chapter 4 — fiber reinforced composite repairs

CHAPTER 4 — FIBER REINFORCED COMPOSITE REPAIRS
FIBER REINFORCED COMPOSITE REPAIRS
Fiber Composites........................................................................................... 4-27 4-2-7 Removal of Honeycomb Core in Glass or Carbon Fiber Honeycomb
Reinforced Composite Parts ............................................................................... 4-54 4-3-1 Wet Layup Repairs — General...................................................................... 4-54 4-3-2 Approved Processes (Process Sheets) ......................................................... 4-55 4-3-3 Wet Layup Impregnation Process.................................................................. 4-56 4-3-4 Installation of Copper Wire Mesh in Wet Layup Repairs ............................... 4-61 4-3-5 Wet Layup Bagging Process ......................................................................... 4-63 4-3-6 Curing Process for Wet Layup Using Epoxy Resin ....................................... 4-67 4-3-7 Honeycomb Core Plug Installation and Splicing Using Epoxy Resin ............ 4-73 4-3-8 Preparing a Precured Wet Layup Doubler or Edge Filler .............................. 4-76 4-4 Typical Wet Layup Repair Procedures for Glass Reinforced Composites.......... 4-82 4-4-1 Glass Monolithic Laminate — Surface Damage ............................................ 4-84 4-4-2 Glass Monolithic Laminate — Damage up to Half of Laminate Structural
Plies ............................................................................................................... 4-90 4-4-3 Glass Monolithic Laminate — Damage Through Full Thickness ................... 4-96 4-4-4 Glass Monolithic Laminate — Laminate Edge Damage ................................ 4-107 4-4-5 Glass Honeycomb Panel — Surface Damage............................................... 4-115 4-4-6 Glass Honeycomb Panel — Damage up to Half of Skin Structural Plies ...... 4-121 4-4-7 Glass Honeycomb Panel — Skin to Core Disbond........................................ 4-127 4-4-8 Glass Honeycomb Panel — Smooth Dent..................................................... 4-137 4-4-9 Glass Honeycomb Panel — Single Skin and Core Damage ......................... 4-142 4-4-10 Glass Honeycomb Panel — Hole Through Panel (Both Skins) ..................... 4-154 4-4-11 Glass Honeycomb Panel — Core Bevel Damage ......................................... 4-169
14 DEC 2010 4-1ECCN EAR99
BHT-ALL-SRM TC/FAA APPROVED
4-4-12 Glass Honeycomb Panel — Panel Edge Damage ........................................ 4-177 4-4-13 Aluminum Facing Honeycomb Panel — Core Bevel Damage....................... 4-188 4-5 Typical Wet Layup Repair Procedures for Carbon Fiber Composite Parts......... 4-193 4-5-1 Carbon Monolithic Laminate — Damage Through First Ply .......................... 4-195 4-5-2 Carbon Monolithic Laminate — Damage up to Half of Laminate Structural
Plies ............................................................................................................... 4-201 4-5-3 Carbon Monolithic Laminate — Damage Through Full Thickness ................ 4-207 4-5-4 Carbon Monolithic Laminate — Laminate Edge Damage.............................. 4-218 4-5-5 Carbon Honeycomb Panel — Surface Damage ............................................ 4-226 4-5-6 Carbon Honeycomb Panel — Damage up to Half of Laminate Structural
Plies ............................................................................................................... 4-232 4-5-7 Carbon Honeycomb Panel — Skin to Core Disbond ..................................... 4-238 4-5-8 Carbon Honeycomb Panel — Single Skin and Core Damage....................... 4-248 4-5-9 Carbon Honeycomb Panel — Hole Through Panel (Both Skins) .................. 4-260 4-5-10 Carbon Honeycomb Panel — Core Bevel Damage....................................... 4-275 4-5-11 Carbon Honeycomb Panel — Panel Edge Damage...................................... 4-283
FIGURES
4-1 Wet Layup Repair Flow Chart............................................................................. 4-8 4-2 Damage Outlining ............................................................................................... 4-20 4-3 Acceptance Criteria for Protruding Head Fastener Holes................................... 4-24 4-4 Acceptance Criteria for Countersunk Fastener Holes......................................... 4-26 4-5 Copper Wire Mesh — Installation ....................................................................... 4-62 4-6 Epoxy Cure without Heat Blanket — Bagging Process ..................................... 4-64 4-7 High Temperature Epoxy Cure with Heat Blanket — Bagging Process ............. 4-66 4-8 Core Plug — Installation ..................................................................................... 4-75 4-9 Precured Wet Layup Doubler — Fabrication ...................................................... 4-78 4-10 Precured Wet Layup Edge Filler — Fabrication.................................................. 4-81 4-11 Damage through Surface Finish of Glass Laminate — Negligible Damage ....... 4-85 4-12 Damage Through First Ply of Glass Laminate — Repair.................................... 4-89 4-13 Damage through Half of Glass Laminate Thickness — Negligible Damage....... 4-91 4-14 Damage Through Half of Glass Laminate Thickness — Repair ......................... 4-95 4-15 Small Puncture Through Glass Laminate — One Sided Repair ......................... 4-98 4-16 Small Puncture Through Glass Laminate — Double Sided Repair .................... 4-101 4-17 Large Puncture Through Glass Laminate — Repair........................................... 4-106 4-18 Small Edge Damage in Glass Laminate — Repair ............................................. 4-109 4-19 Wide Edge Damage in Glass Laminate — Repair.............................................. 4-114 4-20 Damage through Surface Finish of Glass Panel Skin — Negligible Damage..... 4-116 4-21 Damage Through First Ply of Glass Panel Skin — Repair ................................. 4-120 4-22 Damage through Half of Glass Panel Skin Thickness — Negligible Damage .... 4-122 4-23 Damage Through Half of Glass Panel skin Thickness — Repair ....................... 4-126 4-24 Disbond between Glass Panel Skin and Core — Negligible Damage ................ 4-128 4-25 Glass Panel Skin to Core Disbond — Repair ..................................................... 4-132
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TC/FAA APPROVED BHT-ALL-SRM
Figure Page Number Title Number
4-26 Large Glass Panel Skin to Core Disbond — Repair ........................................... 4-136 4-27 Dent in Glass Panel Skin — Negligible Damage ................................................ 4-138 4-28 Smooth Dent in Glass Panel Skin — Repair....................................................... 4-141 4-29 Small Puncture Affecting One Glass Panel Skin and Core — Repair ................ 4-145 4-30 Puncture Affecting One Glass Panel Skin and Core — Repair .......................... 4-149 4-31 Large Puncture Affecting One Glass Panel Skin and Core — Repair ................ 4-153 4-32 Small Puncture Affecting Both Glass Panel Skins and Core — Repair .............. 4-157 4-33 Puncture Affecting Both Glass Panel Skins and Core — Repair ........................ 4-162 4-34 Large Puncture Affecting Both Glass Panel Skins and Core — Repair.............. 4-167 4-35 Small Edge Damage Affecting One Glass Panel Skin and Core — Repair........ 4-172 4-36 Large Edge Damage Affecting One Glass Panel Skin and Core — Repair........ 4-176 4-37 Small Edge Damage in Glass Panel — Repair................................................... 4-179 4-38 Large Edge Damage Affecting Both Glass Panel Skins and Core — Repair ..... 4-185 4-39 Small Edge Damage Affecting One Glass Panel Skin and Core — Repair........ 4-192 4-40 Damage through Surface Finish of Carbon Laminate — Negligible Damage..... 4-196 4-41 Damage Through First Ply of Carbon Laminate — Repair ................................. 4-200 4-42 Damage through Half of Carbon Laminate Thickness — Negligible Damage .... 4-202 4-43 Damage Through Half of Carbon Laminate Thickness — Repair....................... 4-206 4-44 Small Puncture Through Carbon Laminate — One-sided Repair....................... 4-209 4-45 Small Puncture Through Carbon Laminate — Double-sided Repair .................. 4-212 4-46 Large Puncture Through Carbon Laminate — Repair ........................................ 4-217 4-47 Small Edge Damage in Carbon Laminate — Repair .......................................... 4-220 4-48 Wide Edge Damage in Carbon Laminate — Repair ........................................... 4-225 4-49 Damage through Surface Finish of Carbon Panel Skin — Negligible Damage .. 4-227 4-50 Damage Through First Ply of Carbon Panel Skin — Repair............................... 4-231 4-51 Damage through Half of Carbon Panel Skin Thickness — Negligible Damage.. 4-233 4-52 Damage Through Half of Carbon Panel Skin Thickness — Repair .................... 4-237 4-53 Disbond between Carbon Panel Skin and Core — Negligible Damage ............. 4-239 4-54 Carbon Panel Skin to Core Disbond — Repair................................................... 4-243 4-55 Large Carbon Panel Skin to Core Disbond — Repair......................................... 4-247 4-56 Small Puncture Affecting One Carbon Panel Skin and Core — Repair.............. 4-251 4-57 Puncture Affecting One Carbon Panel Skin and Core — Repair........................ 4-255 4-58 Large Puncture Affecting One Carbon Panel Skin and Core — Repair.............. 4-259 4-59 Small Puncture Affecting Both Carbon Panel Skins and Core — Repair ........... 4-263 4-60 Puncture Affecting Both Carbon Panel Skins and Core — Repair ..................... 4-268 4-61 Large Puncture Affecting Both Carbon Panel Skins and Core — Repair ........... 4-273 4-62 Small Edge Damage Affecting One Carbon Panel Skin and Core — Repair ..... 4-278 4-63 Large Edge Damage Affecting One Carbon Panel Skin and Core — Repair ..... 4-282 4-64 Small Edge Damage in Carbon Panel — Repair ................................................ 4-285 4-65 Large Edge Damage Affecting Both Carbon Panel Skin and Core — Repair .... 4-290
Table Page Number Title Number
4-1 Repair Selection Guide for Rivet Pattern Discrepancy ....................................... 4-9 4-2 Repair Selection Guide for Bonded Panels With Aluminum Face Sheets
and Glass Edging................................................................................................ 4-9 4-3 Repair Selection Guide for Glass or Carbon Monolithic Part.............................. 4-10 4-4 Repair Selection Guide for Bonded Panel .......................................................... 4-11 4-5 Process Sheets................................................................................................... 4-15 4-6 Drilling Damage Limits for Protruding Head Fastener Holes .............................. 4-23 4-7 Drilling Damage Limits for Countersink Side of Holes ........................................ 4-25 4-8 Process Sheets................................................................................................... 4-55 4-9 Number of Repair Plies....................................................................................... 4-60 4-10 Standard Cure at Room Temperature ................................................................ 4-69 4-11 Room Temperature Cure with Post-cure ............................................................ 4-71 4-12 High Temperature Cure ...................................................................................... 4-72
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4-1. INTRODUCTION
Many of the repairs for the Bell Helicopter Textron products are a function of the type of construction to be repaired. Some repairs are in fact similar in nature and may be applicable to various parts of the structure. Those repairs are therefore considered typical for a certain type of construction. This chapter describes fiber reinforced composite repairs using glass or carbon fiber fabrics. This chapter provides “typical repairs” for the following types of structure:
NOTE
A monolithic laminate is defined as a unit formed or composed of material without joints or seams consisting of or constituting a single unit.
