Presented at PowerGen International 2019. Introduction of residual compression to mitigate fatigue failure in the leading edge of the 7FA R0, First Stage, Compressor Blade

1. REDUCE GAS TURBINE OPERATIONAL COSTS
WITH ENGINEERED RESIDUAL COMPRESSION
Introduction of Residual Compression to Mitigate Erosion
Initiated Fatigue Failure in the Leading Edge of the
7FA R0, First Stage, Compressor Blade
Authors: Kyle Brandenburg, Dr. N. Jayaraman,
Douglas Hornbach, Michael Prevéy
November 20, 2019

2. Review of 7FA R0 Compressor Blade Cracking
• Gas Turbine Rotor Life: 144,000 hours or 5,000 starts
• Life-Limiting Factor: R0 compressor blade failure
• Mechanism: Leading edge HCF cracking and liberation
• Root Cause: Erosion due to fogging and compressor washing
Opportunities for Cost Reduction:
• Reduce erosion inspection
• Eliminate catastrophic failures
• Reduce blade replacement
• Reduce downtime
source: https://www.epri.com/#/pages/product/3002008863/?lang=en-US

3. Crack Erosion Damage Monitoring Using Molds
Rare axial crack in the leading edge,
above the platform, of a standard R0
blade for a 7FA compressor
http://www.ccj-online.com/rare-leading-edge-crack-found-in-7fa-r0-
blade-just-above-the-platform/
Molds applied on leading edge of
R0 blades to detect <0.010 inch
erosion damage
http://proceedings.asmedigitalcollection.asme.org/proceeding
.aspx?articleid=1602066
Cracked leading edge of R0 blade
https://www.modernpowersystems.com/features/featurecracki
ng-the-fa-r0-problem//featurecracking-the-fa-r0-problem-
412766.html
Erosion related damage tolerance is limited to 0.010 inch on the leading edge,
requiring frequent inspection, blending, and costly downtime

4. Historical Review of R0 Blade Cracking – Corrective Actions
Corrective actions attempted (with significant costs and limited success):
• Changing fogging and compressor washing methods and frequency
• More frequent inspections to detect erosion damage
• Blending of leading edge required for 0.008 inch erosion damage with a total
blend limit of 0.040 inches
• Blade design changes (P-cut, thickening the LE, etc.)
• Material change (from GTD 450 to Inconel 718)
• Laser peening
EPRI estimated cost of a compressor failure is nominally $10-20 million*
*source: http://www.modernpowersystems.com/features/featurecracking-the-fa-r0-problem/

5. Examples of Mitigating Turbine Airfoil Fatigue Cracking with
Residual Compression
Through-thickness compressive residual stress on airfoil blade edges
completely mitigates cracking from preexisting damage
10
4
10
5
10
6
10
7
10
8
0.0
0.2
0.4
0.6
0.8
1.0
Baseline+0.02in.Damage
LPB+0.02in.Damage
1000X Improvement
FailureOccurrences
Stage 1 Compressor Vane
Ti-6Al-4V, 2 LEFM Analysis
Cycles
10
3
10
4
10
5
10
6
0.0
0.2
0.4
0.6
0.8
1.0
>100X Improvement
FailureOccurrences
Cycles
LPB+0.10in.Damage
Baseline+0.03in.Damage
Compressor Blade Airfoil Stages 2 & 3
Ti-8-1-1, 2 LEFM Analysis
10
4
10
5
10
6
10
7
10
8
0.0
0.2
0.4
0.6
0.8
1.0
FailureOccurrences
Cycles
LPB+0.01in.Damage
Baseline+0.01in.Damage
Ground Based Turbine Blade
17-4PH Stainless Steel, 2 LEFM Analysis
>600X Improvement

6. Objectives of Applying Residual Compression
to the 7FA R0 Blade Leading Edge
• Document designed through-thickness residual compression
introduced in the erosion prone leading edge regions of R0 blades
• Document the improvement in damage tolerance by fatigue testing
• Minimize the need for frequent inspections
• Extend the operational life of R0 blades to match the original design
life of the gas turbine rotor

7. Designing Residual Stress to Mitigate
HCF Cracking from Erosion Damage
Fatigue Design Diagram Objective:
Determine the minimum residual
stress required to mitigate HCF
cracking from erosion damage in
the R0 blade
Leading edge damage induced
fatigue cracking can be completely
mitigated with -100 ksi compressive
residual stress-200 -100 0 100 200
0
50
100
150
200
Minimum Required
Through-thickness Compression
 -100 ksi
kf
=1.67
R0 Leading Edge
Nominal
Operating
Stresses
kf
=3
kf
=
1
SAFE
FDD for Custom 450 Stainless Steel
(H1050; YS = 150 ksi)
Salt
,ksi
Smean
, ksi
-1000 -500 0 500 1000
0
250
500
750
1000
1250
MPa
MPa
Nf
= 10
7
Cycles

