S — Aircraft StructuresChapter 5 · 58 practice questions

Chapter 5: Structural Inspection and NDT - Visual, Dye Penetrant, Eddy Current, Ultrasonic, MPI, Crack Detection

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Structural Inspection and NDT - Visual, Dye Penetrant, Eddy Current, Ultrasonic, MPI, Crack Detection

Overview

NDT Methods — Selection by Defect Type NDT Methods — Selection by Defect METHOD PRINCIPLE / DEFECTS MATERIALS ADVANTAGES LIMITATIONS VISUAL INSPECTION (VI) Naked eye + optical aids. Detects: Surface only (dents, corrosion, leaks). Cannot see under paint. All aerospace materials Fast, economical, no special equipment. Basis of all inspection. Visible surface defects only. Fine cracks invisible. Depends on lighting and AME experience. LIQUID PENETRANT (LPI) Capillarity: penetrant + developer. Detects: Surface-breaking defects (cracks, porosity). Ineffective if defect contaminated. Non-porous: Aluminum, steel, titanium, ceramic. Simple, portable, low cost. Complex shapes possible. Results directly visible. Clean surface required (critical). Porous materials excluded. Over-cleaning = false negative. Chemicals involved (safety). MAGNETIC PARTICLE (MPI) Flux leakage + particles. Detects: Surface AND subsurface (cracks, inclusions). Sensitive to defect orientation. Ferromagnetic only: Steel, iron, nickel. Highly sensitive to cracks. Detects subsurface defects. Immediate results. Steel/iron only (not Al/Ti). Demagnetization mandatory. Arc strike risk (prods). Field/defect orientation critical. EDDY CURRENT (ECT) AC coil → induced currents. Detects: Surface and subsurface (cracks, corrosion, thickness). Depth linked to frequency. Conductors: Aluminum, stainless steel, titanium, copper. Fast, highly sensitive. Ideal for rivet holes. Measures conductivity/thickness. No consumables. Conductors only. Sensitive to lift-off and edges. Complex signal interpretation. Frequent calibration required. ULTRASONIC (UT) High-frequency sound waves. Detects: Deep internal, delaminations, thickness. Best NDT penetration. Metals, composites, plastics, ceramics. Wide material range. Excellent deep penetration. Precise defect location. Accurate thickness measurement. Couplant required (gel/water). Smooth surface necessary. Extensive operator training. High equipment cost. Note: VI/LPI = Surface | MPI/ECT = Subsurface | UT = Deep Internal

This chapter covers the fundamental principles, procedures, and interpretation of non-destructive testing (NDT) methods used in aircraft structural inspection. The primary NDT methods discussed include visual inspection, dye penetrant inspection (DPI), magnetic particle inspection (MPI), eddy current testing (ECT), and ultrasonic testing (UT). Each method has specific applications, limitations, and procedural requirements that aircraft maintenance engineers must understand to ensure structural integrity and airworthiness.

The chapter emphasizes the relationship between defect types, material properties, and appropriate NDT method selection. It also addresses regulatory requirements under CAR 571 and FAA AC 43.13-1B, which govern inspection procedures and acceptance criteria.

Key Concepts

Visual Inspection

Visual inspection is the most fundamental NDT method and serves as the first line of defense in structural integrity assessment. It relies on direct observation using appropriate lighting and magnification tools.

Surface Condition Assessment:

  • Smooth surfaces facilitate easier detection of defects
  • Paint bubbling typically indicates underlying corrosion, as corrosion products occupy greater volume than the original metal
  • Dents require measurement of both depth and diameter against SRM limits
  • Faint lines near rivet lines or repaired areas are classic indicators of fatigue cracks

Limitations:

  • Cannot detect tight cracks, cracks under paint, or subsurface flaws
  • Requires adequate lighting and access
  • Mirrors can access hidden areas but cannot overcome the fundamental limitation of detecting non-visible defects
Diagram — Structural Inspection and NDT - Visual, Dye Penetrant, Eddy Current, Ultrasonic, MPI, Crack Detection Structural Inspection & NDT Methods Non-Destructive Testing — Crack Detection & Material Evaluation Aircraft Structure Aluminum / Steel / Composite Visual Borescope / Mirror Surface cracks > 0.1mm Dye Penetrant Red dye / Developer Surface defects only MPI Magnetic particles Ferromagnetic only Eddy Current Conductive materials Surface & sub-surface Ultrasonic Pulse-echo / Through Thickness & depth Crack Detection Capabilities Surface Near-surface Sub-surface Through-thickness ▬ Visual/DPI: surface only ▬ MPI/Eddy: surface & near-surface ▬ Ultrasonic: full thickness Inspection Decision Process Material Type? Defect Type? Accessibility? Select Method Interpret / Report TC Approved — Chapter 5: Structural Inspection & NDT Methods

