Material Technology 2_chapter 4 Failure_Analysis

8/2/2019 Material Technology 2_chapter 4 Failure_Analysis

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Failure Analysis

Dr. Zainuddin SajuriJabatan Kejuruteraan Mekanik & Bahan

Fakulti Kejuruteraan & Alam Bina

Universiti Kebangsaan Malaysia([email protected])

What, Why and How ofFailure Analysis


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Failure Analysis 4

Failure Analysis 5

What is Failure?

Definition of Failure'

o a device or structure is no longer able tofunction as intended.

o system or part of a system fails to perform upto the expectations for which it was created.


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Failure

Failure is often the end result of unexpectedfactors:

o Unforeseen vibrations develop;o unanticipated temperatures are encountered;o manufacturing tolerances can not be held;o components do not quite mate as intended;o the product is used or serviced in unintended

ways.

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Failure is a human concept

Materials do not fail in and of themselves.

o They follow the laws of nature perfectly.o If a part is loaded beyond its tensile strength,

it breaks.o Until that stress level is reached, it does not

break.


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Failure Analysis 8

When a part fails in service

o under-designedo bad engineeringo poor manufacturingo inadequate quality controlo fraudo typographical errors

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"Just because you can,

does not mean you should"


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Failure Analysis 10

Failures Are Caused byHuman Errors

Three general types of error:

o Error of knowledgeo Error of performanceo Error of intent

Tidak boleh menyalahkan Tuhan atas kemalanganyang berlaku:- Ini kehendak Tuhan- Ini takdir Ilahi..

Failure Analysis 11

Error of knowledge

Involve insufficient knowledge, education,training, and/or experience.

o Hydrogen Embrittlement (HE) causesotherwise stable high strength steel

components to fail.


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Failure Analysis 12

Error of performance

Result from lack of sufficient care or fromnegligence. Negligence involves such things asmisreading of drawings, inadequate specifications,and defective manufacturing and workmanship.

o Recent NASA failures in a Mars missioninvolved the incorrect conversion from the

English to the Metric System of measurementin a computer program.

Failure Analysis 13

Error of intent

Very commonly involve greed. Greed leads toactions usually carried out with a conscious orunconscious denial of full knowledge of thepotential consequences.

o Cost reduction driving design of military

vehicles causing premature failures.

o Aloha stadium superstructure corrosionfailures were caused by lack of surfacepreparation and poor materials and coatingselection.


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Failure Analysis 14

What is Failure Analysis?

'forensic engineering'

- the investigation of failed components with aview to ascertaining the cause of failure in orderto prevent further failure, injury or loss of life,and thus reduce any associated financial loss.

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Why Undertake a FailureAnalysis?

Most important: To prevent future failures

Sometimes: To find who to be blamed


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Failure Analysis 16

What Are Some of TheEffects of Failure?

o Loss of profito Injuryo Loss of life

o If a detailed failure analysis is carried outand appropriate ameliorating action takenrepetition of such failure can often be

avoided.

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What Are Some of TheAgents of Failure?

Four major failure agents

o Forceo Material

o Timeo Environment

This means that components will only fail dueto one, or perhaps a combination of several,of these four failure agents.


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What are Some of the Causeof Failure of Machinery?

At least one or more of the following sevencause categories:

o Faulty designo Material defectso Fabrication and/or processing erroro Assembly or installation defectso Off-design or unintended service conditions

o Maintenance deficiencies, including neglectand procedures

o Improper operation

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Some Other Causes ofFailure

o Misuse or Abuseo Manufacturing defectso Improper maintenanceo Fastener failureo Improper material

o Improper heat treatmentso Unforeseen operating conditionso Inadequate quality assuranceo Inadequate environmental protection/controlo Casting discontinuities


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Failure Analysis 20

What Can Be Done toPrevent Failure?

An understanding of possible causes of failure,allows the risk of premature failure to beminimized.

o Kept acceptably low stress levelo Materials chosen are able to withstand

severe environmento Quality control standards need to be imposed

o Manufactured components meet the therequirement of the design

o System needs to be operated as designed,

without abuse or overload.

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What Can Be Done toPrevent Failure?

o If, using appropriate techniques, a potentialfailure and its cause can be identified toprevent complete/final failure may, in somecases, be taken.

o In other cases, if accurate loading data isavailable and the material properties areknown, the potential for catastrophic failuremay be assessed using fracture mechanicsprinciples, such as life prediction.


