Integrity Service Excellence

Air Force Research Laboratory

Structural Adhesive Bonding

in the United States Air Force

ASC Fall Convention & Expo

19 OCT 2016

Jim Mazza

AFRL/RXSA

Materials & Manufacturing Directorate

Air Force Research Laboratory

2

Outline

• Air Force Research Laboratory

• USAF adhesive bonding background

• Materials & processes

• Testing

• Certification issues

• Metal bonding problems/challenges

• Rework and repair

• Related research

• Summary

3

Air Force Research Laboratory

Employees Civilian Military

Total 6,050 82% 18%S&Es 3,567 83% 17%

Mission

LEADING the discovery, development,

and integration of affordable warfighting

technologies for our air, space, and

cyberspace force.

Wright-Patterson AFB, OH

• AFRL HQ

• 711 Human Performance

Wing

• Sensors

• Aerospace Vehicles

• Materials and Manufacturing

Rome Research Site,

NYInformation

Arlington, VA

Office of

Scientific Research

Eglin AFB, FL

Munitions

Kirtland AFB, NM

• Space Vehicles

• Directed Energy

~$4.5 BillionAnnual Funding

Core ~$2B

Customer ~$2.5B

Maui Research Site, HI

London, UK

Santiago, Chile

Tokyo, Japan

Edwards AFB, CA

Fort Sam, TX

Arnold AFB,

TN

4

AFRL in AFMC

Air Force Materiel Command (AFMC)

5

AFRL/RX(Air Force Research Lab Materials & Manufacturing Directorate)

100 nm

6

Systems Support Division (RXS)

Merge systems engineering application expertise with materials and processes (M&P)

technology expertise to provide timely, effective solutions to user needs

Keep AF Systems Safe, Available, and Affordable

• MATERIALS INTEGRITY

BRANCH (RXSA)

Adhesives & Composites Team

Structural Materials Evaluation

Team

Electrical & Electronic Materials

Evaluation Team

• ACQUISITION SYSTEMS

SUPPORT BRANCH

(RXSC)

• MATERIALS DURABILITY

AND SUSTAINMENT

BRANCH (RXSS)

7

What is Structural Bonding?

• “Structural” bonding and “structural” adhesives are difficult to define

– Definitions differ between industries or end applications

– Some consider an adhesive “structural” if it has >1,000 psi shear strength

– No specific definition for structural bonding in the U.S. Air Force (USAF)

– No clear lines as to what is structural versus nonstructural, but obvious

examples of both can be readily given (the gray area is harder to define)

• Aircraft can contain a wide variety of adhesive bonding applications

– Joining load-carrying components (critical for airworthiness or noncritical)

– Bonds related to electronics or electrical applications

– Adhering items associated with interior trim or decorative features

– Attachment of brackets, clips, name plates, etc.

– Decals

• For this briefing: structural bonds are for load carrying components

– But not those bonded with sealants, silicones, pressure sensitive adhesives,

contact cements, etc.

8

USAF Metal Bonding

• USAF has a long history with structural metal-bonded joints

– B-58 (1950s design), F-111, C-141, C-5 with significant adhesive usage

– Much associated with honeycomb panels (aluminum core and/or skins)

– Also bonding on some missiles and munitions

• Little new construction involves metal bonding

– Metal honeycomb core virtually eliminated from new designs (corrosion prone)

• Very few safety-of-flight-critical metal-bonded joints

• Significant repair bonding for legacy platforms

– Repair of originally bonded structure (much of it honeycomb sandwich)

– Bonded doublers for fatigue & corrosion repair (not previously bonded)

• State-of-the-Art metal bonding not much changed since 1970s

C-5 Galaxy

C-5 aircraft contains ≈25,000 ft2 bonded structure

(flaps, ailerons, engine pylon panels, floorboards,

torque deck, fuselage, bulkheads, and ramps)

