R. Craig McClung Michael P. EnrightSouthwest Research Institute®

San Antonio, Texas, USA

The DARWIN® Computer Program for Probabilistic Damage Tolerance Analysis

of Engine Rotors

ADF Aircraft Engine Symposium29-30 October 2003

Engine Disk Safety

Modern aircraft engineshave excellent safetyand reliability recordsNevertheless, uncontaineddisk failures do occasionally occurRecent experience has shown that the primary causes of uncontained failures are out-of-condition “anomalies”

Inherent (but rogue) material defectsInduced by out-of-condition manufacturingInduced by out-of-condition maintenance or usage

These “anomalies” are not addressed by conventional “safe-life” design methods (based on nominal material)The engine industry is currently addressing this issue

Driving Force – Sioux City

Driving Force – Sioux City

July 19, 1989 accidentUAL 232 (DC-10)In-flight separation of Stage 1 Fan DiskCrashed on landing112 fatalitiesLife Limit: 18,000 cyclesFailure: 15,503 cycles

Crack started at material anomalyHard Alpha phase produced during melting

Driving Force – Pensacola

July 6, 1996 accidentDL 1288 (MD-88)Stage 1 Fan Diskseparation on take-off rollTake-off abortedCabin impacted2 fatalitiesLife Limit: 20,000 cyclesFailure: 13,835 cycles

Crack started at abusively machined bolt hole

Responses to Sioux City

FAA Review Team Report (1991) recommended:Changes in titanium melt practices and quality controlsImproved inspections (manufacturing and in-service)Lifing practices based on damage tolerance

Aerospace Industry Association (AIA) Rotor Integrity Subcommittee (RISC) formed to develop new lifing strategies

Address industry policy on rotor safetyAssist FAA in implementing Titanium Review Team ReportAddress other materials and anomaly types later

GE Aircraft EnginesHamilton SundstrandHoneywellMTUPratt & Whitney

Pratt & Whitney CanadaRolls RoyceRolls Royce CorpSNECMAFAA

Enhanced Life Management Process

Damage Tolerance Complements Safe Life

Safe Life Damage Tolerance

Nominal Conditions Anomalous Conditions

Retirement Life Inspection Requirements

Hard Alpha Defects inTitanium Components

Initial FAA and RISC focus on HA defects in titanium

Small brittle zone in microstructureAlpha phase stabilized by N accidentally introduced during melting

Probabilistic Fracture Mechanics Methodology

Size DistributionInclusion

Cycles

Prob

abili

ty

Life Prediction

Thermal & StressAnalysis

InclusionFrequency

Probabilistic Analysis

Fracture MechanicsStressed volume/areaInclusion incubationStatistical Integration

Mission Analysis

Prob

abilit

y

Size

Crack Growth

Stress intensity

Gro

wth

rate

MISSYDD

Cyclic Usage

Anomaly Distribution- Size and Frequency

Probability of Fracture

Inspection POD

Part Inspection Distribution

Probabilistic Fracture Mechanics

Statistical Integration

Advisory Circular 33.14(Initial Version)

ApplicabilityAll critical Ti Rotors in engines applying for new Type Certification under FAR33

Safe Life ProcessBrief description of the traditional approach

to engine lifing and certification

Enhanced Life Management ProcessAdds a new element, Damage Tolerance,

to traditional process

Damage Tolerance ImplementationRequires FM-based probabilistic

assessments of threat posed by Hard Alpha

DT Field Inspection StrategiesIn-service inspections are examined as

potential risk reduction actions

Advisory Material

Framework for Titanium DT Assessments

Risk

Components

A B

MaximumAllowable Risk

Risk ReductionAction Required

C

• Inspection• Redesign

Mission

Materials

Field History

Manufacturing

Service LifeInspection

Plan

Anomaly Dist

LifeAnalysis

NDE/POD

ProbabilisticRisk

Assessment

StressAnalysis

Conduct fracture mechanics-based risk assessments for all new Ti rotors

Must satisfy new industry standard Design Target Reliability

Damage Tolerance Advisory Circular

Enhanced Life Management

Process

Materials

Safe Life

Testing

AssuranceDT

RISC Vision: Comprehensive DT Assessment

Inherent Flaws(Melt related, etc)