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Important

Many failures are preventableif weunderstand the materials and their intendedapplications well enough and are willing to paythe required costs for safety and durability.

Root CauseDetermination


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Failure Analysis 24

Force/Materials/

Time/Environment

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Product Specifications andFailure

Service-life-expectancy of a product

The level of degradation that will bedesignated as failure.

Definition


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Product Specifications andFailure

Perfect materials do not exist

o Presence of defectso Corrosion loss

o Establishing limits Type Size Location distribution

Create allowancesin design,

fabrication &Application

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Root Cause Determination

Product fails gradually, suddenly orcatastrophically

o A method of evaluationmust to understand why

the failure occurred.

o Valuable guidance to avoidfuture failure

Root-cause failureanalysis provides


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Primary Cause vs. Root Cause

o The primary cause is the set of conditionsor parameters from which the failure began.

What is fundamentally responsible for the failurein performance and determine the sequence ofevents that led to the final failure.

o The root cause of a failure is a process or

procedure which went wrong.

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Example:

o The finish on a machine part was not asspecified.

o The heat-treatment on a rail was not uniform.

o The angle on screw-threads was too steep.

Identification of that process is the key tocreating a procedure by which future failurescan be avoided. Most failure analysis stopsshort of this final step.


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Failure Analysis 30

The poor finish, the incorrect heat treatment,the shape of the screw threads in the paragraphabove are the primary causes of thosefailures, not the root causes.


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The root causes would be:

o The failure to check the finish after the partwas machined,

o The failure to ensure that the heat treatment

furnace had sufficient control of changes intemperature to produce the desiredmicrostructure in the rails, or

o The failure to enter the proper informationinto the thread-cutting process.



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Failure Analysis 32

A failure analysis is much like the work of adetective.

o Discover important clues through investigation

o Discover the contributing factors of failure

How to Conduct FailureAnalysis

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Success of failure analyst

o Broad knowledge of materials in general

o Aware of the failed materials mechanical andphysical properties and its fabrication andhistorical performance characteristics.

o Possess a working knowledge of structuraldesign and stress behavior.



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Failure Analysis 34

1) Preliminaries: Determine when, where andhow the failure occurred.

2) Collect samples for laboratory examination.

3) Take on-site photographs.

4) Visually examine the sample.

5) Identify defects Non-Destructively.

6) Conduct appropriate chemical analyses.7) Confirm material composition


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8) Analyze via Fractography

9) Analyze via Metallography

10)Conduct Appropriate Mechanical and

Materials Testing and Analysis as Necessary11) Determine the type of failure

12)Synthesize and summarize the data,determine and report the root-cause of thefailure.



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a) Determine whether or not destructivetesting is permitted or if the testing must belimited to non-destructive approaches.

If the failure is or may be subject to litigation,opposing counsels must agree on this point beforeany sampling begins. Witnessed testing (thepresence of parties from both sides in a law suit)may be called for.

1. Preliminaries: Determinewhen, where and how the

failure occurred.Before beginning any failure analysis,

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b) Visit the failure site in the field if possible. All operators involved in the failure should be

interviewed personally.

Determine what the conditions were at the timeof failure.

Were there prior indications suggesting failurewas about to occur?

Was the failure gradual or catastrophic?

Was the part protected after failure? How was the fracture handled?

Did the failure involve any fire or other conditionwhich could have altered the microstructure ofthe base metal or of some part of the sample suchas a weld?

These and all other appropriate questions shouldprovide a basis for the investigation.


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Failure Analysis 38

c) To obtain documentation on maintenanceprocedures

during the lifetime of the equipment that failed

maintenance personnel,

records of scheduled maintenance, and

suppliers and products used.

obtain the physical and chemical specifications for

the product which failed, against whichperformance may be measured.

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a) Characteristic of the material

b) Contain a representation of the failure or

corrosive attack.

c) Taken from a sound and normal section (forcomparative purposes)

2. Collect samples forlaboratory examination

Samples selected should be:


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Failure Analysis 40

Sample handling

The whole remaining analysis depend on samplehandling

a) Fracture surfaces must be protected fromdamage during shipment by careful packaging.

b) Surfaces should not be touched, cleaned orput back together.

c) Surface chemistry must not be contaminatedby careless handling.