• Monolithic laminate made from glass fibers and epoxy resin
• Honeycomb panel with skin(s) made from glass fibers and epoxy resin (Refer to Chapter 3 for honeycomb panels made with aluminum facings)
• Monolithic laminate made from carbon fibers and epoxy resin
• Honeycomb panel with skin(s) made from carbon fibers and epoxy resin
4-1-1. CONSIDERATIONS PRIOR TO REPAIRING FIBER REINFORCED COMPOSITES
In the past, the majority of the helicopter structure was made out of metal. However, the use of fiber reinforced composite structure is increasing and will continue to do so. The following are characteristic differences between the composite and metallic structures that need to be considered when evaluating damage to and developing repairs for fiber reinforced composite structure.
1.0 DAMAGE
Fiber reinforced materials (composites) do not exhibit plastic deformation prior to failure. Even if they resist impact loads, internal damage such as matrix cracking or delamination may be induced. Propagation of damage in composite structure occurs by growth of a delamination rather than by through thickness cracks, as normally observed with metals. Inspection methods shall be appropriate to detect these damage.
2.0 LIGHTNING STRIKE PROTECTION
In most cases, carbon fiber reinforced composites require a protective layer against potential lightning strikes. The most common form of protective layer is a copper wire mesh that has been applied, using adhesive, to the outboard side of the structure. When a part with this protection has been damaged, the lightning strike layer must be restored during the repair using instructions detailed in paragraph 4-3-4.
3.0 MOISTURE ABSORPTION
Over time, fiber reinforced composite materials will absorb moisture from the surrounding environment (typically about 1 to 2% of their original weight). Removal of entrapped moisture, by drying the structure, is critical prior to carrying out any repair that requires a cure or post-cure at or above 200°F (93.3°C). Failure to dry the laminate before performing a repair can lead to a reduction in the strength of the repair or to additional damage in the original panel. In a solid laminate, the moisture will tend to migrate into the bondline adhesive and reduce its
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strength. In a honeycomb panel, the water entrapped in core cells may turn to steam during the cure cycle and ‘blow the repair doubler off’ or damage the honeycomb core due to the buildup of internal pressure.
4.0 MATERIAL COMPATIBILITY
When carbon fiber composite structure is in contact with various metals (aluminum and some steels), corrosion to the adjacent metal can result (for a complete list of compatible materials, refer to Appendix A-7). Whenever carbon fiber composite structure is required to contact a susceptible material, a glass ply serving as a barrier is used as the last ply. In addition to the glass barrier ply and the epoxy polyamide primer, the faying surfaces (mating surfaces) are coated with a sealant before the joint is assembled. The material compatibility requirements also restrict the choice of fastener type to be used. If no clear direction is provided in the Structural Repair Manual (SRM), please contact Product Support Engineering for the specification of compatible fasteners.
5.0 INSTALLATION OF MECHANICAL FASTENERS
The following are guidelines for mechanically fastened joints in carbon fiber laminates:
5.1 The part has to be properly braced and a backup material (piece of hard wood or equivalent material) has to be used to prevent fiber breakout on the backside of the part during drilling of fastener holes (paragraph 4-2-5).
5.2 Edge distance requirement is greater for composites than for metals. Typically 2.5D is used for both protruding head and countersunk fasteners (Chapter 3, paragraph 3-3-1).
5.3 Only fasteners that are acceptable for use in contact with composite materials may be used. Refer to Appendix A-6 for a list of approved fasteners for use with carbon fiber reinforced composites.
5.4 Blind rivets are only allowed if there is a metal backing on the tail side to avoid damaging the fibers.
5.5 Interference fit fasteners such as blind bolts and Hi-Lites are to be used.
5.6 The use of fasteners that expand to fill the hole shall be avoided as much as possible to prevent fiber breakout and delamination at the periphery of the hole during the installation of these fasteners.
5.7 In some cases it is permissible to squeeze solid rivets. Never use a rivet gun and bucking bar to install rivets as the repetitive impact may damage the composite material.
5.8 Wet installation of fasteners using a corrosion inhibitor (primer or sealant) is generally required.
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4-1-2. WET LAYUP REPAIR FLOW CHART
Figure 4-1 provides a flow chart for the wet layup repair process. The major steps are as follows:
WARNING
REFER TO YOUR MODEL-SPECIFIC STRUCTURAL REPAIR MANUAL FOR REPAIRS OF RESTRICTED AREAS.
1.0 DAMAGE INSPECTION (Chapter 2, paragraph 2-13)
2.0 DAMAGE CLASSIFICATION AND EXTENT (Chapter 2, paragraph 2-13-1)
3.0 SELECTION OF TYPICAL REPAIR PROCEDURE
3.1 Glass fiber reinforced composites (paragraph 4-4).
3.2 Carbon fiber reinforced composites (paragraph 4-5).
4.0 VERIFICATION OF RESTRICTIONS
4.1 Specific Restrictions: Per typical repair procedure in this manual and per part specific requirements in the model-specific SRM.
4.2 General Restrictions: Per typical repair procedure in this manual.
5.0 PERFORM REPAIR
5.1 Using process sheets (paragraph 3-2, paragraph 4-2 and paragraph 4-3).
5.2 Using typical repair procedures
5.2.1 Glass fiber reinforced composites (paragraph 4-4).
5.2.2 Carbon fiber reinforced composites (paragraph 4-5).
6.0 INSPECTION OF REPAIR
DAMAGE INSPECTION
– Section 2-13
DAMAGE CLASSIFICATION
– Section 2-13-1 – Type – Location – Size – Material (Glass or Carbon) – Construction Type (monolithic or Sandwich)
SELECTION OF TYPICAL REPAIR PROCEDURES
– For Glass Composites: Section 4-4 – For Carbon Composites: Section 4-5
VERIFICATION OF RESTRICTIONS
SUITABLE REPAIR FOUND?
YES NO
PERFORM REPAIR
– Use of Specific Repair Procedures: Model-Specific SRM – Use of Typical Repair Procedures: Section 4-4 and Section 4-5 – Use of Process Sheet(s): Section 4-2 and Section 4-3
ASSISTANCE BY PRODUCT SUPPORT ENGINEERING (PSE)
– Damage Evaluation
YES NO
4-1-3. REPAIR SELECTION GUIDE
As this chapter contains repairs for various types of structures with various repair methods based on damage type, size, and location, a repair selection guide is included to facilitate the identification of the appropriate repair. The following tables list the different generic repairs given in this chapter. These repairs are generic and apply to unrestricted repairable areas of the helicopters. Refer to the model-specific Structural Repair Manual (SRM) and to Chapter 1, paragraph 1-20 for a list of restricted areas. If damage is in a restricted area or beyond the limitations given in this chapter, the model-specific SRM may have a suitable specific repair. If the damage cannot be repaired using instructions given in this manual or the model-specific SRM, contact Product Support Engineering for a disposition. Refer to Chapter 2, paragraph 2-21 for the procedure to follow to request an approved structural field repair from Product Support Engineering.
The repair guide in this chapter is divided in several tables. Each table provides a selection guide based on the type of structure or material being repaired: Monolithic glass structures, glass facing(s) on honeycomb panels, monolithic carbon fiber structures, or carbon fiber facing(s) on honeycomb panels. Also included are tables for rivet pattern discrepancies.
Table 4-1. Repair Selection Guide for Rivet Pattern Discrepancy
Maximum Damage
Pitch(1) Repair Para. Appl.
Monolithic Glass or Carbon
50 for 1st oversize
25 for 2nd oversize
Low edge distance 1/5
50 for 1st oversize
25 for 2nd oversize
4D 4D
3-4-3 B
1) Top value is for protruding head fasteners and bottom value is for flush head fasteners.
Table 4-2. Repair Selection Guide for Bonded Panels With Aluminum Face Sheets and Glass Edging
Maximum Damage
Para. Appl.
Punctures Affecting Core Bevel
4-4-13 -
1) All dimensions are in inches. Values between parentheses are in millimeters (mm).
E
Table 4-3. Repair Selection Guide for Glass or Carbon Monolithic Part
Maximum Damage
r Para.
3 0.050
(0.323) 1/2
3 0.050
(0.323) Full
1 2
Fasteners + ED
1.5 (38)
4-4-4 B
3 0.050
(0.323) 1/2
3 0.050
(0.323) Full
1 2
Fasteners + ED
4-5-4 B
1) All dimensions are in square inches. Values between parentheses are square centimeter (cm2).
