determining fastener pull-through...

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

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

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.

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.

MECHANICALLY FASTENED JOINT FAILURE MODES

Hart-Smith

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

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

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

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

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

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

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

RECOMMENDED TESTING METHODOLOGY• Symmetric Pull Test

g

Bending support

RECOMMENDED TESTING METHODOLOGY• Asymmetric Pull Test (off‐centre)

P

Welded bracketfor induced

torque

Supporting Fixed Frame

P

Laminated Structure

g

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.

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.

Questions