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Information Proprietary to Lambda
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
Information Proprietary to Lambda
2Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
Acknowledgement
LPB Design, Implementation, and RS Measurement Conducted at Lambda Technologies
Part Design and Fatigue Testing Conducted at AFRL
Information Proprietary to Lambda
3Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
4Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
5Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
RESIDUAL STRESSDESIGN METHOD
Information Proprietary to Lambda
6Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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.
Information Proprietary to Lambda
7Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
8Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
LPB PROCESS
Information Proprietary to Lambda
9Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
•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)
Information Proprietary to Lambda
10Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
Single Pass FEA Model of LPB Process Showing the Development of Surface and Subsurface
Compression
Information Proprietary to Lambda
11Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
12Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
13Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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.
Information Proprietary to Lambda
14Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
15Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
16Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
17Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
LPB MITIGATES FATIGUE AND PRE-CRACKING
DAMAGE IN AA2024-T851
Information Proprietary to Lambda
18Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
19Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
PART DESIGN, FATIGUE TEST ARTICLES AND VARIABLES
Information Proprietary to Lambda
20Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
21Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
22Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
RESIDUAL STRESS DESIGN,
IMPLEMENTATION & MEASUREMENT
Information Proprietary to Lambda
23Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
24Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
LPB DESIGN – FDD METHODCompressive RS magnitude & locations to mitigate damage
are determined by FDD (Fatigue Design Diagram)
Information Proprietary to Lambda
25Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
RESIDUAL STRESS MEASUREMENTS
Information Proprietary to Lambda
26Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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)
Lambda Research1354-12750
7/25/06
Residual S
tress (MP
a)
Information Proprietary to Lambda
27Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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)
Lambda Research1354-12750
2/20/06
Residual S
tress (MP
a)
Information Proprietary to Lambda
28Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
29Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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)
Information Proprietary to Lambda
30Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
102 103 104 105 106 107
AA2024-T851 Structural Test Panel - Part A (Complex)
LPB - Pre-crackedao = 0.05 in.
LPB - Smooth
Baseline - Pre-crackedao = 0.05 in.
Baseline - Smooth
Nf, Cycles to Failure
Spec
imen
Con
ditio
ns
R = 0.01 Pmax = 5000 lbs Smax ≈ 12.5 ksiPre-cracking at Pmax = 3600 lbs, R = 0.01
Information Proprietary to Lambda
31Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
102 103 104 105 106 107
LPB - Pre-crackedao = 0.05 in.
LPB - Smooth
Baseline - Pre-crackedao = 0.05 in.
Baseline - Smooth
Nf, Cycles to Failure
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)
Information Proprietary to Lambda
32Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
102 103 104 105 106 107
AA2024-T851 Structural Test Panel - Part B (Simple)
LPB - Pre-crackedao = 0.05 in.
LPB - Smooth
Baseline - Pre-crackedao = 0.05 in.
Baseline - Smooth
Nf, Cycles to Failure
Spec
imen
Con
ditio
ns
R = -1 Pmax = 6000 lbs Smax ≈ 12.5 ksiPre-cracking at Pmax = 3600 lbs, R = 0.01
Information Proprietary to Lambda
33Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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
Information Proprietary to Lambda
34Mitigation of Fatigue and Pre-Cracking Damage Through LPB - ASIP2007
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