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Maximum Damage
Glass
3 0.050
thickness
Deep Smooth Dent, Sharp Dent or Puncture Through a Skin
3 0.050
3 0.050
1 2 Fasteners
(12.7) 4-4-12 B
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3 0.050
0.25 (6.4)
4-5-6 A
per Model- specific SRM
Deep Smooth Dent, Sharp Dent or Puncture Through a Skin
3 0.050
3 0.050
Maximum Damage
Size(1) Depth(2) Length(2) Width(2) Repair Para. Appl.
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1 2 Fasteners
Maximum Damage
Size(1) Depth(2) Length(2) Width(2) Repair Para. Appl.
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4-2. COMMON PROCEDURES FOR GLASS OR CARBON FIBER REINFORCED COMPOSITE PARTS
4-2-1. GLASS OR CARBON FIBER REINFORCED COMPOSITE PARTS — GENERAL
CAUTION
NOTE THAT BOTH THE GENERAL AND SPECIFIC RESTRICTIONS MUST BE MET BEFORE PERFORMING A COMPOSITE REPAIR. IF THE RESTRICTIONS CANNOT BE MET, CONSULT PRODUCT SUPPORT ENGINEERING. IN CASE OF CONFLICT BETWEEN A GENERAL AND A SPECIFIC RESTRICTION, THE SPECIFIC RESTRICTION OVERRULES THE GENERAL RESTRICTION.
NEVER USE MEK, ACETONE OR ALIPHATIC NAPHTHA INSTEAD OF THE INDICATED ALCOHOLS ON GLASS OR CARBON FIBER REINFORCED COMPOSITES. MEK IS ONLY TO BE USED FOR CLEANING TOOLS.
WEAR APPROPRIATE SAFETY EQUIPMENT (GLOVES, GOWNS, RESPIRATORS, ETC.) WHEN HANDLING OR WORKING WITH MATERIALS AND MAKING REPAIRS. CONSULT MATERIAL SAFETY DATA SHEETS (MSDS) FOR POTENTIAL HAZARDS AND FOLLOW ALL APPLICABLE SAFETY PROCEDURES.
NOTE
Determine material and tooling requirements and ensure materials are at hand before proceeding with any repair.
• Consumable materials and standards: Materials needed to accomplish a particular repair are listed in the “REQUIRED” section of each repair procedure. Each item is accompanied by a description of the material and a numerical code (C-xxx). This code references a consumable item, which is further described in Chapter 13 of the Standard Practices Manual (BHT-ALL-SPM).
• Required number of plies of repair doubler: Unless otherwise indicated in a repair procedure, the number of plies of a glass fiber repair doubler and their orientation shall be as defined in paragraph 4-3-3. The number of plies of a carbon fiber repair doubler and their orientation may vary and is specific to the region being repaired, and shall be provided by Product Support Engineering unless explicitly specified in the repair.
• Stop drilling: Although stop drilling relieves the stresses in the extremity of a crack in sheet metal parts, Bell Helicopter Textron does not permit stop drilling cracks in fiber reinforced composite structures.
• Finishing: All repairs shall be sealed against moisture intrusion and then finished in accordance with the original finish specifications.
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4-2-2. APPROVED PROCESSES (PROCESS SHEETS)
This section describes the typical processes and repair procedures involving glass or carbon fiber reinforced composite parts. Composite repairs are similar in nature and may be applicable to various parts of the structure. However, the restrictions on repairability may vary from one composite part to another. Therefore, the general and specific restrictions must be verified for each composite part before using a typical repair procedure.
This section describes the processes used in the repair of glass or carbon fiber reinforced composites. These process sheets cover topics such as machining, cleaning and surface preparation prior to bonding. Repair procedures throughout this manual and the model-specific Structural Repair Manual (SRM) make reference to the applicable process sheets in the “REQUIRED” section for each repair.
The applicable processes that are covered in this section are listed in Table 4-5:
Table 4-5. Process Sheets
4-2-3 Page 4-16
Provides a method for sanding glass or carbon fiber composites.
4-2-4 Page 4-18
Provides a method for Cutting/Routing Glass or Carbon fiber composites.
4-2-5 Page 4-21
Provides a method for drilling glass or carbon fiber composites.
4-2-6 Page 4-27
Removal of Paint, Primer and Sanding Surfacer on Glass or Carbon Fiber Composites.
Provides a method of removing paint, primer and sanding surfacer from composite parts.
4-2-7 Page 4-30
Removal of Honeycomb Core in Glass or Carbon Fiber Honeycomb Panels.
Provides a method for removing honeycomb core in glass or carbon fiber honeycomb panels.
4-2-8 Page 4-32
Removal of Copper Wire Mesh from Glass or Carbon Fiber Composites.
Provides a method for removing honeycomb core in glass or carbon fiber honeycomb panels.
4-2-9 Page 4-34
Surface Preparation for Bonding on Glass or Carbon Fiber Composites.
Provides a method for removing copper wire mesh layer from glass or carbon fiber composites.
4-2-10 Page 4-36
Drying Composites Parts Prior to Bonding.
Provides a method for drying glass or carbon fiber composite parts prior to bonding.
4-2-11 Page 4-42
Finish Process Following a Composite Repair.
Provides a method for finishing a glass or carbon fiber composite repair.
4-2-12 Page 4-46
Preparation of Molding Tool for Composite Repair.
Provides a method for fabricating molds to be used in repairs of glass or carbon fiber reinforced composites.
4-2-13 Page 4-51
Preparation of Tooling Surfaces.
Provides a method for cleaning the surface of tools in preparation for bonding wet layup repairs of glass or carbon fiber reinforced composites.
4-2-14 Page 4-53
Preparation of Filled Epoxy Resin.
Provides a method for preparing filler material by mixing epoxy resin with chopped glass fiber.
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4-2-3. SANDING GLASS OR CARBON FIBER COMPOSITES
This process sheet describes the different equipment and procedures used to sand glass or carbon fiber reinforced composites.
CAUTION
WHEN SANDING COMPOSITES, WEAR SAFETY GLASSES, DUST MASK, AND LONG SLEEVED PROTECTIVE CLOTHING. GLASS OR CARBON FIBER DUST CAN CAUSE IRRITATION TO EYES, SKIN, AND LUNGS.
1.0 EQUIPMENT (Use as required)
1.1 Pneumatic high speed sander capable of 20,000 RPM with 3M Roloc sanding disc, 2.0 inches (51 mm) maximum diameter, 80 to 240 grit silicon carbide abrasive.
1.2 Pneumatic vibrating sander capable of approximately 10,000 RPM with self-adhesive sanding disc, 5.0 inches (127 mm) maximum diameter, 220 to 400 grit silicon carbide abrasive.
1.3 Personal protection equipment: safety glasses, dust mask, and long sleeved clothing.
2.0 REQUIRED (Refer to BHT-ALL-SPM for C-xxx consumable materials.)
2.1 Silicon carbide abrasive paper (C-423) of 80 to 400 grit.
3.0 PROCEDURE
3.1 Determine type of sanding to be performed (i.e., rough cuts, edge smoothing, or surface finish).
CAUTION
DO NOT OVERHEAT COMPOSITES DURING MATERIAL SANDING BY USING ORBITAL MOTION.
DO NOT USE WATER SOLUBLE COOLANT ON HONEYCOMB OR SYNTACTIC CORE PANELS.
DO NOT USE ABRASIVE PAPER THAT HAS BEEN USED TO SAND METAL.
ALWAYS COVER WIRE BUNDLES TO PREVENT ENTRAPMENT OF PARTICULES.
3.2 For rough cuts, use high speed sander specified in 1.1 with appropriate grit abrasive. Finish by hand sanding with 400 grit abrasive paper specified in 2.1.
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S A
N D
IN G
G L
A S
S O
R C
A R
B O
N F
IB E
R C
O M
P O
S IT
E S
3.3 For edge smoothing, use abrasive paper of appropriate grit specified in 2.1. Sand by hand in direction along long dimension, not across edge of part, whenever possible. When sanding across edge of part, use proper backups to prevent fiber breakout or splintering.
3.4 For surface finish, use vibrating sander specified in 2.1 for large surface or sand by hand using abrasive paper of fine grit. Generally, finish with 400 grit abrasive specified in 2.1.
14 DEC 2010 4-17ECCN EAR99
4-2-4. CUTTING/ROUTING GLASS OR CARBON FIBER COMPOSITES
This process sheet describes the different equipment and procedures used to cut/rout out damaged area in glass or carbon fiber reinforced composites.
CAUTION
WHEN CUTTING/ROUTING COMPOSITES, WEAR SAFETY GLASSES, DUST MASK, AND LONG SLEEVED PROTECTIVE CLOTHING. GLASS OR CARBON FIBER DUST CAN CAUSE IRRITATION TO EYES, SKIN, AND LUNGS.
1.1 Pneumatic high speed cutter capable of 20,000 RPM.
1.1.1 Reinforced cutting disc, 3 inch (76.2 mm) diameter, 60 to 80 grit abrasive, or
1.1.2 Diamond cutting wheel of appropriate size, 40 to 80 grit abrasive.
1.2 Pneumatic router capable of 12,000 to 23,000 RPM for parts made entirely of composite or 12,000 RPM maximum for composite parts with metallic components or syntactic core embedded in the laminate.
1.2.1 Diamond plated or solid carbide router bits, 0.1875 inch (4.763 mm) diameter, 40 to 80 grit.
2.1 Air vacuum or liquid coolant (lubricant (C-569)).
2.2 Process Sheet(s): Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3)
3.0 PROCEDURE
CAUTION
DO NOT MARK THE COMPOSITE SURFACE USING ANY METHOD THAT WILL INDENT OR OTHERWISE DEFORM THE SURFACE.
