Mitigation of Fatigue and Pre-Cracking Damage in Aircraft ...

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1Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007

Mitigation of Fatigue and Pre-Cracking Damage in Aircraft Structures Through Low

Plasticity Burnishing (LPB)

N. Jayaraman, Douglas J. Hornbach, Paul S. PréveyLambda Technologies

Kristina Langer, Jeffrey Hoover, Scott Van HooganAFRL/VASM

ASIP 2007Palm Springs, CA

December 4 – 6, 2007



Acknowledgement

LPB Design, Implementation, and RS Measurement Conducted at Lambda Technologies

Part Design and Fatigue Testing Conducted at AFRL



Susceptible Material

Types of Damage:

CrackingCorrosionFretting

FOD

Tensile Stress≥ σThreshold

Failure

Damage/Failure Susceptibility Diagram

Residual Stress Approach: “Mechanical Suppression” of Tensile

Stresses – No need to change material/design & Improved damage

tolerance

Damage Prevention, Detection & Control

Approach:• Use Protective Coatings• Inspect for FOD, Cracks• Repair, Blend Damage• Replace Parts

Materials Approach:Develop new,

tougher, damage resistant alloy



Outline• Residual Stress Design Method• LPB Process

– Technology– Tools– Design Protocol– Production and Turnkey Installation

• Example of LPB to Mitigate Fatigue/Pre-Cracking Damage in AA2024-T851 Aircraft Structures

• Conclusions• List of Current LPB Applications



RESIDUAL STRESSDESIGN METHOD



Residual Stress Design Method• RS Design based on FDD (Fatigue Design Diagram –

Lambda Patent Pending)

• FDD is a novel adaptation of Haigh Diagram

• SWT model is used to extend Haigh Diagram into compressive mean stress regime

• Neuber’s kt or kf is used to account for damage

• Predicts RSmin to restore performance and RSmax to enhance performance

• RS optimization based on other design factors like part-distortion, location/magnitude of compensatory tension, etc.



Residual Stress Design Method

-200 0 2000

100

200

300

LPB-RSmax

ENHANCE

RESTORECorrosionFatigue Strength

NominalFatigueStrength

LPB-RSmin

YS = 240 ksiUTS = 285 ksi

kf = 5.4

0.2% Offset YS

Nf = 107 Cycles

SWT, kf = 1

SAFE

Fatigue Design Diagram for 300M HSLA SteelApplication to MLG - Mitigation of Corrosion Fatigue

S alt,

ksi

Smean, ksi

FDD Predicts thatCompressive RS

Restoresor

EnhancesPerformance

No Mode I Crack Growth in“SAFE” Region



LPB PROCESS



•High-hardness ball is rolled, under pressure, over surface•Single pass provides deep compression •Patented hydrostatic bearing with constant volume flow•Low cold work provides stable compression

LPB Technology

0 500 1000 1500

-1500

-1000

-500

0 SP GP LPBLSP

8A SHOT PEEN GRAVITY PEENED LASER SHOCKED, 3X LPB

PERCENT COLD WORK DISTRIBUTION

RESIDUAL STRESS (MPa)

0 500 1000 15000

5

10

15

20

IN718

40%

DEPTH (x10-3 mm)PERCENT COLD WORK

0 10 20 30 40 50 60

-200

-150

-100

-50

0

50

DEPTH (10-3 in.)

RESIDUAL STRESS (ksi)

3 LSPcycles

WORK PIECE

NORMAL FORCE

Lateral Motion

Supporting fluid

Spherical FluidBearing Tool

Residual Stress

Compression Tension

Constant Volume

Flow Bearing (patented)



Single Pass FEA Model of LPB Process Showing the Development of Surface and Subsurface

Compression



LPB Tool Technology

LPB ball

LPB ball

Blade edge

LPB ball

Work Piece

Single-Point Tool for thick pieces or one-sided application

Caliper Tool for thin pieces, providing through thickness compression

Through-thickness compression in compressor blade LE

Disk slot tools and inside calipers for ID bores built in 2006



LPB Causes No Surface Damage

• No metallographically detectable damage at 500x

• Improved Surface Finish <10 μin.

• Finish varies with LPB parameters: force, feed, ball type and size.

Parallel to lay 500x

Perpendicular to lay 500x

LPB Generated Surface in IN718



Residual Stress Stability

• Thermal Relaxation– Cold work increases dislocation density – High dislocation density increases both rate and

amount of relaxation

• Overload (Mechanical) Relaxation– Cold work creates yield strength depth gradient – Subsequent deformation is not uniform

• Cyclic Relaxation– Not significant in HCF at R = Smin/Smax > 0

Low Cold Work = Stable Compression

Fatigue benefit is lost if residual compression relaxes.



