Corrosion 5: NDI/NDT: Visual Inspection & LPI C5

Week 5 NDI/NDT (Visual Inspection and LPI)

• Before any Non-Destructive Inspection (NDI), it's crucial to follow specific preparatory steps for the inspection type.
• Corrosion inspections begin with cleaning the area
• Some aircraft areas require paint removal before inspection.
• Cleaning is critical before crack inspections, whether using visual or other NDT methods.
• Abrasive blasting, scraping, heavy brushing, or sanding should not be used to remove paint/coatings/dirt
• These methods can hide defects by closing discontinuities on the surface.

Visual Inspection

• Visual inspection is widely used and effective for detecting and evaluating corrosion
• This involves looking directly at the aircraft surface or at a low angle
• Basic tools for visual inspection: flashlight, mirror, magnifying glass

Flashlights

• Incandescent or dull LED flashlights are better for showing cracks and surface defects due to less reflection
• Bright LED flashlights are suitable for general inspections from a distance (pre/post-flight checks).
• When looking for surface cracks with a flashlight, aim the light beam at a 5 to 45 degree angle.
• Flashlights come in different types: Explosion Proof, LED and Headlamp
• Explosion proof flashlights are used in confined spaces with volatile fumes
• LED flashlights are the most common, are dimmable and have multiple settings

Inspection mirrors

• Inspection mirrors are used to see areas not in the normal line of sight.
• Different sizes may be needed for proper inspection
• 2" to 3" mirror is recommended for a first inspection mirror
• Mirrors should be free of dirt, cracks, and worn coatings
• High polished stainless steel mirrors may scratch

Magnifying Glass

• Shine a flashlight at right angles to any crack an use a 10x power magnifying glass to confirm suspected cracks
• Multiple angles and light sources may be required.

Borescope

• A borescope enables inspection of areas that would not otherwise be accessible without disassembly.
• Types include: video scope, rigid Borescope and Fiber Optic Borescope
• Aviation video borescopes can record video and audio, saved to an SD card for computer viewing
• Borescopes in aircraft and engine maintenance reduce or eliminate costly teardowns
• They efficiently identify defects or discontinuities, and aircraft turbine engines have specific access ports for borescope use.
• Borescopes are used for critical inspections like engine inspections
• Inexpensive video borescopes are available for AMEs.
• Borescopes can inspect the inside of piston engines, checking for corrosion, erosion, and cracking.
• Engine borescope training focuses on the borescope's operation
• Information and inspection requirements/limits available in the engine/inspection manual

Bonded Structures

• Bonded structures consist of material layers sandwiched and bonded with epoxy and are located in aircraft wings, fuselage, engine nacelles and floorboards
• Bonded or composite structures are inspected for: delamination, debonding of the skin from the core, and moisture/corrosion
• Visual inspection is insufficient for bonded structures and is limited to surface damage and difficult to identify any internal problems.

Tap Test

• The tap test (ring/coin test) is a simple method for testing composite structures quickly
• Testing is used for a quick evaluation of any accessible surface to detect the presence of delamination or debonding that is close to the surface.
• A lightweight hammer (2oz max) or coin is used to lightly tap the surface
• Procedure: tap laminated part with coin or small metallic object and listen for sharp even pitch
• Delaminated areas produce a dull sound.
• Compares the acoustic response to known good area. A "flat/dead" response indicates a problem.
• Tap testing is limited to defects in thin skins (less than 0.080" thick)
• Honeycomb structures require testing on both sides
• Ultrasonic inspection is the most widely used electronic inspection method for composites.

Weld Inspections

• Weld inspections are performed through detailed visual inspections when receiving parts with repairs
• A well designed joint weld is stronger than the base metal, uniform in width
• Ripples are even and feathered into the base metal, with no burn due to overheating.
• Penetration is the depth of fusion is the most important and characteristic contributing to a sound weld.
• Butt weld penetration must be 100% of the base metal thickness
• Fillet weld penetration needs to be 25-50% of the base metal thickness
• Welds should have uniform width and even ripples
• Smooth transition into base metal
• Lack burn marks/overheating signs
• No oxides on base metal >1/2" from weld
• No porosity

Inspecting Plastic Windows

• Small aircraft windshields and windows are made of acrylic while larger aircraft windshields are made of glass-faced acrylic bonded with urethane. Each has anti-ice and anti-fog systems.
• Boeing 787 cabin windows have thin layers of stretched acrylic, measuring between one and three inches depending on the aircraft.
• Crazing is hairline cracks in plastic due to age exposure to sun and stress. Usually the cause for window removal
• Crazing is "micro-cracking" that is typically visible
• Crazing can occur due to Stress, UV degradation, airborne volcanic acids, or chemical damage.
• A glass or transparent object in prism form is used for window/windshield inspections.
• Applying a prism on the window surface with a couplant inspects for defects.
• A window inspection kit with prism and couplant is used, along with inspecting holes on the aircraft windshields for cracks.

Liquid Penetrant Inspection (LPI)

• LPI is a nondestructive test for surface defects in nonporous materials like aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium,.
• LPI detects surface cracks or porosity caused by fatigue, shrinkage cracks, cold shuts, grinding/heat treat cracks, seams, forging laps, and bursts and indicates lack of bond between joined metals (bad welds).
• The main disadvantage is that the defect must be open to the surface.
• The penetrating liquid enters a surface opening, becoming visible with added dye (visible or fluorescent)
• Fluorescent dye is more visible and less messy.
• The liquid dye penetrant kit contains dye penetrant, dye remover/emulsifier, and developer.
• The fluorescent penetrant inspection kit contains a black light assembly (ultraviolet light), as well as spray cans of penetrant, cleaner, and developer.
• Nonaqueous type developer is used, which is suspended in solvent in a spray can.
• Sufficient time is required for the penetrant to fill the defect
• Dwell time is when liquid penetrant remains on test piece before removal.
• Small cracks require longer dwell times, and larger cracks have shorter dwell times.
• Dwell time depends on the temperature of the part
• Penetrant is applied by spraying, brushing, or submerging the parts
• Dye penetrant is most commonly applied using a spray can.
• Three main types of developer: Dry, Wet and Nonaqueous
  • Dry developer: a loose powder removed with low air pressure.
  • Wet developer: a white powder mixed with water, flowed over the surface or submerged.
  • Nonaqueous developer: white chalk-like developer suspended in a solvent in a spray can.