Applications of Adhesive Bonding To Enable Joining Of Multi-Materials

© Lotus Cars Limited © Lotus Cars Limited

Global Automotive Lightweight Materials

Applications of Adhesive Bonding To Enable Joining Of Multi-Materials

April 25th 2013

Daryl Greig

© Lotus Cars Limited 2

Introduction – Lotus Cars


Overall Construction

Elise / Exige:

Volume: 1500 – 2000 p.a.

Tub: Anodised aluminium

Upper: Composite Panels

Rear: Steel subframe – bolted

Front: Composite front module – bonded

Evora:

Volume: 1000 – 1500 p.a.

Tub: Anodised aluminium

Upper: Composite Panels

Rear: Steel subframe – bolted

Front: Aluminium front module – bolted


Tub Construction •  Semi monocoque tub concept

•  Multi material construction

•  No primer or overall paint finish

•  Bolted and bonded structural interfaces

•  3 types of Bonded joints:

• Al / Al, Elise/Exige

• Dow XD4600 single part heat cure

• 1.95kg adhesive, Evora 3.5kg

• Al / composite

•  Dow Betamate 2810

• Composite / composite

• Dow Betamate 2810

•  Panels bonded and bolted to complete BIW

•  Possible common features to higher volume multi material BIW designs – depends on E-coat & paint provision

Kerb weights:

Elise: 720kg (Mk1)

Exige: 1176kg

Evora: 1390kg


Evora - Torsion and Bending Stiffness Contribution

Full GBIW

Stiffness Torsion kNm/deg

Bending kN/mm

Baseline Roadster 14 8

Coupe (GBIW) 26 10.5

Bare Tub


Evora Structural Composite Panels

•  Bonded composite body panels are structural unlike Elise •  Bonded to tub with Betamate 2800 series and large bond

areas

Windscreen surround (WSS) ICS/RTM 3mm-5mm Foam cored CSM and 0/90 UD Stitch mat in A pillar and header. 23% FV

Body Side ICS/RTM 2.5mm-5mm. Foam cored cant rail CSM and 0/90 UD Stitch mat in A pillar and cantrail. 22% FV

Rear Bulkhead RTM 3.5mm CSM 23% FV

Roof panel ICS/RTM 2.5mm CSM and +/-45 UD Stitch Mat. 23% FV

Key CSM – Chopped strand mat FV - Fibre volume RTM - Resin transfer moulding ICS - injection compression system UD - Unidirectional


Bonding Similar / Dissimilar Materials – Challenges

1. Joint design for functionality – Stiffness and strength

•  Design for load bearing joints

•  Predictive analysis of joints – static and dynamic behaviour at extremes of performance envelope

•  Testing to back up engineering – static / dynamic / high / low temp.

2. Pretreatment / Surface prep for Durability

•  Prediction of useful life through accelerated test

•  Component tests

•  Chamber tests – whole structures

•  Vehicle Test

•  Correlation to recognised standards of life – Welded steel to bonded aluminium

3. Manufacturing Process control & Repeatability

•  Substrate surfaces and surface preparation

•  Adhesive contact

•  Manufacturing variables – Extrusion, pretreatment, curing, open time, contaminants, transport…….

•  Tracability


1. Design - Composite Front Module- Multi Functional Component

Clamshell Support

Radiator duct intake / exit

Cooling Module support

Front impact energy absorber –

0 deg, offset and 30 deg

Heater Intake Tow Bar Mount

Replaceable in service

Protects tub up to speeds of 25 mph 7.5kg, 75kJ absorbtion

420mm crushable length

Auxiliary light mounting

Variable wall thickness front to rear


Bonded Joint Design – A Closer Look

Section1 – Al/Al

Section 3 – Comp / Comp

Section2 – Al/ Composite


Section 1 – Al / Al

Mechanical Fasteners – Riv

bonding

Cutback detail for fastener access and progressive failure mode from front

Bracing ribs

0.2mm Ribs - Bond gap

control

Interlock detail for adjoining

extrusions

Spew fillet wiped for moisure

expulsion

Surfaces SAA anodised for pretreatment, corrosion protection and visual finish


Section 2 – Al / Composite

Design features aligned correctly for load bearing

Mechanical attachment plane

Shear / compression planes for joint

Bond gaps controlled by

assembly fixture

Corner load distribution braces

Composites Surfaces primed

for bonding


Section 3 – Composite / Composite

Bond gaps controlled by assembly fixture

Load bearing corners – Hex made of 2 mating sections

Low modulus adhesive with large bond areas

Composites Surfaces primed for bonding


Dynamic Strength – Test data

Dynamic Strength – FE Model Correlation

Upper Bound


Lower Bound

Static Strength – Test Data

Lap Shear Strength Analysis and Test

1. CAE - Riv Bonding Development – Adhesive Strength Correlation

Dynamic Test Drop Rig


T Peel Strength Analysis and Test

Dynamic Strength – Test data

Dynamic Strength – FE Model

Correlation Upper Bound


Lower Bound

Static Strength – Test Data

1. CAE - Riv Bonding Development – Adhesive Strength Correlation

•  Upper and Lower bound values used in crash FE models to envelope possible failure modes


1.Tests for Joint Functionality – Box Joint Tests


2.Durability and Life prediction

•  Technology project to develop bonded aluminium tub.

•  Set up to work with partners:

•  Suppliers: Hydro, Ciba

• Technologists: TWI, AEA, Alcan,

•  Key Universities UK and Europe

•  Others OEMs, Professors……..

•  Preliminary testing for adhesives, fastener and pretreatment selection

•  8 Major phases of core testing to Explore effect of manufacturing variables on performance

•  Confirmatory corrosion tests to complete correlation of life prediction and confirm production specs for anodising, coatings , fixings, interfaces for galvanic protection and transport packaging materials


2.Durability and Life prediction Stress Humidity & SWAAT

Proving Ground Rig Tests


2. Durability - Link Proving Ground to Test exposure (Steel)

Component test

SWAAT / Stress Humidity / Dynamic impact

Proving Ground

System test

Proving ground

Vehicle test

Proving Ground

Correlation

SWAAT / Proving ground

87 d

ays

expo

sure


2.Durability and Life prediction

Correlation – SWAAT to Proving Ground


3.Manufacture

•  Supplier is a wholly owned subsidiary of Group Lotus - LLS

•  Supply chain for extrusions and surface treatment is tightly controlled e.g. SAA chemistries controlled by manufacturing specification. Test parts bonded and tested with every batch

•  Adhesive contact – Adhesive robot applied to approved patterns for complete coverage

•  Tracability – Trace on adhesive, aluminium, pretreatment and curing. Parallel tests of parts with every assembly made.


The future at Lotus

•  Functional integration – Careful design

•  Sandwich materials – Lightweight and quiet

•  Overall mix of materials

•  Reasonable cost of ownership – service / repair

•  Credible supply partners

•  Tooling to allow adventurous design


Conclusions

•  Multi material constructions are possible, even without e-coating and painting the final assembly

•  Bonding durability life can be simulated to an extent by accelerated ageing

•  Durability of bonded joints is achieved in real time exposure

•  Innovation in automotive architectures involves design innovation as well as materials technology


END

Applications of Adhesive Bonding To Enable Joining Of Multi-Materials

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