9

USAF Composite Bonding

• Bonding of advanced composites on USAF aircraft began with their

introduction 40+ years ago and has continued to varying extents

– F-15, F-16, F-22, F-35, B-1B, B-2, C-17

– Co-bonded structure as well as secondarily bonded structure

– Significant bonding with paste adhesives on remotely piloted aircraft (RPAs)

• Safety-of-flight-critical bonds (mostly RPAs)

• Repair bonding for legacy platforms

– Repair of composite structure

– Composite patches/doublers used to repair/reinforce metallic structure

• Significant push for new construction involving composite bonding

– Seen as enabling (necessary?) for efficient, cost-effective structures

10

Materials & Processes (M&P)

• M&P typically dictated by original equipment manufacturers

• Metallic and nonmetallic substrates

– Aluminum – typically 2000 and 7000 series

– Some titanium (usually Ti-6Al-4V) and even less steel/other

– Many composite substrates with carbon, fiberglass or other reinforcements

and any number of resins (mostly epoxy)

– Honeycomb core (metallic & non metallic); foam cores (usually closed cell)

• Adhesive types/forms

– Predominantly elastomer-modified epoxies (toughened epoxies) currently

– Nitrile-phenolic and epoxy-phenolic for older applications (some current)

– Bismaleimides and polyimides for higher temperature service currently

– Film, paste, and foaming (core splice) forms

– Significant use of epoxy paste adhesives for RPAs

11

Materials & Processes (M&P)

• Prebond surface preparations for composites

– Abrasion/cleaning with or without peel ply

– Emerging energetic methods (laser and ambient-temperature plasma)

• Prebond surface preparations for metals (typically with bond primer)

– Acid etches and anodization processes (production & major rework)

– Conversion coatings and coupling agents (mostly repair)

– Simple abrasion and cleaning processes (some repairs – undesirable)

– Phosphoric acid anodize (PAA) desired for aluminum prep

– Processes using Boeing-developed sol-gel solution (3M’s AC-130-2) for

aluminum repair, Ti, and stainless steel

• Many suppliers and products

– Most film adhesives from Cytec-Solvay Group, Henkel, 3M

– Most paste adhesives from Henkel (also Magnolia Plastics & others)

– Bond primers predominantly from Cytec-Solvay and 3M

– Specific products too numerous to mention

12

How Good is Good Enough?

1.00

1.10

1.20

1.30

1.40

1.50

0 200 400 600 800

Cra

ck

Len

gth

(in

)

Hours at 120°F & 98% RH

PAA / BR 127

GB/Silane / BR 127

GB/Sol-Gel / BR 6747-1

1.00

1.10

1.20

1.30

1.40

1.50

0 200 400 600 800

Cra

ck L

en

gth

(in

)

Hours at 140°F & 98% RH

100% coh

100% coh

0% coh

95% coh

• Wedge test conditioning at 140°F (60°C) and 95-100% RH can

be discriminating when the 120°F (49°C) wedge test is not

• Grit-Blast/Silane “fails” 140°F wedge test but performed well in

service for C-141 weep hole repairs for 10+ years

13

Composite Surface Preparation

black arrow – cohesive facture of adhesive

white arrows – fiber imprints

(unbonded – no fracture)

Polyester peel ply removal w/o abrasion

~15% void; >70% unbonded area

Peel ply imprint on ~57% of doubler surface

Peel

Ply

ImprintLight

Abrade

“Proper”

Abrade

Grit Blast

Surface preparation is key for

bonded repairs; typically

abrasion & cleaning with or

without peel ply; much more

difficult to conduct on-aircraft

14

Testing & Evaluation

• Many test methods per ASTM International & other standards

– Shear and peel tests for initial screening (ASTM D1002, ASTM D3167, more)

– Wedge test to screen metal bonds for moisture durability (ASTM D3762)

– Thick adherend shear stress-strain key for design/analysis (ASTM D5656)

