Untitled Quiz

Nondestructive Testing (NDT) - Dye Penetrant Inspection

• Liquid penetrant method detects discontinuities open to the surface of solids and essentially porous materials.
• The method applies a penetrating liquid to the cleaned surface of the component.
• Liquid enters discontinuities via capillary action.
• Excess penetrant is removed by solvent or water.
• A developer layer is applied to draw out any remaining penetrant in the discontinuities.
• Indications appear on the developer layer indicating the presence of discontinuities.
• Penetrants consist of a tracer dye and a carrier fluid (vehicle).

Penetrant Characteristics

• Easily spread over the surface for complete coverage.
• Drawn into surface imperfections by capillary action.
• Easily removed from the surface of the part.
• Remaining penetrant can be drawn back to the surface through drying and developing steps.
• High visibility (visible or fluorescent) for easy indication recognition.
• Non-harmful to the material or inspector.

Penetrant Types

• Type 1 - Fluorescent Penetrants: Contain dyes that fluoresce under UV light. More sensitive than visible penetrants due to the pronounced glow of the fluorescing indication.
• Type 2 - Visible Penetrants: Contain a red dye offering high contrast against the white developer background. No need for dark area or UV light.

Penetrant Removal Methods

• Method A - Water Washable: Excess penetrant can be removed by rinsing with water alone. Penetrants contain an emulsifying agent.
• Method B - Post-Emulsifiable, Lipophilic: Penetrant is oil-soluble and interacts with oil-based emulsifier for removal.
• Method C - Solvent Removable: Solvent is used to remove the penetrant from the part.
• Method D - Post-Emulsifiable, Hydrophilic: Water-soluble detergent emulsifier lifts excess penetrant from the part.

Developers

• Developers increase the brightness intensity of fluorescent and visible penetrant indications.
• Developers blot the penetrant and enlarge the flaw appearance.
• Developers provide contrast between penetrant and the surface, enhancing flaw visibility.
• Speed up the inspection process.

Required Developer Properties

• Adsorptive: Maximizes blotting.
• Fine grain size and specific particle shape: Effective dispersal and exposure of indications.
• Contrast background: Distinguish indications when using color-contrast penetrants.
• Easy to apply uniformly as a thin coating.
• Easily wetted by the penetrant: Effective extraction of penetrant from the flaws.

Dry Powder Developer

• Least sensitive, but inexpensive and easy to apply.
• Applied by: dipping, puffing, or placing in a dust cabinet.
• Powder sticks only to wet areas around indications.
• Seldom used for visible inspections.

Water Soluble Developer

• Chemicals dissolved in water form a developer layer when the water evaporates.
• Applied by spraying.
• Dipping, pouring, or brushing is less desirable but sometimes used.
• The part should be wet or dry.

Water Suspendable Developer

• Insoluble particles suspended in water.
• Requires frequent stirring or agitation to prevent settling.
• Applied like water soluble developers; dried using warm air.

Nonaqueous Developer

• Developer suspended in a volatile solvent.
• Applied using a spray gun.
• The solvent tends to pull the penetrant out of the indications.
• Forced drying is usually not required. (due to volatile solvent)

Special Application Developers

• Primarily used for permanent inspection records.
• Used with plastics or lacquer.

Surface Preparation

• Critical step in liquid penetrant testing.
• Defects must be open to the surface for penetrant entry.
• Contaminants like scales, flakes, grease, paint, and dirt must be removed to prevent false or masking defects.
• Cleaning and drying the surface are essential.

Penetrant Application

• Penetrant fluid is applied to the cleaned surface.
• Fluid spreads freely and evenly, moving into the cracks.
• Dwell time varies depending on crack size and environmental conditions.
• Methods: Dipping, spraying, or brushing.
• Minimum dwell time is 20-30 minutes.

Excess Penetrant Removal

• Delicate step: Removing excess penetrant avoids masking defects.
• Methods include cleaning with solvent, rinsing with water, or emulsifier prior to rinsing.
• Insufficient cleaning leaves a background of penetrant, obscuring defect indications.

Developer Application

• Developer is applied after excess penetrant removal and draws the penetrant from the cracks to the surface.
• Contrast between developer and penetrant is important for defect visibility.
  • Longer dwell is necessary for tight cracks.
• Developers come in various forms: dry powders, dipping solutions, or spraying.

