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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