Kay and Pete FAA Wksp Rev A.ppt5/28/2004, 1
Critical Materials & Processes Bonded Joint Issues
Pete Van Voast425-237-3178
[email protected] Blohowiak
425-237-4657 [email protected]
FAA Bonded Structures WorkshopJune 16-18, 2004
Kay and Pete FAA Wksp Rev A.ppt5/28/2004, 2
Topics
• Overall philosophy• Metal Bonding
Composite Bonding– Surface preparation– Primers– Adhesives– Repair– Inspection
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Philosophy
• The substrate (e.g. specific alloy or composite), surface preparation, and adhesive are a unique system
• Boeing takes a system approach to the qualification and verifies all new products/processes by extensive compatibility tests
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Metal BondingAluminumTitaniumStainless
Other
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Boeing’s Historical Bonding Methods
PAA core to carbon skins
PAA corepour coatCR core
Acid etched foil
next gen. adhesives EA9696,
FM94AF555
BMS 5-137 better
toughness
toughened epoxy nitrileBMS5-101BMS 5-104
250 F cure epoxies
BMS 5-70 to BMS5-51
and 5-80
BMS5-10 cold bond;350 F cure
nitrilephenolicsBMS 5-42
non-chromeLow VOC 0 VOC CIAP
CIAP BMS 5-89 (‘69)
dissolvedadhesive
resin
CWR;Non-Cr, 0V deox; sol gel repair
PADPAA; PANTA;Wedge/
DCB
FPL
2000+1990’sD681704
1980’s1970’s1960’sD16925
1950’s
surf
ace
prim
erad
hesi
veco
re
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Adhesive Bond Failures
737 body skin waffle doublerwith unanodized area
Corroded core
Really corroded coreCorrosion products on repair doubler
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Types of Prebond Surface Preparations
• Mechanical Deoxidation– Grit Blasting, Sanding
• Anodizing– PAA, CAA
• Chemical Etching– FPL, CAE, HF/Alodine, Pasa Jell 105/107
FCHAE, Phosphate fluoride• Functional Coatings
– GBS, Sol-Gel
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Manual Abrade / Solvent Wipe
• Initial adhesion often very good• Abrasion or roughening can give some mechanical
interlock• Deoxidation exposes fresh metal for bonding• Fails at interface under hot/wet conditioning
– Hydration of aluminum oxides– Corrosion– Secondary bonding interactions
- Grit Blasting- Sanding- ScotchBrite Abrasion
Abraded repair area
Die grinder/sanded surface
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Anodizing
• Phosphoric Acid Anodize• Chromic Acid Anodize• Boric Sulfuric Anodize• Thin Film Sulfuric Anodize
• Microstructure promotes wetting of adhesive resins• Mechanical interlock by flow of primer/adhesive into
pores resulting in composite interphase• Good under hot/wet conditioning
– Phosphoric acid anodizing stabilizes the hydration of aluminum oxides
– Corrosion control at interface
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Phosphoric Acid Anodize
• BAC 5555 issued in 1974– US Patents 4,085012 & 4,793,903
• Early issues– Disbonded PAA waffle doublers
(tear straps) on 737– “sporadic escapements” of panels
with insufficient PAA– 747 Section 41 internal doubler
disbonding
POLYMER
OXIDE
METAL
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Chemical Etching
• Deoxidizes the metal surface• May leave chromates that
retard corrosion at the interface
• May provide some surface roughening
• Forest Products Laboratory Etch• Chromic Acid Etch• Pasa-Jell 105• HF/Alodine
• Health/environment• Corrosion due to
entrapped acid• Embrittlement of high
strength steel• Performance• Temp
Issues:
Chromic acid etching process on underwing repair
Pasa-Jell treated surface
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Boeing Service HistoryOriginal Bonding Process– FPL Etch
• FPL etch - industry standard until mid 1970s• Numerous service bulletins and Airworthiness
Directives (ADs) due to bond durability problems– 747 Section 41 flat-sided skin panel (SB 747-
53A2321), L/N 1-430– 747 tear straps in section 46 (SB 747-53A2279),
L/N 1-230– 747 Section 41/42 disbond inspection of large
radius areas (SB 747-54-2406), L/N 1-430
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Functional Coatings
• Chemically bond adhesive to surface• Crosslinking of resin to functionalized silane• May be enhanced by surface roughening• Performance highly dependent on
pretreatment
• Silane• Australian silane process• Grit-blast/silane (GBS)
• Sol-Gel• Boegel-EPII / AC-130
Sol-gel coated metal surface
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Sol-Gel Process Robustness
• Application procedure simple• Sol-Gel Process will “work” under
a variety of pretreatment conditions
• Best results achieved when using recommended abrasion processes
• Works with and without primer• Paste and film adhesives
• Must have clean surface• Only optimized for use with
certain bond primers• Better results with recommended
abrasive procedures
Issues:
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Issues in Qualifying New Surface Preps
• Scope of qualification– Cost– Meet all design scenarios
• Consistent process among OEM and suppliers• What kind of specification to write
Qualification Issues
• What criteria do new surface preps have to meet?• How do you demonstrate 30 year durability in the lab?
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Adhesive Bond Primer• Compatibility with
surface preparations• Bake vs. No-bake• Primer thickness
window• Low VOC bond primer
development• Non-Chrome bond
primer development
• How good is good enough?• What are the real criteria for implementation?• Extent of implementation
Qualification Issues
•Nitrile rubber based primers (e.g. BMS 5-42) do not contain any chrome•Epoxy based primers (e.g. BR127, BR6747-1) do contain chrome•Long term exposure of DCB specimens in various environments resulted in equivalent behavior of chrome and non-chrome.•Chrome bond primer has a benefit in paint applications without additional primer.
