14.1 NDT Methods: Liquid Penetrant, Magnetic Particle, Eddy Current, Ultrasonic & Radiography

14.1 NDT Methods: Liquid Penetrant, Magnetic Particle, Eddy Current, Ultrasonic & Radiography

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Non-Destructive Testing (NDT) comprises specialized engineering inspection techniques designed to assess the structural integrity, material properties, and component health of aircraft structures and engine assemblies without causing physical damage or impairing future serviceability. In modern aviation maintenance, NDT is the cornerstone of damage-tolerant design principles and continuous structural airworthiness monitoring. For the certifying maintenance engineer under EASA Part-66 Module 7 (Maintenance Practices), mastering the physics, equipment setups, operational parameters, and defect detection limitations of each certified NDT method is essential.

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Qualification Standards & Regulatory Framework (EN 4179 / NAS 410)

In aerospace manufacturing and maintenance, Non-Destructive Testing cannot be performed by uncertified personnel. Personnel qualification and certification are strictly regulated under EN 4179 in Europe and NAS 410 in North America, which are harmonized international standards. These standards establish three distinct qualification levels:

• Level 1 (Technician / Operator): Qualified to perform specific calibrations, specific NDT tests, and specific evaluations for acceptance or rejection according to written instructions under the direct guidance of Level 2 or Level 3 personnel. A Level 1 cannot independently write inspection procedures or select inspection methods.
• Level 2 (Inspector): Qualified to set up and standardize equipment, interpret and evaluate results per applicable codes, standards, and specifications, organize and report test results, and provide on-the-job training to Level 1 personnel. Level 2 inspectors possess deep knowledge of the scope and limitations of the methods for which they are certified.
• Level 3 (Specialist / Examiner): Qualified to develop, validate, and verify NDT procedures and work instructions, evaluate new inspection technologies, interpret standards, design specific test techniques, and administer qualification examinations. Level 3 specialists hold overall technical responsibility for the maintenance organization's NDT operations.

Furthermore, all NDT personnel must undergo mandatory annual near-vision acuity examinations (e.g., Jaeger 1 at not less than 30 cm) and color perception testing to ensure reliable defect identification.

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Visual Optical Inspection & Borescopy

Visual Optical Inspection (VOI) remains the most widely utilized and foundational NDT technique in aviation, accounting for over 80% of all maintenance inspections. Visual inspection is subdivided into direct and remote/indirect optical inspection.

1. Direct Visual Inspection

Direct visual inspection involves the unassisted human eye, supplemented by low-power magnification (typically 10x maximum), hand mirrors, and surface texture comparators. Proper illumination is mandatory: ambient lighting must provide a minimum illumination of 1,060 lux (100 foot-candles) at the inspection surface for critical structural assessments.

2. Remote Visual Inspection (Borescopes & Videoscopes)

When internal engine gas paths, wing fuel bays, or enclosed structural cavities cannot be viewed directly, technicians deploy borescopes. Three distinct instrument designs are utilized:

                     BORESCOPE OPTICAL CONFIGURATIONS

   [Rigid Borescope]        [Flexible Fiberscope]       [Flexible Videoscope]
   - Glass relay lenses     - Coherent fiber bundles    - Micro CCD/CMOS chip
   - Superb optical clarity - Broken fibers = black dots- Electronic articulation
   - Straight access paths  - Flexible guide paths      - 3D Phase measurement

• Rigid Borescopes: Consist of an outer stainless steel tube housing a series of precision achromatic optical relay glass lenses and fiber-optic light-guide fibers. They offer superior optical resolution, image clarity, and depth of field, but require straight-line access. Viewing heads provide forward (0°), forward-oblique (45°), side (90°), or retro-viewing (110°) perspectives.
• Flexible Fiberscopes: Utilize two separate bundles of optical glass fibers: an incoherent bundle to carry illumination light from an external source to the target, and a coherent bundle (where thousands of fibers maintain identical geometric alignment at both ends) to transmit the optical image back to the eyepiece. Fiberscopes can negotiate curved paths inside engines, but individual fiber breakage over time causes permanent black dots across the visual field.
• Flexible Videoscopes (Video Borescopes): The modern industry standard. Videoscopes replace coherent fiber bundles with a miniature solid-state charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS) digital image sensor mounted directly at the probe tip. Features include:
  • Four-way motorized joystick articulation of the probe tip (deflection up to 180° in pitch and yaw).
  • Integrated digital freeze-frame, image capture, and video recording.
  • Optical measurement technologies: Stereo Measurement and 3D Phase Measurement. By projecting dual-optical images or structured phase patterns onto damage sites, internal processors calculate exact defect dimensions (crack length, pit depth, missing corner area, and blade tip thermal erosion).
  • Engine maintenance applications: In-situ evaluation of compressor and high-pressure turbine (HPT) blade damage, thermal barrier coating (TBC) spallation, nozzle guide vane (NGV) burning, and foreign object damage (FOD). Defect dimensions are compared directly against the Aircraft Maintenance Manual (AMM) and Engine Maintenance Manual (EMM) allowable blend-out limits without disassembling the engine.