Total Engineering Solution: Ti-6-4 Vane

Providing Total Solutions

For Fatigue & Stress Corrosion Cracking

Applied Stresses

-200 -100 0 100 2000

50

100

150

200

RS =25 ksi

RS = -60 ksi

Se

Tensile Yield Limit

LPB + 0.025 in. FODSmax = 100 ksi

Baseline - no FODSmax = 110 ksi

R=0.1

Fatigue Elastic LimitR=-1.9

RR's Materials DataSy = 130 ksiSe (@ R = -1) = 72 ksi

kf =2

kf =1

SAFE

Fatigue Design Diagram for Ti-6Al-4V AlloyApplied to F402 LPC1 Vanes

S alt,

ksi

Smean, ksi

-1000 -500 0 500 1000

0

250

500

750

1000

1250

Smean, MPa

Sal

t, M

Pa

Fatigue Design

Residual Stress Modeling

Compression Verification

Processing Tools & Code

Turn Key Production

Designed Surface Enhancement

Fatigue Life Validation

103 104 105 106 107

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

LPB

Base Base + 20 mil FOD LPB LPB + 20 mil FOD

Lambda Research230-10352

CYCLES TO FAILURE

MAX

IMU

M S

TRES

S (M

Pa)

Ti 6-Al-4V Vanes, 4-point Bending, R=0.1, 30 Hz, RT

0102030405060708090100110120130140150160170180

MAXIM

UM

STRE

SS (ksi)

Statistical Quality Control

Component Processing Performance Record

0.00%0.10%0.20%0.30%0.40%0.50%0.60%0.70%0.80%0.90%

1 10 19 28 37 46 55 64 73 82 91 100 109

Time (milliseconds)

Perc

ent D

evia

tion

Component

+3 Sigma

+2 sigma

+1 sigma

Average

-1 sigma

-2 sigma

-3 Sigma

Compressive Stress Field Design Protocol



LPB Production

• Machining-like operation using typical CNC machine tools or robots• Highly automated…minimal operator intervention• Low capitalization costs…use existing CNC machines• Shop floor compatible…no specialized facilities

Hydraulic Cabinet

LPB Control

CNC Control

LPB Tool LPB ControlCNC ControlLPB Tool

Mazak Quick Turn 6T LatheHaas VF3B MillFanuc S420W Robot

6-axis 4- and 5-axis 3-axis



Field Location

Web Server SPC ServerOperations

LPB Control System

Immediate Pass/Fail

Determination

QAField

Lambda Technologies

Automatic Analysis &Plot GenerationData File

Uploads

• Maintenance• Calibration

Support

Continuous Performance Data Monitoring

Software Updates& Troubleshooting

E-mail Reports

QALambda

View Progress

E-mail Reports

SoftwareUpdates &

Troubleshooting

101010101010101010101010

Turn Key Field Installation



LPB MITIGATES FATIGUE AND PRE-CRACKING

DAMAGE IN AA2024-T851



Objective of the Test Program

To mitigate pre-cracking and fatigue damage through low plasticity burnishing

(LPB) treatment in AA2024-T851 parts simulating two different features of

airframe structure



PART DESIGN, FATIGUE TEST ARTICLES AND VARIABLES



Part A (Complex)• Material: Al 2024 T851• Loading (uniaxial) – Two load cases

1. Design stress: Constant amplitude, Max stress 11.4 ksi (approximately 30,000 cycles to failure)

2. 10% over design stress: Constant amplitude, Max stress 12.5 ksi • R = 0.01 (ratio of min to max stress)• Pre-crack status (0.05 in.) = yes, no• 3-6 repetitions per test case

4”

12”

0.10” thick

Max Stress in X-direction +72 ksi

Part A – Applied Stress @ 4500 lb Uniaxial Load



Part B (Simple)• Material: Al 2024 T851• Loading (uniaxial) – Two load cases

1. Design stress: Constant amplitude, Max stress 11.5 ksi (approximately 30,000 cycles to failure)

2. 10% over design stress: Constant amplitude, Max stress 12.5 ksi • R = -1 (ratio of min to max stress)• Pre-crack status (0.05 in.) = yes, no• 3-6 repetitions per test case

4”12”

