DETERMINING FASTENER PULL-THROUGH...
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DETERMINING FASTENER PULL‐THROUGH FAILURE MODE IN
ADVANCED COMPOSITE MATERIALS
Rikard B. Heslehurst, PhDHeslehurst & Associate P/LUniversity of NSW, ADFA
Canberra ACT
Composites Australia and CRC – ACS Australian Composite Conference 2015
Gold Coast, QueenslandApril 2015
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Outline• Introduction• Mechanically Fastened Joint Failure
Modes• Fastener Pull‐Through Failure Mode
Mechanics• Complex Failure Issues• Composite Laminate Material
Property Estimation• Parametric Study• Recommended Testing
Methodology• Conclusions
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INTRODUCTION
• Are mechanically fastened joints in highly loaded structures a concern?
• Mechanically fastened joint efficiency is typically less than 100%.
• Metallic components have mechanically fastened joint efficiency ≈ 70%,
• Composite components have mechanically fastened joint efficiency < 40%.
• Of the several mechanically fastened joint failure modes the less common failure modes are: – cleavage failure, – bolt bending and – bolt pull-through.
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INTRODUCTION
• For bolt pull-through the material thickness and through-the-thickness properties are important.
• For composites the through-the-thickness strength interlaminar properties become very relevant.
• This paper addresses the issues, stress analysis and testing of pull-through failure of bolted composite structures.
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MECHANICALLY FASTENED JOINT FAILURE MODES
Hart-Smith
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FASTENER PULL‐THROUGH FAILURE MODE MECHANICS
• Effective Fastener Head/Washer Diameter
• Interlaminar Shear Strength Determination
tskin
P/2
d (dia)
P/2
P
head/washer laminateAllowable
TTTTTT
D tPFoS
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COMPLEX FAILURE ISSUES
P
P
M
t1
t2Dhead
D
F
F
e
Assuming the fastener head diameter is twice the fastener shank diameter and the two components being joined have the same thickness then the TTT force (F) is given by:
3
5PF
Dt
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COMPLEX FAILURE ISSUES• The fastener head rotation TTT
force is distributed over an effective arc (arc of influence) on the structures surface. The larger the fastener head relative to the fastener diameter the smaller is the arc of influence. The arc of influence has an assumed relationship with the fastener diameter‐to‐structure thickness ratio of:
• The arc of influence has been demonstrated through the application of holographic interferometry on fastener tipping
4
arcD
t
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COMPLEX FAILURE ISSUES
• The initial TTT shearing stresses are based on the arc of influence subscribed arc length (larc) and the structure thickness (t).
• The arc length is defined by:
• The TTT area of shear is thus:
• TTT Shear Stress is then:
8arc
DlD
t
38TTTshearA Dt
38
TTTP
Dt
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0
2
4
6
8
10
12
0.5 1 1.5 2 2.5 3 3.5
TTT
SH
EA
R S
TRE
SS
/BO
LT S
HE
AR
STR
ES
S
D/T RATIO
COMPLEX FAILURE ISSUES
3
2
3
8
4
2
TTT
bolt
TTTbolt
P
DtPD
Dt
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COMPOSITE LAMINATE MATERIAL PROPERTY ESTIMATION
T-T-T shear failure in the x-z plane
T-T-T shear failure in the y-z plane
1 1xy
mxy xz f f m
f
GS S V V SG
1 1
1 1
yz
yz
mf
fyz m
mf
f
GVG
S SGVG
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COMPOSITE LAMINATE MATERIAL PROPERTY ESTIMATION
Effect on Shear Strength with Fibre Volume Ratio
0.00
0.20
0.40
0.60
0.80
1.00
0 0.2 0.4 0.6 0.8 1
Nor
mal
ised
She
ar S
treng
th
Fibre Volume Ratio
Sxy/Sm Syz/Sm
0.22 0 0.6134
TTT m
TTT m
P S
S
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RECOMMENDED TESTING METHODOLOGY• Symmetric Pull Test
g
Bending support
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RECOMMENDED TESTING METHODOLOGY• Asymmetric Pull Test (off‐centre)
P
Welded bracketfor induced
torque
Supporting Fixed Frame
P
Laminated Structure
g
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CONCLUSION• Simple bolt pull-through easy to define and test.• Pull through strength complex but relative simple
relationship determined through micromechanics.• Tipping bolt stress state is more complex. Closed
form solution difficult to develop. FEA typically best approach for accurate bolt tipping stresses.
• Experimental testing very difficult to eliminate large secondary stresses
• Material property estimation very difficult to determine, but micromechanics relationship can be used in early design.
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CONCLUSION• Simple analysis will get reasonable values and apply
large Factor of Safety to eliminate potential pull-through failure.
• Biggest issue with thin laminates (< 8 plies)• Where bolted joints are to be used 12 plies minimum
is highly recommended to obtain highest joint structural efficiency.
• Remember – composite structures have a lower structural efficiency with a bolted joint than do metal structures.
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Questions