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Liquid Penetrant Inspection (PT / LPI)

Liquid Penetrant Inspection (PT), also known as Fluorescent Penetrant Inspection (FPI), is a sensitive NDT method designed to detect discontinuities open to the surface in non-porous materials.

1. Physical Principle & Scope

Penetrant testing relies entirely on capillary action—the ability of a liquid with low surface tension and low contact wetting angle ($\theta < 90^\circ$) to flow spontaneously into tight surface cavities against gravity. When developer is subsequently applied, reverse capillary action draws the entrapped liquid back to the surface, where it spreads into a visible indication.

                       LIQUID PENETRANT PRINCIPLE

    (1) Penetrant Dwell          (2) Excess Removed          (3) Developer Action
    Penetrant fills crack        Surface cleaned gently       Developer draws dye out
     [Liquid Penetrant]                 [Clean]                  [Developer Powder]
    -------------------          -------------------          -------| | | |-------
         |   |                        |   |                     \   BLEED-OUT   /
    =====[   ]=====              =====[   ]=====              =====[   CRACK   ]=====
         Crack                        Crack                        Crack Reservoir

• Capabilities: Detects surface-breaking fatigue cracks, grinding cracks, thermal stress checks, forging laps, porosity, and seam defects in metallic (aluminium, titanium, stainless steel, nickel alloys) and non-metallic (ceramics, glazed glass, dense plastics) materials.
• Critical Limitation: Penetrant inspection is completely blind to internal or subsurface defects. It cannot detect defects that do not breach the exterior surface. Furthermore, if surface defects are peened shut, plugged with carbon, or coated with paint, anodize, or plating, penetrant cannot enter, resulting in false-negative indications.

2. Penetrant Systems & Sensitivity Levels (AMS 2644 / ISO 3452)

Aerospace penetrant systems are classified into standard categories:

• Penetrant Types:
  • Type I: Fluorescent Dye (viewed in a darkened booth under UV-A black light; highest sensitivity).
  • Type II: Visible Dye (red contrast dye viewed under standard white light; portable field use, lower sensitivity).
• Sensitivity Levels (Type I Fluorescent):
  • Level 1/2: Very Low Sensitivity
  • Level 1: Low Sensitivity
  • Level 2: Medium Sensitivity (general structural airframe fittings)
  • Level 3: High Sensitivity (critical landing gear components, structural forgings)
  • Level 4: Ultra-High Sensitivity (rotating turbine disks, single-crystal turbine blades)
• Removal Methods:
  • Method A (Water-Washable): Penetrant contains built-in emulsifying agents; excess surface penetrant is washed away directly with a gentle water spray.
  • Method B (Post-Emulsifiable Lipophilic): Oil-based emulsifier applied separately to surface penetrant before water rinsing.
  • Method C (Solvent-Removable): Excess penetrant removed manually with clean, lint-free cloths moistened with approved solvent cleaner. Ideal for line maintenance field spot-checks.
  • Method D (Post-Emulsifiable Hydrophilic): Water-based emulsifier applied following a pre-rinse cycle; provides highest control and process repeatability on critical engine components.
• Developer Forms:
  • Form a: Dry powder developer (applied in dust storm chambers onto dry parts).
  • Form b: Water-soluble developer (dissolved in water bath).
  • Form c: Water-suspendable developer (particles suspended in water agitated continuously).
  • Form d: Non-aqueous wet developer for Type I (solvent suspension sprayed from aerosol cans; creates a thin, uniform white coating; most sensitive).
  • Form e: Non-aqueous wet developer for Type II visible dye.

