metallurgy as a tool for root cause failure investigation · 2016. 8. 22. · • fitted with a...

Metallurgy as a Tool for Root Cause Failure Investigation

Dr Janet Cotton

BSc(eng) PhD(eng) UCT

Director – One Eighty (Pty) Ltd

8th June 2011

Root Cause of a Failure

• Design • Materials Defects • Environment • Manufacturing • Service Life

Tools

• Failure Mode Effect Analysis • Review Sources of Input • People Interviews • Laboratory Investigations

– Metallographic examinations

– Mechanical Testing

– Compositional analysis

– Scanning Electron Microscopy • Fracture Surfaces

• Analysis of Corrosion Products

• Stress Analysis – Effects of notches – Extent of an extraneous load

• Fracture Mechanics Analysis – Fatigue life calculations

Case Study: Big End Failure

• MFV “ Freesia” • Fitted with a Watsilla 6L26 Diesel Engine • Failed Catastrophically 1175 hours after

ext 24 000 hour service • Failure occurred without warning


Remaining Connecting Rod

Starboard Port

Remaining Components – Big End

Starboard Studs

Port Studs

Fatigued Surfaces

Overloaded Surfaces

Case Study: Big End Failure Connecting Rod

Small end journals

Big end journals

Crank Shaft

Gudgeon pin

Pistons

Cylinder Liners

Bolts

Stb Fore & Aft Pt Fore & Aft

No sign of wear

Damage associated with failure

No sign of wear No sign of wear

No sign of wear

Fatigue Fracture

Overload


Top Fracture Surface Bottom Fracture Surface Initiation Site

Starboard


Overload Failure

Necking of stud

Port fore and aft stud


C Si Mn P S Cr Mo Ni 0.38 0.38 0.86 0.01 0.01 1.17 0.24 0.23

•No Flaws

•Expected Microstructure


• Failure – Initiation of fatigue crack • Measure length ~ 10e7-10e8 cycles • High Stress

– Over torque of studs – Insufficient bedding – Transient event


• Conclusion – Most probably cause for failure

• Hydraulic Knock • Corresponded with the crack length • Fracture mechanics • Maintenance records • Interviews

Case Study: Push Rod Failure

• Fishing Vessel MFV “Sette Marie” • CAT D399 V16 engine • 1250 RPM variable pitch propeller • Pain and Brink Gear Box 1:4.7 • Problem:

– Series of push rods failed – Failures occurred after shorter service life


Propeller Hub

Stern Tube Bearings

Forward Bearing

Main Engine

Aft Bearing

Thrust Bearing

Gear Box

Push Rod

Stern Tube


Propeller Blade Hub


Wear Marks from Blade Assembly


Fracture through thread


Ratchet marks ~

Fatigue initiation

Final Fracture

Direction of engine rotation

Rotating Bending Fatigue


Wear of the forward stern tube bearing


• Summary of Physical Evidence – Fracture – Rotating Bending Fatigue – Thread worn – Wear on Blade Hub – Wear on forward bearing


Bending stress

on push rod

Rotating Bending Fatigue

Shudder on Blades

Role of the Worn Thread

Misalignment ?

Where ?


• Fatigue Stress Required – ~ 160MPa – Worn thread reduced strength

• Propeller blade shudder not feasible • Misalignment more feasible

– Laser measurements – 1.5mm misalignment stb direction along stern

tube


• Conclusions – Interpretation of fracture surface

• Type of loading • Magnitude of the stress

– Stress Analysis & Fracture Mechanics

• Feasibility various scenarios • Root cause – misalignment

Case Study: Aluminium Hull

• Allures 44 ft Sailing Yacht • Aluminium Hull AA 5083 6.0mm Plate • Drop Keel System • Fixed Ballast System • Built 2003 • New Ownership Nov 2007 • Corrosion problem observed Jan 2008


Allures 44 ft Aluminium Sailing Yacht


Lead Bars

Resin

Hull

Centre Board Assembly


• Metallurgical Techniques – Energy Dispersive X-Ray Analysis (EDS) on

corrosion deposits – Cut sample of the polyester resin, sectioned,

microscopy


Mg

Na

Al

Cl

0 2 4 6 8 10 12 14 16 18 20keVFull Scale 625 cts Cursor: 0.000

Spectrum 2


Sample Elements Present Peak Elements

Gel deposit 1 Na, Cl, Al Al

Gel deposit 2 Al, S, Cl, Ni, O Al, Cl

White gel deposit Cl, Al, Na, Cl, C, O Al, Cl, Na

White deposit Al, S, Cl, K, Na Al, Cl

Brown deposit Al, Cl, S, Na, Mg, Ca, Cl Al, Cl, Na

Yellow deposit Al, Na, S, Cl, O Al, Cl

Resin sample Cl, O, Al, Si, Cl Al, Si

Resin sample near lead Al, Si, K, C, Mg, Fe Al, Si

Strong evidence that the hull was corroding


porosity

Polyester Resin

Sample mount

material

Polyester resin acting like a sponge


• Conditions: – Stagnant water in contact with AA 5083 – Water was acidic – Rich in Cl- – Effect of the lead bars


Lead Bars

Resin

Hull

Centre Board Assembly


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

302928272625242322212019181716

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

908988878685848382818079787776

91 92 93 94 95 96 97 98 99 100101102103104105

PLATE A

PLATE B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

302928272625242322212019181716

31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75C

OUTBOARD SIDE

KEEL SIDE

FWDAFT


0

10

20

30

40

50

60

70

80

90

100

A1-A36 A37-A72 A73-A105 B3-B38 B39-B75

Port Starboard

Pe

rce

nt

This chart shows the percentage of readings in each group of 100mmx100mm sections that shows an under thickness measurement of greater than 0.2mm.


Cross member

Pitting

Starboard - aft section

Excessive pitting

Port – forward section


• Root Cause for Damage • Solution ?

– Lead ballast must be insulated from the hull – Water ingress must be prevented

• Coat hull with epoxy resin, a few mm thick • Coat each lead bar with epoxy resin • Place bars • Fill up the space with resin

Conclusions

• Root Cause Investigations – Find out how it broke ! – How to fix it, prevent further failure / damage – Whose fault was it ?

• Metallurgical Expertise – Experimental work – Understanding of materials

• Stress Analysis – Couple failure mode to stress origins

metallurgy as a tool for root cause failure investigation · 2016. 8. 22. · • fitted with a...

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