failure_last_eng_2013_bw.pdf

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FAILURE ANALYSIS

Introduction

Definition

Failure Analysis and Prevention

Objectives of Failure AnalysisObjectives of Failure Analysis

Part one

The causes of fracture

I Tupes of fracture

Il Test methods

Part two

Some examples

Conclusions

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Failure causes

Failure causes %

Design 10

Material/Process 60

Use 30Use 30

carenze di progetto

utilizzo non adeguato

non conformit

materiale/processo

INTRODUCTION

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WHAT IS FAILURE ANALYSIS?

The FAILURE ANALYSIS is the critical method adopted toidentify the physical reasons that originate the fracture or thefunctional anomaly.functional anomaly.

The word FAILURE does not referred exclusively toBREACKAGE but to any phenomenon that induces aperception of a not regular operation of a mechanical part.

It is a technical instrument to improve the quality and thereliability of product.reliability of product.

It evidences the critical situations of a component (part orcomplex product) that lead to economic decline, object orpeople damages or accidents.

It s an instrument of continuous improvement of the product.

FAILURE ANALYSIS E PREVENZIONEQualit ed Utilizzo Atteso di un Prodotto

CUSTOMER SATISFACTIONIn the early 80s, by opening of markets, emerged

the need to improve

Product Quality

Greater attention to Product, resulted in

development of activities designed to:

Manufacture products immediately

MAJOR SAFETY

MAJO RELIABILITY

MORE PERFORMANCE

HIGHER EFFICIENCY

EASY MAINTENANCE

Product Quality

And orient business strategy to

Customer Satisfaction

comply

(do the right thing, right the first time)

This choice has generated quality products,able to satisfy greater expectations ofconsumers and users .

EASY MAINTENANCE

LOW OPERATING COSTS

LOW ENVIRONMENTAL IMPACT

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FAILURE ANALYSIS E PREVENZIONEProblem Solving, Qualit e Soddisfazione del Cliente

Objectives of quality improvement can be reached with the activity ofFailure Analysis, by applying the methods of :

PROBLEM SOLVINGPROBLEM SOLVING

analyzing and evaluating fully technically relevant properties of a product:

SHAPE ADAPTABILITY

CAPABILITIES SERVICE LIFE

When a functional problem arises its necessary pay great attention andavoid mistakes such as: :avoid mistakes such as: :

Do nothing and hope that the problem will go away

Deny the existence of the problem and minimize its importance

Considering the issue accidental

Follow false trails

FAILURE ANALYSIS E PREVENZIONEModel of Problem Solving

Identify

Standardization Determining the causes

Identify

Identify the problem

Collect data

Make assumptions

Test hypothesis

Validate and verify the corrective actions

Corrective actions

development

Test hypothesis

Develop conclusions

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FAILURE ANALYSIS AND PREVENTION

Definition of Failure

Failure:

Occurrence of an event or a situation undesired . Occurrence of an event or a situation undesired .

When a component does not carry out its function properly.

Levels of Rupture:

1. Loss of efficiency

2. Loss of efficacy

3. Loss of operation

FAILURE ANALYSIS AND PREVENTION : Design life

There is no mechanicalcomponent that can endure andwork forever.

Breaks can not be totally avoided .

Can be evaluated, anticipated andcontrolled .

The premature failure (InfantMortality) are often associatedwith problems of quality andworkmanship.

Breaks during the life cycle(Design Life) are less frequentand often random.and often random.

Breaks at the end of the life cycle(Wearout Failures) are due tonatural wear of the components.

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AIM OF FAILURE ANALYSIS

Its carried out to identify the causes of failure Its carried out to identify the causes of failure and prevent the phenomenon happen again.

It can, but does not necessarily, lead to the identification of the causes of fracture.

It is used to activate corrective actions for recovery or improvement.

FAILURE ANALYSIS: Levels of investigations

Three levels to identify:

Physical causes: emergingfrom the laboratory analyzes,

For example:

Rupture of a pressure vessel Physical causes: emergingfrom the laboratory analyzes,calculations, simulations andrelated materials.

Human causes: humanfactors, operating errors,improper use, etc..