3. The 6-Step Standard Operating Procedure

Strict adherence to the sequential six-step protocol is mandatory:

1. Pre-Cleaning: The part surface must be completely free of oil, grease, scale, paint, and moisture. Approved volatile solvents, ultrasonic degreasing, or vapor degreasing must be used. Mechanical blasting, wire brushing, or scraping is strictly forbidden, because abrasive impact peens malleable metal over crack edges, physically sealing the crack mouth.
2. Penetrant Application & Dwell Time: Applied by immersion, spraying, or brushing. The part must remain wetted throughout the entire dwell time. Standard dwell times range from 10 to 30 minutes (up to 120 minutes for tight stress-corrosion cracks). Standard operating temperature is 10°C to 52°C (50°F to 125°F); applying penetrant below 10°C drastically increases viscosity and degrades capillary action.
3. Removal of Excess Surface Penetrant: For Method C, wipe the surface with clean, dry lint-free cloths until the bulk of penetrant is removed. Then wipe with a cloth lightly dampened with solvent remover. Never spray solvent directly onto the test surface! Direct spraying dilutes and flushes penetrant out of crack reservoirs, an error known as over-washing, which completely destroys test sensitivity.
4. Developer Application: Apply developer immediately after drying. Form d non-aqueous developer must be sprayed as a light, translucent, uniform dusting. A thick, heavy blanket of developer obscures fine bleed-out indications.
5. Developing Time & Inspection: Developing time begins immediately upon application (typically 10 to 60 minutes, with a minimum of 10 minutes). Under 365 nm UV-A light, penetrant bleeds out laterally, magnifying crack width. The rate of bleed-out indicates crack volume and depth: deep cracks bleed out rapidly and spread widely; shallow scratches bleed out slowly with minimal spread.
6. Post-Cleaning: Residual developer must be removed with water or solvent to prevent moisture absorption, corrosion initiation, or interference with subsequent service coatings.

4. UV-A Inspection Booth Environmental Standards

Type I fluorescent penetrant inspection requires a controlled optical environment:

• UV-A Black Light Wavelength: Peak output at 365 nm (long-wave ultraviolet).
• Minimum UV-A Intensity: Minimum 1,000 µW/cm² (10 W/m²) at the test surface, measured with a calibrated ultraviolet radiometer at a working distance of 38 cm (15 inches).
• Ambient White Light: Inspection booth ambient white light must not exceed 20 lux (2 foot-candles).
• Dark Adaptation: The inspector must remain in the darkened booth for at least 1 to 5 minutes prior to inspecting parts to allow retinal rhodopsin regeneration for optimal dark adaptation.

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Magnetic Particle Inspection (MT / MPI)

Magnetic Particle Inspection (MT) is a rapid, highly reliable NDT method designed to detect surface and slightly subsurface discontinuities in ferromagnetic materials.

1. Physical Principle & Material Limitation

When a ferromagnetic material is magnetized, magnetic lines of force (magnetic flux) flow through the interior of the part. If a crack, void, or non-metallic inclusion interrupts the magnetic path, the flux cannot easily jump the high-reluctance gap. Magnetic flux lines are forced out of the part into the surrounding air, creating a Magnetic Flux Leakage (MFL) field. This localized leakage creates distinct north and south magnetic poles that attract and hold finely divided ferromagnetic particles applied to the surface, forming an immediate visible indication.

                    MAGNETIC FLUX LEAKAGE (MFL) AT CRACK

            Flux Lines Travel Internally Through Ferromagnetic Steel
            ===================\              /===================
                                N            S    <-- Flux Leakage Field
            ===================/   [ CRACK ]  \===================
                                    Air Gap
                              Attracts Iron Particles

• Strict Material Limitation: MT is applicable ONLY to ferromagnetic materials—predominantly carbon steels, low-alloy high-strength steels (e.g., 4340, 300M landing gear steels), and ferritic/martensitic stainless steels (e.g., 17-4PH).
• Non-Applicable Alloys: MT cannot inspect non-ferromagnetic materials such as aluminium alloys, magnesium, titanium alloys, or austenitic 300-series stainless steels, because they possess no ferromagnetic properties and cannot sustain magnetic flux leakage.

