Presented by: Louis C Dorworth Abaris Training Resources, Inc

Louis C. (Lou) Dorworth Division Mgr.-Direct Services Abaris Training Resources, Inc. Reno, Nevada – USA

• Material & Process Engineer • Manufacturing Engineer • Tool Design Engineer • Educator

This is an Interactive Session-Please Feel Free to Ask Questions

as We Go!

Correctly select, mix and/or apply

adhesive

Apply uniform clamping pressure 5-50 psi

Control the bondline thickness

Properly prepare substrate surfaces

Properly cure the adhesive

Terminology…

• Adhesion: the forces or mechanisms that keep the adhesive attached to a substrate, the term refers to all adhesion mechanisms or forces located in a thin layer (boundary layer) between the substrate and the adhesive itself.

• Cohesion: all the forces or mechanisms that

hold together the adhesive itself.

Illustration: www.adhesiveandglue.com

• Chemical Bond: the attraction and attachment between atoms, defined by the behaviors of their outermost electrons.

• Covalent Bond: sharing of electrons in which the positively charged nuclei of two or more atoms simultaneously attract the negatively charged electrons that are being shared between them. • Polar and non-polar covalent arrangements

• Ionic Bond: a chemical bond in which one atom loses an electron to form a positive ion and the other atom gains an electron to form a negative ion. • The term "ionic bond" is given to a bond in which the ionic

character is greater than the covalent character

Two atoms with equal electro-negativity will make non-polar covalent bonds such as H−H.

An unequal electronegative relationship creates a polar

covalent bond such as with H−C.

Illustration: www.adhesiveandglue.com

Ionic bonds are formed due to the attraction between an atom that has lost one or more electrons and an atom that has gained one or more electrons.

Image Courtesy of Garland Science

• Technical definition: Bonding that involves a mechanical constraint preventing two parts of a molecule from separating, rather than a chemical linkage based on transfer or sharing of electrons.

• Practical definition: A bond formed by keying or interlocking into mechanical features on a surface that bind an adhesive in a manner such that it is physically resistant to adhesive failure.

• Cocuring: The act of curing a composite laminate and simultaneously bonding it to some other uncured material, or to a core material such as balsa, honeycomb, or foam core • All resins & adhesives are cured during the same

process

• Cobonding: The curing together of two or more elements, of which at least one is fully cured and at least one is uncured • Requires careful surface preparation of the previously-

cured substrate • Additional adhesive may be required at bondline

interface

• Secondary Bonding: The joining together, by the process of adhesive bonding, two or more pre-cured composite parts, during which the only chemical or thermal reaction occurring is the curing of the adhesive itself.

• Requires careful preparation of each previously cured substrate at the faying surfaces

• Usually requires well designed fixturing to align & clamp parts during processing

• Re-heating previously cured substrates can be risky

Surface Preparation of Composites

• Goal:

• Raise the surface-free energy of the composite substrate to enhance wetting of the surface and to facilitate molecular (chemical) attachment to the surface

• Raise surface energy without damaging fibers at the laminate surface

If the surface tension value of the liquid is greater than the surface-free energy value of the substrate

the liquid molecules stay bound together

Poor wetting means a poor bond!

When the surface free energy value of the substrate is higher than that of the liquid it

allows the liquid to uniformly wet the surface

Wetting is important to achieving a good bond

OR

If contact angle is > 90° then poor wetting is achieved If contact angle is < 90° then good wetting is achieved

• Engineered for use on the shop floor • Quantifies surface treatment level • Immediate indication of surface energy level • Replaces water-break test methods • Minimal risk of surface contamination • Logs data for statistical process control • Uploadable

• Not the Goal:

• Creating a mechanical or rough surface! • Scratches or grooves can be sites for gas

entrapment and/or voids that are susceptible to post-process moisture uptake.

• A rough mechanical surface can actually effect uniform adhesive wetting!

