BMFB 4283 NDT & FAILURE ANALYSIS

BMFB 4283NDT & FAILURE ANALYSIS

Lectures for Week 8

Prof. Qumrul Ahsan, PhD Department of Engineering MaterialsFaculty of Manufacturing Engineering

8. Introduction to Failure Analysis 8.0 Objective of failure analysis. 8.1 Approach to failure analysis 8.2 Tools of Failure Analysis8.3 Failure Mode and Effect Analysis (FMEA)

Failure Analysis

Outline of Failure Analysis

• Vast topic, many failure modes and mechanisms, overlap, disputes

• Always exceptions• Focus on:

– Failure analysis process– Some common failures

• Highlight some interesting failures• Primarily steel, but present in most materials

Objectives of Failure Analysis• Objective investigation of material facts

associated with a part or system failure– To understand the failure mechanism of the failed

component– To determine the primary/root cause of the failure– To recommend the corrective actions

• Determine:– Timeline, chain of events– Root-cause of incident / contributing factors– Post incident fitness for purpose– Repair options– Mitigating future failures

What is failure?• Part and/or system no longer complies with design intent for

part or system• Subjective definition based on operation• Not always structural

– Leaking hydraulic seals– Inappropriate stiffness in component– Rate of corrosive decay/breakdown– Part/system lifetime– Operating/maintenance costs– Aesthetics

• Any design parameter

Facts of Failures

• Manifestation of failure (elastic & plastic deformation, rupture or fracture, change of metals)

• Failure inducing agent (force, time, temperature, reactive environment)

• Failure locations (body type, surface type)• Failure Mode (ductile, brittle, fatigue, creep, wear,

weld etc)

Where to Apply • Industrial / Manufacturing

– Manufacturing equipment (presses, fixtures)• Consumer Products

– Design improvement, validation• Civil Structures / Infrastructure

– Structural stability– Utilities (inspection, testing, failure analysis)

• Insurance / Legal cases– Root cause, forensics

Tools of the Trade• Instrumentation

– Strain gage, accelerometer, data acquisition• Inspection

– NDT, weld inspection, cracking, corrosion • Analysis

– Failure analysis, Fitness for purpose, Stress analysis, Finite Element Analysis (FEA), Metallurgical

• Testing– Load testing, cycle testing, instrumented tests, impact test,

tensile test

Contributing Factor Areas

• Original Design• Material Properties• Manufacturing and processing• Service Factors

– Loading– Environment

• Repair Procedures– Weld Repair

Frequency of Causes of Failure Analysis

Causes of failures %

Improper Material Selection 38

Fabrication Defects 15

Faulty Heat-Treatment 15

Mechanical Design Fault 11

Unforeseen Operating Conditions 8

Inadequate environment Control 6

Improper or lack of inspection 5

Material Mixup

Failure Analysis Process• Description of the failure situation (Background

information)• Visual Examination (Record Keeping)• Mechanical Design Analysis (NDT and Destructive

Testing)• Macroscopic examination( Appearance,

Fractography)• Microscopic examination (Metallography)• Properties• Chemical Design Analysis• Failure Simulation

Investigator Requirements• OBJECTIVENESS• Visual Cues• Verbal Cues• Documentation

– Names, dates, times, quantities, history• Questioning Attitude

– Vagueness of Language– Opposing views of an incident

• Broad background in failure mechanisms

Technological Tools

• Photography and lighting• Optical Microscopy up to 600X• Scanning Electron Microscopy (SEM) over

10,000X• Chemical Analysis

– SEM/EDS– Spark Emission Spectroscopy– Fourier Transform Infra Red (FTIR)

Optical Microscopy (7)

• Use polished and etched specimens

• Limited depth of field• Shows individual grain

structure

SEM Image (7)

• Individual grains

• Large depth of field

• Vacuum chamber

• Charging

SEM-EDS (3)

• Elements give distinct peaks, often primary and secondary

FTIR (4)

• Measurement of frequencies that are absorbed by organic media

Technological Tools• Mechanical Testing

– Hardness, micro-hardness, tensile, shear, physical testing

– NDT (UT, MT, PT, RT ET etc)• Stress Analysis / FEA

– Magnitude, principal direction, sensitivity

• Instrumentation– Strain Gages, accelerometers,

thermocouples, ect.• Non-Destructive Testing (NDT)

Finite Element Analysis

• Used to answer particular questions

• Stress, strain deflection, principal directions, mode shapes, thermal, impact, etc.