Dye Penetrant Inspection (DPI)

Dye penetrant inspection sequence Liquid Penetrant Inspection Sequence 6-Step Procedure — Detection of Surface-Breaking Defects 1. CLEANING Initial Cleaning Remove oil, grease, corrosion, and contaminants from the surface to inspect. ⚠ Contamination = false negative 2. PENETRANT APPLICATION Apply Penetrant By spraying, immersion, or brushing onto the surface. The liquid penetrates defects by capillary action. 3. PENETRATION TIME Dwell Time Allow penetrant to act 5 to 30 min depending on type and ambient temperature. ⏱ 4. REMOVAL Remove Excess Solvent-washable: cloth soaked with solvent. Water-washable: rinsing. ⚠ Excessive cleaning = false negative 5. DEVELOPER Apply Developer Fine powder or suspension. Acts as a "wick" to draw penetrant out of the defect to the surface. 6. INDICATION & CLEANING Interpretation Sharp line = crack Regular dots = porosity Diffuse area = unremoved excess Final cleaning mandatory Reminder: Penetrant testing only detects surface-breaking defects. Porous materials (sintered metals) = unsuitable. Magnesium alloys = approved products required.

DPI detects surface-breaking flaws in non-porous materials. The process involves applying penetrant, allowing dwell time, removing excess penetrant, applying developer, and interpreting indications.

Indication Interpretation:

Dye Penetrant — Indication Interpretation Dye Penetrant Testing: Interpretation of Indications Valid Indications Sharp Line Surface crack Sharp red indication Capillary action: penetrant is drawn out by the developer Small Round Dots Surface porosity Evenly spaced dots False Indications / Errors Diffuse Red Area Incomplete removal of penetrant Excess penetrant not removed Immediate Reddening of the developer Excessive residue Developer turns red immediately Causes of False Negatives 1. Contaminated Crack Oil, grease, corrosion Penetrant blocked Penetrant cannot enter the crack → no indication 2. Over-Cleaning Penetrant removed by excessive washing Excessive solvent/washing removes penetrant from shallow cracks → No visible indication, defect not detected Pre-cleaning mandatory — CAR 571 / AC 43.13-1B
  • Small, round, evenly spaced indications: surface porosity (common in castings and welds)
  • Sharp, distinct linear indications: cracks
  • Large, diffuse red areas: incomplete removal of excess penetrant
  • Immediate reddening of developer: excessive residual penetrant on surface

Critical Procedural Factors:

  • Smooth surfaces allow complete penetrant removal, reducing false indications
  • Rough surfaces trap penetrant, creating false indications
  • Porous materials (e.g., sintered metals) absorb penetrant, making inspection impractical
  • Contamination in cracks prevents penetrant entry, causing false negatives
  • Solvent-based cleaners risk over-cleaning and removing penetrant from cracks
  • Water-washable penetrants risk over-washing, especially for shallow cracks

Safety Considerations:

  • Magnesium alloys require approved, non-reactive penetrant materials due to flammability concerns
  • Proper ventilation is essential when using solvent-based systems

Magnetic Particle Inspection (MPI)

Magnetic field leakage at cracks Magnetic Field and Leakage at Cracks — Magnetic Particle Inspection (MPI) Sound part — Parallel flux lines Flux lines magnetic uniform No flux leakage — no attraction of magnetic particles Suspended particles Cracked part — Flux leakage at defects Particle accumulation Flux leakage magnetic Crack Magnetic Particle Inspection (MPI) Principle — Detection of Fatigue Cracks 1. Magnetization I Magnetizing current 2. Application of particles Magnetic suspension 3. Indication Visible indication 4. Eval. Evaluate Note: Particles accumulate at flux leakages — a sharp indication indicates a surface crack; a broad, blurred indication may indicate a sub-surface defect or a change in permeability.

MPI detects surface and near-surface flaws in ferromagnetic materials. It requires magnetization, application of magnetic particles, and interpretation of particle patterns.