• Damaging fibers! • Damaged fibers = damaged structure

• Peel ply fabrics • Dry or prepreg versions applied during layup

• May not Be viable for high rate (automotive) production

• Abrasion Methods • ScotchBrite™ or sandpaper abrasion • Grit-blast with alumina grit or other abrasive media

• High risk method - somewhat operator dependent • May be adapted to robotic process to mitigate human factor

• Laser Ablation

• Plasma Surface Treatment

Six different depths of ablation demonstrated on sample panel

Samurai linear system by DPSS for production line applications

• Chemically modifies the outermost surface of the composite substrate

• Clean the surface without damaging fibers

• Creates select chemical

groups on the surface of the material that actually enhance chemical bonding

• Different gases are used to achieve different surface chemistries.

Photo by Eric Hamburg-www.aerospace.org

Hand held plasma wand used to treat localized composite surfaces *Can be automated or attached to a robot for high rate production

• Non-coated Nylon or polyester fabrics • Leaves no trace contaminates on part surface • Does not always peel off easily

• Release treated Nylon or polyester fabrics • Can transfer release agent to part surface

• P.T.F.E. Coated Glass Fabrics • Easy to remove from part surface with low risk of

damage to the part • Produces a low energy surface on the composite

due to inwardly-directed polarity of the developing matrix surface during cure

From report: DOT/FAA/AR-06/28 The Effect of Peel-Ply Surface Preparation Variables on Bond Quality

Wettability envelope for PFG Nylon (blue) and Polyester (yellow) peel ply surfaces on BMS 8-276 CFRP laminate surfaces

Cross-section through peel ply on surface of laminate

Heavy Yarns of Peel Ply Fabric Brittle Resin Matrix

Remove the peel ply from the surface

Peel ply leaves small fractured peaks of resin

on the surface

Would you want to bond to this surface?

• Prepreg Peel Plies

• Coated with a medium modulus adhesive Vs. a high modulus matrix resin • Creates a desirable surface for adhesive bonding

• Made with a fine, tightly woven plain weave polyester fabric • Reduces size of resin “peaks” on the surface • Results in smaller, more uniform peaks that are not

fractured

Prepreg peel ply leaves smaller un-fractured peaks of medium modulus (not brittle) adhesive on the surface

You may want to bond to this surface!

You would rather bond to this surface, but at the risk of damaging fiber!

Abrading the peel ply surface with ScotchBrite or other abrasive media can raise the surface free energy of the surface

Total micro-surface of plastic matrix available to prepare for adhesive bonding

Surface detail

Composite Laminate

Abrasion through the available matrix results in a greater amount of fiber exposed at the surface

Loss of fiber = loss of structure, however, the bigger question is what effect does exposing fiber have when subsequently trying to adhere to this surface with an

adhesive? What about wettability?

Abrasion through the outer surface finish on filaments reduces the adhesion potential due to removal of sizing and exposure of raw fiber at the resulting surface

• Objective: Remove dust and debris from bonding surface without inducing contamination

• Solvent wipe with clean cheesecloth or approved wipes • Double wipe method often specified to remove residue • High risk of inducing moisture or other contaminants onto freshly

energized/slightly porous composite surface

• What effect does wiping with solvent have on the surface-free energy of the freshly prepared composite surface?

• Alternative: Dry wipe with clean cheesecloth or approved wipes • Multiple wipes may be required to remove dust sufficiently • Low risk of inducing moisture or other contaminants

Surface Preparation of Metals

• The same as secondary bonding except with metal substrates instead of cured composite substrates.

• Sometimes composites are bonded directly to metals using a cobond or a secondary bonding process • Metals require very stringent surface preparation including

potentially applying a corrosion inhibiting primer prior to bonding to obtain long term bond-durability at the metal oxide interface

• Care must be taken when bonding carbon to metal as galvanic corrosion can occur in the metal substrate • Aluminum has a very high corrosion potential with carbon as it is

very far apart on the galvanic scale

Material Galvanic Scale Anodic Clad Aluminum 7075 10 Clad Aluminum 2024 9 Aluminum 7075 – T6 9 Cadmium 8 Aluminum 2024 – T4 7 Wrought Steel 6 Cast Steel 6 Tin 4 Brass 2 Copper 2 Nickel 1 300 Series 18-8 CRES 0 Titanium 0 Carbon Fiber 0 Cathodic