3.1 Locate damaged area and mark section to be removed. This section shall be at least 1/32 inch (0.79 mm) greater than existing damage in all directions. For honeycomb panels, remove only damaged skin(s), unless otherwise specified in applicable repair.
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CAUTION
ENSURE THAT NO OTHER PART INTERFERES WITH SECTION TO BE CUT OR ROUTED OUT.
DO NOT OVERHEAT COMPOSITES DURING MATERIAL CUTTING/ROUTING.
FOR SYNTACTIC CORE PANELS, REDUCE ROUTER SPEED TO 12,500 RPM.
DO NOT USE SAME ROUTING BIT FOR METAL AND COMPOSITE PARTS.
ROUTERS FITTED WITH DUST EXTRACTION ARE RECOMMENDED.
A TEMPLATE IS RECOMMENDED TO GUIDE ROUTER TO OBTAIN REQUIRED SHAPE.
3.2 Crack damage: Rout out crack using router specified in 1.2 equipped with router bit specified in 1.2.1, and using a maximum feed rate of 12.0 inches (305 mm) per minute.
3.3 All types of damage (including excessive crack damage): Cut out damaged area using high speed cutter specified in 1.1 or router specified in 1.2 equipped with router bit specified in 1.2.1. Cutout radius to be a minimum of 0.50 inch (12.7 mm). Maintain a maximum feed rate of 12.0 inches (305 mm) per minute.
3.4 If applicable, sand edges to required dimensions using instructions detailed in paragraph 4-2-3.
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4-2-5. DRILLING GLASS OR CARBON FIBER COMPOSITES
This process sheet describes the different equipment and procedures used to drill glass or carbon fiber reinforced composites.
CAUTION
WHEN DRILLING COMPOSITES, WEAR SAFETY GLASSES, DUST MASK, AND LONG SLEEVED PROTECTIVE CLOTHING. GLASS OR CARBON FIBER DUST CAN CAUSE IRRITATION TO EYES, SKIN, AND LUNGS.
1.1 Pneumatic high speed drill or drill press capable of the RPM specified below.
1.1.1 C2 carbide drill bits of appropriate diameter.
1.1.2 C2 carbide reamer of appropriate diameter.
1.1.3 C2 carbide dreamer (drill/reamer) of appropriate diameter.
1.2 Locally fabricated metallic fixture, workaid or jig.
2.2 Lubricant (C-569).
2.3 Backup material (of same contour or shape), particle board or phenolic (other than glass base) at least 3/16 inch (4.76 mm) thick.
2.4 Wet layup adhesive (C-363).
2.5 Process Sheet(s): Preparing and Mixing Two-part Epoxy Resin by Weight (paragraph 3-2-25) Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Curing Process for Epoxy Resin (paragraph 4-3-6)
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ENSURE THAT DRILLING DEPTH WILL NOT AFFECT OTHER PARTS.
3.1 Locate center of hole to be drilled and use metal fixture, workaid, or jig to start hole.
3.2 To prevent splintering and delamination, use a backup workaid such as a hard wood block against tool exit surface. Backup workaid shall be held in intimate contact with workpiece at all time during drilling process.
CAUTION
DO NOT OVERHEAT COMPOSITES DURING MATERIAL DRILLING PROCESS.
DO NOT USE LUBRICANT SPECIFIED IN 2.2 ON HONEYCOMB OR SYNTACTIC CORE PANELS.
NOTE
Drill bits shall be changed often so that a sharp cutting edge is maintained at all time. Life of sharp carbide drill bits are limited to approximately 30 holes per bit. Bits that exceed this limit must be either resharpened, or discarded and replaced.
Step 3.3 and step 3.4 can be performed in a single operation using a dreamer (combined drill and reamer), and using a speed between 2400 and 2700 RPM.
3.3 Drill hole using a speed between 1600 and 2700 RPM. To prevent fiber breakout, reduce feed rate near end of cut. For solid laminate thicker than 1/16 inch (1.59 mm) use lubricant specified in 2.2.
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DO NOT OVERHEAT COMPOSITES DURING MATERIAL REAMING.
DO NOT USE LUBRICANT SPECIFIED IN 2.2 ON HONEYCOMB OR SYNTACTIC CORE PANELS.
NOTE
Reamers are limited to approximately 10 holes. Reamers that exceed this limit must be discarded and replaced.
3.4 Ream hole using a hand drill operating at a maximum speed of 250 RPM and a feed rate of 4.0 to 6.0 inches (102 to 152 mm) per minute.
3.5 If required, countersink hole. Countersink and counter bore shall be concentric within 0.003 inch (0.08 mm) and parallel to the fastener hole direction.
3.6 Inspect hole for fiber breakouts, delamination and chipping. Damage shall not exceed the following limits:
3.6.1 Minor/negligible damage is allowed on exit side of drill only.
3.6.2 Acceptance criteria are provided in Table 4-6. Both negligible and repairable limits are provided in second and third column respectively as are repair procedures.
3.7 Deburr, if necessary, by hand sanding using silicon carbide abrasive paper specified in 2.1. Refer to instructions detailed in paragraph 4-2-3.
Table 4-6. Drilling Damage Limits for Protruding Head Fastener Holes
MATERIAL TYPE
LIMIT(1)(2)(4)(5)(6) REPAIR PROCEDURE
Fabric Either • 0.16 times hole diameter, or • 0.03 inch (0.76 mm)
0.5 times hole diameter in any direction
• Prepare wet layup adhesive specified in 2.4 according to paragraph 3-2-5. • Coat with wet layup adhesive. • Cure according to paragraph 4-3-6. • Redrill hole as necessary • Sand smooth according to paragraph 4-2-3. • No fiber damage allowed during corrective action.
Unidirectional Tape
Either • 0.50 times hole diameter, or • 0.10 inch (2.54 mm) whichever is smaller.
1.0 inch (25.4 mm) in length, no more than 90° of hole periphery
NOTES:
2. Distance measured radially from edge of the hole.
3. No rework required.
4. Rework in accordance with procedures provided in fourth column.
5. Damage must not extend within 0.5 inch (12.7 mm) of another fastener hole.
6. Breakout from any two separate fastener holes shall not meet.
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Table 4-7. Drilling Damage Limits for Countersink Side of Holes
MATERIAL TYPE
LIMIT(1)(2)(4)(5)(6) REPAIR PROCEDURE
Fabric Either • 0.16 times hole diameter, or • 0.030 inch (0.76 mm)
0.25 inch (6.4 mm) in length, no more than 60° of hole periphery
• Prepare wet layup adhesive specified in 2.4 according to paragraph 3-2-25. • Coat with wet layup adhesive. • Cure according to paragraph 4-3-6. • Redrill hole as necessary. • Sand smooth according to paragraph 4-2-3. • No fiber damage allowed during corrective action.
Unidirectional Tape
Either • 0.50 times hole diameter, or • 0.100 inch (2.54 mm) whichever is smaller.
NOTES:
2. Distance measured radially from edge of the hole.
3. No rework required.
4. Rework in accordance with procedures provided in fourth column.
5. Damage must not extend within 0.50 inch (12.7 mm) of countersink of another fastener hole.
6. Breakout from any two separate fastener holes shall not meet.
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4-2-6. REMOVAL OF PAINT, PRIMER, AND SANDING SURFACER ON GLASS OR CARBON FIBER COMPOSITES
This process sheet describes the different equipment and procedures used to remove paint, primer, and sanding surfacer on glass or carbon fiber reinforced composites.
CAUTION
CHEMICAL PAINT STRIPPERS ARE NOT TO BE USED TO REMOVE PAINT FINISHES ON BONDED PARTS OR PANELS. CHEMICAL PAINT STRIPPERS MAY CONTRIBUTE TO CONTAMINATION OF CORE, DETERIORATE ADHESIVE BOND LINE AND GLASS OR CARBON FIBER COMPOSITES. THIS MAY ULTIMATELY CAUSE PART OR PANEL TO BE REJECTED.
MEK, ACETONE, TRICHLOROETHYLENE, AND VAPOR DEGREASERS ARE NOT TO BE USED TO CLEAN OR STRIP SURFACES ADJACENT TO A DAMAGED AREA FOR THE SAME REASONS.
METHYL ALCOHOL, ETHYL ALCOHOL AND ISOPROPYL ALCOHOL ARE ACCEPTABLE SOLVENTS FOR REMOVAL OF PAINT FROM SOLID LAMINATE COMPOSITE PARTS OR BONDED PANEL WITH COMPOSITE SKINS. HOWEVER EXCESSIVE APPLICATION OF ALCOHOL MAY AFFECT ADHESIVES USED IN A BOND. IT IS PREFERABLE TO WIPE SURFACE TO BE STRIPPED USING A MOISTENED CHEESECLOTH RATHER THAN BY SOAKING.
BEFORE HANDLING A SOLVENT, EXTINGUISH ALL FLAMES AND PILOT LIGHTS. KEEP PRODUCT AND ITS VAPORS AWAY FROM HEAT, SPARKS, AND FLAME. DURING APPLICATION AND UNTIL VAPORS HAVE DISSIPATED, AVOID USING SPARK PRODUCING ELECTRICAL EQUIPMENT SUCH AS SWITCHES, APPLIANCES, ETC. AVOID PROLONGED BREATHING OF VAPORS AND REPEATED CONTACT WITH SKIN.
1.0 EQUIPMENT
1.1 Pneumatic high speed sander or drill equipped with a variable pressure regulator and 3M Roloc white bristle brush, 3.0 inches (76 mm) maximum diameter, or material specified in 2.1 or 2.2.
2.1 Silicon carbide abrasive paper (C-423) of 80 to 400 grit, or
2.2 Nylon web abrasive pad (C-407), maroon and green.
2.3 Cleaner: ethyl alcohol (C-339), isopropyl alcohol (C-385), or toluene (C-306).
2.4 Cheesecloth (C-486).
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3.0 PROCEDURE
3.1 Mask around area to be reworked using plastic film or Kraft paper specified in 2.6, and masking tape specified in 2.5. If a repair doubler is required, masked area shall be 1.00 inch (25.4 mm) further away from periphery of repair doubler.