0.12” thick

AXIAL APPLIED STRESS FOR 11 KSI FAR FIELD



RESIDUAL STRESS DESIGN,

IMPLEMENTATION & MEASUREMENT



Compressive RS is designed using Lambda’s FDD (Fatigue Design Diagram) method

Both controlled magnitude and depth of compression introduced at critical locations through LPB treatment

RS measured by x-ray diffraction method



LPB DESIGN – FDD METHODCompressive RS magnitude & locations to mitigate damage

are determined by FDD (Fatigue Design Diagram)



RESIDUAL STRESS MEASUREMENTS



Residual Stresses – Part A (Complex)

0.0 0.2 0.4 0.6 0.8 1.0-75

-50

-25

0

25

50 Surface0.0198 in. Depth 0.0403 in. Depth0.0596 in. Depth

TANGENTIAL RESIDUAL STRESS DISTRIBUTION

Res

idua

l Str

ess

(ksi

)

Distance (in.)

0 5 10 15 20 25

-400

-200

0

200

400

Distance (mm)


7/25/06

Residual S

tress (MP

a)



Residual Stresses – Part B (Simple)

0.0 0.1 0.2 0.3 0.4 0.5-75

-50

-25

0

Surface 0.02 in. Depth 0.04 in. Depth 0.06 in. Depth

(Mid-Thickness)

LONGITUDINAL RESIDUAL STRESS DISTRIBUTION

Res

idua

l Str

ess

(ksi

)

Distance (in.)

0 1 2 3 4 5 6 7 8 9 10 11 12

-400

-200

0

Distance (mm)


2/20/06

Residual S

tress (MP

a)



Summary of Fatigue Test Results

• Fatigue life of the smooth undamaged part was decreased by a factor of 20 due to 0.050 in. precracking damage

• LPB improved fatigue life of smooth undamaged part by nearly a factor of 5

• LPB completely mitigated the precracking damage by restoring fatigue performance to that of a smooth undamaged part



102 103 104 105 106 107

Run-out

LPB - Pre-crackedao = 0.05 in.

LPB - Smooth

Baseline - Pre-crackedao = 0.05 in.

Baseline - Smooth

Nf, Cycles to Failure

Spec

imen

Con

ditio

nsR = 0.01 Pmax = 4550 lbs Smax ≈ 11.4 ksiPre-cracking at Pmax = 3600 lbs, R = 0.01

AA2024-T851 Structural Test Panel - Part A (Complex)



102 103 104 105 106 107

AA2024-T851 Structural Test Panel - Part A (Complex)


LPB - Smooth


Baseline - Smooth


Spec

imen

Con

ditio

ns

R = 0.01 Pmax = 5000 lbs Smax ≈ 12.5 ksiPre-cracking at Pmax = 3600 lbs, R = 0.01



102 103 104 105 106 107


LPB - Smooth


Baseline - Smooth


Spec

imen

Con

ditio

ns

R = -1 Pmax = 5500 lbs Smax ≈ 11.5 ksiPre-cracking at Pmax = 3600 lbs, R = 0.01

AA2024-T851 Structural Test Panel - Part B (Simple)



102 103 104 105 106 107

AA2024-T851 Structural Test Panel - Part B (Simple)


LPB - Smooth


Baseline - Smooth


Spec

imen

Con

ditio

ns

R = -1 Pmax = 6000 lbs Smax ≈ 12.5 ksiPre-cracking at Pmax = 3600 lbs, R = 0.01



Summary• The magnitudes and locations of needed

compressive RS were determined by the FDD (Fatigue Design Diagram) Method

• LPB treatment was designed to introduce the intended compressive RS into the locations chosen for Parts A and B

• RS distribution in the treated parts was verified by x-ray diffraction method– In Part A (Complex) nominally uniform

compressive RS of –30 ksi was achieved up to mid-thickness at critical locations

– In Part B (Simple) nominally uniform compressive RS of –45 ksi was achieved up to mid-thickness at critical locations



Summary (cont’d)• Fatigue test results validated predictions

– LPB almost doubled the fatigue life of both smooth parts A & B

– In both Parts A & B, pre-cracks (0.05 in. long) reduced the fatigue life by nearly an order of magnitude

– In both Parts A & B, LPB fully restored the fatigue life of the pre-cracked (of length 0.05 in.) parts to that of smooth baseline parts

– The benefits of LPB were consistently evident at both stress levels of 11.5 and 12.5 ksi

– The benefits of LPB were consistently evident at both stress ratios (R) of 0.01 and -1

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