RESIDUAL STRESSES: TOOLS FOR OPTIMIZING COMPONENT PERFORMANCE

Transcription

1 RESIDUAL STRESSES: TOOLS FOR OPTIMIZING COMPONENT PERFORMANCE Doug Hornbach Lambda Technologies 1

2 Residual Stresses: Tools For Optimizing Component Performance Outline: Residual Stress Effect on Fatigue Processes That Produce Residual Stress Surface Treatments Review of LPB Process Example LPB of Ti-6Al-4V Neck Segment LPB Process Control Conclusions 2

3 Residual Stress Definition Stresses That Exist In An Elastic Solid Body With All External Loading Removed 3

4 Residual Stress Effect on Fatigue Manufacturing Processes Create RS RS Can Be As Large As Yield Level Surface Is Weaker in Fatigue Than Subsurface Compressive RS Beneficial in Fatigue Tensile RS Detrimental in Fatigue 4

5 STRESS MAX STRESS Residual Stress Effect on Fatigue Alternating Applied Stress S-N Curve S max R = Smin/Smax = -1 S alt t R = -1 S min N 5

6 - STRESS MAX APPLIED STRESS Residual Stress Effect on Fatigue Fatigue Strength Reduced + +S mean t -S mean -S mean S mean = +S mean S mean = (S min + S max )/2 Fatigue Strength Increased N 6

7 Residual Stress Effect on Fatigue HCF strength increases with subsurface compression Shot Peened 17 Spring Steel Endurance limit increases linearly with compression: 4X Surface compressive layer retards crack initiation and growth. Mattson, R.L. and Roberts, J.G., The Effect of Residual Stresses Induced by Strain Peening upon Fatigue Strength, Internal Stresses and Fatigue in Metals, Edited by G.M. Rassweiler and W.L. Grube, New York, NY: Elsevier Publishing Co., 1959, pp

8 S alt, ksi 2 15 FATIGUE DESIGN DIAGRAM Ti-6Al-4V STA SWT k f = 3 SWT k f = 1 Yield Stress N f = 1 7 cycles 8 1 Enhance 5 Safe No Mode I Cracking Possible RS -2-1 min 1 2 RS max S mean, ksi RS max and RS min are the upper and lower bounds RS optimum can be determined by taking into account part distortion and compensatory tension 8 Restore

9 How Are Residual Stresses Formed RS Result From Strain Gradients (Differences in Strain From One Location to the Next) Most Manufacturing Processes Produce RS Machining Milling, Turning, Grinding, EDM Heat Treatment Carburizing, Nitriding, Induction Hardening Joining Welding, Brazing Surface Treatment Shot Peening, Burnishing, Laser Shock Processing (LSP), Low Plasticity Burnishing (LPB) 9

10 Residual Stress Measurement LINEAR ELASTIC METHODS Mechanical 1) Various dissection methods 2) Ring core 3) Center-hole drill (ASTM E837) Diffraction 1) X-ray 2) Neutron NON-LINEAR ELASTIC METHODS Magnetic 1) Barkhausen noise 2) Eddy Current Ultrasonic 1

11 Residual Stress (ksi) Residual Stress (MPa) Residual Stress (ksi) Residual Stress (MPa) Residual Stress (ksi) Residual Stress (MPa) Machining Residual Stresses TURNING MILLING EDM Depth (x 1-3 mm) CIRCUMFERENTIAL LONGITUDINAL Depth (x 1-3 in.) Ti-6Al-4V ROUND BAR Mid-Length of Turned Region Depth (x 1-3 mm) Dry With Cutting Fluid Depth (x 1-3 in.) Ti-6Al-4V TEST COUPON Center of Face Milled Region Depth (x 1-3 mm) Depth (x 1-3 in.) Ti-6Al-4V TEST COUPON Finish EDM Test Face

12 Residual Stress (MPa) Residual Stress (ksi) Welding Residual Stresses Distance from Weld Center (in.) Retreating Side.1 in. Advancing Side -4 Surface Distance from Weld Center(mm) Ti-6Al-4V Friction Stir Weld Longitudinal Direction

13 Surface Treatments Shot Peening Heat Treatment Hole Expansion LPB Burnishing Laser Peening 13

14 Residual Stress (ksi) Residual Stress (MPa) Shot Peening Impact of shot at high speed creates surface deformation in the form of a dimple Deformed in Tension Dimple SHOT PEENING RESIDUAL STRESS DISTRIBUTIONS Depth (x 1-3 mm) A -25 1A A 4A 6A Ti 6Al-4V Depth (x 1-3 in.) 14

15 Heat Treatment Carburizing Nitriding Induction Hardening Heat & Quench Hoop RS in Turbine Disk Following Heat Treat and Quench 15

16 Residual Stress (ksi) Residual Stress (MPa) Hole Expansion Circumferential Residual Stress Depth (mm) % Interference % Interference Depth (in.) 775 ALUMINUM TEST COUPON Inside Diameter Mid-Length of Expanded Hole 16