Inspection and Evaluation

• Indications are scanned under visible (or UV) light for recognition with human eye or automated optical scanners.
• Each indication is evaluated.
  • False, non-relevant, unacceptable, and acceptable indications are possible.
• Acceptance/rejection of the component based on relevant standards.

Procedure Flow Chart (Visual Aid)

(A flowchart showing the sequence of steps).

Interpretation of Results

• Table showing typical indications for different defect types (e.g., cracks, porosity) under visible and fluorescent penetrant methods.
• Images of typical defect indications (e.g., coarse cracks, porosity, shrinkage).

Penetrant Removal Methods

• Method A: Water Washable: Penetrants removed using water rinse.

• Method B: Post-Emulsifiable, Lipophilic: Penetrants are removed using an emulsifier, then rinsing with water

• Method C: Solvent Removable: Penetrants are removed by hand-wiping with a solvent-moistened cloth.

• Method D: Post-Emulsifiable, Hydrophilic: Penetrants requiring a hydrophilic agent /water-soluble detergent, followed by water rinse are removed.

Advantages of Dye Penetrant Inspection

• High sensitivity for detecting small discontinuities.
• Applicable to many materials (metallic, nonmetallic, etc.).
• Inspection of large areas and complex-shaped parts.
• Portable and relatively inexpensive.

Disadvantages of Dye Penetrant Inspection

• Limited to surface-breaking defects.
• Pre-cleaning is critical for effective inspection (prevent contaminants masking defects).
• May require additional processes (like post-cleaning/demagnetization).

Safety Precautions (NDT)

• Ventilation, appropriate protective gear like gloves and eye protection, and following manufacturer's instructions ensure safety during inspection procedures.

Magnetic Particle Inspection

• Used for testing easily magnetized materials.
• Detects open-surface and just below-the-surface flaws.
• Ferromagnetic materials are typically tested (e.g., iron, nickel, cobalt, and some steels).
• Equipment is relatively cheap and robust, handled by semi-skilled personnel..
• Theory of Magnetism: -Magnets are permanent or temporary—they retain magnetism as long as magnetization force is applied.

Theory of Magnetism

• Ferromagnetism is the ability of materials, like iron, nickel, and cobalt alloys, to attract other ferromagnetic materials.
• Diamagnetic materials are repelled weakly by magnets.
• Paramagnetic materials are weakly attracted by magnets.

Magnetizing Force (H) and Flux Density (B)

• Magnetizing force (H): Sets up magnetic flux in a material.
• Flux density (B): Flux (magnetic lines of force) per unit area in a material (measured in gauss).
• Reluctance: Resistance offered by a material to magnetic flux establishment; comparable to electric resistance.
• Permeability: Ease with which a material can be magnetized. (Often expressed numerically as B/H) High permeability means low reluctance.

Magnetization Methods (Using Electric Current)

• Magnets can be used for creating magnetic field on the part to be inspected

• Induction Method (used for ring-shaped parts): AC or DC passed through a coil, inducing a current and field in the component; no chance of damaging component surfaces from arcing.

• Circular Field Method (used for multiple types of parts): Current is passed through the part itself, and the current magnitude is systematically reduced to zero.

• Longitudinal Field Method (used for multiple types of parts): Current passed through a coil, magnetizing the part lengthwise or longitudinally.

Demagnetization

• Components often retain some residual magnetism after inspection, and demagnetization is necessary for further processing or preventing interference with subsequent operations (e.g., welding, machining).
• Methods for removing residual magnetism include: -Coil withdrawal
  • Step-down reduction of current
  • Reversing DC current

General Procedure for Magnetic Particle Testing

• Surface preparation (degreasing, removing contaminants) is crucial.
• Parts with magnetic paints require removal of those too to ensure good contact with the magnetic field.
• Magnetic particles, in appropriate concentration, are dispersed in a carrier solution.
• The magnetized part is inspected.
• Appropriate illumination/lighting is used to view the indications produced by the dispersed particles on the part, locating discontinuities.

Viewing/Detecting

• Black/red paste is observed with an appropriate light source.
• Fluorescent particles under UV light.

Marking Defects

• Marking defects is essential for documentation and future reference. -Using a permanent marker or transparent film to mark indicated areas/flaws is useful.

Removal of Ink/Residue

• Use of a stripper like paraffin oil wash for removing the magnetic ink.

Residual Magnetism Issues

• Parts retain some magnetism after testing, which can interfere with subsequent operations; and requires demagnetization.