Failure Analysis

1Ludwig & Associates Engineering Ltd.

Metallurgical Failure Analysis:Techniques and Case Studies

Prepared By:Derrick Sarafinchan, M.Eng., P.Eng.


Failure Analysis Objectives Primary objective

! Identify the root cause for a particular failure

1. Metallurgical Evaluation

2. Root Cause Analysis

Failed Component(s) Failure Analysis Investigation Reporting: Examination Findings Failure Mechanism(s) Probable Root Cause


Failed Component

Failure: A term which infers that a component or assembly is in a state of damage that renders it no longer capable of reliably performing its intended function.

Definitions


Failed Component

! Prior Damage: damage that may have contributed to the failure

! Consequential Damage: damage that resulted due to continued operation post failure

! Mechanical/Environmental Damage: subjected to the failed component!Handling

!Cleaning

!Storage

Definitions

Secondary Damage: Inadvertent changes to a component that may or may not affect functionality.


Fracture (full section) Cracking (partial section) Distortion (bending, elongation, plastic collapse) Corrosion (pitting, through wall perforation) Wear (material wastage)

Failed ComponentGeneral Failure Categories


Failure Analysis Investigation

A. Examination and Testing WHAT physical evidence can be attained from the

failed component(s)?

B. Identify Failure Mechanism(s) HOW did the damage/failure occur, in metallurgical

terms?



Metallurgical Evaluation

1. Characterize Failure Identify Failure Origin Characterize fracture textures

A. Examination and TestingPrimary Objectives:


Fracture Examples

! Ductile shear lip Fracture Macro Textures:

Metallurgical EvaluationA. Examination and Testing

Primary Objectives:



Fracture Examples Fracture Macro Textures:

!Chevron markings

1. Characterize Failure

A. Examination and Testing


Fracture Examples Fracture Macro Textures:

! Ratchet markings! Beach markings 1





Fracture Examples Fracture Micro Textures:

!Intergranular






! Transgranular ! Dimple (MVC)






! Transgranular ! Cleavage






! Transgranular ! Striated! Fibrous textured





2. Characterize and categorize other forms of visual damage present. Secondary cracks along component or adjacent to fracture Anomalies associated with the region of initiation or otherwise

potentially contributory to the failure


Primary Objectives:

3. Characterize the material. Microstructure Composition Mechanical Properties


1. Visual examination and NDE (by eye)

2. Macroscopic examination (stereomicroscope) provides depth of field magnification to about 20X

3. Microscopic examination Fractures - examined by SEM to characterize fracture textures

(20X to 3000X magnification) Prepared cross section - optical microscopy (no depth of field

magnifications between 50 and 1000X)! Characterize microstructure and damage/fracture profiles


Tools and Techniques:


4. Chemical Analysis Generally for grade verification or impurity evaluation Examples:

!Stainless steel corrosion/oxidation resistance!Alloy steel hardenability, temper embrittlement resistance,

weldability




4. Chemical AnalysisTypical Methods

Bulk Analysis !Spark Emission!Wet Analysis

Microanalysis !SEM EDX

!XRF/XRD (non-metallic scales and friable substances)




5. Mechanical Testing Generally for purposes of grade determination/verification,

or estimation of component fracture properties

Examples:! Material strength and ductility! Fatigue properties! Fracture resistance




5. Mechanical TestingTypical Methods

Hardness testing (micro and macro) Tensile testing (yield, ultimate, and elongation) Charpy V-notch impact testing (temperature transition, value

conversion for Level 1 fracture mechanics assessment) Fatigue testing (axial or bending) Fracture toughness testing (CTOD)




Definition Specific series of events that describe both how the

damage was incurred and the resulting consequences.


Examples Metal fatigue Stress corrosion cracking Hydrogen embrittlement

Ductile or brittle fracture

High temperature creep, oxidation, sulphidation

B. Identification of Failure Mechanism(s)


General Categories of Failure Root Cause

Metallurgical deficiencies Mechanical overload Service environment Manufacturing deficiency Remanufacturing/repair deficiency Design deficiency

Failure Analysis Investigation2. Root Cause Analysis


Root Cause Analysis

Castings - Porosity (shrinkage)

Common Metallurgical Deficiencies


Root Cause Analysis

Castings - Poor homogeneity



Root Cause Analysis

Castings - Microsegregation (Al-N embrittlement)

2

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Wrought Materials (rolled, forged, etc.) Inadequate heat treatment

! Inadequate case depth!Poor transformation

Poor chemical homogeneity

Root Cause AnalysisCommon Metallurgical Deficiencies


Root Cause Analysis

Wrought Materials (rolled, forged, etc.) Poor material cleanliness (eg. sulphur, phosphorus,

tramp elements)



Root Cause Analysis

Wrought Materials (rolled, forged, etc.) Forging flaws (seams, laps)



Root Cause Analysis

Lack of fusion, lack of penetration Slag Insufficient preheat (elevated hardness, residual stresses) Elevated hydrogen Under matched consumable selection

Welds



Root Cause Analysis

Metallurgical deficiencies Remanufacturing/

repair deficiency Mechanical overload Service environment Manufacturing deficiency Design deficiency

Metallurgy determines cause

Metallurgy alone is not sufficient

General Categories


1. Assess for mechanical overload relative to rated capacity.

2. Assess for a critical manufacturing deficiency relative to component design.

3. Examine component design for deficiencies relative to intended service conditions.

Root Cause Analysis

Example: Mechanical Failures

General Approach

Solving the Problem when the Metallurgy Appears Sound


Attain background information regarding the failure and equipment

Understand the mechanics of the equipment and the operating conditions

1. Attain additional information and knowledge:

Root Cause Analysis


Detailed Approach



2. Application of Metallurgical Findings

Root Cause Analysis


Detailed Approach


Failure mechanism

Failure origin

Direction and mode of loading

Condition of loading (single event or cyclic)


3. Perform a component stress assessmentA. Metallurgical Approach

Estimate applied loading causing failure

Root Cause Analysis


Detailed Approach


! Ductile failure (yield, ultimate strength)

! Brittle failure from flaw precursor (fracture toughness)

! Fatigue failures (estimation of material endurance limit for conditions of loading based on microstructure, tensile strength, or fatigue test data)


Estimate nominal service stresses and critical failure loads

3. Perform a component stress assessmentB. Mechanical Approach

Root Cause Analysis


Detailed Approach


! Closed form analysis (moment, force balance)

! Finite element analysis

! Instrumented strain gauge analysis



Failure Analysis Investigation

! Failure mechanism! Material compliance and properties

2. Root Cause Analysis! Background information! Analytical analysis! Rationalization

Summary

Failure Analysis

Documents

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