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Material specification

a) Materials specifications and service historyreveal much about the nature of failure.

b) Background information will need to be

provided for analysis.c) Take copious notes.

d) Do not rely on memory


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Failure Analysis 42

Samples removal

a) Acetylene torch,

b) air-arc,

c) saw,

d) trepan, or

e) drill.

All cuts by torch or air-arc should be at least six

inches away from the area to be examined toavoid altering the microstructure or obscuringcorrosive attack.

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3. Take on-site photographs

a) The failed pieces and the samples to beremoved and their surroundings.

b) Show the relationship of the questioned areato the remainder of the piece of equipment.

c) The samples after removald) Proper designation, location and dimension of

the sample

e) The date the failure occurred, and the dateof the photographs

f) Use video recording if necessary

Photographs should be taken of


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Failure Analysis 44

4. Visually examine thesample

a) Examine the sample with unaided eye, handlens and/or low magnification fieldmicroscopes.

b) Note all anomalies, searching for cracks,corrosion damage, the presence of foreignmaterial, erosion or wear damage, or evidenceof impact or other distress.

c) The condition of protective coatings.d) Manufacturing defects.

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4. Visually examine thesample

a) Measure wall thickness both at the failuresite and some distance away from it.

b) Note the presence of any corrosion and map

its general distribution.

If pipe failure


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Failure Analysis 46

5. Identify defects Non-Destructively

o radiography,

o magnetic particle,

o ultrasonic,

o liquid/dye penetrant,

o eddy current,

o leak,

o acoustic emissions

Search for material imperfections with

Magnetic Particle Testing doneby inducing a magnetic field in aferro-magnetic material anddusting the surface with ironparticles.

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5. Identify defects Non-Destructively

Radiography Fluorescent Liquid DyePenetrant viewed in black light.


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Failure Analysis 48

6. Conduct appropriatechemical analyses

a) material was of proper type and grade,

b) Material met appropriate standards,

c) Whether deviation from the specificationscontributed to

o fracture,

o wear,

o breaks corrosion and

o failure.

Chemical analysis to determine

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Method of chemical analysis

o Wet chemical analysis,

o Atomic Absorption,

o X-ray Photoelectron,

o Auger Electron and

o Spectroscopy


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Failure Analysis 50

7. Confirm materialcomposition

a) In conjunction with scanning electronmicroscopy (SEM) to identify the elements

b) Advantages of EDS are

Easily performed

reliable as a qualitative method.

c) Limitations

Only marginally useful as a quantitative method.

Use EDS (Energy-Dispersive Spectroscopy) toidentify material composition.

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EDS

Line scanning isolates an area of thespecimen. The red line indicates thelocation of the scan.


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Failure Analysis 52

8. Analyze via Fractography

a) the mode of fracture

o intergranular,

o cleavage, or

o shear

b) the origin of fracture,

c) location andd) nature of flaws that may have initiated

failure.

Fractography is used to determine

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8. Analyze via Fractography

a) Determine why a part failed can usually bedetermined.

b) Reveal the relationship between physical and

mechanical processes involved in the fracturemechanism.

c) The size of fracture characteristics rangefrom gross features to a few micrometersacross.

The importance of fractograph


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Failure Analysis 54

Common Technique inFractography

a) Light microscopy

b) Scanning electron microscopy

the depth of field in the SEM is muchhigher; thus the SEM can focus on allareas of a three-dimensional objectidentifying characteristic features such

as striations or inclusions..

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9. Analyze via Metallography

a) not to remove inclusions, erode grainboundaries

b) Study structural characteristics in relation

to its physical and mechanical propertiesc) note of grain size, shape, and distribution of

secondary phases and nonmetallic inclusions.

Prepare a laboratory specimen


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Failure Analysis 56


Intergranular StressCorrosion Cracking (ISCC) inturbine component

Cross-section of copper lancecomponent exposed to excessive

temperatures showing graingrowth

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a) pit depth,

b) intergranular corrosion,

c) hydrogen attack and embrittlement,

d) caustic embrittlement,

e) stress corrosion cracking (intergranular ortransgranular),

f) corrosion,

g) mechanical or thermal fatigue.

Metallography for the analyst may be concernedwith


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10. Conduct AppropriateMechanical and MaterialsTesting and Analysis asNecessary

1. Physical Testing

2. Finite Element Analysis

3. Fracture Mechanics

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Physical Testing

o Hardness,

o tensile strength,

o impact,

o fatigue resistance,

o wear,

o Flexibility etc.