• 3500 ton forging press frame, cracking problems

Strain Gages

• Attach to surface to measure surface strains

• Available in hundreds of configurations

• Used to calibrate FEA models, measure loads, dynamic

• Signal conditioning, error elimination, ground loops etc.

ASM Failure Analysis

• American Society of Metals (ASM) outline• Experts in each area• Remain OBJECTIVE• Important to be thorough, information will be

lost• Failure analysis becomes less reliable with less

information

ASM Failure Analysis• 1. Background

– Location, name, P/N, description, manufacturer, fabricator– Function of item– Maintenance / cleaning history– Operational history– Operational documentation– Normal stress orientations– Extent of incident– Precipitating events– Drawings, photographs, reports, inspections– Service deviations– Opinions of related personnel

ASM Failure Analysis• 2. Visual Examination

– Survey the entire region– Macroscopically classify the fracture– Estimate manner of loading– Determine associated equipment– Observe colors, contaminants, corrosion products, grinding

marks, weld progression, other structures in region– Base material quality, uniformity, coatings– Document and record


• 3. Fractographic Examination– Often necessary to ascertain failure mode– Identify microstructure– Note fracture progression– Note deformation– Isolate contaminants– Note colors– Anomalies– Distinguish post incident damage


• 4. Chemical Analysis– Base metal composition– Contaminant composition– Presence of coating on fracture surface– Corrosion product composition


• 5. Mechanical Properties– Bulk material properties– Anomalous material properties– Hardness– Ductility– Tensile strength– Corrosion susceptibility– SCC susceptibility


• 6. Macroscopic Examination– Overall homogeneity– Uniform cracking– Any differences


• 7. Metallographic Examination– Section polish and etch

• 8. Microhardness– Traverse across crack, HAZ. Determine gradients– Inclusions, metallographic phases


• 9. SEM analysis– Identify microscopic fracture modes– Ductile dimpling, inter-granular, cleavage– Presence of contamination on fracture surface

• 10. Microprobe– SEM-EDS of individual areas– Graphite, carbides, precipitates

ASM Failure Analysis• 11. Residual Stresses and phases

– X-Ray diffraction, strain gage methods• 12. Simulation / Tests

– FEA, stress analysis– Testing– Consistent with findings?

• 13. Summarize findings• 14. Report and Distribute• 15. Follow-up


• 16. Preserve Evidence– Package carefully– DO NOT put fracture faces together– Use desiccant– Other experts, new information– Sometimes destructive testing is required

Failure Mechanisms

• Parts “Fail” for many reasons (Deficiency)• Operational

– Plastic deformation (permanent set, buckling)– Excessive deflection (floors, beams, shafts)– Excessive vibration (machine mounts)– Acoustics issues

• Fracture– Ductile, Brittle, Fatigue, Thin Lip…

Failure Mechanisms

• Corrosion– Thinning– Stress Concentrators (risers)– Corrosion Products

• Wear– Thinning– Wear Products

• Welding

Synergistic Effects

• All failures have components of multiple failure mechanisms associated

• Simultaneous presence, interacting• Task is to determine the important ones from

the auxiliary modes

Some Techniques in FA• Cleaning Fracture Surfaces and SEM Observation

– Clean the surface using acetone, alcohol, ultrasonic cleaner and then dry

– Replicating surfaces by • Wetting the fracture surface• Wetting the adhesive side of the acetate tape• Rub the wet surface of tape to fracture surface

to put impression on the tape and collecting debris

• Tape is coated with gold/carbon and carried out SEM and EDAX

Some Techniques in FA• Preparation of Replicas for the TEM

– Take the tape replica– Put a thin(200 A) layer of a carbon on it in a

vacuum coater– Alternatively carbon can be coated on the fracture

surface and then removed by the adhesive tape– Put a thin(200 A) layer of a heavy metal (Cr, Pt) to

enhance contrast– Acetate tape is dissolved in a solvent (acetone) ;

freeing the thin and fragile replica– Replica is removed onto a screen or grid

Some Techniques in FA• Steremicroscopy

– Stereo imaging involves recording a given field of view twice at slightly different orientations and simultaneously viewing the stereo pair such that a three dimensional image is percieved

– The four methods to record stereo pairs• The tilt method- where angle is applied between the the

two micrographs• The lateral shift method - where there is a horizontal

displacement between the two micrographs• The rotation methods-where the specimen is rotated

between exposure• Electromagnetic deflection of the electron beam

between two images•

Failure Mode & Effects Analysis (FMEA)

• Defined: FMEA is a systematic tool for identifying:– effects or consequences of a potential product or process

failure.– methods to eliminate or reduce the chance of a failure

occurring.• Ideally, FMEA’s are conducted in the product design or

process development stages[conducting an FMEA on existing products or processes may also yield benefits]

• FMEA generates a living document that can be used to anticipate and prevent failures from occurring. (note: documents should be updated regularly.)

