Composite Joining and Joints

7/17/2019 Composite Joining and Joints

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19 COMPOSITE JOINING AND JOINTS



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Practical structures generally require joints to transfer loads from member to member. The jointsmay be between similar or dissimilar materials. Joints in composite structures present a greater

challenge than for homogeneous, isotopic materials since anisotropic materials do not easily

accommodate stress concentrations and have intrinsic weak directions. Composite joints can be

mechanical attachments such as bolts and rivets or adhesive attachments using glue.

MECHANICAL JOINTS

Mechanical joints are used when repeated disassembly and reassembly is required or when

surface preparation is not practical. Mechanical joints require that bolt or rivet holes are drilled

into the composite that reduced the net cross sectional area of the structure and introducelocalized stress concentration. These stress concentrations can cause ply delamination since they

will include through thickness tensile and shear stresses (see Chapter 15). Mechanical joints add

weight to the structure from the added weight of the bolt or rivet. They also pose a risk forcorrosion since the laminate and fastener may comprise dissimilar materials and moisture can be

trapped in the crevices inherent in such joints. However mechanical joints can be readilyinspected before assembly and while in service. Examples of two typical bolted joints are thesingle lap joint and double strap joints. These joints and the relevant dimensions are shown in

Fig.19-1.

Bolted Single Lap

Bolted Double Strap

W

e

DP

P

PP

AxialPitch

t

Figure 19-1 Basic types of mechanical joints.



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The single lap joint is the simplest and most weight efficient but the load results in a moment due

to off-set load. The double lap joint eliminate the moment but adds additional weight from thestraps and additional bolt.

Failure modes in mechanical joints

The composite designer must consider four relevant stresses in mechanical joints. The bearing

stress, bσ is the load, P divided by the projected transverse cross sectional area of the

hole b P Dt σ = . The shearout stress is determined by the longitudinal shear surfaces, and is

given as 2SO P et σ = . The net section stress is ( ) N P t W Dσ = − . The transverse splitting stress

is a localized stress normal to the applied load. When any of these four stresses reach a criticalvalue the joint will fail with a characteristic mode, shown in Fig.19-2. The gross stress defined as

P Wt σ = is used to rate the effectiveness of the joint. Joint efficiency is the ratio of the gross

section stress at failure to the strength of the laminate in the gross section. For metals single

fastener joints can have efficiencies as high as 80%. Polymer matrix-fiber composite laminateshave efficiencies generally less than 50% due to their strength anisotropy and inability to

redistribute stress. Net section failures can be prevented by increasing the ratio of

a) Net section failure b) shearout c) transverse splitting d) bearing failure

Figure 19-2. Failure modes in bolted joints

the plate width to hole diameter,W D . Generally 6W D

> is sufficient to prevent net section

failures. Shearout failures can be eliminated the ratio, e D to 3 or greater. Transverse splitting is

rare but will occur if there is a high fraction of the fibers in the load direction such as would be

the case in a unidirectional composite. Bearing failure is the preferred failure mode since the joined members are not catastrophically separated. Bearing failures are associated with localized

hole damage such as local delamination and matrix crazing. Some examples of bearing damage

are shown in Fig.19-3. An extreme case of damage induced by the counter-sunk head fastener,composite plates can be completely pulled out of the fasteners. To promote bearing failures, A



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lower stiffness material is placed in the bearing region. Placing ±45º plies in the bearing region is

also used to induce bearing failures. Local delamination can be induced near the hole region by

using interference fit fasteners.

Single Lap Joint

Double Lap Joint

Counter-Sunk Head Fastener

Figure 19-3 Examples of bearing damage

Bolted joint failure analysis

Chang, Scott and Springer offered the following composite bolt failure analysis. A schematic

diagram of the bolt hole specimen and the critical parameters used in the analysis are shown in

Fig. 19-4. The bolt shank is loaded uniformly and symmetrically along its length in the 2 X

direction. The angle α is measures from the load direction. The failure criterion used in this

analysis was originally proposed by Yamada,

22

1 12 1c X S

σ τ + = (19.1)

where X is the strength of a longitudinal and cS is the shear strength of a symmetric cross-ply.

This criterion is a simplified Tsai-Hill criterion with the transverse and biaxial strength terms

eliminated.



