Managing Fatigue Failures of Longwall Hydraulic Equipment

26th Mechanical Engineering Safety Seminar 4th August 2016

Russell Smith Daniel Carpenter

Your Machinery has been performing flawlessly for years

Your Job is to keep the machinery running

Suddenly, your key machine starts failing, and no one

seems to know why

You find the failures are due to fatigue, and design effective

remedial strategies.

Learn about fatigue and how to manage it

Familiarise yourself with the causes and characteristics of fatigue failure.

Overload failures require just a single load application.

Overload failure occurs at less than the theoretical bond

strength.

Dislocations can move at very low stresses, and pile-up at stress concentrations and form cracks.

Overload failures often display bend-before-break

characteristics

Necking Elongation

Fatigue failures often display ratchet marks, beach marks, and striations.

Ratchet Marks Beach Marks

Striations (microscopic)

Final Fracture

80 - 90% of component fractures are due to fatigue.

Normal Distribution

Mean = 3

-1 0 1 2 3

Standard deviation = 1

Fatigue failures follow a lognormal distribution. Median

Median = 2

Geometric SD = 2

1 2 4 8 16

Mean

Number of Cycle

Fatigue lives for identical components can vary by a factor of 10.

Understand the key inputs needed for reliable fatigue

design.

Cycle counts are required for all repetitive events.

Dynamic responses add to the planned behaviour.

Fatigue damage from cycles of different magnitudes is not

a simple addition.

1 + 2 3

Difficult inspection and repair conditions demand low failure probabilities.

Large populations make matters worse.

Intolerable failure consequences demand very small failure probabilities.

Welded and non-welded components behave differently

Pre

ssur

e

Cycles

Not all standards use the same catalogue of fatigue details

1000

Stre

ss

100 1.E+04 1.E+05 1.E+06 1.E+07

Cycle s

Different standards adopt different failure probabilities

AS 4100, EN1993 5% probability of failure

BS7608 (default) 2.3% probability of failure

BS7608 (user defined) ?% probability of failure

Assess your available remedial options.

Trend do you have a problem?

Repair

Replace

Quantify & Analyse what causes the most damage? can it be eliminated or minimised?

Minimise the Worst Contributions

Can the peaks be clipped?

Operator Training

Different Material

Material Strength = Fatigue Life Toughness = Critical Crack Length

But not for welded components Welded and non-welded components

Pr

essu

re

Cycles

Pres

sure

Cycles

1000

Different Detail St

ress

Stre

ss

100 1.E+04 1.E+05 1.E+06 1.E+07

1000

100 1.E+04 1.E+05 1.E+06 1.E+07

Cycles Cycles

Different Owner

Acknowledge Wrong to Make it Right - Staples

Spring Steel 420 Stainless

Acknowledge Wrong to make it Right - Sample Size

Acknowledge Wrong to Make it Right Leg Cylinders

THANK YOU - THE END

Russell Smith Daniel Carpenter

Managing Fatigue Failures of Longwall Hydraulic EquipmentYour Machinery has been performing flawlessly for yearsYour Job is to keep the machinery runningSuddenly, your key machine starts failing, and no one seems to know whyYou find the failures are due to fatigue, and design effective remedial strategies.Learn about fatigue and how to manage itFamiliarise yourself with the causes and characteristics of fatigue failure.Fatigue failure requires repetitive loading and unloading Overload failures require just a single load application.Overload failure occurs at less than the theoretical bond strength.Dislocations can move at very low stresses, and pile-up at stress concentrations and form cracks.Fatigue failure requires repetitive loading and unloading Fatigue failures are different to overload failureOverload failures often display bend-before-break characteristicsFatigue failures often display ratchet marks, beach marks, and striations.80 - 90% of component fractures are due to fatigue.Fatigue failure requires repetitive loading and unloading Fatigue failures are different to overload failureFatigue lives scatter statistically within large component populationsNormal DistributionFatigue failures follow a lognormal distribution. Fatigue lives for identical components can vary by a factor of 10.Understand the key inputs needed for reliable fatigue design.Identify the repetitive loads of the applicationCycle counts are required for all repetitive events.Dynamic responses add to the planned behaviour.Fatigue damage from cycles of different magnitudes is not a simple addition.Identify the repetitive loads of the applicationEstablish your tolerable failure rateDifficult inspection and repair conditions demand low failure probabilities.Large populations make matters worse.Intolerable failure consequences demand very small failure probabilities.Identify the repetitive loads of the applicationAdopt your tolerable failure rateNot all fatigue design standards are equal choose carefullyWelded and non-welded components behave differently Not all standards use the same catalogue of fatigue details Different standards adopt different failure probabilities Assess your available remedial options.Do nothingTrend do you have a problem?RepairReplaceDo nothingChange the operationQuantify & Analyse what causes the most damage? can it be eliminated or minimised?Minimise the Worst ContributionsOperator TrainingDo nothingChange the operationChange the designDifferent MaterialDifferent DetailDifferent OwnerDo nothingChange the operationChange the designWatch out for trapsAcknowledge Wrong to Make it Right - StaplesAcknowledge Wrong to make it Right - Sample SizeAcknowledge Wrong to Make it Right Leg CylindersSuddenly, your key machine starts failing, and no one seems to know whyYou find the failures are due to fatigue, and design effective remedial strategies.Learn about fatigue and how to manage itSlide Number 53