The Effect of Overload of a Bolted Connection on its Fatigue Life: A

Case Study

Presenter: Leon D. Wegner

Authors: Shahin Shadlou, Leon D. Wegner, Timo Tikka

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PresenterPresentation NotesWill talk about the overload, but in some ways Ill present a bit of a play-by-play of the study and provide some lessons learned along the way.

Plan View of Typical Mine Shaft

Sample 2

Sample 1Background

Static Design: Concentric + torsional components

Design for Fatigue Limited guidelines Conservative approach Accelerated fatigue testing

Cage

Skip Skip

PresenterPresentation NotesPut the presentation into context need to describe the steelwork in a typical mineshaft. In a typical mineshaft, ore, people, and equipment is carried between the mine level and the surface by conveyances. Skips carry the ore. The cage carries people and equipment. The conveyances are guided by vertically oriented HSS sections, which are supported periodically by horizontal steel sets bunton sets. In this case, a bunton set consists of a main bunton beam, to which is connected shorter beams supporting the guides. Double gusset plate bolted connections are used, designed for slip-critical conditions. As a conveyance travels, if there is any mis-alignment in the guide, the conveyance will shift from side to side and exert repetitive horizontal slam loads on the bunton sets. Leads to eccentrically loaded connections. So the connections must be designed for fatigue over the life of the mineshaft.Static design of eccentrically loaded connections good guidelines distribute forces by considering a superposition of concentric and torsional components.

Outline Description of test program Results of initial fatigue tests Overload incident and immediate effects Finite element analysis and results Post-overload fatigue tests & results Conclusions

Test Program - Summary

Sub-assembly of Sample 2

Subjected to simulated cyclic slam forces from payload skips

Accidently subjected to 1 cycle of overload (5 times the design load)

PresenterPresentation NotesSlip critical design

Test Setup

1782

629

2083

296

2292

1003

Load applied with 250 kN hydraulic actuator

6 LVDTs to monitor displacement 32 strain gauges used to monitor

strains on the gusset plate

PresenterPresentation NotesMore complete view of the setup.Interest is the performance of the gusset plate connection.

Test Stages

Test Stage Load (kN) Number of cycles Events

Stage 1 42.6 1 million --Stage 2 85 1 million --

OVERLOAD 215 1 --Stage 3 93 1 million Bolt breakageStage 4 103 1 million --Stage 5 113 0.5 million Bolt breakage

PresenterPresentation NotesFocus on effects of the overload, but will also summarize results at other stages.

Results Stages 1 & 2

Load-point Displacement (mm)

(positive value is downward)

Stage 1 (42.6 kN)Support bolts re-tightened

PresenterPresentation NotesDisplacements varied from 0.6 or 0.7 mm to 2.7 to 2.8 mmShift associated with re-tightening of support boltsGradual increase in displacement, range constantStage 2 similar to Stage 1

The Overload Incident

0

50

100

150

200

250

0 5 10 15 20 25

Forc

e (k

N)

Displacement (mm)

Force-displacement of overload incident

215 kN5 times the design load!

Permanent Displacement

PresenterPresentation NotesSoftware error transfer from load-control to displacement control (lesson!)215 kN (5 x the design load)Permanent displacement observed at load-point

Damage Due to the Overload

Clip angle 2

Clip angle 1

PresenterPresentation NotesDetailed investigation conductedSlippage between gusset plates and boltsSlippage of clip angles

Damage Due to the Overload

Bolt slip Contact between bolt shank

and bolt-hole

PresenterPresentation NotesScratches on the bolt shank and damaged threads confirm that the bolt came into contact with the hole.

Finite Element Analysis x=y=z=0

Rx=Ry=Rz=0

Applied load

Purpose: to estimate share of load carried by each bolt

Linear-elastic analysis using ABAQUS

Symmetric model

Tie constraint at bolt locations

Hard contact elsewhere, with 0.5 friction coefficient

Finite Element Results

0

20

40

60

80

100

0 1 2 3 4 5

Forc

e (k

N)

Load-Point Displacement (mm)

Experimental data

FEA data

Comparison of FE and experimental results

Exaggerated deformation (20 times) under 81.6 kN load

Finite Element Results

Finite Element ResultsThe share of load carried by each bolt Critical load for slip = 26%

PresenterPresentation NotesBased on FE results, 5 bolts experienced load close to slip valueAll except bolt 2 had slippedOther bolts also slipped. Believed to be secondary to slip of other bolts.Coating of some contact surfaces was partially worn off, which could lead to premature slippage.

Experimental Results Stage 3 (93 kN)

Bolt #19 broke Four bolts were loosened

Broken bolt

Half of the broken bolt

Loosened bolt

Loosened bolt

Loosened bolts

PresenterPresentation NotesNote that the broken bolt was carrying a tensile load.

Experimental Results Stage 3 (93 kN)

Vertical load-point displacement

Increase in rangeLoosening of bolts

ShiftBreakage of bolt

Vertical load-point displacement

Fracture Surface of the Broken Bolt

Ratchet marks- Crack initiation

Higher roughness- Fast fracture

Smooth surface- Fatigue crack growth

Experimental Results - Stages 4 & 5 (103 kN & 113 kN)

No incident during Stage 4 Stage 5 stopped at 500,000 cycles, failure of two bolts at the support

outside the region of interest

Conclusions Good fatigue behaviour at design load and double the design load Immediate effect of over-load:

Bolt slip, leading to contact between bolt and edge of hole Damaged coating of slipped surfaces

Effect of over-load on fatigue performance Bolt contact possible reduction in fatigue resistance Bolt loosening and fatigue failure of bolt

Confirmed (in approximate manner) critical load for bolt slip Pay attention to components not necessarily considered most critical or

even along the main load path

PresenterPresentation NotesWhat did we learn?

Acknowledgements

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BackgroundOutlineTest Program - SummaryTest SetupTest StagesThe Overload IncidentDamage Due to the OverloadDamage Due to the OverloadExperimental Results Stage 3 (93 kN)Fracture Surface of the Broken BoltExperimental Results - Stages 4 & 5 (103 kN & 113 kN)Conclusions