelines, buildings.

3. Manufacturing: Welds, castings, forgings.

4. Power Generation: Turbines, pressure vessels.

5. Transportation: Rails, automotive components.

6. Oil & Gas: Pipelines, storage tanks.

1.4 NDT Personnel Certification

1. Level I: Perform tests under supervision.

2. Level II: Set up and calibrate equipment, interpret results.

3. Level III: Develop procedures, oversee programs, train personnel.

4. Certifying Bodies: ASNT, PCN, ISO 9712.

2. Ultrasonic Testing (UT)

2.1 Basic Principles

1. Physics Behind UT:

   • High-frequency sound waves (0.5-25 MHz) propagate through materials.

   • Waves reflect at interfaces (flaws, boundaries).

   • Reflection time indicates flaw depth.

2. Wave Types:

   • Longitudinal (Compressional): Particle motion parallel to propagation.

   • Shear (Transverse): Particle motion perpendicular to propagation.

   • Surface (Rayleigh): Travel along surface.

   • Lamb (Plate): In thin materials.

2.2 Equipment and Setup

1. Main Components:

   • Pulser/Receiver: Generates and receives signals.

   • Transducer: Converts electrical to mechanical energy.

   • Display Unit: Shows A-scan, B-scan, or C-scan.

2. Couplant Requirement:

   • Liquid (gel, oil, water) eliminates air gap.

   • Air causes 99.9% signal loss at interface.

2.3 Testing Techniques

1. Pulse-Echo Method:

   • Single transducer sends and receives.

   • $Depth = \frac{Velocity \times Time}{2}$

   • Velocity depends on material (steel: ~5900 m/s longitudinal).

2. Through-Transmission:

   • Separate transmitter and receiver.

   • Measures signal attenuation.

3. Immersion Testing:

   • Component and transducer in water tank.

   • Improved coupling, automated scanning.

2.4 Data Presentation

1. A-Scan:

   • Amplitude vs time.

   • Shows signal peaks indicating flaws.

2. B-Scan:

   • Cross-sectional view.

   • Depth vs lateral position.

3. C-Scan:

   • Plan view.

   • Color-coded amplitude map.

2.5 Applications and Limitations

1. Advantages:

   • Deep penetration (several meters in steel).

   • High sensitivity to small flaws.

   • Accurate depth measurement.

   • Single-sided access sufficient.

2. Limitations:

   • Requires smooth surface.

   • Couplant needed.

   • Training-intensive interpretation.

   • Difficult with coarse-grained materials.

3. Typical Uses:

   • Weld inspection.

   • Thickness measurement.

   • Detection of cracks, inclusions, laminations.

3. Dye Penetrant Testing (PT)

3.1 Principle of Operation

1. Capillary Action: Liquid penetrant drawn into surface-breaking defects.

2. Three-Step Process:

   • Penetration.

   • Development.

   • Observation.

3.2 Testing Procedure

1. Step 1: Surface Preparation:

   • Clean thoroughly (degrease, remove contaminants).

   • Dry completely.

2. Step 2: Penetrant Application:

   • Apply by spray, brush, or immersion.

   • Dwell time: 5-30 minutes (depends on material, defect size).

3. Step 3: Excess Removal:

   • Water-washable: Rinse with water.

   • Post-emulsifiable: Apply emulsifier then rinse.

   • Solvent-removable: Wipe with solvent.

4. Step 4: Developer Application:

   • Draws penetrant from defect to surface.

   • Types: Dry powder, wet suspension, non-aqueous.

5. Step 5: Inspection:

   • White light or UV light (fluorescent penetrants).

   • Indications show defect location and shape.

3.3 Penetrant Types

1. Fluorescent:

   • Visible under UV light (365 nm).

   • High sensitivity.

   • Requires dark area for inspection.

2. Visible (Color Contrast):

   • Red dye visible in white light.

   • Lower sensitivity.

   • Suitable for field use.

3. Sensitivity Levels:

   • Level ½: Ultra high sensitivity.

   • Level 1: High sensitivity.

   • Level 2: Medium sensitivity.

   • Level 3: Low sensitivity.

   • Level 4: Ultra low sensitivity.

3.4 Applications and Limitations

1. Advantages:

   • Simple, inexpensive equipment.

   • Portable for field use.

   • Detects very fine surface cracks.

   • Works on complex shapes.

2. Limitations:

   • Surface-breaking defects only.

   • Porous materials unsuitable.

   • Surface preparation critical.

   • Chemical handling required.

3. Common Applications:

   • Castings and forgings.

   • Welds.

   • Aircraft components.

   • Turbine blades.

4. Magnetic Particle Testing (MT)

4.1 Fundamental Principles

1. Magnetism Basics:

   • Ferromagnetic materials only (iron, nickel, cobalt, some steels).

   • Defects cause magnetic flux leakage.