CAUTION
DO NOT WET SAND.
DO NOT SAND INTO GLASS OR CARBON FIBERS. PAY SPECIAL ATTENTION NOT TO DAMAGE FIBERS WHEN MULTIPLE SANDING OPERATIONS ARE TO BE PERFORMED AS EACH STEP MAY SLIGHTLY DAMAGE FIBERS.
DO NOT OVERHEAT COMPOSITES DURING MATERIAL SANDING. USE OF ORBITAL MOTION IS PREFERABLE.
DO NOT USE PNEUMATIC TOOL, OR ABRASIVE CLOTH OR PAPER ON THIN COMPOSITE SKIN OF BONDED PANEL WHERE HEXAGONAL SHAPE OF CORE CELLS ARE VISIBLE. USE NYLON WEB ABRASIVE PAD INSTEAD.
NOTE
Some primer residue may remain in recesses of peel ply and vacuum bag impression on surface of laminate. This condition is acceptable, but all efforts shall be made to keep it to a minimum while taking care not to damage glass or carbon fibers.
3.2 Determine type of sanding to be performed (i.e., sanding with pneumatic tool, by hand with abrasive cloth/paper, or with nylon web abrasive pad).
3.3 For hand sanding with abrasive paper specified in 2.1, remove paint, primer and sanding surfacer using orbital motion. When primer is or becomes visible, use 180 to 240 grit or finer abrasive paper to continue sanding operation. Vacuum area frequently to reduce paint/primer residue buildup on abrasive paper.
3.4 For hand sanding with green or maroon nylon web abrasive pads specified in 2.2, remove paint, primer, and sanding surfacer using orbital motion. When primer is or becomes visible, use green pads only to continue sanding operation. Vacuum area frequently to reduce paint/primer residue buildup inside nylon web abrasive pad.
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3.5 For pneumatic tool sanding using equipment specified in 1.1, remove paint, primer, and sanding surfacer using orbital motion. Always begin with lowest speed setting to prevent damaging the fibers, and slowly increase speed until bristle brush stops gripping. Maintain bristle brush perpendicular to surface to be reworked. Vacuum area frequently to reduce paint/primer residue buildup inside brush.
3.6 Accomplish surface cleaning by wiping with a clean cheesecloth moistened with cleaner specified in 2.3. Change cheesecloth often. Repeat operation until all evidence of residue is removed and wipe dry using a clean cheesecloth.
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4-2-7. REMOVAL OF HONEYCOMB CORE IN GLASS OR CARBON FIBER HONEYCOMB PANELS
This process sheet describes the different equipment and procedures used to remove honeycomb core from glass or carbon fiber reinforced honeycomb panels.
CAUTION
WHEN REPAIRING COMPOSITES, WEAR SAFETY GLASSES, DUST MASK, AND LONG SLEEVED PROTECTIVE CLOTHING. NOMEX CORE DUST CAN CAUSE IRRITATION TO EYES, SKIN, AND LUNGS.
1.1 Pneumatic router capable of 12,000 to 23,000 RPM.
2.0 REQUIRED
2.1 Diamond plated or solid carbide router bits.
2.2 Process Sheet(s): Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Cutting/Routing Glass or Carbon Fiber Composites (paragraph 4-2-4)
3.0 PROCEDURE
3.1 If not already accomplished, remove damaged section of skin(s) using instructions detailed in paragraph 4-2-4.
CAUTION
UNLESS A SECTION OF OPPOSITE SKIN MUST ALSO BE REMOVED, ENSURE THAT OPPOSITE SKIN WILL NOT BE DAMAGED.
3.2 Adjust depth of the cut as required.
CAUTION
DO NOT OVERHEAT COMPOSITES DURING CORE REMOVAL.
3.3 Rout out section of core to be removed using router specified in 1.1 equipped with router bit specified in 2.1, and using a speed between 12,000 and 23,000 RPM.
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3.4 Sand edges of skin(s) around cavity using instructions detailed in paragraph 4-2-3.
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4-2-8. REMOVAL OF COPPER WIRE MESH FROM GLASS OR CARBON FIBER COMPOSITES
This process sheet describes the different equipment and procedures used to remove the copper wire mesh layer from glass or carbon fiber reinforced parts.
CAUTION
1.0 EQUIPMENT
1.1 None.
2.1 Silicon carbide abrasive paper (C-423) of 400 grit.
2.2 Process Sheet(s): Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Removal of Paint, Primer and Sanding Surfacer on Glass or Carbon Fiber (paragraph 4-2-6)
3.0 PROCEDURE
3.1 If not already accomplished, remove paint, primer and sanding surfacer from affected area using instructions detailed in (paragraph 4-2-6).
CAUTION
DO NOT OVERHEAT COMPOSITES DURING MATERIAL SANDING.
3.2 Carefully sand off adhesive to expose copper wire mesh taking care not to damage glass or carbon fibers.
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CAUTION
CARE MUST BE TAKEN TO PREVENT DAMAGING GLASS OR CARBON FIBERS WHILE CUTTING COPPER WIRE MESH.
NOTE
Do not remove more copper wire mesh than required by applicable repair.
3.3 Use a small knife blade to carefully cut copper wire mesh along periphery of section to be removed.
3.4 Slowly peel off section of copper wire mesh to be removed.
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4-2-9. SURFACE PREPARATION FOR BONDING ON GLASS OR CARBON FIBER COMPOSITES
This process sheet describes the different equipment and procedures used to prepare glass or carbon fiber reinforced composites surfaces for bonding.
NOTE
In order to provide optimal conditions for bonding, it is recommended that delays between surface preparation detailed in this process sheet and bonding of repair plies be kept to a minimum.
1.0 EQUIPMENT
1.1 None.
2.1 Silicon carbide abrasive paper (C-423) of 100 grit or finer, 240 grit preferred.
2.3 Kraft paper (C-254).
2.4 Masking tape (C-426).
2.6 Process Sheet(s): Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Removal of Paint, Primer, and Sanding Surfacer on Glass or Carbon Fiber (paragraph 4-2-6) Removal of Copper Wire Mesh from Glass or Carbon Fiber Composites (paragraph 4-2-8)
3.0 PROCEDURE
NOTE
When performing structural repair, surfacing film and copper wire mesh must be removed to allow bonding of repair plies to existing structural plies.
3.1 If not already accomplished, remove paint, primer, sanding surfacer, and surfacing film (if required) using instructions detailed in paragraph 4-2-6.
3.2 If required and not already accomplished, remove copper wire mesh using instructions detailed in paragraph 4-2-8.
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CAUTION
BEFORE HANDLING A SOLVENT, EXTINGUISH ALL FLAMES AND PILOT LIGHTS. KEEP PRODUCT AND ITS VAPORS AWAY FROM HEAT, SPARKS, AND FLAME. DURING APPLICATION AND UNTIL VAPORS HAVE DISSIPATED, AVOID USING SPARK PRODUCING ELECTRICAL EQUIPMENT SUCH AS SWITCHES, APPLIANCES, ETC. AVOID PROLONGED BREATHING OF VAPORS AND REPEATED CONTACT WITH SKIN.
3.3 Wipe surface to be bonded with a clean cheesecloth moistened with cleaner specified in 2.2.
CAUTION
GLASS OR CARBON FIBERS MUST BE SANDED UNTIL FIBERS ARE SLIGHTLY ABRADED.
DO NOT SAND INTO GLASS OR CARBON FIBER PLIES.
3.4 Sand surface(s) to be bonded with silicon carbide abrasive paper specified in 2.1. Refer to instructions detailed in paragraph 4-2-3.
3.5 Wipe surface with a clean cheesecloth moistened with cleaner specified in 2.2. Change cheesecloth often. Repeat operation until all evidence of residue is removed and wipe dry using a clean cheesecloth.
3.6 Allow surface to air dry for at least 30 minutes before bonding.
3.7 Use clean Kraft paper specified in 2.3 and tape specified in 2.4 to protect surface from contamination until ready for bonding.
NOTE
If fibers have been exposed during a previous operation and this process sheet must be performed again, substitute abrasive paper specified in 2.1 with green web abrasive pad (C-407).
Surface must be bonded within 72 hours following abrading and alcohol wipe. If not, surface preparation using this process sheet must be performed again.
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4-2-10. DRYING COMPOSITE PARTS PRIOR TO BONDING
This process sheet describes the different equipment and procedures used to dry solid laminates or honeycomb panels.
NOTE
Laminates or honeycomb panels made with organic matrix material such as epoxy must be dried before performing any repair that requires a cure or a post-cure at or above 200°F (93.3°C). If moisture is not removed from laminate or honeycomb panel, there is a possibility of delamination or skin to core disbond due to steam buildup.
APPLICATION A: DRYING OF SOLID LAMINATES
1.0 EQUIPMENT
1.1 Temperature monitoring equipment with a minimum of two thermocouples.
1.2 Device capable of applying heat such as:
1.2.1 An oven (if part can be removed from helicopter).
1.2.2 A Bell Helicopter Textron approved hot-bonding unit, or equivalent, in conjunction with a heat blanket capable of maintaining a minimum of 250°F (121.1°C).
1.2.3 Heat lamps.
1.2.4 Heat guns.
2.1 Bagging film (C-257) or bagging film (C-564), if required.
2.2 Pressure sensitive masking tape (C-260), if required.
2.3 Breather felt (C-258), if required.
2.4 Glass fabric (C-404), if required.
2.5 Vacuum bag sealant tape (C-259) or sealant tape (C-566), if required.
2.6 Pressure sensitive masking tape (C-260) or tape (C-462) respectively, if required.
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NOTE
Drying time depends on material type, part thickness, drying temperature, and relative humidity inside part. Ensure that part is dried before performing repair.