8. LPB Treated
LE
CONVEX SIDE
LPB Treated
LE
CONCAVE
SIDE
LPB Treated
LE
CC-Close-up
View
LPB Treated
LE
CV-Close-up
View
LPB Treated Leading Edge Region of a Typical R0 Blade
Features of LPB: Deep, stable compression and mirror-like surface finish

9. LPB Induced R0 Blade LE Residual Compression
Untreated R0 blades showed compressive
residual stress to a shallow chordwise depth of
0.01 inch
EPRI 2015: “Low plasticity … burnished blade’s
residual stress measurements met or exceeded
the compressive layer depth and magnitude of the
originally applied … compressive patch.”
Source: https://www.epri.com/#/pages/product/3002006059/?lang=en-US
Deep compression exceeding -100 ksi was achieved at depths >0.025 inch
0.000 0.050 0.100 0.150 0.200
-150
-100
-50
0
SpanwiseResidualStress,ksi
Chordwise Distance From LE, inch
0 1 2 3 4 5
-1000
-750
-500
-250
0R0 Blade Leading Edge Mid-Thickness RS as a
function of Chordwise Distance from Blade Edge
RS Measurement Location:
0.5 inch From the Point of Tangency Near Blade Root
Baseline
LPB Treated
Airfoil Mid-Thickness Locations
mm
MPa

10. Prediction of Effects of LPB Induced RS to Mitigate
HCF Cracking from Erosion Damage
FDD Predictions: LPB Treated R0 Blade
-100 ksi compression achieved at the
mid-thickness location would
completely mitigate fatigue cracking
with erosion damage of 0.025 inch
Better than -70 ksi compression
achieved would still lead to significantly
improved damage tolerance and life
improvement with erosion damage of
0.050 inch
-200 -100 0 100 200
0
50
100
150
200
kf
=
1.67
With LPB and
0.050 inch
(1.27 mm)
Erosion
Damage
With LPB and
0.025 inch
(0.635 mm)
Erosion
Damage
R0 LE
Operating
Stresses
kf
=
3.7
kf
=
1
SAFE
FDD for Custom 450 Stainless Steel
(H1050; YS = 150 ksi)
Salt,ksi
Smean, ksi
-1000 -500 0 500 1000
0
250
500
750
1000
1250
MPa
MPa
Nf = 107
Cycles

11. High Cycle Fatigue Test Fixture
R0 Blade Leading Edge Loaded in Cantilever Tension
Fatigue testing conducted with EDM notches
to simulate deep erosion damage
Tested at EPRI estimated
maximum operating
stresses:
Smean = 57 ksi (390 MPa)
Salt = 35 ksi (240 MPa)
R = Smin / Smax = 0.24
AppliedLoad

12. EDM Notch Simulating Erosion Damage – R0 Blade LE
View of EDM notch to simulate erosion damage on the leading edge
and cross sectional view of the fracture surface

13. HCF Test Results and Damage Tolerance Analysis
LPB treatment
completely mitigated
fatigue cracking and
failure from deep
erosion damage
LPB with 0.025 inch
Deep Damage
LPB with 0.035 inch
Deep Damage
LPB with 0.050 inch
Deep Damage
Baseline with
0.008 inch
Deep Damage
10
4
10
5
10
6
10
7
>15 million cycles
All Run-Out
None Failed
All Failed Under 150,000 Cycles
Cycles

14. CONCLUSIONS
• LPB introduced through-thickness LE compression of
-100 ksi to a depth of 0.025 inches
• Damage tolerance improvement achieved:
• >15 million cycle life for damage <0.025 inch
• >3X improvement over current baseline 0.008 inch limit

15. COST-BENEFIT ANALYSIS
Factors and Assumptions for Cost-Benefit Analysis:
• Originally projected lifetime of the turbine 5,000 starts
• Assume a current inspection and blending sequence once every 500
starts with the 0.010 inch erosion damage
• Conservatively, the 3X improvement in damage tolerance from designed
compression would reduce inspection/blending requirement to about
1/3 of the current frequency
• 100X fatigue life improvement reduces operational risk
Downtime, blade erosion inspection, repair, and replacement
costs are estimated to be 1/3 of the current costs

16. EPRI 2015: “Increasing the damage tolerance of the flow path using
compressive layer patches is of increasing interest, particularly for engines
used in combined cycles. The plant owner could readily implement this
enhancement on spare components. Typically, components would be
shipped to a specialized shop to implement the compressive patch.”
Kyle Brandenburg
kbrandenburg@lambdatechs.com
(513) 561-0883
www.lambdatechs.com
source: https://www.epri.com/#/pages/product/3002006059/?lang=en-US
CONCLUDING REMARKS