   • Particles accumulate at leakage fields.

2. Magnetization Methods:

   • Direct Contact: Current through part.

   • Indirect: Magnetic field from coil or yoke.

   • Multidirectional: Rotating magnetic field.

4.2 Testing Procedure

1. Step 1: Surface Preparation:

   • Clean surface, remove coatings if necessary.

2. Step 2: Magnetization:

   • Apply magnetic field.

   • Direction important: perpendicular to expected defects.

   • Use right-hand rule for field direction.

3. Step 3: Particle Application:

   • Dry powder or wet suspension.

   • Apply while magnetized (continuous method) or after (residual method).

4. Step 4: Inspection:

   • Visible particles: White light.

   • Fluorescent particles: UV light.

5. Step 5: Demagnetization:

   • Required if residual magnetism problematic.

   • Methods: AC current, reversing DC with decreasing amplitude.

4.3 Particle Types

1. Dry Method:

   • Colored particles (gray, red, yellow).

   • For rough surfaces.

   • Less sensitive than wet method.

2. Wet Method:

   • Particles in liquid carrier.

   • Better for fine defects.

   • Fluorescent or visible.

3. Particle Size:

   • Fine: 5-25 microns (high sensitivity).

   • Coarse: 25-150 microns (deep defects).

4.4 Applications and Limitations

1. Advantages:

   • Fast and relatively simple.

   • Detects subsurface defects near surface.

   • Immediate results.

   • Portable equipment available.

2. Limitations:

   • Ferromagnetic materials only.

   • Directional sensitivity.

   • Demagnetization often required.

   • Surface preparation needed.

3. Typical Uses:

   • Welded joints.

   • Castings and forgings.

   • Railway components.

   • Pressure vessels.

5. Radiographic Testing (RT)

5.1 Basic Physics

1. X-ray Generation:

   • Electrons accelerated toward target.

   • Bremsstrahlung radiation produced.

   • $E_{max} = eV$ (electron charge × voltage).

2. Gamma Radiation:

   • From radioactive isotopes (Ir-192, Co-60, Se-75).

   • Natural decay process.

3. Attenuation Law: $I = I_0 e^{-\mu x}$ Where μ = linear attenuation coefficient.

5.2 Equipment and Setup

1. X-ray Machines:

   • Energy range: 10 kV to 30 MV.

   • Portable and stationary units.

2. Gamma Sources:

   • Ir-192: Most common (0.3-1.5 MeV).

   • Co-60: Higher energy (1.17, 1.33 MeV).

   • Safety: Shielding, remote handling.

3. Film Radiography:

   • Silver halide film.

   • Density measured in optical density units.

4. Digital Radiography:

   • Computed Radiography (CR): Phosphor plates.

   • Digital Detector Arrays (DDA): Direct digital.

   • Real-time radiography.

5.3 Image Quality Indicators (IQI)

1. Purpose: Verify sensitivity and quality.

2. Wire Type: ASTM E747.

   • Series of wires of different diameters.

   • Sensitivity = smallest visible wire.

3. Hole Type: ASTM E1025.

   • Plaque with different thickness holes.

4. Placement: On source side of specimen.

5.4 Safety Considerations

1. Radiation Protection:

   • Time: Minimize exposure time.

   • Distance: Inverse square law.

   • Shielding: Lead, concrete, steel.

2. Dosimetry:

   • Personal dosimeters.

   • Area monitors.

3. Regulations:

   • Licensed operators.

   • Controlled areas.

   • Emergency procedures.

5.5 Applications and Limitations

1. Advantages:

   • Permanent record (film/digital).

   • Works on most materials.

   • Detects volumetric defects.

   • Established standards.

2. Limitations:

   • Radiation safety concerns.

   • Access to both sides needed.

   • Expensive equipment.

   • Orientation sensitivity.

3. Common Applications:

   • Weld inspection.

   • Castings.

   • Aerospace components.

   • Pipeline girth welds.

6. Eddy Current Testing (ET)

6.1 Basic Principles

1. Electromagnetic Induction:

   • AC current in coil generates alternating magnetic field.

   • Field induces eddy currents in conductive material.

   • Defects alter eddy current flow.

2. Impedance Changes:

   • Coil impedance affected by material properties.

   • Displayed on impedance plane.

6.2 Testing Variables

1. Frequency Selection:

   • Standard depth of penetration (δ): $\delta = \frac{1}{\sqrt{\pi f \mu \sigma}}$

   • Where: f = frequency, μ = permeability, σ = conductivity.

   • Higher frequency → less penetration.

2. Probe Types:

   • Absolute: Single coil measures absolute properties.

   • Differential: Two coils compare adjacent areas.

   • Reflection: Separate transmit and receive coils.

3. Lift-off Effect:

   • Distance between probe and surface.

   • Affects signal significantly.