Cr vs. Non-Cr Primers
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Adhesive Issues- Engineering
• Thermal Capability• Toughness• Shear• Filleting• Moisture effects (hot/wet)• Bondline Thickness
Qualification Issues
• Cost of allowables• Long-term durability
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Metal Bond Process Inspection
• Surface Prep Verification• First Part Qualification
correlations– Prefit– Verifilm– TTU– Destructive Test
• TTU• Tap Testing
C o lo r p a r a m e t e r s b * v s . a *7 0 7 6 - T 6 , G BS /BR- 1 2 7
- 2 5
- 2 0
- 15
- 10
- 5
0
5
10
15
2 0
2 5
-6 -4 -2 0 2 4 6
a *
0 .3 9 m i l0 .3 1 m i l
0 .2 5 m i l0 .13 m i l
0 .12 5 m i l
0 .0 6 3 m i l0 .0 6 0 m i l
Wet vs. Dry Scotch-Brite Scuff(Sol-Gel, BR-6747)
0.0
0.5
1.0
1.5
2.0
2.5
MEK,WS,W
B,SG
MEK,DS,W
B,SG
MEK,DS,D
ryWipe
,SG
MEK,Rolo
c,MEK,S
G
Tota
l Cra
ck E
xten
sion
[inc
h]
1
10
100
1000
OSE
E, %
Coh
esiv
e Fa
ilure
ExtensionOSEE (pre SG)% Cohesive Failure
Colorimeter to measure primer thickness
OSEE to measure surface preparationQuestions
• How do you measure success of the process steps
• Kissing bonds
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Repair vs. OEM
• Typically better controls at manufacturing level– Environmental controls– QC/inspection methods
• Fewer tools/materials available in field
• Training/certification• New clean parts vs.
dirty old parts• Access to repair area• Potential damage to
areas adjacent to repair
Why we repair…
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Composite Bonding
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Philosophy
• The substrate (e.g. specific alloy or composite), surface preparation, and adhesive are a unique system
• Boeing takes a system approach to the qualification and verifies all new products/processes by extensive compatibility tests
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Composite vs. Metal bond durability1500 psi Cyclic Fatigue in 140F/100% RH
0
500
1000
1500
2000
2500
3000
PAA/Aluminum
Peel ply/composite
Cycles to failure
Test stopped
1500 psi lap shear cycles 140F/100% RH
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Composite Surface Preparation Methods
• Peel Ply (baseline)• Grit Blast• Peel Ply + additional
surface preparation
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Peel Ply Surface Preparation
• Widely used at Boeing Commercial• No “universally” functional peel ply• Sensitivity to using the wrong peel ply • Cleanliness and max delay requirements• Excellent durability• Specification and PCD control for critical
applications
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Mechanical Properties of Composite Bonds (Optimal vs. not optimal peel ply)
Optimal Peel Ply Not Optimal Peel Ply
RT Lap Shear (ksi)DCB (in-lb/in2)
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Peel Ply Affects Failure Mode
OptimalPeel PlyCohesive FailureMode
Not OptimalPeel PlyAdhesive FailureMode
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Using the Wrong Peel Ply Results in Adhesive Failure Mode
Peel Ply PatternReplicated in Adhesive
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Grit Blast Surface Preparation
• Provides an excellent surface for bonding• Process control issues (pressure, distance,
grit control etc.)• Economics (process time, grit
management)
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Effect of Additional Surface Treatmenton Mode I Fracture Toughness
as is as is + additionaltreatment
GIC
(in-
lb/in
2 )
Bad peel ply 1Bad peel ply 2No peel ply (smooth)Good peel plyAdhesive
failures
Interlaminarfailure
Mixed failure modes
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Adhesive Issues - Manufacturing
• Stability – Long Out-time and Storage• Appropriate Tack• Rheological compatibility and stability• Compatibility with autoclave and vacuum
curing
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Process Control Document
• Controls formulation, manufacturing and testing of product
• Controls changes to product• Statistical process control
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Adhesive Issues- Engineering
• Prebond humidity• Thermal Capability including hot/wet• Toughness• Shear• Filleting• Bondline Thickness
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Effect of Prebond Humidity on Adhesive performance
• Different adhesives have different responses to prebond humidity–Morphology –GIC–Lap shear–Tg–Kinetics
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Moisture Content vs. Environment10 ply carbon epoxy laminate
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 10 20 30
Days
Perc
ent M
oist
ure
72F/0%72F/50%92F/50% 160F/100%RH
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Effect of Prebond Humidity on Adhesive Morphology
Adhesive A bonded after 28 days at 92F/50% of composite prebond exposure
Adhesive A bonded immediately
10 µm
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Effect of Pre-bond Humidity on GIC
11.5
22.5
33.5
44.5
55.5
0 0.2 0.4 0.6 0.8 1 1.2
Prebond Moisture Content (%)
GIc
(In-
lb/in
ch2) A
BCPoly. (B)Poly. (A)
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Effect of Pre-bond Humidity on 270oF Lap Shear
0500
10001500200025003000350040004500
0 0.4 0.6
Shea
r (p
si)
ABC
Prebond Moisture Content (%)
-42%-44%-37%
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Summary => New Directions
• Surface Preparation– Boegel-EPII/AC-130 for metals– Grit Blast or enhancements to peel ply for
composites• Adhesives
– Stability in out-time and prebond humidity• Inspection
– Methods of detecting weak bonds