Underlying causes:procedural errors,environmental problems, or

Rupture of a pressure vessel

Corrosion with thicknessreduction

Inadequate inspectioncontrols

Insufficient training ofinspection staff

environmental problems, orimponderable causes

Insufficient training ofinspection staff

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OBJECTIVES OF FAILURE ANALYSIS: Requirements

Failure Analysis requires an interdisciplinary approach for Failure Analysis requires an interdisciplinary approach for

an appropriate choice of corrective actions .

Many breaking involve factors crossing several disciplines,

such as metallurgy, mechanical engineering, quality control,

maintenance, human factors, etc..maintenance, human factors, etc..

Teamwork

PART ONE

Causes of RuptureCauses of Rupture

Failure ModesFailure Modes

Investigation methodsInvestigation methodsInvestigation methodsInvestigation methods

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CAUSES OF RUPTURE

PRIMARY CAUSES

Design deficiency

Material defects

Production and/or installation defects

Improper application state

Environment

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DESIGN

COMPROMISE PROJECT DEVELOPMENT

Idea

General arrangement

PRICE

QUALITY

PERFORMANCES

RISKS

General arrangement

Detail:

materials selection

production process

product objectivation

required performances

application conditionsRISKS

application conditions

constrains

criticalities and fracture

risks

CONSTRAINS

Structural computationStructural computation

Stress analysis

and

component sizing

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MATERIAL

SELECTION

Identification of the optimal

combination among:

SELECTION

Materials

Heat Treatments

Technologies

CONSTRAINS

Functional

SpecificationsSpecifications

Object Analysis and

MISSION Settlement

(performance,

duration,

operative cycles)

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CONSTRAINS

Product

specificationspecification

Quality standard,

definition of acceptability,

limits of defects,

dimensional and shape dimensional and shape

tolerances .

DESIGN DEFICIENCY

NOTCHESNOTCHES

VARIATIONS ON THE CHEAP

(ABATEMENT-cost reduction)

TAKE OFF WEIGHT VARIATIONS

CARRY OVER

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DESIGN DEFICIENCY

NOTCHES

To place notches in high stresses zone increases theoperational failure risk.operational failure risk.

(e.g. reduced junction radius in change of sections of ashaft undergone torsionals or bending stresses)

DESIGN DEFICIENCY

ABATEMENTS VARIATIONS and/or TAKE OFF WEIGHT

It is necessary to have a good evaluation of the ratioeconomical advantages / risk factor increases.economical advantages / risk factor increases.

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DESIGN DEFICIENCY

TECHNOLOGICAL TRANSFER or CARRY OVER:

Carefully consider the possible increase of theCarefully consider the possible increase of thestresses and the Mission of the new component

Material Defects

Forged and Moulded

components

Flashes

Extrudes and drawn

components

Flakes

Melting

Porosity, gas

Flashes

Burnt parts

Flakes

Segregations

Shrinkage pores

Band distributions

Inclusions

Flakes

Dechoesions

Signs

Burnt parts

Porosity, gas

Shrinkage pores

Segregations

Cold joints

Inclusions

Plates and sheets

Flakes

Peeling

Scales

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Material defects

They are generally described as Discontinuties which reduce the

performances of products

Despite the controls and continous improvement of productionDespite the controls and continous improvement of production

processes , defects are always present in materials.

It may be necessary to make strict and frequent controls,

depending on the criticality of the product.

Manufacturing

Technological process

aimed to the realization of

a product as described by

Manufacturing/Installation

a product as described by

technical documentation,

using raw materials.

InstallationInstallation

Construction process on site

(plants, structures,

buildings, ecc.)

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Manufacturing defects

Machining

Cracks

Vibrations

Microstructural damages due to worn tools

Re-melting due to electrostatic discharges

Intergranural - electrochemical attack

Residual stress

Grinding defects


Plastic deformation (Molding)

Cracks, rips, failed filling Cracks, rips, failed filling

Thread folding (rolling)

Die tracks

Surface ripps

Residual stress

Stress corrosion cracking (lubricants) Stress corrosion cracking (lubricants)

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Mechanical processing

Steel hub made of 42CrMo4.