Indication Characteristics:

  • Sharp, distinct lines: surface cracks creating strong magnetic flux leakage
  • Broad, fuzzy indications: changes in magnetic permeability (e.g., heat-affected zones near welds)
  • Patterns parallel to shaft axis with circular magnetization: transverse cracks
  • Patterns along weld beads: permeability changes in heat-affected zone

Method Considerations:

MPI Magnetization Methods — Circular vs Longitudinal Magnetic Particle Inspection: Circular and Longitudinal Magnetization Methods Circular Method (prods / direct current) + − SOURCE B I (direct current) Longitudinal crack Sensitivity: Detects longitudinal defects (parallel to the axis) ⚠ Danger: electric arc Poor prod contact can create an arc, causing heating and fusion points ✓ Demagnetization mandatory Residual magnetism attracts debris and interferes with onboard instruments Longitudinal Method (coil) B Transverse crack Sensitivity: Detects transverse defects (perpendicular to the axis) Application: Continuous: particles during current (more sensitive) Residual: after shutdown, for high-remanence materials ✓ Demagnetization mandatory Prevents attraction of foreign particles and prevents interference with future NDT SUMMARY — METHOD SELECTION BASED ON DEFECT ORIENTATION Circular current → longitudinal defects Coil → transverse defects ⚠ Arc danger (prods) ✓ Demagnetization required
  • Prod-type contact method: hazard of arcing from poor contact, causing localized heating and potential crack initiation
  • Residual method: less sensitive to small cracks due to weaker residual magnetism
  • Wet fluorescent method: higher sensitivity due to smaller particles and better contrast under UV light

Demagnetization Requirements:

  • Removes residual magnetism that attracts ferromagnetic debris
  • Prevents interference with painting and future NDT
  • Required after all MPI procedures per FAA AC 43.13-1B

Eddy Current Testing (ECT)

Eddy current frequency and depth Eddy Currents: Frequency and Depth Basic Principle Excitation coil Variable magnetic field Induced currents Skin Effect High density zone Depth Current density δ = 1 / √(π·f·μ·σ) δ = standard depth of penetration High Frequency (1 – 6 MHz) Crack Signal: sharp deflection ✓ Sensitive to small surface cracks Low Frequency (100 Hz – 1 kHz) Sub-surface crack Signal: weaker deflection ✓ Penetrates deeper Applications and Probes Pencil probe • Small coil, high resolution • Ideal for rivet holes • Detects fatigue cracks around fasteners Reflection probe • Transmitter/receiver coils • Detects cracks on far side • For thin sheets • Thickness measurement Signal Interpretation • 1 peak per revolution: localized crack • 2 peaks at 180°: two opposite cracks • Continuous signal: lift-off or probe wobble ⚠ Critical Points • Probe too large: loss of resolution and sensitivity • Clean and recalibrate if signal is uncertain near noise floor NDT — Non-Destructive Testing | CAR 571 | AC 43.13-1B

ECT detects surface and near-surface flaws in conductive materials. It uses electromagnetic induction to generate eddy currents that are disrupted by flaws.

Frequency Effects:

  • Higher frequencies: concentrate eddy currents near the surface, sensitive to surface cracks but limited depth penetration
  • Lower frequencies: penetrate deeper but with reduced sensitivity to small surface cracks

Probe Selection:

Eddy Current — Probe Selection Eddy Currents: Probe Selection PENCIL PROBE High resolution Inspection around rivet holes CRACK Characteristics: • Small concentrated coil • Detects fine cracks • Ideal for rivets • Manual or motorized rotation • Clear signal at one point = localized crack REFLECTION PROBE Transmit/receive coils Detects cracks on the far side of thin sheets FAR-SIDE CRACK T R Characteristics: • Transmit (T) + receive (R) coil • Penetrates through the sheet • Detects sub-surface defects • Screen indication: characteristic phase shift • Used on thin sheets OVERSIZED PROBE Reduced resolution Loss of sensitivity to small cracks SMALL CRACK NOT DETECTED Characteristics: • Coil too large for the area • Extended magnetic field • Dispersed eddy currents • Poor spatial resolution • Small cracks masked • Diluted signal, false negative RULE: The probe must be matched to the size of the target defect
  • Pencil probes: small coil, high sensitivity to localized cracks around fastener holes
  • Reflection probes: detect cracks on far side of thin skins using separate transmit and receive coils
  • Probe size affects spatial resolution; oversized probes reduce sensitivity to small cracks

Signal Interpretation:

Eddy Current — Fastener Hole Signal Interpretation Eddy Currents: Interpretation of Signals in Holes Probe rotation animation — NDT signal interpretation 1. Signal at a Point Crack Single peak Localized crack detected at a precise angular position during probe rotation 2. Two Points at 180° Crack 1 Crack 2 Two peaks Two opposing cracks High bearing loads (often on rods or shafts) 3. Continuous Signal (lift-off) Clearance / wear Continuous signal Lift-off: probe loses contact with the hole wall → excessive clearance / oval hole 4. Rivet Row Crack Probe movement → Regular peaks = rivet heads Abnormal peak = crack between rivets 5. Subtle Signal Near Noise — Essential Cleaning and Recalibration Subtle signal (fine crack) Background noise Problem: A subtle signal can be masked by noise if the surface is dirty or if the equipment is not properly calibrated. ✓ Solution: Cleaning and Recalibration 🧹 Clean the surface and the hole ⚙ Recalibrate on a standard test piece → Clear signal → Reliable detection ⚠ Important: A false negative (undetected crack) can have catastrophic consequences in aviation. Always clean and calibrate before inspection. S-STRUCTURES ch5 — Structural Inspection and NDT | Eddy Current Testing (ECT) | Transport Canada — AME Training
  • Signal at one point in rotation: crack at that location
  • Signals at two points 180° apart: two separate cracks from high bearing loads
  • Signal throughout rotation: probe lift-off or wobble
  • Signal along entire rivet line: lift-off from protruding rivet heads
  • Subtle signals near noise band: requires cleaning and recalibration before further action

Ultrasonic Testing (UT)

Ultrasonic echo Ultrasonic Echo — Pulse, reflection from flaw, back-wall echo and A-scan reading Principle of ultrasonic inspection Inspected structure (e.g. fuselage) PROBE Emitted pulse Flaw (crack) Flaw echo Back-wall echo Signal legend Emitted pulse (outgoing) Flaw echo (return) A-scan reading (amplitude vs time) Pulse Flaw echo Back- wall Time (distance in part) → Amplitude → Surface Flaw position Back of part Interpretation of A-scan signals Without flaw • Initial pulse • Back-wall echo only • No intermediate echo With flaw • Flaw echo visible • Reduced back-wall echo • Position = depth Interpretation • Distance = time × speed • Amplitude = flaw size • Compare to SRM criteria Transport Canada — AME Training (TEA) — Chapter: Structural Inspection and NDT

UT uses high-frequency sound waves to detect internal flaws. It can be performed using straight-beam, angle-beam, or through-transmission methods.

Straight-Beam Method:

  • Strong back-wall echo: sound structure with no major defects
  • Series of equally spaced echoes after back-wall echo: multiple reflections between front and back surfaces (normal for sound material)
  • Small echoes between initial pulse and back-wall echo: small reflectors (inclusions, porosity, small cracks)
  • Large echo at depth with reduced back-wall echo: large defect reflecting significant sound energy
  • Loss of back-wall echo with normal front echo: delamination or disbond in composites

Shear Wave (Angle Beam) Method:

  • Primary application: detecting cracks oriented at angles to the surface (e.g., weld toes, fastener holes)
  • Less effective for thickness measurement or detecting perpendicular cracks

Through-Transmission Method:

Ultrasonic Through-Transmission Method Ultrasonics: Transmission Method Cross-section — Transmission Principle Composite Structure (carbon laminate / honeycomb) Delamination PROBE TRANSMITTING piezo CX PROBE RECEIVING piezo CX reflected / attenuated wave ⚠ Signal loss (receiver screen) Receiver A-scan Signal received: high amplitude (healthy zone) Weak / no signal Characteristics 2 probes: transmitting and receiving on opposite sides High sensitivity to delaminations and disbonds Loss of received signal = defect (shadow zone) Used primarily for composites Requires access to both faces of the part Limitation: no single-side access possible Access required: top face and bottom face Transmitted ultrasonic wave Delamination (defect) Reflected wave NDT Method — Ultrasonic Transmission | AME Training — Transport Canada | S-STRUCTURES ch5
  • Uses two probes on opposite sides of the part
  • Highly sensitive to delaminations and disbonds
  • Requires access to both sides of the structure
  • Primary method for composite inspection

Composite Inspection:

  • Delamination: loss of back-wall echo with normal front surface echo
  • Core crush: depressed area on surface indicating internal core failure
  • Tap test: dull or hollow sound indicates delamination or disbond