Titanium Hard Alpha

Ni/Powder Metals

Analytical Method:

Probabilistic FMRisk Calc <DTR

Induced Flaws

Manufacturing Maintenance/ Service

Analytical Method:

Probabilistic FMRisk Calc <DTR

Current RISC Focus

•Analysis Tool calibrated by Test Case

•Criteria Calibrated by Experience

Surface Damage Tolerance

• Following same industry process as with titanium HA

• Industry data collected to prioritize threat• Machined Holes - Current Focus • Broached Slots• Turned Surfaces

20%23%

27%

Turned Surfaces Holes Broaching

1990s Manufacturing Related Events

RISC ScheduleSioux City 7/19/89 Pensacola 7/6/96 AC33.14

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 20011989 1990 2002 2003

Draft Advisory Material to FAA 11/96

InherentAnomalies

InducedAnomalies

Ti

Ti

Ni

Ni

RISC Kickoff 10/91 Damage ToleranceFramework for all Future Work

FAA/SMPC Data GatheringOEM Data Gathering Started

Draft Advisory MaterialAvailable by 4Q2003

Origins of TRMD Program

RISC noted that emerging life management process could be significantly enhanced by further R&D to address shortfalls in technology and dataSwRI (guided by RISC) proposed to the FAA and was awarded an R&D grant to address these shortfalls

Enhanced predictive tool capabilitySupplementary material/anomaly behavior characterization and modeling

“Turbine Rotor Material Design” (TRMD) programGoal is to provide direct supportfor implementation of AC 33.14and for additional improvementsin those guidelines

TRMD Program Team

SwRI is program managerU.S. engine companies are steering committee, major subcontractors

General ElectricHoneywellPratt & WhitneyRolls-Royce Corp.

Activities coordinated with RISC

Overview of TRMD Program

Defect distribution modelingUpdated anomaly distributions for HAModeling of HA diffusion zone evolution

Modeling of hard alpha deformation during forgingConstitutive and damage behavior of hard alphaDevelopment and validation of DEFORM™ microcode

Crack nucleation and growth data and modelingVacuum FCG testingHA crack nucleation and growth coupon testingSpin pit testingNickel anomaly nucleation and growth coupon testingTMF crack growth testingDevelopment of fracture mechanics module

Development of DARWIN® software tool for reliability assessmentTechnology transfer to FAA and industry

DARWIN® OverviewDesign Assessment of Reliability With INspection

Probabilistic Fracture Mechanics

Probability of DetectionAnomaly Distribution

Finite Element Stress Analysis

Material Crack Growth Data

NDE Inspection Schedule

Pf vs. Cycles

Risk Contribution Factors

Zone-Based Risk Assessment

1

2 3 4

m

5 6 7

Define zones based on similar stress, inspection, anomaly distribution, lifetimeTotal probability of fracture for zone:

(probability of having a defect) x (POF given a defect)Defect probability determined by anomaly distribution and zone volumePOF assuming a defect computed with Monte Carlo sampling or advanced methods

POF for disk obtained by summing zone probabilitiesAs individual zones become smaller (number of zones increases), risk converges down to “exact” answer

Zoned Impeller Model

Fracture Mechanics Model of Zone

m

7

Retrieve stresses along line

Fracture Mechanics Model

hx

hy

x

Ygr

adie

nt d

irect

ion

1

2

3

4

5

Defect

Finite Element Model

(Not to Scale)

Integrated Flight_Life FM module • K solutions for cracks in plates• FCG life calculations

Stress Processing

FE Stresses and plate definition

stress gradient

Stress gradient extraction

FE Analysis

0.0 0.2 0.4 0.6 0.8 1.0Normalized distance from the notch tip, x/r

-0.8

-0.4

0.0

0.4

0.8

1.2

1.6

2.0

σ/σo

(σz)relax

(σz)residual

(σz)elastic

Shakedown module

Computed relaxed stressσelastic - σresidual

σ0/σ

Residual stress analysis

3 4 5 6 7 0 1 2 3Load Step

01020304050607080

Hoo

p St

ress

(ksi

)