Explosion during broaching due to folding of molding


Mechanical processing

Molded water trap of brass OT58:

Tool contaminated by particles of Fe which oxidizes forming

blotches

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Heat treatment

Grain coarsening

Incomplete phase


Incomplete phase

transformations

Quenching cracks

Decarburizing

Residual stresses


Welding

Lack of melting

Cracks and brittlenes in HAZ

Residual stresses

Porosities

Slag inclusions

Craters

Hydrogen embrittlement

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Residual stresses

They are stresses present in the component after heattreatment, mechanical working and / or plastic strain.

They are caused by not uniform deformation for processes thatinduce high temperature or stress gradient in thecomponents.

Residual stresses can show a positive or negative effect:

Positive if compressive stresses (fatigue life)

Negative if tensile (cracks and deformation after mechanicalworking (cutting, turning, drilling, grinding).

Residual stresses releaving

Stress Relieving is carried out at temperature of about 75 Cabove the transformation temperature (A in Fe-C diagram).above the transformation temperature (A1 in Fe-C diagram).

In practice, the treatment is carried out at about 650 C for anhour or until the whole piece reachs the treatment temperature.

In this way about 90% of residual stresses are released .

Alloy steels are heat treated at higher temperatures.

At the end of the cycle, the components are removed from theAt the end of the cycle, the components are removed from thefurnace and cooled in still air.

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Installation defects

Assembly/Installation

Misalignment

Component lack

Wrong choice of component

Wrong positioning

Excessive/scarce tightening Excessive/scarce tightening

torque

Not suitable tools

Unauthorized modification

Unsuitable conditions of use / stress

Not always a priorexpected

They depend on theThey depend on themanner of usage.

No more allowable

Use in nonstandardconditions in respect toproducer specificationsproducer specifications

Working environment moreaggressive than expected

Insufficient maintenance

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Stresses

Main types of

stresses and

cracking mode:

overloaded

Environment

Not always evaluable in advance

(humidity, temperature,(humidity, temperature,

contamination, etc..).

Difficult to assess in retrospect,

downstream of the failure.

Often its synergistic with other

critical factors (i.e. pressure

variability, internal stress, etc..)

Triggers failure modes peculiar,

often unexpected

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Erosion

Seat pump for food mixture stainless steel AISI406

Cratering surface erosion by solid

METHOD OF RESEARCH

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There is an old saying of American carpenters that analysts haveadopted for Failure Analysis :

MEASURE TWICE BEFORE CUT

The failure analysis can be considered as a surveyinvestigation of an inanimate body.

Before start, well its necessary organized mentallyabout the procedure that have be adopted.

Its therefore essential working with: Its therefore essential working with:

METHOD

ORGANIZATION FOR FAILURE ANALYSIS

COLLECTING OF FUNDAMENTALINFORMATION

VISUAL INSPECTION

NOT-DESTRUCTIVE TESTING

RESIDUAL STRESS RELIEF

DESTRUCTIVE TESTING

ANALYSIS OF RESULTS

GUESSES AND REPORT OF ANALYSIS

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DATA COLLECTION: what information are to be collected?

Acquire all the information Acquire all the information relating to events that occurred before and at the time of rupture

Acquire all the information relating to the manufacturing procedures of the particular Brokenof the particular Broken

operational requirements

DATA COLLECTION: what information are to be collected?

operational requirements

Running time (hours, km,)

Temperature e pressure

Materials specifications

Static and dynamic loadingconditions

Environmental conditions(corrosion, erosion)

Vibrations and cyclic loads appliedVibrations and cyclic loads applied

Maintenance applied

Other data deemed useful to theinvestigation

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VISUAL ANALYSIS

It 's the first and mostimportant step in ruptureanalysis: from everythingthat it can be record, it willinfluence the correctinfluence the correctcontinuation of theinvestigation

Examining the scene,sketching, take measures,notes and photographs,whenever possible

Selecting parts to removeand analyze in the laboratoryand analyze in the laboratory

Carefully examine all the finds

VISUAL ANALYSIS

Carefully examine all the findswith the aid of a magnifying

Take pictures and micrography

Assess the presence ofabrasion, overheating, stressstates, corrosion.

Remove samples of corrosion orforeign material for necessaryanalysisanalysis

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When the size of the affected component is relevant, the choice of thesampling areas is decisive on the outcome.

VISUAL ANALYSIS

VISUAL ANALYSIS

Dimensionalmeasurements, if deemednecessaryAnalyze the fracture:triggers, appearance offracture at the starting, inpropagation and in the finalstages of breaking.stages of breaking.

