fatigue bolted connections
Post on 27-Nov-2014
289 Views
Preview:
TRANSCRIPT
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
2
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 Failure
Locations
Anchor BoltsDesign Codes
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
3
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 clip broken by ~6 cycles of repeated bending (80 X)
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
4
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
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
5
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 cycles required 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
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
6
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 of the bolt and nut.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
7
Bolted Connections in Tension – Influences
The magnitude of the load on the bolt depends on:
1. The magnitude of the external tension load.
2. The bolt-to-joint stiffness ratio (KB/KJ).
3. Whether or not the external tension load exceeds the critical load required to separate the joint (depends on initial preloading).
4. Location of the contact area.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
8
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.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
9
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.
04/09/23 Fatigue Failure of Bolted Connections Chris 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 bolt until preload is exceeded.
Contact area at edges: KJ < KB
High variation of load on bolt.
Contact areas located at edges are more susceptible to fatigue.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
11
Bolted Connections in Tension – Contact Area
Contact force in centre Contact forces located at flange edges
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
12
Bolted Connections in Shear
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.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
13
Bolted Connections in Shear
Non-Preloading Stress
concentration at hole.
Fatigue cracks near hole, or shearing of bolt.
Preloaded: Low stress
concentration near hole.
Fatigue cracks at gross section of plate.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
14
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.
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
15
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: = 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 threshold range
FsrnN
1
3
04/09/23 Fatigue Failure of Bolted Connections Chris Meisl
16
Code Requirements – AISC LRFD 1999
Based on detail categories, number of load cycles, and the corresponding S-N curve.Considers fatigue failure to occur in the connecting material and bolt.Parameters:
Fsr = design stress range
Cf = constant [Table A-K3.1]N = number of stress range fluctuations
Fth = threshold fatigue stress range [Table A-K3.1]
At = net tensile areaP = pitch
db = nominal diameter
Shear
Fsr Fth
TensionCategory E’ – Cf = 3.9E8
Fth = 48MPa
Include prying effects
At4
db 0.9382P( )2
top related