FMEA/FMECA History• The history of FMEA/FMECA goes back to

the early 1950s and 1960s.

– U.S. Navy Bureau of Aeronautics, followed by the Bureau of Naval Weapons:

– National Aeronautics and Space Administration (NASA):

• Department of Defense developed and revised the MIL-STD-1629A guidelines during the 1970s.

FMEA/FMECA History (continued)

• Ford Motor Company published instruction manuals in the 1980s and the automotive industry collectively developed standards in the 1990s.

• Engineers in a variety of industries have adopted and adapted the tool over the years.

Published Guidelines

• J1739 from the SAE for the automotive industry.

• AIAG FMEA-3 from the Automotive Industry Action Group for the automotive industry.

• ARP5580 from the SAE for non-automotive applications.

FMEA is a Tool - When to Use

• FMEA is most effective when it occurs before a design is released rather than “after the fact”.– focus should be on failure prevention not detection.

FMEA is often a standard process used in the development of new products.

System Design ProcessComponentsSubsystems

Main Systems

ComponentsSubsystems

Main Systems

ManpowerMachineMethodMaterial

MeasurementEnvironment

Machines

Tools, Work Stations,

Production Lines,Operator Training,

Processes, Gauges

Focus:Minimize failure

effects on the System

Objectives/Goal:Maximize System Quality, reliability,

Cost and maintenance


effects on theDesign

Objectives/Goal:Maximize Design Quality, reliability,



effects on the Processes

Objectives/Goal:Maximize

Total Process Quality, reliability,


What tools are available to meet our objective?

• Benchmarking• customer warranty reports• design checklist or guidelines• field complaints• internal failure analysis• internal test standards• lessons learned• returned material reports• Expert knowledge

What are possible outcomes?

• Actual/potential failure modes• customer and legal design requirements• duty cycle requirements• product functions• key product characteristics• Product Verification and Validation

FMEA Roadmap

FMEA Variables

Design FMEA Format

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken

Action ResultsResponse &

TargetComplete

Date

RecommendedActions

RPN

Detec

CurrentDesignControls

Occur

PotentialCause(s)/

Mechanism(s)Of Failure

Class

Sev

PotentialEffect(s) of

Failure

Potential FailureMode

Item

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


TargetComplete

Date

RecommendedActions

RPN

Detec

Current

Controls

Occur

PotentialCause(s)/


Class

Sev


Failure


Function

General

•Every FMEA should have an assumptions document attached (electronically if possible) or the first line of the

FMEA should detail the assumptions and ratings used for the FMEA.•Product/part names and numbers must be detailed in the

FMEA header•All team members must be listed in the FMEA header•Revision date, as appropriate, must be documented in the

FMEA header

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


TargetComplete

Date

RecommendedActions

RPN

Detec


Occur

PotentialCause(s)/


Class

Sev


Failure


Item

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


CompleteDate

RecommendedActions

RPN

Detec

Current

Controls

Occur

PotentialCause(s)/


Class

Sev


Failure

Function

Severity

• Definition: assessment of the seriousness of the effect(s) of the potential failure mode on the next component, subsystem, or customer if it occurs

• Severity applies to effects• For failure modes with multiple effects, rate

each effect and select the highest rating as severity for failure mode

Severity

•EXAMPLE:•Cannot see out of front window – severity 9•Air conditioner makes cab too cold – severity 5•Does not get warm enough – severity 5•Takes too long to heat up – severity 4

