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ReliabilityOctober 26, 2004

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Today

• DFDC (Design for a Developing Country)

• HW November 2 – detailed design– Parts list – Trade-off

• Midterm November 4

• Factory Visit November 16th

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Midterm

• Presentation Purpose- a midcourse correction– less than 15 minutes with 5 minutes discussion– Approx. 7 power point slides- all should participate in

presentation– Show what you have done– Show what you are going to do– Discuss issues, barriers and plans for overcoming

(procedural, team, subject matter, etc.– Scored on originality, candor, thoughtfullness, etc. not

on total amount accomplished – Schedule today from 1:00 to 4:00 (speaker at 4:00 PM)

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Reliability The probability that no (system) failure will occur in a given time interval

A reliable system is one that meets the specifications Do you accept this?

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What do Reliability Engineers Do?

• Implement Reliability Engineering Programs across all functions– Engineering– Research– manufacturing– Testing– Packaging– field service

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Reliability as a Process module

• Reliability Goals

• Schedule time

• Budget Dollars

• Test Units

• Design Data

Reliability Assurance

Module

Internal Methods•Design Rules

•Components Testing

•Subsystem Testing

•Architectural Strategy

•Life Testing

•Prototype testing

•Field Testing

•Reliability Predictions (models)

INPUT

ProductAssurance

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Early product failure

• Strongest effect on customer satisfaction– A field day for competitors

• The most expensive to repair– Why?– Rings through the entire production system– High volume– Long C/T (cycle time)

• Examples from GE (but problem not confined to GE!)– GE Variable Power module for House Air Conditioning– GE Refrigerators– GE Cellular

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Early Product Failure

• Can be catastrophic for human life– Challenger, Columbia– Titanic– DC 10– Auto design– Aircraft Engine– Military equipment

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# of componentsin Series

ComponentReliability =99.999%

ComponentReliability =99.99%

100 99.9 99.01250 99.75 97.53500 99.50 95.12

1000 99.01 90.4810,000 90.48 36.79100,000 36.79 0.01

Reliability as a function of System ComplexityWhy computers made of tubes (or discrete transistors)

cannot be made to work

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Three Classifications of Reliability Failure

Type

• Early (infant mortality)

• Wearout (physical degradation)

• Chance (overstress)

Old Remedy- Repair mentality

• Burn-in

• Maintenance

• In service testing

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Bathtub Curve

Infant Mortality

Useful lifeNo memoryNo improvementNo wear-outRandom causes

Wear out

Failure Rate#/million hours

Time

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Reliability

Age

Probof dyingin the nextyear (deaths/1000) 0

10

20

30

40

50

60

70

80

90

0 2 5 12 16 19 30 50 70 86

From the Statistical Bulletin 79, no 1, Jan-Mar 1998

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Early failure causes or infant mortality (Occur at the beginning of life and then disappear)

• Manufacturing Escapes– workmanship/handling– process control– materials– contamination

• Improper installation

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Chance Failures (Occur throughout the life a product at a constant rate)

• Insufficient safety factors in design

• Higher than expected random loads

• Human errors

• Misapplication

• Developing world concerns

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Wear-out(Occur late in life and increase with age)

• Aging

• degradation in strength

• Materials Fatigue

• Creep

• Corrosion

• Poor maintenance

• Developing World Concerns

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Failure Types

• Catastrophic

• Degradation

• Drift

• Intermittent

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Failure Effects(What customer experiences)

• Noise• Erratic operation• Inoperability• Instability• Intermittent operation• Impaired Control• Impaired operation • Roughness• Excessive effort requirements• Unpleasant or unusual odor• Poor appearance

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Failure Modes• Cracking

• Deformation

• Wear

• Corrosion

• Loosening

• Leaking

• Sticking

• Electrical shorts

• Electrical opens

• Oxidation

• Vibration

• Fracturing

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Reliability Remedies

• Early

• Wearout

• Chance

• Quality manufacture/Robust Design

• Physically-based models, preventative maintenance, Robust design (FMEA)

• Tight customer linkages, testing, HAST

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Reliabilitysemi-empirical formulae

22 2/)(

2

1)(

MTeTf Wear out

Chance Failure

Early failure

TmT e

meTf

11)(

=constant failure rate

m=MTBF

)(12

1)()( kTekTkTf =pdf

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Failures Vs time as a function of Stress

High Stress

Medium Stress

Low Stress

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Highly Accelerated Stress Testing

• Test to Failure

• Fix Failed component

• Continue to Test

• Appropriate for developing world?

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Duane PlotReinertson p 237

LogFailuresper 100hours

Log Cumulative Operating Hours

xxxx xx

xx xx xxxx

x

Actual Reliability

Required Reliabilityat Introduction

Predicted

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Integration into the Product Development Process FMEA- Failure Modes and Effects Analysis

Customer Requirements

Baseline data fromPreviousProducts

Brainstormpotential failures

Summarize results (FMEA)

UpdateFMEA

Baseline data fromPreviousProducts

Feed results to Risk Assessment

Process

Use at Design Reviews

Develop Failure Compensation

Provisions

Test ActivityUncovers newFailure modes

Failure prob-through test/field

data

Probabilitiesdeveloped

through analysis

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Risk Assessment process

Assess risk

• Program Risk

• Market Risk

• Technology Risk– Reliability Risk

• Systems Integration Risk

Devise mitigation Strategy

Re-assess

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Fault Tree analysisSeal Regulator

Valve Fails

Valve Fails Openwhen commanded

closed

Fails to meet response time

Excessiveleakage

Regulates High

RegulatesLow

Fails closedwhen commanded

open

Excessive hysteresis

or

oror

Excessiveport leakage

Excessivecase leakage

Fails to meet response time

Fails to meet response time

1 5432

6 7 8 9

NextPage

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Fault Tree analysis (cont)Valve Fails Open

when commanded closed

1Valve Fails Open

when commanded closed

orMechanical

Failure Selenoid

ElectricalFailure ofSelenoid

or or

OpenCircuit

or

Coil shortInsulation

Solder JointFailure

WireBroken

corosion

or

Armature

or

sealsMaterialselection

wearMaterialselection

Contamination

orValve

orientationInsuff

filteringWire

Broken

Transientelectro mechanical

force

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FMEA

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FMEA Root Cause Analysis

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Fault Tree Analysis- example

Example: A solar cell driven LED

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Reliability Management• Redundancy

– Examples• Computers• memory chips?• Aircraft

– What are the problems with this approach• 1. Design inelegance

– expensive– heavy– slow– complex

• 2. Sub optimization– Can take the eye off the ball of improving component and system reliability by reducing defects

– Where should the redundancy be allocated• system • subsystem• board• chip• device• software module• operation

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Other “best practices”• Fewer Components

• Small Batch Size (why)

• Better material selection

• Parallel Testing

• Starting Earlier

• Module to systems test allocation

• Predictive (Duane) testing

• Look for past experience– emphasize re-use

• over-design– e.g. power modules

• Best: Understand the physics of the failure and model– e.g. Crack propagation in airframes or nuclear reactors

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Other suggestions?