30 Questions
Why is compressed air not recommended for drying surfaces?
Because it may contain oil that can degrade the repair's durability
What is the main purpose of hand abrasion or grit blasting in surface preparation?
To generate a fresh, chemically active surface
What should be used to dry surfaces before bonding?
All of the above except compressed air
Why is it important to apply subsequent treatments soon after hand abrasion or grit blasting?
To prevent unwanted contamination or oxide formation
What is the consequence of inadequate drying of the surface before bonding?
Micro-voiding in adhesive joints
What should be done after water break testing?
Heat the surface to a temperature specified in the repair procedure or SRM
What is the primary purpose of using Methyl Ethyl Ketone (MEK) in composite component repair?
Removing dust, grease, and mould release agents
What is a critical requirement for solvents used in repair surface preparation?
They must produce no residue on the surface
Why should chlorinated solvents not be used on titanium alloys?
They may cause stress corrosion cracking
What type of wipe cloths should be used for wiping the surface with solvents?
Clean, lint-free white facial tissues or approved aircraft wipes
Why must wipe cloths be changed frequently during solvent degreasing?
To prevent the transfer of contamination over the surface
What is the final step in the solvent wiping process?
Performing a final solvent wipe with a new cloth
What is the primary purpose of a hot patch bonding machine?
To perform two functions: applying atmospheric pressure and heat
What is the purpose of a temperature recording unit in hot bonding units?
To record the temperatures of the curing process
Why is it crucial to control the temperature during the curing process?
To achieve effective bonding results
What is the purpose of a thermocouple in the curing process?
To sense the temperature of the part
What is the purpose of the controller in hot patch bonding equipment?
To control the temperature at a constant set point and specific rate of change
Why is it necessary to include permanent records of the cure cycle in the aircraft log book?
To maintain a record of the repair
What is the purpose of applying a special UV-blocking dark paint to a component?
To reduce the degradation of resins by UV radiation
What is the primary benefit of applying a white or light-coloured paint finish to fibreglass-reinforced plastics?
To reduce temperature rise through heat soakage
What is necessary for polyurethane paint to provide effective protection against erosion?
A waterproof undercoat
What is the effect of UV radiation on cured epoxy?
It causes the resin matrix to become more brittle and prone to cracking
Why is it recommended to keep an aircraft out of the sun while it is on the ground?
To reduce the effect of heat soakage
What is the benefit of using an opaque paint with a UV barrier to protect composites?
It protects against UV radiation
What is the primary purpose of the water break test?
To evaluate the surface condition of prepared metals and composites
What happens if the surface fails the water break test?
Solvent degreasing must be repeated before continuing with the preparation process
What is the consequence of inadequate drying prior to application of adhesives?
Serious degradation of adhesive strength due to micro-void formation in the adhesive
How is the water break test performed?
By spraying, pouring or squirting distilled water onto the surface
What should be observed during the drying process after water break testing?
Water breaks
Why is solvent degreasing repeated if contamination is detected on the final wipes?
To ensure the surface is properly cleaned by solvent action
Study Notes
Solvent Properties
- Solvents used for repair surface preparation must meet specific requirements, including:
- Readily dissolve surface contaminants
- Have a rapid evaporation rate to prevent re-contamination
- Produce no residue on the surface
- Have no deleterious effect on the material being cleaned
- Be of a guaranteed high grade of purity
- Chlorinated solvents should not be used on titanium alloys to prevent stress corrosion cracking
Solvents Used
- Methyl ethyl ketone (MEK) is used for removing dust, grease, and mould release agents from composite components
- Acetone is used for general clean-up of tools and equipment and to clean composite parts after sanding as a pre-bond prep
Solvent Wipe Cloths
- Only clean, lint-free white facial tissues or approved aircraft wipes should be used for wiping the surface with solvents
- Wipe cloths must be changed frequently during solvent degreasing to prevent re-contamination
- The frequency of changing cloths increases as the degreasing process progresses
Water Break Test
- The water break test is used to evaluate the surface condition of prepared metals and composites
- The test assesses the surface tension of water placed on the surface
- The test is performed after solvent degreasing and prior to generating a fresh, chemically active surface
- If the surface is properly clean, the water break test should pass; if not, solvent degreasing must be repeated
Drying Procedures
- Drying is performed after water break testing and before application of adhesives
- Inadequate drying can lead to serious degradation of adhesive strength and durability
- Drying methods include:
- Using hot air from a handheld hot air gun or an external hot air blower fitted with air filters
- Placing the item in a recirculating air oven
- Compressed air must not be used, as it can degrade the repair’s durability
Exposing Fresh Material
- Following solvent degreasing, water break testing, and drying, surfaces are prepared for adhesive bonding by:
- Hand abrasion or grit blasting to generate a fresh, chemically active surface
- Applying subsequent treatment as soon as possible to prevent unwanted contamination or oxide formation
- The procedure must be thorough, and the exposure time during this step is critical to the bonding process
Hot Patch Bonding Equipment
- Hot patch bonding equipment applies atmospheric pressure and heat to the repair area
- Heat blankets are used to control the temperature, and the rate of temperature rise and fall is critical
- The controller allows for slow temperature rise, holds the temperature constant, and then allows a slow decline
- Recording the temperatures of the curing process is essential, and permanent records must be kept in the aircraft log book
Learn about the use of solvents like Methyl Ethyl Ketone (MEK) and Acetone in cleaning and preparing composite components for repair. Understand their properties and engineering requirements.
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