joining techniques

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Joining composites

14/11/2013 1CompositesIves De Baere and Joris Degrieck – 2013‐2014

Introduction

• Joints should be avoided as much as possible, it always causes stress concentrations and quite often, failure occurs at the joint or close to itoccurs at the joint, or close to it.

• With composites, it is possible to manufacture very complex structures in one piece. However, in some cases, this is not possible, or dissassembly is necessary

• For complex joints, with 3D loads, a metal connection element can be preferred, because of its isotropic nature.


Why are mechanical fasteners used?

• Usually to join composite parts with other (metal) parts

• When it is absolutely necessary to allow disassembly

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Possibilities for connecting composite parts

• Mechanical fastening

– Bolts

– rivets

• Adhesive bonding

• Combined mechanical‐adhesive bonding

• Fusion bonding (thermoplastics only)


Types of mechanical fasteners

• Bolts

– With or without chamfered head (countersunk)


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Example of bolts in (segmented) turbine blades



Carrot T‐bolt


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Connection using imbedded bolts


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Bron: SANDIA REPORTSAND2008‐4648Unlimited ReleasePrinted July 2008Blade System Design Studies Phase II:Final Project ReportDerek S. Berry

Mounting a turbine blade, using the T‐bolts


http://youtu.be/xFI3Dy2k6oQ

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Types of mechanical fasteners

• Rivets

– Normal rivets(access to both sides is necessary)

– Blind rivets(access to only one side suffices)


Rivets to attach stiffeners in an airplane wing


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Most common test methods for mechanical fasteninga) Unstabilized single shear b) double‐shear test c) Stabilized single shear


Mechanical fastening: other possible geometries


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Most common failure modes for mechanical fastening




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Mechanical connections

Advantages

• Higher resistance against the environment (temperature and moisture)

• Higher resistance against fatigue (lower stress concentrations)

• High reliability

• Possibility for repetitive assembly and disassembly


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• High reproducibility

• Easy to inspect

• No specific surface preparations necessary

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Mechanical connections

Disadvantages

• Necessity to drill/provide holes– Specific tools to drill

– Significant wear on the drilling tools (unless waterjet or lasercutting is considered)

– Possibility of inducing (significant) damage in the vicinity of the holes (both drilling and waterjet)

– Creating extra free edges (delaminations)

• Stress concentrations in the vicinity of the holes

• Combining orthotropic material and isotropic


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• Combining orthotropic material and isotropic material results in stress concentrations

• Different thermal expansion

• Possible galvanic corrosion

What about sandwich materials?

Inserts, embedded during or after manufacturing


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Sandwich materials: Blind insert


19/01/04 21

Sandwich materials


19/01/04 22

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– Bolts

– rivets





Adhesive bonding

Advantages

• Fairly high ratio of strength to mass

• Possibility of smooth surfaces (depending on the geometry)Possibility of smooth surfaces (depending on the geometry)

• Low number of parts (two + adhesive layer)

• No need to provide holes

• No stress concentrations due to holes or resulting damage are present

• Galvanic corrosion of metal fasteners is not present– When composite parts are joined with metal parts this off course


When composite parts are joined with metal parts, this off course remains an issue

• Can be quite cheap (but not necessarily)– To lower the production cost, the adhesive can be co‐cured with the

pre‐cured parts

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Adhesive bonding

Disvantages

• Sometimes extensive surface preparation necessary– Roughening/grinding/sandblasting of the surfaces– Roughening/grinding/sandblasting of the surfaces

– Cleaning/degreasing of the surfaces

– Applying extra chemical agents

• Large stress concentrations, depending on the used geometry

• Curing/hardening can be time consuming

N t l k h th h i l ti ill


• Not always known how the mechanical properties will change over time– Ageing due to UV‐light, temperature

• Can be susceptive to creep

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Commonly used adhesive joint geometries


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Some design criteria for adhesive joints

• Main objective:

– Adhesive joint should have longer life than its constituents

• Consequences

– Surface of the elements should be prepared properly

– Failure due to creep in the adhesive is not


Failure due to creep in the adhesive is not tolerated

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Some design criteria for adhesive joints