– Flatwise tension & peel for honeycomb structure (ASTM C297, D1781)

– Other methods for composite bonded joints

• Qualification of structural adhesives can be very expensive

– Typically B-basis allowables (3 batches, 6 specimens per batch, minimum)

– Testing at temperature extremes (incl. hot/wet) and with fluid immersions

– Bonded joints in component and full-scale (static & fatigue) test articles

Honeycomb Climbing Drum Peel Tensile Lap Shear Metal DCBs (outdoor exposure) Stressed Bonded Doublers

15

Adhesive Bonding Based on Trust

• Develop validated designs and processes

• Assure integrity of materials prior to processing

• Assure validated processes are followed: trained personnel & quality control (QC)

• Nondestructively check prebond surfaces

• Nondestructive inspection (NDI) for disbonds

• Process control (witness) coupons (somewhat valuable at times)

• Proof tests (perhaps for certain applications)

This Approach Does Work When Properly Applied

Failures likely if first three steps above are not properly executed

Remaining steps help reduce risk and build confidence

Trust the bond is good even though it cannot be proved

16

JSSG-2006 Requirements for

Bonded Structures

Paragraph 3.10.5 Static strength

• Sufficient static strength shall be provided in the airframe

structure for reacting all loading conditions loads without

degrading the structural performance capability of the airframe.

Sufficient strength shall be provided for operations, maintenance

functions, and any tests that simulate load conditions, such that:

a. No detrimental deformations at 115% of design limit load (DLL)

b. No rupture or collapsing failures at design ultimate load (DUL)

c. Use nominal dimensional values

d. Bonded structure shall be capable of sustaining the residual

strength loads of 3.12.2 (i.e. DLL) without a safety of flight

failure with a complete bond line failure or disbond.

JSSG-2006: Tailorable Joint Service Specification Guide for Aircraft Structures (contains

requirements, rationale, guidance, lessons learned, and verification)

The USAF Certification Authority sees continuing airworthiness as the most

difficult issue facing certification of safety-of-flight-critical bonded joints

17

USAF Bonding Problems

• Many metal bond failures have occurred in service

– Interfacial failures due to inadequate surface preparation

– Corrosion of core due to moisture ingress

– Poorly bonded repair doublers combine the above

• Some adhesive degradation for older metal bonds

– Discolored, dry, cracking

– May not fail prior to interface degradation

• Adhesive availability for legacy platforms

– Long lead times, large minimum purchase reqmts, discontinued products, etc.

– Affects ability to repair components in accordance with technical order data

– Sole source issues (drove supply chain change from GSA to DLA)

• Concern regarding impact of environmental regulations

Few composite bonding problems in service; more stable interfaces

18

Additional Adhesive Bonding Issues

• Training and certification of mechanics is not cohesive or thorough

– Human element is critical for successful bonding; often the cause of failure

– No standards for certification of mechanics (ad hoc for some applications)

• Sensitive bonding processes require extreme process control to ensure

good bonds in a production environment*

• A reliable NDI technique does not exist to validate good bonds*

– Some progress being made; a practical method would be helpful

• Effects of long-term environmental exposure on bond strength is not

easily predicted*

– Moisture attack at the polymer-metal interface is key

– Wedge test qualitatively assess moisture durability; no quantitative correlation

with service life

• Difficult to meet USAF damage tolerance requirements*

* Certification challenges per USAF certification authority

19

Rework and Repair

• Most adhesive bonding in the Air Force is for rework and repair

– Depots often rebuild components (especially honeycomb panels)

– Depots and operational units conduct bonded repairs on and off aircraft

• On-aircraft repair is especially challenging

– Few good metal surface preparation choices (AC-130-2 processes are best)

– Adhesive cure temperatures limited and hard to achieve/maintain (RT cure?)