6.3 Signal Analysis

1. Impedance Plane Display:

   • X-axis: Resistance component.

   • Y-axis: Reactance component.

   • Defects cause characteristic movements.

2. Phase Analysis:

   • Differentiates defect types.

   • Distinguishes between conductivity, thickness, defects.

6.4 Applications and Limitations

1. Advantages:

   • No contact required (except lift-off).

   • High speed inspection possible.

   • Sensitive to surface/near-surface defects.

   • Measures conductivity, thickness, coating.

2. Limitations:

   • Conductive materials only.

   • Limited penetration depth.

   • Sensitive to many variables.

   • Complex interpretation.

3. Common Uses:

   • Tube and pipe inspection.

   • Aircraft skin inspection.

   • Conductivity sorting.

   • Coating thickness measurement.

7. Other NDT Methods

7.1 Visual Testing (VT)

1. Basic Method: Direct observation.

2. Enhanced VT: Borescopes, fiberscopes, videoscopes.

3. Applications: Surface condition, alignment, cleanliness.

7.2 Acoustic Emission Testing (AE)

1. Principle: Detect stress waves from growing defects.

2. Applications: Structural monitoring, pressure testing, leak detection.

7.3 Thermographic Testing (IRT)

1. Principle: Detect temperature differences.

2. Applications: Building insulation, electrical systems, composites.

7.4 Leak Testing (LT)

1. Methods: Bubble test, pressure decay, halogen diode, mass spectrometer.

2. Applications: Pressure vessels, pipelines, sealed components.

8. Method Selection Criteria

8.1 Factors Influencing Selection

1. Material Type: Ferromagnetic, conductive, thickness.

2. Defect Type: Surface, subsurface, volumetric.

3. Access: Single-side, both sides, geometry.

4. Environment: Field or laboratory, hazardous conditions.

5. Cost: Equipment, personnel, time.

6. Sensitivity Requirements: Defect size to detect.

8.2 Comparative Matrix

Method	Material	Defect Type	Penetration	Advantages	Limitations
UT	Most solids	Internal, surface	Deep	Depth sizing, portable	Couplant, training
PT	Non-porous	Surface-breaking	Surface only	Simple, inexpensive	Surface only
MT	Ferromagnetic	Surface, near-surface	Shallow	Fast, portable	Material limited
RT	All	Volumetric	Full	Permanent record, established	Radiation safety
ET	Conductive	Surface, near-surface	Shallow	Fast, non-contact	Conductive only

8.3 Complementary Methods

1. Multiple Methods: Often used together for comprehensive inspection.

2. Example Sequence: VT → PT/MT → UT/RT for critical components.

3. Follow-up: NDT indications may require destructive testing confirmation.

9. Standards and Codes

9.1 International Standards

1. ASTM: American Society for Testing and Materials.

   • E317: Ultrasonic testing.

   • E165: Liquid penetrant testing.

   • E709: Magnetic particle testing.

   • E94: Radiographic testing.

   • E566: Eddy current testing.

2. ISO: International Organization for Standardization.

   • ISO 17635: NDT of welds.

   • ISO 3452: Penetrant testing.

   • ISO 9934: Magnetic particle testing.

3. ASME: American Society of Mechanical Engineers.

   • Boiler and Pressure Vessel Code, Section V.

9.2 Industry-Specific Codes

1. API: American Petroleum Institute.

2. AWS: American Welding Society.

3. EN: European Norms.

10. Recent Advances in NDT

10.1 Digital Transformation

1. Automated Systems: Robotic scanning.

2. Data Management: Cloud storage, AI analysis.

3. Real-time Monitoring: Continuous inspection systems.

10.2 Advanced Techniques

1. Phased Array UT: Multiple elements, beam steering.

2. Time-of-Flight Diffraction: Accurate sizing of defects.

3. Digital Radiography: Faster, less chemical waste.

4. Remote Field ET: For tube inspection.

10.3 Integration with Other Technologies

1. NDT with IoT: Sensor networks, predictive maintenance.

2. Augmented Reality: Real-time visualization of defects.

3. Machine Learning: Pattern recognition, automatic defect classification.

11. Summary

11.1 Key Points

1. NDT is Essential: For safety, quality assurance, and cost savings.

2. Method Selection: Based on material, defect type, and requirements.

3. Trained Personnel: Critical for reliable results.

4. Multiple Methods: Often needed for complete assessment.

5. Documentation: Essential for traceability and analysis.

11.2 Future Trends

1. Increased Automation: Reduced human error.

2. Advanced Sensors: Higher sensitivity and resolution.

3. Data Analytics: Predictive maintenance capabilities.

4. Integration: With design and manufacturing processes.

5. Sustainability: Reduced chemical use, energy efficiency.

NDT continues to evolve with technology while remaining rooted in fundamental physical principles, ensuring structures and components meet safety and performance requirements throughout their service life.