Important Formulas, Regulations, and Procedures

Regulatory Framework

  • CAR 571.02: Inspection must follow approved data (SRM, manufacturer's instructions)
  • CAR 571.05: Damage tolerance principles govern inspection intervals
  • CAR 571.06: Repairs must use approved data (SRM, STC, or repair design approval)
  • CAR 605.86: Inspection intervals established by approved maintenance schedule
  • FAA AC 43.13-1B: Acceptable methods, techniques, and practices for aircraft inspection and repair

Inspection Interval Determination

  • Fatigue-critical structures: intervals based on flight cycles (pressurization cycles) rather than flight hours
  • Visual inspections: intervals defined by manufacturer's maintenance manual or approved CAMP
  • NDT methods: intervals specified in SRM or maintenance planning document

Defect Classification

  • Fatigue damage: cracks from cyclic loading, typically at stress concentrations (rivet holes, lap joints, access panel cutouts)
  • Corrosion damage: material loss from environmental exposure, indicated by paint bubbling or white powdery deposits
  • Accidental damage: dents, gouges from discrete events (hail, bird strike, hard landing)
  • Environmental damage: UV degradation, chemical attack

Damage Assessment Procedure

  1. Identify defect location and type
  2. Measure defect dimensions (depth, length, diameter)
  3. Compare to SRM allowable limits
  4. If within limits: document and continue in service
  5. If exceeds limits: perform structural repair using approved data

Common Relationships Between Concepts

Method Selection Based on Defect Type

NDT method selection NDT Method Selection Surface / sub-surface flaw → inspection technique selection FLAW & MATERIAL Material Ferromagnetic (steel) Non-ferromagnetic (Al) Flaw type • Surface (fine crack, porosity, pitting) • Sub-surface (inclusion, delamination, corrosion) Accessibility • Visible / direct area • Hidden / restricted area Depth • Breaking the surface • Sub-surface (deep) Breaking visible Non-porous Ferromagnetic Conductive Sub-surface deep NDT METHOD 1. Visual Inspection Naked eye + lamp, mirror, magnifier Dents, corrosion, leaks, FOD 2. Penetrant (LPI) Penetrant + developer Surface-breaking cracks, porosity 3. Magnetic Particle (MPI) Magnetic particles Steel: surface + sub-surface 4. Eddy Current (ECT) Coil + alternating current Al, stainless: surface/sub-surface 5. Ultrasonic (UT) High-frequency sound waves Deep internal flaws DETECTION PRINCIPLE Light reflection Shadow contrast Visual deformation Capillarity: penetrant enters the flaw then is drawn out by developer Magnetic flux leakage Particles accumulate at field leakages Coil impedance variation caused by disturbance of the currents Wave reflection at the flaw interface Time-of-flight measured TYPICAL INDICATION Dent, blister, visible crack leak trace Compare to SRM Sharp line = crack Round dots = porosity Diffuse = excess penetrant Sharp line = crack Wide blurred = sub-surface Demagnetize after Point signal = crack 2 points 180° = 2 cracks Continuous = lift-off / gap Clear echo = internal flaw Echo loss = attenuation Position = depth Typical inspection sequence

Relationship Between NDT Method and Material

  • Ferromagnetic materials: MPI, Eddy Current, Ultrasonic
  • Non-ferromagnetic conductive materials: Eddy Current, Ultrasonic, Dye Penetrant
  • Non-conductive materials: Ultrasonic, Dye Penetrant (surface only)
  • Composites: Ultrasonic (primary), Tap Test (preliminary)
  • Porous materials: Limited DPI applicability; prefer ultrasonic or radiographic methods

Relationship Between Frequency and Detection Capability

  • Eddy Current: Higher frequency = shallower penetration, better surface sensitivity
  • Ultrasonic: Higher frequency = better resolution, shallower penetration
  • Selection: Must balance depth of interest against required sensitivity

Relationship Between Surface Condition and NDT Effectiveness

  • Smooth surfaces: Better for DPI (easier penetrant removal), better for UT (good coupling)
  • Rough surfaces: Increased false indications in DPI, reduced coupling in UT
  • Painted surfaces: Eddy Current can penetrate thin paint; DPI requires paint removal
  • Contaminated surfaces: Must be cleaned before any NDT to avoid false negatives

Relationship Between Crack Orientation and Detection

  • Perpendicular to sound beam: Readily detected by straight-beam UT
  • Parallel to sound beam: May not produce detectable echo; requires angle-beam or shear wave
  • Transverse to magnetic field: Detected by MPI with appropriate magnetization direction
  • Circumferential cracks: Detected by circular magnetization in MPI
  • Longitudinal cracks: Detected by longitudinal magnetization in MPI

Diagram

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