– Pressure application typically via vacuum bag (volatile issues)

– Often restricted access for all operations

• Operational units have additional considerations

– Limited facilities, equipment, and trained personnel

– Standard repair sizes usually quite small

Scarf 30 Plies Deep

20

AFRL/RX Research

• No real adhesives formulation research for well over 30 years

• Process development; M&P and equipment test/evaluation

– Led DoD/Industry team that developed sol-gel surface preps

– Ongoing evaluations for bond primers, new adhesives, repair bonding

• Currently supporting industry efforts to modify ASTM D3762 standard &

replace ASTM D5656 (KGR-1 extensometer) for stress-strain

• Other related efforts

– PABST program (1970s)

– Composites Affordability Initiative (CAI)

– Evaluations of bonded repairs retired from service

– Surface Analyst device for prebond surface prep quality control (BTG Labs)

– Laser bond inspection

– SERDP Project WP-2144 related to the role of bond primers

– New nonmetallic removal tools (adhesive removal)

ASTM D5656 Specimen

Ball Chain Leaf Spring

KGR-1 Extensometer

21

USAF PABST Program

• Primary Adhesively Bonded Structure Technology

– USAF Flight Dynamics and Materials Labs; McDonnell Douglas

– Mid to late 1970s; approximately $20M (then-year dollars)

• Objective: dem/val significant improvements in cost, weight

reduction, integrity, and durability of primary fuselage structure by

application of adhesively bonded joints

• Multidisciplined approach to validation of bonding “primary” structure

– Design philosophy

– Al surface prep, adhesive & primer system (PAA/BR 127/FM 73M)

– Tests for validating design and processes

• Technically successful, but did not raise confidence to the level

needed to bond safety-of-flight-critical structure

• In many ways, still defines metal bonding state of the art (SOA)

22

Composites Affordability Initiative

• Roughly 10-yr ManTech effort (AF, Navy, Industry) completed in 2007

• Significantly reduced risk for using vacuum assisted resin transfer

molding (VARTM) to produce aerospace quality parts (no autoclave)

• Showed robustness of “Pi” bonded joint

– Two bondlines – robust, strong, structurally redundant

– Takes advantage of adhesive’s excellent shear strength

– Reduced assembly times (vs. fay surface bonds)

• Conducted bonded structures demonstrations

• Developed tools

– Laser bond inspection device - feasibility shown to quantify bond quality

– Validated improved structural analysis tools for bonded joints (static & DaDT)

• Helped understand barriers to composite usage via certification plans

and generated huge database & tech guideline manuals

Pi Joint

23

C-141 Weep Hole Repair Evaluation

Outer Mold Line

(OML) Patch

Riser Patches

Installed Inside Wing

(over weep holes)

Test Specimen

• About 120 aircraft repaired (~770 repairs & ~2300 bonded patches)

• 52 valid test specimens with service history (2097 - 7574 flight hours)

• 47 specimens had detectable cracks; none appeared to grow in service

• No evidence of degradation due to service environment

• No specimens failed at loads below design ultimate stress (DUS)

– Two specimens began to disbond or delaminate just below DUS; one due to

peel ply in patch near outer ply, but cause for other is unknown

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C-141 Weep Hole Repair Evaluation

C-141 Bonded Repair Residual Strength

0

50

100

150

200

1 3 5 7 9

11

13

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

47

49

51

53

Specimen Number

Pe

rce

nt

De

sig

n U

ltim

ate

Str

es

s (

%D

US

)

Requirement (Design Ultimate Stress)

Failure Locations & Modes

Panel @ discontinuity away from repaired weep hole (fracture or net section yield)

Repair @ Bondline or within patch (prior to overall specimen failure at panel discontinuity)