• Clean Metal Surfaces • Vapor or solvent degrease • Acid or alkaline cleaner

• Obtain a fresh metal oxide layer • Abrasion or chemical etch/prep

• Increase Surface Area/Mechanical Surface • Chemical or acid-etch bonding surfaces

• Phosphoric Acid Anodization (PAA) for aluminum

• Chemical Coupler Surface Treatment • Conversion coatings

• Sol-Gel technology

• Corrosion Resistant Primer • Primer is necessary to preserve the freshly treated surface

• Provides resistance to hydrolysis

• Example - steps to aluminum anodization:

• Clean in (non-etching) alkaline chemical cleaner to remove all dirt, oils, and other contaminants.

• Rinse thoroughly to remove residual contaminants. • Deoxidize or treat with chromic-sulfuric acid etch. • Rinse again to remove residual contaminants. • Anodize with phosphoric acid etch.

• 10-12% phosphoric acid solution, 6-10V DC, 2-7 amps per sq ft of area.

• Rinse again to remove residual contaminants. • Dry and apply primer within 2 hours. • Cure primer and cover to prevent contamination.

Photo courtesy www.Linetec.com

Typical Phosphoric acid tank line

Courtesy ASM Materials Handbook, Vol 3

Sol Gel Chemistry

Illustration courtesy of Kay Blohowiak

Adhesive Selection & Application

• Epoxies • Wide range of high-strength adhesives available

with a variety of curing & service temperatures

• Polyurethanes • Tough/abrasion resistant • Good low-temperature adhesion properties

• Modified Acrylics/Methacrylates • High strength and elongation properties • Bonds well to thermoplastics

• Liquids:

• Viscosities typically range between 100-6000 cps

• Generally works best in thinner bondlines and provide for a higher degree of direct load transfer than pastes • Effective thickness range: .002-.010 inch • Can run out of thicker bondlines with too low of a viscosity

• Liquids tend to be more brittle and less resistant to peel and cleavage loads than pastes or films

• Often “liquid” adhesives are categorized as “pastes” without distinction by the various adhesive manufacturers

• Pastes

• Paste adhesive viscosities typically are > 10,000 cps

• Generally work better in slightly thicker bondlines • Effective thickness range: .005-.020 inch • Thicker shim or gap filling applications are not necessarily

considered structural – sometimes used with fasteners

• Different fillers offer a wide range of properties • Minerals, rubbers, thermoplastics, & metals are common

• Pastes usually do not wet-out on plastic substrates as well as liquids, due to the influence of the added filler

• Film Adhesives

• High-performance structural prepreg film adhesives • Stored frozen & thawed to room temperature before use • Requires an elevated temperature cure cycle

• Different carriers for maintaining bondline thickness control • Woven scrim cloth • Knit carrier • Non-woven (mat)

• Typically carriers are made of treated Polyester or Nylon fibers

Applying Film Adhesives

Simple to apply along the faying surface of one or both substrates that are to be joined Heat may be required to form some films to complex

shapes Knit carriers may form easier than non-woven carriers

Applying Liquid & Paste Adhesives

Goal: to apply slightly more adhesive than required and close the joint in a timely fashion Provide enough adhesive along the joint to do the job

*Refer to application template design sketch Excess adhesive = excess weight

• A freshly energized surface will try to stabilize over time and subsequently loose the desired effect • The surface takes on H2O and other contaminants when left

exposed to the normal shop/clean-room environment

• Adhesive left open on the surface for extended time may also be affected by the environment (H2O & CO2) • Amine Carbonate formation can inhibit most room-

temperature curing epoxy adhesive systems • *Refer to Hysol EA 9394 Open Time Considerations doc.