To determine if the mechanical properties- conform to specifications.


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Failure Analysis 60

Physical Testing

o excessive elastic deformation (deflectionunder applied loads),

o yielding (permanent material deformation asa result of stress), or

o fracture.

Structural members and machine parts can failto perform their intended functions by:

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Physical Testing

o Deflection of closely mating machine partsdue to surface stresses (elastic deformation)can degrade adjacent parts by increasingwear and in certain cases can promotecomplete failure.

o A study of the mechanical properties of theparts can provide information on load-bearingcapabilities of the system and can minimizesuch failures.

Examples:


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Finite Element Analysis

o Predicting the response of an existingcomponent or assembly to stress

o Assessment of remaining life of a componentor assembly

o Determining the failure mode of a failedcomponent or assembly, e.g. fatigue, creep,

and buckling.o Designing of a new component or assembly as

a part of recommendations for remediationof the problem

A powerful numerical tool for analyzingmechanical components and systems

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Fracture Mechanics

o How the part in question actually failed,

o What the mode of failure was, and

o Where the failure was initiated.

Using the many analytical techniques abovewill help to determine


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Failure Analysis 64

Fracture Mechanics

o How the part in question actually failed,

o What the mode of failure was, and

o Where the failure was initiated.

Using the many analytical techniques abovewill help to determine

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Fracture Mechanics

o The size of flaws in a material, principallycracks, to

o The applied stresses on those cracks, and to

o the fracture toughness of the material, orits resistance to cracking.

Fracture mechanics relates


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Failure Analysis 66

Fracture Mechanics

The relation between flaw size, stress andfracture toughness

aY

KIC

: fracture stress

Y : dimensionless shape factor

a : crack length

KIC : fracture toughness

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Fracture Mechanics

o the determination of permissible flaw size,

o the calculation of the stress necessary tocause catastrophic failure

o the determination of the load on a componentat the time of failure

o the determination as to whether adequatematerials were used in manufacturing

o the determination as to whether a partdesign was adequate.

This calculation will allow


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Fracture Mechanics

o how a particular crack formed at a specificlocation and

o the stress conditions that caused the crackto propagate.

Thus, fracture mechanics can be used to helpus understand

The design engineer will normally includefactors of safety in his design to preventstresses from reaching critical levels

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11. Determine the type offailure

a) Ductile fracture,

b) Brittle fracture,

c) Fatigue fracture,

d) Wear,

e) Fretting,

f) Elevated Temperature and

g) Corrosion

The major types of failures likely to beencountered by metals in service are:


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Failure Analysis 70

12. Synthesize andsummarize the data,

determine and report theroot cause of the failure

Proposed root causes of a failure must bebased primarily on observed facts. Thesefacts, combined with the experience, skill and

knowledge of the analyst will lead to soundconclusions.

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12. Synthesize andsummarize the data,determine and report theroot cause of the failure

All the observed data should be reported,even if some of it seems peripheral. In thefuture, with additional data, it may turn outto be possible to use what seemed peripheralat first to make an even more soundinterpretation.


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Failure Analysis 72

The failure analysis report represents theculmination of the analysis effort and thebeginning of failure elimination. The goal of anyfailure analysis is targeted towards theelimination of identified causes and preventfuture failure.

How to Write FailureAnalysis Report

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The completed failure analysis report includesthe following sections:

a) Description of the failed component

b) Service condition at the time of failure

c) Prior service historyd) Manufacturing and processing history of component

e) Mechanical and metallurgical study of failure

f) Metallurgical evaluation of quality

g) Event Summary of failure causing mechanism

h) Recommendations for prevention of similar failures



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Failure Analysis 74

The final failure analysis report providessolutions with expected returns on investmentsbut also identifies how the failure occurred inthe first place. To accomplish this eventsummary, a description of the failure mechanismand list of recommendations are included in thereport.


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A brief description of:

Event Summary

a) how the failure was first noticed

b) how long it has been going on andc) the method(s) used to isolate or mitigate the

consequences of the failure.


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A summary of the root cause(s) that led tofailure occurrence.

Failure Mechanism

a) characterize the things that must occur inorder for the failure to manifest itself.

b) outline the mechanical and metallurgicalstudy of failure including the metallurgical

evaluation of quality.

Explain what, when and who is going to beresponsible for implementation, and also includea recommendations for prevention of similarfailures.

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Material Technology 2_chapter 4 Failure_Analysis

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