If repair is to be cured or post-cured in an oven, entire part must be dried in an oven.
It is recommended to vacuum bag part or area to be dried using equipment specified in 1.3, materials specified in 2.1, 2.3, 2.4, and 2.5, and applying a minimum of 15 inches (381 mm) HG (vacuum bagging process, paragraph 3-2-26).
3.1 Dry region to be repaired at 190 ±10°F (87.8 ±5.6°C) for carbon fiber reinforced composite parts or 160 ±10°F (71.1 ±5.6°C) for glass fiber reinforced composite parts for a minimum of 4 hours using equipment specified in 1.2. Use a heat up rate and a cool down rate between 1 to 5°F (0.6 to 2.8°C) per minute.
Examples:
3.2 A minimum of two thermocouples must constantly monitor temperature. Thermocouples’ readings must fall within specified temperature range at all times.
3.3 Repair must be performed within 8 hours after drying process is performed. If not, drying procedure must be repeated. The 8 hours limit between drying and repair must be met. It is possible to protect dried part or repair area by covering it with bagging film specified in 2.1 secured in place with tape specified in 2.6.
Carbon fabric: Starting point 75°F (23.9°C)
Ramp up 3°F/minute (1.7°C/min) for 38 minutes
Dwell time 4 hours at 190°F (87.8°C)
Cool down 3°F/minute (1.7°C/min) for 38 minutes
Total Elapsed Time: 316 minutes
Glass fabric: Starting point 75°F (23.9°C)
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APPLICATION B: DRYING OF HONEYCOMB PANELS USING A LOCALIZED HEAT SOURCE.
NOTE
Procedure for drying honeycomb panels depends on equipment used to cure or post-cure repair. If repair is cured or post-cured using a local heat source (heat blanket, heat lamps, or heat guns) use Application B. If repair is cured or post-cured using an oven, use Application C.
1.0 EQUIPMENT
1.2.1 A Bell Helicopter Textron approved hot-bonding unit, or equivalent, in conjunction with a heat blanket capable of maintaining a minimum of 250°F (121.1°C).
1.2.2 Heat lamps.
1.2.3 Heat guns.
2.3 Glass fabric (C-404) if required.
2.5 Pressure sensitive masking tape (C-260) or tape (C-462), if required.
2.6 Process Sheet(s): Drilling Glass or Carbon Fiber Composites (paragraph 4-2-5)
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NOTE
Before drying repair area, inspect for traces of water that has migrated in core. If part has water in a repairable zone, drill holes through skin in area containing water. Use a 0.063 inch (1.60 mm) diameter carbide drill bit, and drill using instructions detailed in paragraph 4-2-5. For each square inch (645 mm2) of water accumulation in repair area, drill two holes through skin to be repaired. Drilled holes to be uniformly distributed over entire area where water has accumulated in core.
Drying time depends on material type, part thickness, drying temperature, and relative humidity inside part. Ensure that the part is dried before performing repair.
It is recommended to vacuum bag part or area to be dried using equipment specified in 1.3, material specified in 2.1, 2.2, 2.3, and 2.4, and applying a minimum of 15 inches (381 mm) HG (vacuum bagging process, paragraph 3-2-26).
3.1 Dry region to be repaired at 190 ±10°F (87.8 ±5.6°C) for carbon fiber reinforced composite parts or 160 ±10°F (71.1 ±5.6°C) for glass fiber reinforced composite parts for a minimum of 8 hours using equipment specified in 1.2. Use a heat up rate and a cool down rate between 1 to 5°F (0.6 to 2.8°C) per minute.
Examples:
3.2 A minimum of two thermocouples must monitor temperature. Thermocouples’ readings must fall within specified temperature range.
3.3 Repair must be performed within 8 hours after drying process is performed. If not, drying procedure must be repeated. The 8 hours limit between drying and repair must be met. It is possible to protect dried repair area by covering it with bagging film specified in 2.1 secured in place with tape specified in 2.5.
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NOTE
Procedure for drying honeycomb panels depends on equipment used to cure or post-cure repair. If repair is cured or post-cured using a local heat source (heat blanket, heat lamps, or heat guns) use Application B. If repair is cured or post-cured using an oven, use Application C.
1.0 EQUIPMENT
1.2.1 An oven (if part can be removed from helicopter).
1.3 Vacuum fitting (if required).
2.3 Glass fabric (C-404) if required.
2.5 Pressure sensitive masking tape (C-260) or tape (C-462), if required.
2.6 Process Sheet(s): Drilling Glass or Carbon Fiber Composites (paragraph 4-2-5)
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NOTE
Before drying repair area, inspect for traces of water that has migrated in core. If part has water in a repairable zone, drill holes through skin in area containing water. Use a 0.063 inch (1.60 mm) diameter carbide drill bit, and drill using instructions detailed in paragraph 4-2-5. For each square inch (645 mm2) of water accumulation in repair area, drill two holes through skin to be repaired. Drilled holes to be uniformly distributed over entire area where water has accumulated in core.
Drying time depends on material type, part thickness, drying temperature, and relative humidity inside part. Ensure that part is dried before performing repair.
It is recommended to vacuum bag part or area to be dried using the equipment specified in 1.3, material specified in 2.1, 2.2, 2.3, and 2.4, and applying a minimum of 15 inches (381 mm) HG (vacuum bagging process, paragraph 3-2-26).
3.1 Dry bonded panel to be repaired in an oven at 190 ±10°F (87.8 ±5.6°C) for carbon fiber reinforced composite parts or 160 ±10°F (71.1 ±5.6°C) for glass fiber reinforced composite parts for a minimum of 16 hours using equipment specified in 1.2. Use a heat up rate and a cool down rate between 1 to 5°F (0.6 to 2.8°C) per minute.
Examples:
3.2 A minimum of two thermocouples must monitor temperature. Thermocouples’ readings must fall within specified temperature range.
3.3 Repair must be performed within 8 hours after drying process is performed. If not, drying procedure must be repeated. The 8 hours limit between drying and repair must be met. It is possible to protect dried part by covering it with bagging film specified in 2.1 secured in place with tape specified in 2.5.
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4-2-11. FINISH PROCESS FOLLOWING A COMPOSITE REPAIR
This process sheet describes the different equipment and procedures used to prepare the composite repair before paint application.
APPLICATION A: INTERIOR SURFACES
CAUTION
BEFORE HANDLING A SOLVENT, PRIMER, AND SANDING SURFACER, EXTINGUISH ALL FLAMES AND PILOT LIGHTS. KEEP PRODUCT AND ITS VAPORS AWAY FROM HEAT, SPARKS, AND FLAME. DURING APPLICATION AND UNTIL VAPORS HAVE DISSIPATED, AVOID USING SPARK PRODUCING ELECTRICAL EQUIPMENT SUCH AS SWITCHES, APPLIANCES, ETC. AVOID PROLONGED BREATHING OF VAPORS AND REPEATED CONTACT WITH SKIN.
FINISH PROCESS MUST BE PERFORMED IN A CLEAN ENVIRONMENT, FREE OF DUST, OIL, AND GREASE.
USE CARE WHEN APPLYING PRIMER AND SANDING SURFACER. FUMES FROM THESE MATERIALS CAUSE BOTH HEALTH AND FIRE HAZARD. HAVE GOOD VENTILATION AND BREATHING PROTECTION. WEAR PROTECTIVE CLOTHING AND EYE SHIELD.
2.2 Epoxy Polyamide Primer (C-204).
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DO NOT SAND INTO GLASS OR CARBON FIBERS.
3.1 Once composite repair is cured per instructions detailed in paragraph 4-3-6, lightly sand surface with abrasive paper specified in 2.1.
3.2 Wipe surface with a clean cheesecloth moistened with cleaner specified in 2.3. Allow to air dry for 30 minutes.
3.3 Apply pinhole filler specified in 2.5 per manufacturer’s instructions. Dry surface per manufacturer’s instructions.
3.4 Once pinhole filler is fully cured, lightly sand surface with abrasive paper specified in 2.1.
3.6 If required, sand to match surrounding structure and within 4 hours of sanding (step 3.4), prime with epoxy polyamide primer specified in 2.2. Allow to dry.
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CAUTION
BEFORE HANDLING A SOLVENT, PRIMER, AND SANDING SURFACER, EXTINGUISH ALL FLAMES AND PILOT LIGHTS. KEEP PRODUCT AND ITS VAPORS AWAY FROM HEAT, SPARKS, AND FLAME. DURING APPLICATION AND UNTIL VAPORS HAVE DISSIPATED, AVOID USING SPARK PRODUCING ELECTRICAL EQUIPMENT SUCH AS SWITCHES, APPLIANCES, ETC. AVOID PROLONGED BREATHING OF VAPORS AND REPEATED CONTACT WITH SKIN.
FINISH PROCESS MUST BE PERFORMED IN A CLEAN ENVIRONMENT, FREE OF DUST, OIL, AND GREASE.
USE CARE WHEN APPLYING PRIMER AND SANDING SURFACER. FUMES FROM THESE MATERIALS CAUSE BOTH HEALTH AND FIRE HAZARDS. HAVE GOOD VENTILATION AND BREATHING PROTECTION. WEAR PROTECTIVE CLOTHING AND EYE SHIELD.
2.2 Epoxy Polyamide Primer (C-204).
2.3 Sanding Surfacer (C-228).
2.8 Process Sheet(s): Curing Process for Epoxy Resin (paragraph 4-3-6)
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3.1 Once composite repair is cured per instructions detailed in paragraph 4-3-6, lightly sand surface with abrasive paper specified in 2.1.
3.3 Apply fairing compound specified in 2.7, per manufacturer’s instructions. Dry surface per manufacturer’s instructions.