19-5

e

D

L

W

P

1

2

X

X

α

P

t

X3

Figure 19-4 Schematic of bolt hole used in Chang, Scott and Springer analysis

According to this analysis, bolt hole failure will occur when the Yamada failure criterion is met

in any ply on the characteristic curve. The distance to the characteristic curve, ( )cr α measured

from the edge of the bolt hole is determined by two length parameters, 0c R and 0t R , that are

experimentally measured material constants independent of geometry and stress distribution.

Theses length parameters are defined schematically in Fig.19-5. The term 0c R is the

α

r c

R

R

X

X

2

1

Dot

oc

c

Figure 19-5 Schematic diagram of bolt hole showing characteristic distances

characteristic distance in compression and 0t R is the characteristic distance in tension. The

characteristic curve is then



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( ) ( )0 0 0 cos2

c t c t

Dr R R Rα α= + + − (19.2)

where the range of

α is

2 2

π π

− ≤ α ≤.

If at any point on the characteristic curve the Yamada criterion is exceeded,

( ) ( )2 2

1 12, ,1

c c

c

r r

X S

σ τα α + =

(19.3)

the joint has failed at that point. The local stresses on the characteristic curve,

( , ), ( , ) and ( , ) x c y c xy cr r r σ α σ α τ α must be determined by numerical method.

Converting the local stresses on the characteristic curve to principal material directions then,

( )

2 2

1

2 212

( , ) ( , ) cos ( , )sin 2 ( , ) cos sin

( , ) ( , ) cos sin ( , ) cos sin ( , ) cos sin

c x c y c xy c

c x c y c xy c

r r r r

r r r r

σ α = σ α θ + σ α θ + τ α θ θ

τ α = −σ α θ θ + σ α θ θ + τ α θ − θ (19.3)

The failure mode of the bolt hole depends on the point on the characteristic curve where the

over-load occurs, as shown in Fig.19-6. The bearing mode occurs over the range of o15± from

the bottom of the hole. Shear-out occurs over the range from o o30 60− . The tension mode failure

occurs over the range from o o70 90− .

R

R

X

X

2

1

D ot

oc Bearing

Shear Out

Tension

Figure 19-6 Regions of hole failure modes.

Ply lay-up and hole diameter relative to the plate width have a significant effect on failure mode

type and strength of the joint, as seen in Fig.19-7. In this figure the maximum joint load, ax P



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5 100

0.3

0.2

0.1

0

W/D

P m a x

f P

tension

bearing

shear out

bearing

shear out

[0 / 45 / 90 ]

[0 / 45 ]

[ 0 /90 ]

+-

+-

2s

s

2 s

Figure 19-7 Ply layup effects on bonded joints

divided by the failure load of the ply without a joint, f P is plotted against the ratio of plate

width to hole diameter, W D . This ratio is a measure of relative hole diameter. The strongest

bolt hole joint occurs for the quasiisotropic lay-up [ ]0 / 45 /90S

± . At large relative hole diameters

the failure mode is net tension failure. For small relative bolt holes bearing failure is the

predominant mode. The cross ply design has the lowest bolt hole strength while the [ ]20 / 45S

±

lay-up has the intermediate strength.

ADHESIVELY BONDED JOINTS

Adhesively bonded joints have high structural efficiency and are used extensively to joincomposites in advanced aerospace structures. Bonded joints can be between two composite

laminates or between a composite laminate and a metal structure. Composite to metal bonds are

used often in airframe structures. Adhesively bonded joints can distribute load over a muchwider area than mechanical joints. Since no holes are required, the risk of local delamination is

practically eliminated. Compared to bolted joints with weight of the joint is significiantly

reduced. On the other hand adhesively bonded joints cannot be disassembled without destroying

the substrate. Some adhesives are susceptible to degradation by temperature and humidity. Themost critical drawback for safety critical structure such as airframes is their inspectability.

Critical joints may require ultrasonic inspection over their entire area. Corrosion can be problemin carbon fiber composite to aluminum joints due to galvanic action. In such cases anintermediate insulating layer can be used. Some of the most common types of adhesively bonded

joints are shown in Fig.19-8. In addition to the lap and strap joints that are used in bolted joints

additional types are possible with adhesive bonding such as the stepped joints and scarf joints.