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SAMPLES TAKING

VISUAL ANALYSIS

identify the sites of taking andphotograph it before cutting

take as much as possible in the area ofrupture

avoid damage of the zones of rupture

clean samples befora examine it

Do not use abrasives or acids forcleaningcleaning

... clean the samples before examine it without using anyabrasives or acids for cleaning (when its enough!).

VISUAL ANALYSIS

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NOT-DESTRUCTIVE TESTING

RADIOGRAPHY

CT (Computerized tomography)

MAGNETOSCOPY

PENETRATING LIQUIDS

ULTRASOUNDS

INDUCED CURRENTS

Modes of fracture

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Modes of fracture

STRAIN

FRACTURE

CORROSION

WEARWEAR

Instantaneous

Fracture can be:

Instantaneous Overcharge (ductile/brittle)

Progressive Fatigue Corrosion Wear Creep Creep

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Modes of fracture

Fracture

It is typically defined as separation of material.

It can ductile or brittle

Any mechanism of structural separation, which

progressively leads to rupture.

Modes of fracture

Plastic strain

It occurs when a geometric modification prevents the component to perform its

function.

Changes in the geometry result in changes involume (bulge, shrinkage) or shape (twisting,bending or crushing).

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Modes of fracture

Corrosion

Decaying of the material for environmentalcauses.

The most common type of corrosion metals isthe electrochemical corrosion.

Other forms comprise brittleness by immersionin liquid, molten salts, heat oxidation, etc..

Modes of fracture

abrasiveabrasive erosiveerosive

Wear

Removal or distribution ofsurface by contact and relativemotion with solid, liquid or gas.motion with solid, liquid or gas.Loss of material and load-bearingcapacity, phenomena ofadhesion, increased friction andformation of metal chips.

adhesiveadhesive

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Instantaneous fracture: Strain Ductility

Ductility

tendency of a material to deformsignificantly plastically before fracturing.

Brittlenes

Tendency of a material to fracture without exhibit before asignificant plastic deformation

Instantaneous fracture: Fracture brittleness

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Instantaneous fracture by overcharging: Ductility and brittleness

Ductile fracture Brittle fracture

Ductile fracture: DIMPLES (micro-ductility)

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Traction

Ductile fracture: DIMPLES orientation

Traction

Twisting

Brittle fracture: CLEAVAGE and INTERGRANULAR

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Brittle fracture: CLEAVAGE

Brittle fracture: INTERGRANULAR

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Ductile / Brittle Transition

Materials with body-centered cubic lattice

Damage produced by repeated stress through time

Fatigue: Progressive Fracture

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Main features of fatigue

It is essentially a surface phenomenon.It depends on the stress value, not onthe frequency.the frequency.

Also the propagation rate depends onthe amount of the stress value.

May synergistically appear with othersources of degradation of materials (eg,thermal fatigue, stress corrosion, etc.)..

The morphology of fracture ischaracteristic.

Fatigue: Morphology of fracture

Flession + traction Twisting

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At higher magnification ...


At higher magnification ...


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Waves of fatigue (beach marks)

Waves of fatigue (beach marks)

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PITTING CORROSION

Nozzle head shot blasting machine by water of C40 with nickel plating

Pitting:contamination of the grit with iron oxidePitting:contamination of the grit with iron oxide

Tube of stainless steel AISI 304

Pitting: perforating corrosion

Interstitial Corrosion: CREVICE

It occurs at the interstices, wherepassive film breaks (e.g., theconjunction of two metallic parts)conjunction of two metallic parts)

- generally its development is due todifferentiated aeration betweenexternal and internal surfaces

- its generally produced in neutralenvironments

- it affects bulk materials and coatings

- its also produced in metal-nonmetalcontacts (eg. seals), obviouslydamaging the metal part.

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CORROSION FATIGUE

Wheel rim of magnesium alloy: fatigue fracture enhanced by corrosion

The wheel rim is not varnished: corrosion starts from edge of thespokes and there nucleates fatigue fracture

WEAR: Abrasive wear

Al 6060 of the body of hydraulic pump:

Abrasion of inner wall: insufficient hydrostatic capacity of oil atlow rpm

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