Part

Template Adhesive

Application with a comb template allows for uniform distribution of adhesive and redistribution of amine carbonate formation that may

occur on open adhesive surface

• Consistent bondline thickness of the adhesive layer is critical, without uniform thickness the joint strength is only as good as its weakest point • Options for thickness control media:

• Micro-Beads (mixed in the adhesive) • Scrim Cloth • Knit Carriers • Non-Woven Carriers (Mat)

Adhesive Carriers

Non-Woven Mat Knit Carrier

Scrim Carrier

Microbeads

Precision .005 inch Diameter Microbeads ~Tolerance±0.0002 inch

• Uniform clamping pressure is required to achieve good wet-out and optimum bond strength

• Added force contributes to free-energy exchange • Typical bonding pressures range from 5-50psi • Mechanical clamping requires sturdy fixturing • Vacuum bagging can provide uniform pressure

• Vacuum bagging can also cause micro-porosity in the joint due to frothing of the adhesive under vacuum

• Used to locate and secure mating parts for co-bond or secondary bonding operations

• Design may include mechanical clamping devices and/or detail locator provisions

• Designed to provide dimensionally accurate assembly at maximum process temperature

Bond Assembly Fixture or Jig

Source: Plasan Carbon Composites

Inner and outer panels are brought together in a fixture and adhesively bonded and cured via hot-air impingement.

Photo courtesy Taylor Design engineering Ltd

Fully automated fixture to bond all component parts to form a complete Aston Martin fender.

To achieve maximum performance and ultimate structural & thermal properties, the adhesive must be properly cured and/or post-cured Room temperature curing systems usually take

several days to achieve good structural properties The ISO standard for room temperature is 77° F (25°C)

Elevated temperatures lower the adhesive viscosity

and enhancing the wet-out (energy exchange) characteristics

High performance adhesives usually require an elevated temperature cure and/or post-cure for best performance

Know the Cure Properties:

Viscoelastic properties of an epoxy specimen

• Unidirectional Tape • Should run directly across the joint for best results

• Normally not recommended as faying layer in joint design

• Bidirectional woven fabrics • Plain & Twill weaves

• Generally good surface materials for faying layer in joint design • Harness-satin weaves

• Warp/fill face orientation dominance must be considered

• Multiaxial stitched fabrics • Functions like a unidirectional tape dependent on faying layer

orientation

• Nonwoven mats • Damping & diminished load transfer through mat surface layer

Effect of UD Fiber Forms at the Faying Surfaces

Uni-directional fibers at faying surfaces: Orient fibers across the bonded joint in primary load axis

Effect of 0° to 90° UD at the Faying Surfaces

Effect of 90° uni-directional fibers at bond joint

90° Fibers tend to “roll” off of the underlying ply of the substrate

Effect of Bi-directional Fiber Forms at the Faying Surfaces

Bi-directional plain-woven fabrics at faying surfaces provide uniform load transfer across bonded joint

Bonding to Core Materials

Relative Stiffness 100% 700% 3700%

Relative Strength 100% 350% 925%

Relative Weight 100% 103% 106%

t 2t 4t

Note the relatively high increase in stiffness and strength compared to the very low increase in weight

• Bonding to honeycomb core

• Reticulation of adhesive is important: • Sufficient flow of adhesive is required to allow

uniform filleting at the core cell walls • Unsupported (film or paste) adhesive or knit carriers

work best to promote reticulation properties

• Prevent core crush during fabrication • Pressure range is typically 5-45 psi in cocure process • Can be higher in secondary bond process

• Well below the compressive strength of the core

Adhesive fillet at core cell walls

• Nomex or glass with

phenolic or polyimide are common • *Densities range from

1.8-10 pounds/ft3

• Other materials types are available • Aluminum • Stainless Steel • Titanium • Carbon fabric • Thermoplastic • Elastomeric • Ceramic

Hex cell

Flex cell

Over-expanded cell

Double flex cell

Common cell types shown ~ “L” direction is typically referred to as “Ribbon” direction

• Bonding to foam core

• Foam has high coefficient of thermal expansion • Foam has a very low thermal transfer coefficient

• Combined, these properties can cause problems in high temperature molding operations causing shearing of core due to thermal stress

• Adequate adhesive required to fill porous surface • Lower density core has larger surface pore sites