▼ Panel @ Saw Cuts (atypical) - INVALID TESTS

▼▼

Design Limit Stress

25

C-141 Weep Hole Repair Evaluation

• Goal: Increase trust in bonded repair technology to permit some “credit”

to be taken for bonded repairs to safety-of-flight-critical structure

– To support the concept of permanence; reduce inspection burden

• All specimens failed in metal away from the patches

– Did not change approach to bonded repair certification

– Did not evaluate patch or bonded joint

• AFRL now conducting additional tests in an

attempt to learn if patches or bonds degraded

while in service

Full-field strereo-optical strain

measurement; 3-D strain mapping

Specimens failed at weakest link:

adjacent unpatched weep holes or

rib clip holes

26

BTG Lab’s Surface Analyst

• Two ongoing AFRL efforts to further develop the Surface Analyst

– One focused on depot applications and the other on manufacturing needs

• Intent is to provide a simple, cost effective, and reliable metric of

surface preparation quality (readiness for bonding); increase confidence

Nozzle Port

Water Drop

Stream

Bond Surface

Unit photographs surface before and after depositing water drop, then performs

image analysis on drop size and correlates it to a contact angle measurement,

which indicates degree of wetting (increased wetting should be better for adhesion)

High

er con

tamin

ation

levels

Hig

he

r co

nta

min

atio

n le

vels

Hig

her

Co

nta

ct A

ngl

e

High

er Co

ntact A

ngle

Wettability is Profoundly Sensitive to Surface Prep Quality

Surface AnalystTM

COTS

27

Laser Bond Inspection (LBI)

OBJECTIVES

• Validate LBI can reliably measure composite bond strength integrity

• Develop strategy for LBI technology transition & its use to support certification

APPROACH

PROBLEM

LBI System

Adhesive Bonding is Based on Trust

• The ability to inspect bonded joints is

considered a HIGH priority within the

composites community to enable

integrated structure

• Nondestructive inspection (NDI)

techniques are not adequate for

determining bond strength to allow for

management of bonded structure

• Two efforts led by Boeing (with LSP Technologies & others)

• Establish laser fluence thresholds and inspection levels for nominal bond strength

• Develop methodology and protocols to validate laser bond inspection as a bond

quality tool for any material system

• Identify tech gaps for implementation; develop procedures; demo

28

SERDP Project WP-2144

• Goal is to understand role of bond primers

– What is required of the inhibitor package and are chromates necessary?

– Better understanding could lead to improved moisture durability testing

– Results could guide nonchromated primer implementations (reduce risk)

• DoD/Industry team led by Naval Air Systems Command (NAVAIR)

– Technical effort completed; report due by end of 2016

– Goal was not met, but significant knowledge was gained (including testing)

SERDP – Strategic Environmental Research and Development Program

Metal DCBs at Outdoor Exposure Site

Surface preparation is by

far a more important factor

than bond primer inhibitor

package for long-term

moisture durability of metal

bonded joints

29

Nonmetallic Removal Tools

Gap Filler Removal (GFR) Bit Removing Fastener Fill

Torlon Scraper Blades (TSB)

Gap Filler Removal (GFR) Blades and Adapter

Discs Used with Oscillating Tool

Tools fabricated from fiberglass-reinforced

Torlon® will be available from Performance

Plastics, Ltd. in Cincinnati OH (Cage Code

01JC6) – GFR bits/blades & TSB in Sep 2016

GFR on COTS Handle (Pneumatic Tool Also Available)

30

Summary

• USAF aircraft have a wide variety of adhesive bonding applications for

both metallic and nonmetallic substrates

– Many different M&P requirements (OEM driven)

– Safety-of-flight-critical bonds are rare

– Failures occur when approved processes are inadequate or not accomplished

properly (surface preparation is biggest concern)

• Several challenges exist

– Developing trust in bonding process to gain certification for critical bonds

– Nondestructive methods do not exist for identifying weak bonds

– Obtaining qualified materials for legacy aircraft

– On-component repair of high-temperature structures

• Ongoing research attempts to address some of the needs

31

Questions?

REVOLUTIONARY · RELEVANT · RESPONSIVE