3.4 Once fairing compound is fully cured, if required, lightly sand surface with abrasive paper specified in 2.1.
3.5 If required, wipe surface with a clean cheesecloth moistened with cleaner specified in 2.4. Allow to air dry for 30 minutes.
3.6 Apply pinhole filler specified in 2.6, per manufacturer’s instructions. Dry surface per manufacturer’s instructions.
3.7 Once pinhole filler is fully cured, if required, lightly sand surface with abrasive paper specified in 2.1.
3.8 If required, wipe surface with a clean cheesecloth moistened with cleaner specified in 2.4. Allow to air dry for 30 minutes.
3.9 Apply sanding surfacer specified in 2.3. Allow to dry.
3.10 Sand to a smooth finish with abrasive paper specified in 2.1.
3.12 Prime with epoxy polyamide primer specified in 2.2 before topcoat is applied.
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4-2-12. PREPARATION OF MOLDING TOOL FOR COMPOSITE REPAIR
This process sheet describes the different equipment and procedures used to fabricate a molding tool that is used to recreate the shape of a part to be repaired.
APPLICATION A: FABRICATION OF MOLDING TOOL USING ADHESIVE ALUMINUM TAPE
This procedure is recommended for damage up to 1.00 inch (25.4 mm) diameter. For larger damage, procedure detailed in Application B is recommended.
1.0 EQUIPMENT
1.1 None.
2.1 Adhesive aluminum tape (C-439).
2.2 release film (C-256).
2.3 Process Sheet(s):
Preparing and Mixing Two-part Epoxy Resin by Weight (paragraph 3-2-25) Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Removal of Paint, Primer, and Sanding Surfacer on Glass or Carbon Fiber (paragraph 4-2-6) Wet Layup Impregnation Process (paragraph 4-3-3)
3.0 PROCEDURE
3.1 If required, sand surface to be molded smooth and free of obstructions using instructions detailed in paragraph 4-2-3.
3.2 Clean surface to be molded using instructions detailed in paragraph 4-2-6.
3.3 Prepare a piece of release film specified in 2.2 dimensioned to same shape and size as cutout in damaged part.
3.4 Prepare a sufficient number of strips of adhesive aluminum tape specified in 2.1 to cover beyond cutout in damaged part. Strips to extend a minimum of 2.00 inches (50.4 mm) from edge of cutout. Alternate tape direction to ensure maximum adhesion to damaged part. Multiple layers of tape may be used to increase stiffness.
3.5 Stick a piece of release film prepared in step 3.3 at the center of sticky side of adhesive aluminum tape strips prepared in step 3.4.
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3.6 Stick tape strips to surface of damaged part. Ensure strips match contour of part and are sufficiently stiff to maintain contour throughout repair. Additional tape strips may be used as required.
3.7 Perform remaining instructions of applicable repair.
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APPLICATION B: FABRICATION OF MOLDING TOOL (SPLASH) USING GLASS FABRIC
This process sheet describes the different equipment and procedures used to fabricate a molding tool that is used to recreate the shape of a part to be repaired.
CAUTION
PRIOR TO PREPARING A MOLDING TOOL, DAMAGED AREA MUST BE THOROUGHLY INSPECTED AND EXTENT OF DAMAGE FULLY DEFINED THROUGH VISUAL INSPECTION, TAP TEST, OR OTHER MEANS SUCH AS ULTRASONIC INSPECTION.
MOLDING TOOL IS DIMENSIONED TO ACCOMMODATE EXPECTED REPAIR DOUBLER SIZE AND BAGGING MATERIALS NECESSARY FOR CURE STEP. MOLDING TOOL SHALL BE APPROXIMATELY 4.0 TO 8.0 INCHES (101.6 TO 203.2 MM) LARGER (ALL AROUND) THAN LARGEST REPAIR PLY.
MOLDING TOOL SHALL BE CENTERED OVER REPAIR AREA AND ITS LOCATION CLEARLY MARKED TO FACILITATE SUBSEQUENT PLACEMENT OF REPAIR DOUBLER ONTO HIGHLY CONTOURED SURFACES WITHOUT ANY GAPING CONDITION.
IF IT IS NOT POSSIBLE TO TAKE MOLDING TOOL DIRECTLY FROM DAMAGED COMPONENT, USE AN UNDAMAGED IDENTICAL COMPONENT.
2.1 Silicon carbide abrasive paper (C-423) of 100 grit or finer, 180 to 240 grit preferred.
2.2 Glass fabric (C-404), style 7781 or glass fabric (C-560), style 120.
2.3 General purpose bonding adhesive (C-317).
2.4 Wet layup adhesive (C-363).
2.5 Glass tape (C-157) or adhesive tape (C-460).
Preparing and Mixing Two-part Epoxy Resin by Weight (paragraph 3-2-25) Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Removal of Paint, Primer, and Sanding Surfacer on Glass or Carbon Fiber (paragraph 4-2-6) Wet Layup Impregnation Process (paragraph 4-3-3)
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3.1 If required, sand surface to be molded smooth and free of obstructions using instructions detailed in paragraph 4-2-3.
3.2 Clean surface to be molded using instructions detailed in paragraph 4-2-6.
3.3 Cover surface to be molded with adhesive backed Teflon coated glass fabric specified in 2.5 to act as a release surface. Release surface shall be 3.0 inches (76.2 mm) larger (all around) than size of molding tool.
3.4 Build up a wax dam of suitable height to contain layup around periphery of release surface material to prevent adhesive from flowing away from release surface.
3.5 Prepare general purpose bonding adhesive specified in 2.3 using instructions detailed in Chapter 3, paragraph 3-2-25.
3.6 Molding tool will be built up from alternating layers of general purpose bonding adhesive specified in 2.3 and dry glass fabric specified in 2.2 as follows:
NOTE
A balanced and symmetrical layup is composed of an even number of plies. The plies are positioned in such a way that plies at a given distance from the mid plane of the laminate (on both sides of the mid plane) have the same fiber orientation, which helps avoid warpage during the curing of the part. For example, a laminate composed of 6 plies having the following orientation (90/45/0/0/45/90) is balanced and symmetrical since the first ply on both sides of the mid plane is at 0°, the second ply is at 45° and the third ply is at 90°. The following laminate (90/45/0/90/ 45/0) is not balanced and symmetrical since the orientation of the first and last plies on both sides of the mid plane are not identical.
3.6.1 Generously apply general purpose bonding adhesive specified in 2.3 as required for a base coat.
3.6.2 Wet layup one glass fabric repair ply using instructions detailed in paragraph 4-3-3 and wet layup adhesive specified in 2.4.
3.6.3 Repeat step 3.6.1 and step 3.6.2 as many times as required until desired thickness is reached. First two and last two plies of glass used may be style 120 in order to provide a smooth surface and then style 7781 plies shall be added to provide strength and stiffness to final tool. Final molding tool shall consist of at least 10 plies of style 7781 fabric to handle vacuum bagging pressure without distortion. Actual number of plies used shall take into account size of molding tool. Plies must be oriented to produce a balanced and symmetrical layup. If molding tool is found to require greater strength and stiffness, a backing structure (e.g., supporting ribs) can be fabricated onto tool using these same materials.
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3.7 Allow splash to cure at room temperature for 24 hours in order to achieve handling strength.
3.8 Remove molding tool from surface of part, and allow to cure at room temperature for an additional 48 hours.
3.9 Sand tool surface lightly with fine grit sandpaper to remove all bubbles.
3.10 Inspect tool surface, any remaining defects must be filled and faired using general purpose bonding adhesive specified in 2.3.
3.11 Verify fit of molding tool to damaged part.
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4-2-13. PREPARATION OF TOOLING SURFACES
This process sheet describes the different equipment and procedures used to clean tooling surfaces prior to layup and bonding.
CAUTION
BEFORE HANDLING SOLVENT, EXTINGUISH ALL FLAMES AND PILOT LIGHTS. KEEP PRODUCT AND ITS VAPOURS AWAY FROM HEAT, SPARKS, AND FLAME. DURING APPLICATION AND UNTIL VAPOURS HAVE DISIPATED, AVOID USING SPARK PRODUCING ELECTRICAL EQUIPMENT SUCH AS SWITCHES, APPLIANCES, ETC.
CLEANING MUST BE PERFORMED IN A CLEAN ENVIRONMENT, FREE OF DUST, OIL, AND GREASE.
HAVE GOOD VENTILATION AND BREATHING PROTECTION. WEAR PROTECTIVE CLOTHING AND EYE SHIELD.
1.4 Release agent (C-555).
Preparing and Mixing Two-part Epoxy Resin by Weight (paragraph 3-2-25) Sanding Glass or Carbon Fiber Composites (paragraph 4-2-3) Preparation of Molding Tool for Composite Repair (paragraph 4-2-12) Wet Layup Impregnation Process (paragraph 4-3-3)
2.0 CLEANING OF MOLD OR TOOL
2.1 Wipe mold or tool surface thoroughly with a clean cheesecloth moistened with cleaner specified in 1.1. Change cheesecloth often. Repeat operation until all evidence of residue is removed and wipe dry using a clean cheesecloth. If scrubbing is necessary to remove residue, use an abrasive pad specified in 1.3 wetted with cleaner specified in 1.1, followed with wiping with cheesecloth.
3.0 APPLICATION OF RELEASE AGENT
3.1 Wipe two coats of release agent specified in 1.4 over entire surface of mold or tool allowing 15 minutes air dry between coats. Baking at 250 to 280°F (121.1 to 137.8°C) for 15 to 20 minutes is recommended. Apply an additional coat of release agent and air dry for 2 hours minimum before beginning layup.