• Foam core is often weaker than the adhesive when loaded in peel, cleavage, tension, and shear • Has good compressive properties

Failure often occurs along the “zip-line” in the weaker foam core surface

Adhesive attaches to porous foam surface

• Foam • Polyvinylchloride (PVC)

• < 200°F service temperature

• Polyurethane (PU) • < 300°F service temperature

• Polymethacrylimide (PMI) • < 600°F service temperature

• *Densities range from 2-30 pounds/ft3

Joints, Loads, Tests, & Failure Modes

Loads on Adhesive Bonded Joints

Tension

Compression

Shear

Cleavage Peel Both parts are rigid One or both parts are flexible

Adhesives perform best in tension, compression and

shear – this should be considered in joint design

Adhesives are inherently weak in peel and cleavage

Common Joint Designs

Single Lap Tapered Single Lap

Single Strap Lap

Single lap joint designs introduce peak shear stresses at the edges of the lap joint. This effect can be minimized with tapered edges.

Common Joint Designs

Double Lap Double Strap Lap

Double Tapered Strap Lap

Double lap joint designs reduce peak shear stresses at the edges of the lap joint. This effect can be further

minimized with tapered edges.

• Single lap coupons • ASTM D1002-01: Standard Test Method for Apparent

Shear Strength of Adhesively Bonded Metal Specimens by Tension Loading (Metal to Metal)1 • Typical Baseline ½ inch lap x 1inch wide coupon

• ASTM D5868-01: Standard Test Method for Lap Shear

Adhesion for Fiber Reinforced Plastic (FRP) Bonding • Somewhat useful for quality assurance testing of adhesives

and surface preparation methodology

• Not useful for generating actual design data*: • Example: 1 inch wide x ½ inch overlap coupon fails at

1500 lbs breaking loads. • Multiply the breaking strength x 2 = 3000 psi. • With a 1 inch wide by 1 inch overlap the breaking

number is significantly less than 3000 lbs. • The joint strength is not doubled with the overlap

length

*Ref. L.J. Hart-Smith, The Bonded Lap Shear Coupon-Useful for Quality Assurance But Dangerously Misleading for Design Data

*SAMPE technical conference proceedings-archives

Stress Distribution: Single Lap Peak stresses in both shear & peel/cleavage concentrated at edges of single lap coupon induced by deformation of specimen when loaded

*Deformation of metal specimen shown – there would be little deformation of comparable thickness CFRP adherends

• ASTM D 3165-07 Standard Test Method for Strength Properties of Adhesives in Shear by Tension Loading of Single Lap Joint Laminated Assemblies • Intended for determining the comparative shear

strength of adhesives in large area joints using single lap shear coupons

• ASTM D 2093-03 Standard Test Method for Preparation of Surfaces of Plastics Prior to Adhesive Bonding • Current ASTM document on plastic surface

preparation. (Not considered useful industry-wide)

• ASTM D 3528-96: Standard Test Method for Strength Properties of Double Lap Shear Adhesive Joints by Tension Loading

• Also useful for quality assurance testing of adhesives although not generally referenced in data sheets from adhesive manufacturers

• More useful for generating design data or predicting shear load capabilities through a bonded double-lap joint

Stress distribution through a double-lap joint

Common Joint Designs

Tapered Scarf Joint

Tapered scarf joints provide for uniform tensional shear strength across the entire bonded joint

Stress Distribution: Taper Scarf

Stress distribution in a tapered scarf joint

Typical cohesive failure: Desirable in test coupons at predicted loads. Less desirable if failure occurs at loads below predicted

values which can be indicative of moisture contaminated adhesive, improper adhesive chemistry (mix ratio) or cure.

Failure Modes in Adhesive Bonded Joints

Adhesive failure in shear at interfacial surface between adhesive and one, or both, substrates. May be due to poor surface preparation, surface contamination, or incorrect adhesive selection with a surface tension

value that does not allow proper wetting of the substrate.

Failure Modes in Adhesive Bonded Joints

Typical adhesive peel failure: occurs often with composite substrates at fairly low loads, whereas the chemical bond of the adhesive to the FRP substrate is

weak when loaded in peel.