2. Magnetization Techniques: Circular vs. Longitudinal

To produce magnetic flux leakage, magnetic flux lines must intersect the flaw at an angle between 45° and 90° (ideally 90°). Flaws running parallel to the magnetic field produce zero flux leakage and remain undetected. Therefore, complete inspection requires two separate magnetization operations:

     CIRCULAR MAGNETIZATION                   LONGITUDINAL MAGNETIZATION
    (Headshot / Central Conductor)                  (Solenoid Coil / Yoke)

          Current In                                Coil Turns
              v                                       | | | |
         +---------+                               +--+-+-+-+--+
         | ( O )   |  Circular Flux                |  ----->   | Longitudinal Flux
         |   |     |  Detects Axial                |  ----->   | Detects Transverse
         +---------+  Cracks                       +--+-+-+-+--+ Cracks
              v                                       | | | |
          Current Out

3. Particles & Carrier Medium

• Wet Fluorescent Particles: Fine ferromagnetic iron oxide particles coated with fluorescent dye, suspended in a treated water or light petroleum distillate bath. Inspected under 365 nm UV-A light (>1,000 µW/cm²). Particle concentration is verified daily using an ASTM centrifuge settling tube (nominal concentration 0.1 to 0.4 mL per 100 mL of bath after 30 minutes of settling).
• Dry Powder Particles: Finely divided magnetic powders applied with a manual powder blower; commonly used with portable electromagnetic AC yokes for on-wing landing gear trunnion inspections.

4. Demagnetization & Residual Field Verification

Following MT inspection, parts retain strong residual magnetism. Residual magnetism causes severe operational hazards: it attracts airborne ferrous debris into bearings, causes electric arcing across moving parts, interferes with cockpit magnetic compasses, and renders subsequent electron-beam welding impossible. Demagnetization is mandatory:

• Method: The part is placed in an alternating current (AC) coil carrying a high initial current, which is then gradually decayed to zero, or the part is slowly withdrawn along the coil axis to a distance of at least 1.5 to 2 meters.
• Verification: The residual magnetic field is measured using a calibrated Gauss Field Indicator (Magnetometer). Acceptable residual field strength must not exceed 3 Gauss (±2 Gauss / 240 A/m) prior to release to service.

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Eddy Current Testing (ET)

Eddy Current Testing (ET) is an electromagnetic NDT method widely utilized for inspecting conductive aircraft alloys without stripping non-conductive paint coatings.

1. Physical Principle: Electromagnetic Induction

ET operates on the principle of electromagnetic induction (Faraday's Law and Lenz's Law):

1. An alternating current (AC) flows through a test coil inside an eddy current probe, creating an oscillating primary magnetic field around the coil.
2. When the probe is brought near an electrically conductive material, the primary magnetic field induces circular, swirling electrical currents within the material. These are eddy currents.
3. The eddy currents generate their own secondary magnetic field, which opposes the coil's primary field (Lenz's Law).
4. The interaction between the primary and secondary fields alters the net electrical impedance ($Z = R + jX_L$) of the probe coil.
5. Any crack, material change, or corrosion void directly interrupts the circular flow of eddy currents. This disruption alters coil impedance, displaying a distinct trace displacement on the instrument's impedance plane screen.

                       EDDY CURRENT PRINCIPLE

            [ Alternating Current ] ---> Probe Coil
                                           |
                              Primary Magnetic Field (Oscillating)
                                           |
                                           v
            ===============================================
            ( ( ( ( ( ( ( ( Eddy Currents ) ) ) ) ) ) ) )
            Induced circular currents in conductive metal skin
            ===============================================
                     Crack interrupts current flow
                     ---> Shifts coil impedance

2. Skin Effect & Frequency Selection

Eddy currents do not distribute evenly through the thickness of a material; they concentrate near the surface and attenuate exponentially with depth. This phenomenon is known as the Skin Effect. The depth at which eddy current density decreases to $1/e$ (approximately 37%) of its surface value is defined as the Standard Depth of Penetration ($\delta$):

$$\delta = \frac{1}{\sqrt{\pi f \mu \sigma}}$$

Where:

• $f$ = Test frequency (Hz)
• $\mu$ = Magnetic permeability of the test material (H/m)
• $\sigma$ = Electrical conductivity of the test material (% IACS or S/m)

Because depth of penetration is inversely proportional to the square root of frequency, frequency selection governs inspection capability:

• High Frequency (100 kHz to 2 MHz): Produces a shallow depth of penetration. Highly sensitive to minute surface-breaking fatigue cracks, fastener hole edge cracks, and non-conductive coating thickness measurement (via the lift-off effect).
• Low Frequency (100 Hz to 10 kHz): Produces deep penetration. Used for inspecting multi-layer airframe lap joints to detect subsurface fatigue cracks in second or third skin layers under fastener heads, and to detect internal corrosion thinning.

3. Aerospace Applications

• Fastener Bolt-Hole Inspection: Utilizing high-speed motorized rotating scanner probes fitted with differential coils to detect radial fatigue cracks as small as 0.5 mm in bolt holes with fasteners removed.
• Conductivity Sorting & Heat Damage: Measuring electrical conductivity in % IACS (International Annealed Copper Standard) to detect structural fire or overheat damage in 2024 and 7075 aluminium alloys (overheating causes loss of precipitation temper, shifting conductivity).
• Subsurface Corrosion Thinning: Dual-frequency phase-discriminated testing through outer aircraft skin panels.

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Ultrasonic Testing (UT)

Ultrasonic Testing (UT) utilizes high-frequency mechanical sound waves (typically 0.5 MHz to 25 MHz, with 2.25 to 10 MHz standard in aerospace) to locate deep internal discontinuities and bond integrity flaws.

1. Acoustic Couplant & Piezoelectric Crystals

Sound waves are generated by piezoelectric crystals (e.g., lead zirconate titanate, PZT) housed inside the transducer probe. Under an alternating electrical voltage, the crystal expands and contracts, converting electrical energy into mechanical acoustic vibrations.

• Acoustic Couplant Mandate: Ultrasound cannot transmit through air at high frequencies due to the immense acoustic impedance mismatch between air and solid metals, which reflects over 99.9% of acoustic energy at the probe boundary. A liquid couplant (demineralized water, petroleum gel, cellulose paste, or light oil) is mandatory to eliminate the air interface and transmit sound energy into the component.

2. Pulse-Echo vs. Through-Transmission

         PULSE-ECHO METHOD                     THROUGH-TRANSMISSION
      (Single Transducer Probe)             (Transmitter + Receiver)

           Transducer                           Transmitter
           [Tx / Rx]                               [Tx]
       -------| |-------                    -------| |-------
       |               |                    |               |
       |    [Flaw]     |                    |    [Flaw]     | Blocks sound
       |               |                    |               |
       -----------------                    -----------------
          Backwall                              Receiver [Rx]
                                                Total loss of signal

• Pulse-Echo Method: A single probe acts as both transmitter and receiver. A short acoustic pulse travels through the material, reflects off the opposite back surface (backwall echo), and returns to the probe. If an internal flaw (void, forging crack, or inclusion) lies in the beam path, an intermediate flaw echo returns earlier. The display (A-scan) plots signal amplitude versus time-of-flight, allowing exact calculation of flaw depth:

$$d = \frac{v \times t}{2}$$

Where $d$ is depth, $v$ is acoustic velocity in the material, and $t$ is round-trip transit time.

• Through-Transmission Method: Utilizes two separate probes on opposite surfaces of the component: a transmitter and a receiver. The receiver monitors the transmitted signal amplitude. An internal defect (such as composite delamination or honeycomb skin debonding) scatters or absorbs sound, causing a severe drop or complete loss of received signal. Requires two-sided access.

3. Display Formats

• A-Scan: 1D plot of ultrasonic signal amplitude (vertical axis) versus elapsed time / depth (horizontal axis).
• B-Scan: 2D cross-sectional profile view displaying defect depth and longitudinal position along a single scan line.
• C-Scan: 2D plan-view color mapping showing defect boundaries and lateral dimensions across an inspected surface area; standard for composite manufacturing quality control.

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Radiographic Testing (RT)

Radiographic Testing (RT) utilizes penetrating, short-wavelength ionizing electromagnetic radiation to produce internal photographic or digital shadow images of aircraft components.