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NOTE
If a problem with release of part from the tool is known or anticipated, the following procedure shall be used instead:
3.2 Generously apply four coats of release agent specified in 1.4. Air dry for 15 minutes between each coat.
3.3 Bake tool at 265 ±15°F (129.4 ±8.3°C) for 15 to 20 minutes.
3.4 Generously apply four coats of release agent specified in 1.4. Air dry for 15 minutes between each coat.
3.5 Bake tool at 265 ±15°F (129.4 ±8.3°C) for 15 to 20 minutes.
3.6 Generously apply two coats of release agent specified in 1.4. Air dry for 15 minutes between each coat.
3.7 Air dry for 2 hours before beginning layup.
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4-2-14. PREPARATION OF FILLED EPOXY RESIN
This process sheet describes the different equipment and procedures used to prepare epoxy resin filled with milled/chopped glass fibers to be used to fabricate tools or fill core cavities.
2.1 Glass fabric (C-404), style 7781.
2.2 Wet layup adhesive (C-363) as required in applicable repair.
2.3 Process Sheet(s): Preparing and Mixing Two-part Epoxy Resin by Weight (paragraph 3-2-25)
3.0 PROCEDURE
3.1 Chop glass fiber cloth specified in 2.1 to length of approximately 0.125 inch (3.18 mm).
3.2 Prepare wet layup adhesive specified in 2.2 using instructions detailed in Chapter 3, paragraph 3-2-25.
3.3 Mix 35 to 50% by weight of chopped glass fibers prepared in step 3.1 to wet layup adhesive.
NOTE
Filled epoxy resin must be used within pot life of unfilled adhesive. Refer to Table 3-16 for pot life.
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4- 3.
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4-3. COMMON PROCEDURES FOR WET LAYUP REPAIRS OF GLASS OR CARBON FIBER REINFORCED COMPOSITE PARTS
4-3-1. WET LAYUP REPAIRS — GENERAL
CAUTION
NOTE THAT BOTH THE GENERAL AND SPECIFIC RESTRICTIONS MUST BE MET BEFORE PERFORMING A COMPOSITE REPAIR. IF THE RESTRICTIONS CANNOT BE MET, CONSULT PRODUCT SUPPORT ENGINEERING. IN CASE OF CONFLICT BETWEEN A GENERAL AND A SPECIFIC RESTRICTION, THE SPECIFIC RESTRICTION OVERRULES THE GENERAL RESTRICTION.
NEVER USE MEK, ACETONE, OR ALIPHATIC NAPHTHA INSTEAD OF THE INDICATED ALCOHOLS ON GLASS OR CARBON FIBER REINFORCED COMPOSITES. THOSE SOLVENTS ARE ONLY TO BE USED FOR CLEANING TOOLS.
WEAR APPROPRIATE SAFETY EQUIPMENT (GLOVES, GOWNS, RESPIRATORS, ETC.) WHEN HANDLING OR WORKING WITH MATERIALS AND MAKING REPAIRS. CONSULT MATERIAL SAFETY DATA SHEETS (MSDS) FOR POTENTIAL HAZARDS AND FOLLOW ALL APPLICABLE SAFETY PROCEDURES.
NOTE
Determine material and tooling requirements and ensure materials are at hand before proceeding with any repair.
Consumable Materials and Standards: Materials needed to accomplish a particular repair are listed in the “REQUIRED” section of each repair procedure. Each item is accompanied by a description of the material and a numerical code (C-xxx). This code references a consumable item, which is further described in Chapter 13 of the Standard Practices Manual (BHT-ALL-SPM).
Required Number of Plies of Repair Doubler: Unless otherwise indicated in a repair procedure, the number of plies of a glass fiber repair doubler and their orientation shall be as defined in paragraph 4-3-3. The number of plies of a glass fiber repair doubler and their orientation may vary and is specific to the region being repaired, but unless it is explicitly specified in the repair, the 0° orientation is parallel to the longitudinal axis of the helicopter (FWD/AFT direction). The number of plies of a carbon fiber repair doubler and their orientation may vary and is specific to the region being repaired, and shall be provided by Product Support Engineering unless explicitly specified in the repair.
Stop Drilling: Although stop drilling relieves the stresses in the extremity of a crack in sheet metal parts, Bell Helicopter Textron does not permit stop drilling cracks in fiber reinforced composite structures.
Finishing: All repairs shall be sealed against moisture intrusion and then finished in accordance with the original finish specifications.
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4-3-2. APPROVED PROCESSES (PROCESS SHEETS)
This section describes the processes used in wet layup repairs of glass or carbon fiber reinforced composite parts. These process sheets cover topics such as wet layup impregnation process, core plug preparation and installation, wet layup vacuum bagging, and curing procedures. Wet layup is a process in which dry woven glass or carbon cloth is impregnated with a semi-liquid epoxy resin and then cured to form a laminate. Repair procedures throughout this manual and the model-specific Structural Repair Manual (SRM) make reference to the applicable process sheets in the “REQUIRED” section.
The following processes that are covered in this section are listed in Table 4-8.
Table 4-8. Process Sheets
4-3-3 Page 4-56
Wet Layup Impregnation Process
Provides a method for impregnating dry carbon or glass fiber fabric with epoxy resin to be used in a wet layup repair.
4-3-4 Page 4-61
Installation of Copper Wire Mesh in Wet Layup Repairs
Provides a method for repairing the copper wire mesh grounding plane following a repair.
4-3-5 Page 4-63
Provides a method for vacuum bagging a wet layup repair.
4-3-6 Page 4-67
Curing Process for Epoxy Resin
Provides a method for curing carbon or glass fiber reinforced wet layup repairs.
4-3-7 Page 4-73
Honeycomb Core Plug Installation and Splicing Using Epoxy Resin
Provides a method for installing core plugs in composite faced bonded panels.
4-3-8 Page 4-76
Preparing a Precured Wet Layup Doubler or Edge Filler
Provides a method for preparing a precured wet layup repair doubler.
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4-3-3. WET LAYUP IMPREGNATION PROCESS
This process sheet describes the different equipment and procedures used to impregnate dry glass or carbon fabric with epoxy resin.
CAUTION
WET LAYUP MUST BE PERFORMED IN A CLEAN ENVIRONMENT, FREE OF DUST, OIL AND GREASE.
TO AVOID CONTAMINATION, MANIPULATE PARTS, EPOXY RESIN, AND FIBERS USING CLEAN COTTON GLOVE.
BEFORE HANDLING A SOLVENT, EXTINGUISH ALL FLAMES AND PILOT LIGHTS. KEEP PRODUCT AND ITS VAPOURS AWAY FROM HEAT, SPARKS, AND FLAME. DURING APPLICATION AND UNTIL VAPOURS HAVE DISSIPATED, AVOID USING SPARK PRODUCING ELECTRICAL EQUIPMENT SUCH AS SWITCHES, APPLIANCES, ETC.
1.0 PRELIMINARY REQUIREMENTS
CAUTION
AN EXCELLENT SURFACE PREPARATION IS ESSENTIAL TO ENSURE INTEGRITY AND DURABILITY OF A BONDED JOINT. EXTRA CARE IS TO BE TAKEN TO ENSURE THAT BONDING SURFACES ARE PROPERLY CLEANED AND PROTECTED FROM CONTAMINATION DURING ALL PHASES OF A REPAIR.
1.1 Cleanliness is to be carefully controlled through all phases of the preparation and bonding operations.
2.1 Glass fabric (C-404) style 7781, glass fabric (C-560) style 120, carbon fabric (C-255), or other material as required by applicable repair.
2.2 Wet layup adhesive as required in applicable repair.
2.3 Polyethylene film of 0.004 inch (0.10 mm) minimum thickness (commercially available), release film (C-256), or bagging film (C-257) or bagging film (C-564). Commercial polyethylene film is inexpensive and is recommended.
2.4 Cleaner: MEK (C-309) or acetone (C-316).
2.5 Process Sheet(s): Preparing and Mixing Two-part Epoxy Resin by Weight (paragraph 3-2-25) Installation of Copper Wire Mesh in Wet Layup Repair (paragraph 4-3-4)
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3.0 PROCEDURE
3.1 Clean working table, equipment, and tools with cleaner specified in 2.4.
NOTE
Refer to the model-specific SRM for the 0° orientation. If no 0° orientation is specified in model-specific SRM for part to be repaired, 0°orientation is parallel to longitudinal axis of helicopter (FWD/AFT direction) for glass fiber repair. For carbon fiber repair contact Product Support Engineering.
3.2 Fabricate templates using cardboard material for each ply of repair doubler maintaining a minimum of 0.75 inch (19.1 mm) overlap, or as specified in applicable repair, between end of damage and first repair ply, and between end of each subsequent repair ply. The 0° orientation and ply number shall be marked on each template to facilitate lay up. Ensure no ply overlaps under rivet, bolt, insert, or any other fastener. Number of plies to be as shown in Table 4-9 unless otherwise specified in applicable repair.
3.3 Get appropriate dry fiber reinforcement (i.e., glass fiber fabric style 7781, glass fiber fabric style 120, carbon fiber fabric, or other material as specified in 2.1).
3.4 Calculate amount of fabric required to fabricate all plies of repair doubler by arranging cardboard templates on work surface with 0° orientation properly aligned with approximately 1.0 inch (25 mm) buffer material all around. Do not use the last 1.0 inch (25 mm) on all sides of fabric since handling may have loosened fibers or strands of fibers. Measure maximum length and width of arranged templates. Cut length of fabric to required size taking into account constraints stated above.
NOTE
For repairs with small repair plies and/or small number of plies, it is possible to wet out all repair plies as a single piece of fabric to be cut as individual plies after fabric is impregnated in step 3.18. In this case, maximum size of fabric sheets should be 24.0 x 24.0 inches (610 x 610 mm) or a maximum weight of 0.265 pound (120 grams) of dry fabric, whichever results in smallest sheet dimensions.
3.5 If required, cut fabric to more manageable dimensions by cutting each individual ply in such way to maintain a minimum of 1.00 inch (25.4 mm) of bu

chapter 4 — fiber reinforced composite repairs

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