Rainflow stress pairing

Anomaly Distribution

# of anomalies per volume of material as function of defect sizeLibrary of default anomaly distributions for HA (developed by RISC)

Probability of Detection Curves

Define probability of NDE flaw detection as function of flaw sizeCan specify different PODs for different zones, schedulesBuilt-in POD library or user-defined POD

Random Inspection Time

“Opportunity Inspections” during on-condition maintenanceInspection time modeled with Normal distribution or CDF table

Output: Risk vs. Flight Cycles

Output: Risk Contribution Factors

Identify regions of component with highest riskRefine zone breakup as needed to achieve convergence

DARWIN Analysis ModesInherent Material Defects Vs. Surface Damage

Inherent Defects Surface Damage

Zone- Based Risk (Volume)

Crack in Plate solutions

2D finite element models

Random Defect

Feature-Based Risk (Area)

Crack in Plate/ Hole solutions

3D finite element models

initial crack location

GUI for Simple Definition ofZones and Fracture Models

Read in FE model and stress resultsDefine zone with mouse by selecting elements

Similar stress, lifeSelect crack location with mouse

Or snap to highest stress location in zone

GUI draws default fracture model

Rectangular plateStress gradient line

Use mouse to adjust fracture model

Automated Model Refinement Zone and Element Subdivision

Zone subdivision from original zone breakup Element subdivision

from original FE mesh

3-D Modeling in DARWIN 5.0

Load 3D Model

Compute Slice

1. Import 3D Finite Element Model2. Select crack location with mouse

and compute cracking plane3. Create 2D slice for fracture modeling

Computationally Efficient Probabilistic Methodologies

0

50

100

150

200

250

CO

MP

UT

AT

ION

TIM

E (

SE

C)

CPU Time % Error

Pf - with inspection

COMPUTATIONAL METHOD

MC100,000

IS100SAMPLES

MC10,000

IS400

ER

RO

R M

AG

NIT

UD

E

5%

10%

15%

20%

25%

0%

DARWIN Use for Conventional Fracture Mechanics Lifing

DARWIN was originally developed to solve a few highly-specialized probabilistic damage tolerance problemsEvolving DARWIN capabilities also enable the code to be used to solve conventional fracture mechanics life problems

GUI facilitates setting up the problemFlight_Life can be run in deterministic mode from a specific initial flaw sizeGUI facilities visualization of the answer

DARWIN Development History

Initial focus on hard alpha anomaliesDARWIN Version 1.0 (April 1997)Version 2.0 (June 1998)Version 3.2 (July 1999)

Current focus on surface damage in all materialsVersion 4.0 (March 2002)Version 5.0 (April 2003)

Planned effort on materials with multiple anomaliesVersion 6.0 (planned January 2005)

Planned effort on inherent anomalies in conventional cast and wrought nickel materials

Use of DARWIN by Industry

FAA Advisory Circular 33.14 requests risk assessment be performed for all new titanium rotor designs

Designs must pass design target risk for rotors

ComponentsComponents

RiskRisk

MaximumMaximumAllowableAllowable

RiskRisk

1010--99

RiskRiskReductionReductionRequiredRequired

CCAA BB

DARWINDARWIN –– AcceptableAcceptableMeans To AssessMeans To Assess

ComplianceCompliance

Other DARWIN Activities

Exploratory DARWIN-NASGRO link for rotorcraft applicationsExploratory study of probabilistic damage analysis for turbine engine prognosis (DARPA/AFRL)

Engine sensor inputs (speed, damage)Crack nucleation + crack growth modeling

Demonstration platform for new probabilistic, physically-based nucleation and growth life models (AFOSR)Demonstration platform for enhanced life prediction methodologies for engine rotor life extension (AFRL)

Explore links between DARWIN and ProDAPS (Smiths)DARWIN fracture mechanics technology transfer to NASGRO

www.darwin.swri.orgwww.darwin.swri.org