Bolted&Bonded joints

8/12/2019 Bolted&Bonded joints

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COMPOSITESJOINTS

M THIRUMALAIMUTHUKUMARANASSISTANT PROFESSORDEPARTMENT OF MECHANICAL ENGGDr N G P INSTITUTE OF TECHNOLOGY


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BOLTED JOINTS:

POINTS FOR:

No surface preparation of components is

required

Disassembly is possible without component

damage

There are no abnormal inspection problem

POINTS AGAINST:

Holes cause unavoidable stress concentrations

They tend to incur a large weight penalty


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Failure modes:

Fig: Bolted Joint Failures


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tensile failure:

Tensile or tearing is related to the net area (w-d)t through the

fastener hole corresponding to the member width in a single

hole joint or pitch in a row of fasteners

t- minimum plate thickness

Ptfailure load

W width of plate

d diameter of hole

tnet tensile load of joint


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SHEAR OUT FAILURE:

Shear out failure is related to shear areas emanating

from the hole edge parallel to the load and determined

by the end ( or edge) distance ,e which is typically about

4d.

The smaller the value of e the more likely we get shear

failure

Ps = s2et

Ps failure load

s shear out strength

t minimum plate thickness


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BEARING FAILURE:

Bearing failure is based on the projected area of the hole

and is determined by the diameter, the relationship of

failure load Pbto bearing strength bbeing

Pb= b dt

Pb- failure load

b- bearing strength

d diameter of hole

The bearing strength is normally greater than the

compressive strength of the composites.


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Cleavage failure:

Cleavage failure is a mixed mode involving tension &

bending , cannot be expressed by a simple formula.

In general, we find that fastener failure can be prevented

if extreme values of d/t are avoided. Small values of d/t( large thickness) leads to bending of

fastener.

Large values of d/t(small thickness) leads to plate cutting

into the fastener.

Generally 1< d/t


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Material:

Bearing strength is determined by the compressive

strength of material and the through thickness

constraints afforded by the fastener.

45 degree or 90 degree fibres have a good bearing

performance

GRP( glass fibre polyester) laminates in the form of

chopped strand materials(CSM) which are effectivelyisotropic are better than more directional woven

roving(WR)materials.


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Repair techniques:

The two bolted repair techniques are

1. external patch

2. flush patch

Bolted repairs can be used in cases

where bonded patch repair of a thicklaminate may result in shear stresses

beyond the limit of the adhesive strength.


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Bonded joints

POINTS FOR:

Stress concentration can be minimized.

They incur a small weight penalty.

POINTS AGAINST:

Disassembly is impossible with component

damage,

They can be severely weaker by thermal effects

They require surface preparation.

Joint integrity is difficult to confirm by inspection.


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Failure modes:

A number of failure modes occur in bonded

composite joints because of their anisotropic

nature.

The different modes of failure are

1.tensile lamina

2.interlaminate or transverse resin or fibre

resin interface

3.cohesive adhesive which occurs at low loads


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Surface pre treatments:

PMC are based upon epoxy or polyester

resin which are highly receptive to adhesive

bonding but form oxides, corrode in a moist

environment. Hence surface penetration is

required.

The two techniques are

1. peel ply method

2. abrasion and solvent cleaning


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PEEL PLY METHOD:

A ply of DACRON is installed at the bonding surfaceand then removed just prior to the surface of bonding,

thus exposing a clean surface for bonding.

But care should be taken that it does not leave

contaminants during removal.

ABRASION AND SOLVENT CLEANING:

A grit blast with alumina particles has been found very

effective removing the contaminants. Hand abrasion by abrasive pads and the solvent

cleaning.


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Theoretical stress analysis:

The classical failure theories approach does not

hold good due to the low values of external

moduli , thickness shear and effect of thermal

strains parameters. Hence linear and non linear

analysis are used.

LINEAR ANALYSIS:

Use identical adherands or if this is not possible

then equalize the in plane bending stiffness.

Use as large as over lap as possible


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Use an adhesive with the lowest possible

tensile shear and elastic moduli.

Use as homogenous a ply lay up as possible

NON LINEAR ANALYSIS: The adhesive shear strain energy in the only

factor determining joint strength. Thus a

weaker ductile adhesive with large stress

strain curve will produce a joint of higher

strength


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Repair techniques:

The objective of repair is to restorestructural integrity to the damaged

component.

The two types of bonded patcheswhich is used to repair structural

damage are

1. external patches2. scarf repairs


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BONDED BOLTED JOINTS: Bonded-bolted joints generally have better performance than either

bonded or bolted joints. The bonding results in reduction of the usual

tendency of a bolted joint to shear out.

The bolting decreases the likelihood of a bonded joint debonding in

an interfacial shear mode.

The usual mode of failure for a bonded-bolted joint is either atension failure through a section including a fastener or an inter

laminar shear failure in the composite material or a combination of

both.

Bonded-bolted joints have good load distribution and are generallydesigned so that the bolts take all the load.

Then, the bolts would take all the load after the bond breaks

(because the bolts do not receive load until the bond slips). The

bond provides a change in failure mode and a sizable margini t f ti f il

Bolted&Bonded joints

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