Failure Modes in Adhesive Bonded Joints

Atypical cohesive peel failure: rare occurrence with composite substrates as non-ductile, low surface energy FRP adherends usually fail at the interfacial surface. This is more likely to occur with metal

adherends with etched and primed interfacial surfaces.

Failure Modes in Adhesive Bonded Joints

Undesirable interlaminar fracture, typically between first and second ply in one or both adherends, whereas the laminate matrix is weaker than the

adhesive bond interface under load

Desirable far-field fracture or failure of substrate occurring outside of the bonded joint at predictable design loads

Modified peel test developed by Boeing The backing adherend is clamped and peeling adherend is removed Qualitative Mode I test for bond quality

•Adhesion failure-poor bond •Cohesive failure-strong bond

Intended to screen out poor adherend surface prep and adhesive combinations Failure modes correlate with DCB test with ~90% less cost and flow time

Courtesy of The Joint Advanced Materials and Structures Center of Excellence

Peeling adherend (0.020” Al PAA+ single ply of composite- peel ply surface)

Adhesive film FEP crack starter

Backing adherend ().063 Al-w/PAA

Courtesy of The Joint Advanced Materials and Structures Center of Excellence

• ASTM D 3762-03 (2010) Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test)

• Best method of assessing surface durability in a short period of time

Four Day Wedge Test Results

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Exposure (Days)

CONTROL GROUP: 70°F at <40%RH

Scotchbrite & Solvent Wipe

Grit Blast & Solvent Wipe

Waterbreak & Alodine

AC-130 Sol-Gel

Phosphoric Acid Anodizing

Four Day Wedge Test Results

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Exposure (Days)

HOT / WET: 140°F at 100% RH

Scotchbrite & Solvent Wipe

Grit Blast & Solvent Wipe

Waterbreak & Alodine

Ac-130 Sol-Gel

Phosphoric Acid Anodizing

Four Day Wedge Test Results

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Exposure (Days)

5% SALT WATER BATH at 70°F

Scotchbrite & Solvent Wipe

Grit Blast & Solvent Wipe

Waterbreak & Alodine

Ac-130 Sol-Gel

Phosphoric Acid Anodizing

Non-Destructive Inspection of Adhesively Bonded Joints

Inspect for continuous adhesive squeeze-out (filleting) along the bonded joint

*Note that this may be an automated laser or photographic inspection process

GE’s robot based automated ultrasonic platform for the inspection of complex composite structures

High Throughput Application Tools: Conventional TTU Squirters available in single or dual frequency configurations Wide Area Phased Array TTU Squirters - Phased Array Skin Bubbler (Shown) - Phased Array Inside Radius Bubbler - Phased Array Outside Radius Bubbler - Phased Array Stringer Bubbler

Robotic Ultrasonic Linier Array

Courtesy of Inspection Technologies, Inc

Photos and images courtesy of Laser Technology, Inc

Laser Shearography • Shearography, detects sub-

surface defects and anomalies by observing the deformation of the surface of a test article as it is acted upon by an applied stress

• Successful detection relies

entirely on how the surface of the test article moves when it is stressed, and how this is imaged by the shearography camera.

Photos and images courtesy of Laser Technology, Inc

Laser shearography can be adapted for rapid

automated inspection of adhesively bonded joints

Laser Shearography

• To achieve an optimum bond with composites:

• Abrade or energize the faying surfaces to be bonded • Clean surfaces free from dust or debris • Use appropriate adhesive for the application • Provide uniform bondline thickness • Provide constant clamping pressure along B/L • Properly cure adhesive to achieve structural properties

• To achieve an optimum bond with metals:

• Clean surfaces free of oils & dirt if applicable • Refresh oxide layer with a suitable process • Chemically etch or couple to fresh oxide layer • Apply corrosion inhibiting primer • Use appropriate adhesive for the application • Provide uniform bondline thickness • Provide constant clamping pressure along B/L • Cure adhesive to achieve structural properties