1. Radiation Sources: X-Rays vs. Gamma Rays

• X-Rays: Generated electrically by accelerating electrons across a high-voltage vacuum tube onto a heavy metal target (tungsten anode). Energy is adjustable by varying tube voltage (kV) and current (mA). Highly controllable; standard for aircraft maintenance.
• Gamma Rays: Emitted continuously by spontaneous radioactive decay of unstable radioisotopes (e.g., Iridium-192 or Selenium-75). Encapsulated in portable lead exposure containers (projectors). Does not require electrical power, but energy cannot be adjusted and isotope source decays over its half-life.

2. Differential Absorption Principle & Applications

As radiation penetrates an object, dense or thick materials absorb more photons, while thinner sections, voids, or cracks absorb fewer photons. The radiation reaching the photographic film or Digital Detector Array (DDA) exposes the medium proportionally:

• Dense areas (heavy spar caps, sound metal) appear lighter / clear.
• Low-density areas (internal cracks, hollow cavities, corrosion thinning) appear darker.
• Aviation Applications:
  • Inspecting internal wing spar caps, bulkheads, and flight control hinge ribs without removing skins.
  • Locating foreign objects, loose tools, and dropped fasteners inside sealed flight control surfaces.
  • Detecting Water Ingress in Honeycomb Core Panels: Water trapped inside aluminium or Nomex honeycomb cells absorbs radiation significantly more than empty air cells, producing sharp, distinct dark fluid outlines on the radiograph.

3. Critical Limitations & Radiation Safety

• Geometric Limitation: Radiography is highly dependent on flaw orientation. A crack must be oriented within ±5° of parallel to the radiation beam axis to cause sufficient differential absorption. Laminar cracks or delaminations perpendicular to the beam cannot be detected.
• Radiation Safety Mandate: Ionizing radiation destroys human biological tissue and DNA. Mandatory precautions include:
  • Establishing physical rope-off safety exclusion zones calculated using the inverse square law.
  • Continuous monitoring with calibrated Geiger-Müller or scintillation survey meters.
  • Mandatory wearing of personal Thermo-Luminescent Dosimeters (TLD) or Optically Stimulated Luminescent (OSL) badges by all personnel.
  • Lead collimators to restrict beam spread.

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Aerospace NDT Method Selection Matrix

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

During a scheduled base maintenance inspection of an Airbus A320 main landing gear trunnion fitting (high-strength 300M steel), a junior technician plans to inspect the fitting for fatigue cracks using Liquid Penetrant Inspection (Type I, Method C). The technician applies a steel wire wheel to clean off loose paint and grime from the trunnion radius.

The certifying Level 2 NDT inspector immediately intervenes and stops the task. First, using a steel wire wheel on high-strength steel smears metal over fatigue crack openings, physically masking defects from penetrant ingress. Second, because 300M steel is a ferromagnetic alloy, the mandatory and far superior method specified in the Nondestructive Testing Manual (NTM) is Magnetic Particle Inspection or High-Frequency Eddy Current. MPI can detect both surface and near-surface flaws without risk of capillary blockage. The inspector strips paint with approved chemical stripper, performs circular and longitudinal magnetization using an AC electromagnetic yoke with wet fluorescent particles, identifies a 2.5 mm transverse fatigue crack, marks the boundary, and demagnetizes the fitting to below 2 Gauss.

Common Exam Traps

• Trap 1: Over-washing penetrant during Method C removal. Spraying solvent directly onto a penetrant-treated part flushes penetrant out of crack reservoirs. The cloth must be moistened with solvent, never the part.
• Trap 2: Attempting Magnetic Particle Inspection on aluminium or austenitic stainless steel. MT works ONLY on ferromagnetic materials. Austenitic stainless steel (300-series), aluminium, and titanium possess zero ferromagnetism and cannot be tested with MT.
• Trap 3: Inverting Eddy Current frequency depth relationships. High frequency = shallow penetration (surface cracks); Low frequency = deep penetration (subsurface cracks and multi-layer corrosion). Remember the inverse square root formula.
• Trap 4: Assuming Radiography easily detects delaminations. RT requires significant volumetric material loss or beam-parallel alignment (±5°). Laminar cracks and composite delaminations perpendicular to the radiation beam cast almost zero shadow and are virtually invisible to RT; Ultrasonic Testing is required.