• Consequences (cont.)‐ Avoid or lower the number of discontinuities in the construction (stress concentrations)the construction (stress concentrations)

‐ Static loading should not exceed the yield stress of the adhesive film

‐ Fatigue: adhesive layer should only take elastic loads

‐ Avoid normal stresses, load the adhesive in shear


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,

‐ In case of a metal constituent: as soon as the metal yields, consider failure of the adhesive/joint

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Stress analysis in adhesive joints

• Theory of Volkersen

• Theory of Goland and Reisner

• Finite element modelling


Stress analysis according to Volkersen

• Simple elastic analysis

• Joining elements are considered as membranes, so they cannot take a bending moment

• Tensile stresses normal to the adhesive layer (=peel stress) is not considered.

• Maximum shear stress are expected near the


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pends. However, these are free surfaces, so stress must be zero singularity

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Volkersen – double lap specimen (no bending)

2PP

t1t2ta

y

x1.cosh

x.cosh.1k

sinhgem

x K.K.K.

K 21

21

21

21

2

1 2.

with anda KG t t

Px


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Ref. : VOLKERSEN, O., « Die Nietkraftverteilung in Zugbeanspruchten Nietverbindungen mit konstanten Laschenquerschnitten. «Luftfahrtforschung, 15, pp. 41 – 47, (1938)

2 1

with and.

a

KEt t t

Influence of the overlap length of the adhesive layer (Volkersen)

(x) [MPa]

max

min

SCF


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Increasing the overlap hardly increases the maximum shear stress, but the stress concentration factor (SCF) increases almost linearly

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Influence of the shear modulus of the adhesive (Volkersen)

(x) [MPa]


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Increasing the shear modulus of the adhesive increases the shear stress and the SCF

Influence of the adhesive thickness (Volkersen)(x) [MPa]


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Increasing the adhesive thickness results in a lower SCF

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Volkersen for single lap shear

With

Where


Lucas F.M. daSilva, Paulo J.C. dasNeves, R.D. Adams, A. Wang, J.K. Spelt. Analytical models of adhesively bonded joints ‐ PartII: Comparative study. int. J. Adhes Adhes 2009; 29: 331‐341.

Stress analysis according to Goland –Reissner

• Limitations:– Thickness of the adhesive layer should be small with respect to the thickness of the adherendsrespect to the thickness of the adherends

– Visco‐elastic behaviour

• Hypotheses:– Stresses in the adhesive layer are constant through the thickness

– Plane strain assumption, deformations perpendicular to the loading direction are allowed


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the loading direction are allowed

– Acting loads: axial load + bending moment

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Goland‐Reisner: adhesive shear stress

tat

2c

x

z

P

P

With

2c


Goland‐Reisner: adhesive Peel stress

With

Where


Lucas F.M. daSilva, Paulo J.C. dasNeves, R.D. Adams, A. Wang, J.K. Spelt. Analytical models of adhesively bonded joints ‐ PartII: Comparative study. int. J. Adhes Adhes 2009; 29: 331‐341.

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Stress distribution for a low shear modulus (Goland ‐ Reissner)

avg


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• Ga= 10,55 MPa

• Ea = 35,85 MPa

• ta = 0,18 mm

Stress distribution for a high shear modulus (Goland ‐Reissner)

• Ga= 1,24 GPa

• Ea = 3,45 GPa

• ta = 0,08 mm


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avg

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Finite element simulation

2 mm

1 mm

• 2D model, to reduce calculation time

• Simple elastic simulation with non‐linear deformations

• No cohesive layers necessary to illustrate the stress distribution


• A very dense mesh is necessary to capture the behaviour, especially near the free edges of the adhesive layer

Finite element simulation


• Both Volkersen and Goland‐Reisner are accurate predictions

• Only the FEM can capture the shear behaviour on the free edge

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Critical length of an adhesive joint

Such a critical lenght exits

Double

overlap

h Such a critical lenght exits

• for every joint configuration

• for every material combination

Single

overlap

Joint strength


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Overlap length

Influence of temperature on the adhesive

[MPa]a


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[mm/mm]a

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General remarks

• The (shear) stress distribution will have higher stress concentrations when:

Th dh i l i hi– The adhesive layer is thinner

– The adhesive layer is longer

– The adhesive is stiffer

• Ageing in general will have an important influence

• There is a critical length


• Assessment of the different parameters:http://users.ugent.be/~sfjacque/FlandersDrive

Common errors in adhesive joints

• Strokes (by hand) on the adhesive surface

• Cold area’s during curing

• Curing time too short

• Porosities – inclusions and other foreign contents

• Dry spots

• Discontinuities in the adhesive layer

• ‘Kissing bond’ surfaces touch each other or the


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• Kissing bond surfaces touch each other or the adhesive layer, but no effective bond has formed. Very dangerous, as this is very difficult to detect.

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– Bolts

– rivets





Combined mechanical‐adhesive bonding

• The mechanical fasteners takes the general load

• The adhesive layer provides a margin against shearfailure

• Design so that the adhesive film introduces the forces into the joint, before the constituents deform/move to load the mechanical fastener difficult

• When properly designed, they are usually better than both mechanical and adhesive joints:

• Failure modes


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– Failure in tension of the constituents

– Interlaminar failure of the constituent

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– Bolts

– rivets





Fusion bonding

• Only for thermoplastics (at least one part)

• Consists of

– Surface preparation (if necessary)

– Heating the polymer to a viscous state

– Physically causing polymer chains to interdiffuse

– Cooling of the joint for consolidation


• Classification

– Usually based on the heat generation (friction, thermal or electromagnetic)

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Fusion bonding techniques

ThermalWelding

Friction WeldingElectromagnetic

Welding

Hot Tool Welding

Hot Gas welding

Extrusion Welding

Spin Welding

Vibration Welding

Ultrasonicldi

Induction Welding

Dielectric Welding

Microwaveldi


Infrared Welding

Laser Welding

Welding

Stir Welding

Welding

ResistanceWelding

Friction Welding

• Generation of heat at a joint interface fromfrictional work under pressure, followed bycooling and consolidation


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Linear vibration welding• Main advantages

– High production rates– Short cycle times– Ability to weld a number of components

Weld interface

Ability to weld a number of componentssimultaneously

– Small to medium sized parts– Easy process control– Insensitive to surface preparation

• Disadvantages– Not suitable for not‐flat specimens– Causes fibre distortion/displacement at the


– Causes fibre distortion/displacement at the interface

– Expensive machinerie

• Applications– Automotive (light assembly, spoilers,

instrument panels, reservoirs, …)

http://www.youtube.com/watch?v=iG3t0Q7UuCU

Spin Welding• Main advantages

– High weld quality– High speed– SimplicitySimplicity– High reproducibility– Little surface preparation necessary

• Main disadvantage– Non‐uniform heat distribution

(heating depends on relative linearvelocity V=R x ), thus yieldingresidual stresses


• Appliciations– Tubes and circular parts (e.g. sealing

a water‐filled compass)– Attaching studs to plastic parts

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Ultrasonic Welding

• Uses a high (ultrasonic, e.g. 20 to 40kHz) frequency mechanical vibration to weld.

• Vertical or parallel oscillation is possible

• Heat generation: surface and intermolecular

• Energy directors are often used/necessary

– Design is important for the quality of the weld


Ultrasonic Welding

• Main advantageFast and clean process– Fast and clean process

• Main disadvantage– Need for energy directors (not always

possible on sheet components)– Risk of fibre disruption at interfaces– Heat conduction in carbon, resulting in

long weld times– It is spot welding (difficult but not


It is spot welding (difficult, but not impossible, to weld larger area’s)

– Expensive equipment

• Application– Automotive parts, floppy disks, medical

deviceshttp://www.youtube.com/watch?v=A2uFi65IWxM

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Friction Stir Welding• Originally developed for aluminium (1991), but also succesfully

applied for composites.