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


TargetComplete

Date

RecommendedActions

RPN

Detec


Occur

PotentialCause(s)/


Class

Sev


Failure


Item

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


CompleteDate

RecommendedActions

RPN

Detec

Current

Controls

Occur

PotentialCause(s)/


Class

Sev


Failure

Function

Occurrence

• Definition: likelihood that a specific cause/mechanism will occur

• Be consistent when assigning occurrence• Removing or controlling the cause/mechanism

though a design change is only way to reduce the occurrence rating

Occurrence

•EXAMPLE:•Incorrect location of vents – occurrence 3•Incorrect routing of vent hoses (too close to

heat source) – occurrence 6•Inadequate coolant capacity for application –

occurrence 2

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


TargetComplete

Date

RecommendedActions

RPN

Detec


Occur

PotentialCause(s)/


Class

Sev


Failure


Item

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


CompleteDate

RecommendedActions

RPN

Detec

CurrentOccur

PotentialCause(s)/


Class

Sev


Failure

Function

Detection

•Detection values should correspond with AIAG, SAE •If detection values are based upon internally defined criteria, a reference

must be included in FMEA to rating table with explanation for use•Detection is the value assigned to each of the detective controls•Detection values of 1 must eliminate the potential for failures due to design

deficiency

•EXAMPLE:•Engineering specifications – no detection value•Historical data – no detection value•Functional testing – detection 3•General vehicle durability – detection 5

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


TargetComplete

Date

RecommendedActions

RPN

Detec


Occur

PotentialCause(s)/


Class

Sev


Failure


Item

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


CompleteDate

RecommendedActions

RPN

Detec

Current

Controls

Occur

PotentialCause(s)/


Class

Sev


Failure

Function

Risk Priority Number(RPN)• Severity x Occurrence x Detection• RPN is used to prioritize concerns/actions• The greater the value of the RPN the greater the

concern• RPN ranges from 1-1000• The team must make efforts to reduce higher RPNs

through corrective action• General guideline is over 100 = recommended action

RPN (Risk Priority Number)

•Risk Priority Number is a multiplication of the severity, occurrence and detection ratings

•Lowest detection rating is used to determine RPN•RPN threshold should not be used as the primary trigger for

definition of recommended actions

•EXAMPLE:•Cannot see out of front window – severity 9, – incorrect vent

location – 2, Functional testing – detection 3, RPN - 54

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


TargetComplete

Date

RecommendedActions

RPN

Detec


Occur

PotentialCause(s)/


Class

Sev


Failure


Item

DetectPrevent

RPN

DET

OCC

SEV

ActionTaken


CompleteDate

RecommendedActions

RPN

Detec

Current

Controls

Occur

PotentialCause(s)/


Class

Sev


Failure

Function

RPN Considerations (continued)

• RPN ratings are relative to a particular analysis. – An RPN in one analysis is comparable to other

RPNs in the same analysis …– … but an RPN may NOT be comparable to RPNs

in another analysis.

1 5 10

Conducting FMEA

• Prior to conducting an FMEA, it is often useful to:

– perform a functional analysis, and– generate FMEA cause-and-effect diagrams.

Conducting FMEA• Basic and Secondary Functions - verb~noun

descriptions of what product (process) does.– Basic Function: ingress to and egress from vehicle– Secondary functions - protect occupant from noise

• Failure Mode - physical description of a failure.– noise enters at door-to-roof interface

• Failure Effects - impact of failure on people, equip. – driver dissatisfaction.

• Failure Cause - refers to cause of the failure.– insufficient door seal.

Continuous Improvement

• Last Columns of FMEA worksheet are used to identify improvement plan.– Recommend action– Identify responsibility to complete action.– Identify target dates to complete action.– List action taken and reassess RPN.

• FMEA also uses a Measure - Analyze - Improve - Control Cycle.

Benefits of FMEA• Contributes to improved designs for products

and processes.– Higher reliability– Better quality– Increased safety– Enhanced customer satisfaction

• Contributes to cost savings.– Decreases development time and re-design costs– Decreases warranty costs– Decreases waste, non-value added operations

• Contributes to continuous improvement

Reasons FMEA’s fail One person is assigned to complete the FMEA

Members of the FMEA team are not trained in the use of FMEA

Rushing through identifying the failure modes to move onto the next step of the FMEA

Listing the same potential effect for every failure i.e. customer dissatisfied.

Stopping the FMEA process when the RPN’s are calculated and not continuing with the recommended actions.

Summary

• Remain OBJECTIVE• Be aware of various failure mechanisms• Design to ACTUAL loads, not estimates• Analyze stress concentrations• Know your material• Design for brittle or ductile fracture

Documents

BMFB 4283 NDT & FAILURE ANALYSIS