fatigue of bolted connections

7/28/2019 Fatigue of Bolted Connections

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Fatigue Failure of Bolted Connections

by: Chris Meisl


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09/06/2013 Fatigue Failure of Bolted ConnectionsChris Meisl

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Presentation Outline

The Fatigue Process Sequence of Fatigue Failure

Behavior of Fatigue Loading

S-N Curves

Factors Affecting Fatigue

Bolted Connections in Tension Location of Failure

Preloading and Contact Area

Prying Action

Bolted Connections in Shear

Preloading

Stress Concentration and FailureLocations

Anchor Bolts

Design Codes


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What is Fatigue?

Caused by the repeated application

of loads that are not large enough to

cause failure in a single application.

Generally caused by repeated

cycles of tensile loading.

Failure occurs suddenly.

Fracture surface of a paper clipbroken by ~6 cycles of repeated

bending (80 X)


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Stages of Fatigue Failure

Failure will only occur if the

following essential conditions are

present:

Cyclic tensile loads

Stress levels above a threshold

value Flaw in the material

Stages of fatigue failure:

1. Crack initiation

2. Crack growth

3. Crack propagation

4. Final rupture


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Behaviour of Fatigue Loading

The fatigue life = the number of

repeated cycles of loading (N) that

a material will undergo before it

fails.

Higher the fatigue stress level, the

fewer number of loading cyclesrequired to cause failure.

Major factors that effect fatigue

life:

1. Shape of the connection

2. Magnitude of stress variations

3. Mean stress level

4. Choice of materialS-N Curve for varying magnitude

of altering stress


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Bolted Connections in Tension Failure Locations

Crack is initiated at areas of high stress concentrations.

Potential failure sites:

a. Head-shank transition

b. Run-out of thread

c. Thread at nut

Failure is most likely to occur at the first engagement of the threads ofthe bolt and nut.


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Bolted Connections in Tension Influences

The magnitude of the load on the

bolt depends on:

1. The magnitude of the externaltension load.

2. The bolt-to-joint stiffness ratio(K

B

/KJ

).

3. Whether or not the externaltension load exceeds the critical

load required to separate the joint

(depends on initial preloading).

4. Location of the contact area.


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Bolted Connections in Tension Non Preloaded Bolts

Flange connection with non-preloaded bolts

The external tensile force Ft applied on the connection will be transferred

directly to the bolts, Fb.


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Bolted Connections in Tension Preloaded Bolts

Flange connection with preloaded bolts

Preload decreases the load variation in the bolt, until the contact forces

Fc are exceeded.


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09/06/2013 Fatigue Failure of Bolted ConnectionsChris Meisl 10

Bolted Connections in Tension Contact Area

If the flange thickness is to

thin bending may occur,

leading to contact areas.

Contact area at the centre:

KJ > KB

Low variation of load on boltuntil preload is exceeded.

Contact area at edges:

KJ < KB

High variation of load on

bolt.

Contact areas located atedges are more susceptible

to fatigue.


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Bolted Connections in Tension Contact Area

Contact force in centre Contact forces located at flange edges


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Non-Preloading

Load transferred

via bearing of

bolt shank.

Can not be used

in variable load

conditions.

Preloaded:

Load transferred

by friction

between plates.

Use HSFG

bolts.

Can be used in

variable load

conditions.


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Non-Preloading

Stress

concentration at

hole.

Fatigue cracks

near hole, or

shearing of bolt.

Preloaded:

Low stress

concentration

near hole.

Fatigue cracksat gross section

of plate.


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Anchor Bolts

Behave in the same way as normal

bolts.

Bolt diameter and thread size has

little effect.

Method of forming thread influences

fatigue strength Rolled threads better than cut due to

residual compressive stress.

Double nut increases fatigue

resistance.

Must consider prying effects.


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Code Requirements CAN/CSA-S16.1-94

Based on detail categories,

number of load cycles, and the

corresponding S-N curve.

Considers fatigue failure to occur

in the connecting material.

Parameters:g = fatigue life constant [Table 4(a)]

n = number of stress range cycles

[Table 4(b)]

N = number of passages of the

moving load

Fsrt = constant amplitude thresholdrange

Fsrg

nN

1

3

Fsrt

2


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Code Requirements AISC LRFD 1999

Based on detail categories, numberof load cycles, and the correspondingS-N curve.

Considers fatigue failure to occur inthe connecting material and bolt.

Parameters:

Fsr= design stress rangeCf= constant [Table A-K3.1]

N = number of stress range fluctuations

Fth = threshold fatigue stress range[Table A-K3.1]

At = net tensile area

P = pitchdb = nominal diameter

Shear

Fsr Cf 327

N

1

3

Ft

Tension

Category E Cf= 3.9E8

Fth = 48MPa

Include prying effects

At

4db 0.9382P( )

2


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Design Tool Formatted Spreadsheet


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Design Tool Formatted Spreadsheet


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Conclusions

Fatigue is an important

consideration in applications

involving repeated loading.

Codes offer little guidance for

fatigue of bolted connections.

Avoid fatigue susceptible details.

Firth of Forth Rail Bridge

fatigue of bolted connections

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