• Two parts are firmly pushed together and a metal tool drills and plunges into the parts The softened polymer is stirred andplunges into the parts. The softened polymer is stirred and forged behind the trailing face of the head pin (HP)

• Great potention for particle‐filled and short‐fibre reinforced thermoplastics

• Careful design of the HP and accurate control of


the process could improve fibre breakage at the interface

• more research needed

Thermal Welding

• Consists of two stages

– An external heating stage

– A forging stage

• A heat source applies heat directly on both surfaces to melt the matrix. Then, the heat source is removed and the bonding surfaces are brought in contact under a forging pressure.


• Limitations lie in the size of the components (entire welding surface heated in one stage), long heating time and high welding pressures needed.

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Hot‐Tool (or plate) Welding

• Main advantages– Surface temperature can be accurately controlled

– Surface inaccuracies can taken into account

– It can handle complex geometries

• Main disadvantage


• Main disadvantage– Melted polymer is likely to stick to the tool and non‐stick materials (teflon) tend to break down at high temperature (teflon: 200°C)

• Appliciations– Automotive (battery cases, fuel tanks, …)

– Infrastructure (gas and water pipelines, …)

Infrared welding

• Surfaces to be bonded are exposed to intense infrared radiotion, usually produced by high intensity quartz lampsintensity quartz lamps.


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Infrared welding

• Main advantage– Fast process– Can join combinationsCan join combinations of flat and curved parts

– Strong welds are possible

– Automization is possible

• Main disadvantage– Possible


Possible deconsolidation

– Warpage of substrates during heating

Hot gas and extrusion welding

• Similar to gas welding of metals• Thermoplastic filler rod is pushed into the groove• Extrusion: molten filler is extruded into the joint• Slow process not suitable for high production


• Slow process, not suitable for high production rates

• Only suited for particle filled or short fibre composites

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LASER welding

• Mostly used for metals, but can be used for composites


• Laser transmission welding is also possible

• Usually only considered for the tape laying process

Electromagnetic welding

• A high frequency magnetic field causes magnetic materials at the weld interface to get hot and start melting the surroundingget hot and start melting the surrounding matrix.

• Then, molten polymer diffuses under the applied pressure to form a bond.

• When necessary, inserts or powders such as


iron oxide, stainless steel, ferrite and graphite are added in the polymer matrix between the parts to be joined.

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Induction welding

• High frequency electromagnetic field (200‐500kHz) heats up an implant in the joint

• Implant can be a foil or particles, typically containing


some polymer for adequate flow and consolidation, e.g. a polymer impregnated metal mesh

Induction welding

• Weld depends on uniform heating which depends on• Weld depends on uniform heating, which depends on the uniformity of the magnetic field. Hence, the coil design is very important.

– Single: magnetic field around the inner diameter

– Multi turn: for remote areas

– Pancake: for large flat areas


Pancake: for large flat areas

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Induction welding

• Main advantage

– Fast and clean process

– Complex geometries

– Weld can be reopened by induction re‐heating

• Main disadvantage

– High cost of insert materials

f h d b h b dl


– Non‐uniform heat distribution in the bondline

– Difficult to weld large parts

Dielectric welding

• Electric field in the MHz region

• Polymers with high dielectric loss factor convert some of the field energy into heat


• Main problem is bulk heating of the entire element

• Mostly used for heating and melting of thermoplastics and curing of thermosets

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Microwave heating

• Intense microwave energy in GHz region applied to electromagnetic absorbent material

• Can also be made as contionuous process, but


p ,shielding is required.

• It is clean and fast, but for smaller size parts (dimensions of the oven)

Resistance welding

• Uses an electrically resistive element (metal mesh or b h)


carbon mesh)

• Electrical current is applied, to generate Joules heat

• Insulation of the mesh improves the weld quality, as it prevents current leakage

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Resistance welding

• Main advantages– Fast, simple, clean and cheap

A li bl f l t t– Applicable for larger structures

• Main disadvantages– Because of power limitations, only narrow welds are possible

– Implant remains inside the weld (can cause stress concentrations)


)

• Applications– Automotive

– Aerospace (leading edge of Airbus A340 and A380)

joining techniques

Documents

imbedded bolts14112013

possible geometries14112013

final projectreportdereks

blade systemdesignstudiesphase

berrymounting aturbineblade