19-8

Double Lap

Double Strap

Stepped Lap

Double Stepped Lap

Single Scarf

Double Scarf

Single Lap

Figure 19-8 Common adhesive joint configurations

A variety of polymer adhesives can be used to bond composites. The specific selection dependsupon the maximum operating temperature of the structure and the nature of the loads expected at



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the joints. Table 19-1 lists some of the more commonly used adhesives. High strength adhesives

tend to be brittle, therefore lower strength, flexible adhesives may be preferred if impact loads

are expected or high displacements are required. Usually the higher the operating temperature ofthe adhesive the greater is the cost.

Table 19-1 Polymer adhesives

Type Temp Limit Cure Temp Use

Epoxy Polyamide 200 RT-200 General

Epoxy Amine 400 RT-300 General

Epoxy Phenolic 600 325 High TempPolyester 300 RT General

Silicone 600 RT HT, Flexible

Polyimide 900 350 HT

Acrylic 300 RT Polyester

Rubber 400 RT FlexiblePolyurethane 250 RT Flexible

Cyanoacrylates 475 RT Strong, Brittle

Strength of adhesively bonded joints

The strengths of E-glass to E-glass and E-glass to aluminum single lap bonded joints as a

function of lap length are shown in Fig 19-9. The adhesive is epoxy in both joints. Increasing the

lap length to greater than 1 inch has only a small effect on the joint strength. The bond strengthof E-glass to aluminum is not as great as E-glass bonded to itself. The effect of joint

configuration on the joint strength for various joint lengths of boron-epoxy bonded to aluminum

are shown in Fig.19-10. For this material pair there is a large difference in strength between thesingle lap and double lap joint strengths. For large joint lengths the stepped lap joint are stronger

than the lap joints. Stepped lap joints are often used for bonding multiply laminates where each

step corresponds to a ply. The highest bond strength can be achieved with a scarf joint if the bond length is great enough.

Adhesive bond strength analysis

Elastic solutions to stresses and strains in adhesively bonded joints are available for simple joint

configurations such as the single lap joint shown in Fig. 19-11. Applying a load to this joint in

the deformations and displacements illustrated in Fig. 19-12. For a single lap joint the loads areoff-set from the midplane of the adhesive causing a moment which produces bending in the



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0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 0.5 1 1.5 2

Lap Length, in.

E-glass/E-glass

E-glass/Aluminum

Figure 19-9 Joint strength for single lap joints. (E-glass t = 0.04 in., aluminum t = 0.063)

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5

Length of Joint, in.

J o i n t S t r e n g t h ,

k s i

Single Lap

Double Lap

Stepped Lap

Scarf

Figure 19-10 Effect of joint configuration on strength of boron-epoxy to aluminum joints



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h

L

t

Figure 19-11 Geometry of adhesively bonded single lap joint

P

P Bending

Tension

MaximumShear

Figure 19-12 Adhesively bonded single lap joint under load

substrate plies. In addition to shear in the adhesive parallel to the plane of the plate, the free edgeof adhesive is under tension. This tension can cause the tear and the tear flaw can propagate by

mode II crack propagation. The stress distribution of a single lap joint over the joint length, L is

shown in Fig. 19-13. The high normal stresses near the free ends of the joint results in bond

peeling. Bond peeling can be reduced by tapering the joint as illustrated in Fig.19-14. Peelingcan

0

+

-

S t r e s s

L

Shear Stress

Normal Stress

Figure 19-13 Stress distribution in a loaded single lap joint.



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Figure 19-14 Tapered adhesively bonded single lap joint

be virtually eliminated by using a scarf joint shown in Fig.19-15. If tapering of the lap joint oruse of a scarf joint is not possible, the ends of the joint can be held mechanically to prevent

peeling as shown in Fig.19-16.

Figure 19-15 Adhesively bonded scarf joint

Figure 19-16 Combined mechanical fastened and adhesively bonded single lap joint

Adhesively bonded joint design guidelines

Good joint design requires that 0º plies are joined together. Bonding 90º to 0º plies or 90º to 90º

plies should be avoided. If these joint matching plies cannot be avoided the scarf joints should be

used. Joint strength is always improved with increased lap length to thickness ratios, L t ,

especially at low L t ratios. Tapering the substrate plate ends in the overlap reduces the normal

stresses that tend to peel the joints apart. Always use equal stiffness substrates for the joined

members. If the Young’s modulus is different between the two members the thickness should be

adjusted such that 1 1 2 2 E t E t = . The ideal adhesive has high shear strength and tensile strength but

low shear modulus and Young’s modulus.

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