17 RESIDUAL STRESS (ksi) RESIDUALSTRESS (MPa) Laser Peening Longitudinal Residual Stress DEPTH (mm) DEPTH (x1-3 in.) Ti-6Al-4V TEST SAMPLE Longitudinal Residual Stress 17

18 How to Choose a Surface Treatment Component Geometry & Operating Environment Part Cross Section Baseline Residual Stress Condition Accessibility of Critical Region/s Operating Temperature Magnitude of Applied Stresses Damage Mechanism Corrosion Pits FOD Fretting Damage Distortion Limits Surface Finish Production Requirement Cycle Time & Part Volume Logistics Quality Control Costs (Capital Equipment, O&M) 18

19 PRODUCTION LPB APPLICATION M-Series Modular Hip System 19

20 PERCENT COLD WORK RESIDUAL STRESS (MPa) RESIDUAL STRESS (ksi) LPB Technology High-hardness ball is rolled, under pressure, over surface Single pass provides deep compression Patented hydrostatic bearing with constant volume flow Low cold work < 3.5-5% provides stable compression RESIDUAL STRESS DISTRIBUTION DEPTH (1-3 in.) SHOT PEEN GRAVITY PEENED LPB % DEPTH (x1-3 mm) PERCENT COLD WORK DISTRIBUTION IN DEPTH (x1-3 mm) 2

21 Features of Low Plasticity Burnishing (LPB) Low magnitude controlled cold working Thermal and Mechanical (Overload) stability Easily performed on CNC tools in a machine shop Applicable to arbitrary shapes and directions Rapid processing (>2 sfm in turning) Deep compressive layer (~1mm, up to 12 mm ) High magnitude compression (~YS) Established QA of a mechanical manufacturing system Improves HCF and SCC by mitigating damage without changing material or design 21

22 LPB Tool Technology 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 26 22

23 LPB CNC Production Tooling CNC Control LPB Control LPB Tool LPB Tool CNC Control LPB Control Hydraulic Control Cabinet Fanuc S42W Robot Haas VF3B Mill Machining-like operation using typical CNC machine tools or robots for precise, reproducible processing Highly automated minimal operator intervention Low capitalization costs use existing CNC machines Shop floor and logistically compatible no specialized or remote facilities required 23 Mazak Quick Turn 6T Lathe 6-axis 4- and 5-axis 3-axis

24 Metaphyseal Baseline Fractography Neck Segment Fatigue Initiations Fretting on O.D. of Neck Multiple Cracks Along Length of Neck Fatigue Initiation 12 o clock 24

25 Fretting Fatigue Overview Fretting occurs when two mating parts, are pressed together, and are subjected to a transverse cyclic loading typically less than.4 in. (.1 mm). (1) Fretting can cause metal-to-metal adhesion or microwelds. Weld edges are susceptible to high cyclic shear stress from which microcracks can propogate. Microcracks are typically on the order of a few thousandths of an inch. In the absence of a sufficient depth of compressive residual stress the microcracks can grow into macrocracks leading to a significant reduction in component fatigue strength and life. Example of Fretting Induced Microcrack In Ti-6-4 Fan Blade Dovetail (1) Frost, N.E. Marsh, K.J. Pook, L.P., Metal Fatigue, Oxford University Press,

26 Finite Element Mesh Force Applied to Femoral Head Neck Segment Constrained Neck Taper 26

27 Finite Element Determination of Applied Stress Maximum Tension Near 12 o clock 27 Maximum Compression Near 6 o clock

28 Finite Element Determination of Applied Stress 28

29 Low Plasticity Burnishing of Neck Stem LPB Tool Neck Stem 29

30 Residual Stress (ksi) Residual Stress (MPa) X-Ray Diffraction Residual Stress Results Depth (mm) X Surface Compression.3 in. Depth of Compression Depth (x 1-3 in.) LPB Roller Burnished Baseline

31 Finite Element Prediction of Residual Stresses Full Model Cross-Section O.D. Compression I.D. Tension < +15 ksi 36 deg. of Compression In Tapered Region 31

32 Stress (ksi) Stress (M Pa) Applied + Residual Stress Distance From Tip of Tapered Neck (m m ) Nom inal Baseline Fatigue Strength Radius Start 8 6 Applied Stress 15 lbs -6 Applied Stress 18 lbs Applied 15 lbs + Residual Stress(LPB) -8 Applied 18 lbs + Residual Stress(LPB) Distance From Tip of Tapered Neck (in.)

33 High Cycle Fatigue Test Set-Up Fatigue Testing Per ISO 726-4,-8 Bending + Torsion R=.1 Frequency = 15Hz Saline Drip Femoral Head Neck Segment Bone Cement 33

34 Force (lbs) Force (N) High Cycle Fatigue Results ~33% Increase in Fatigue Strength ~5X Increase in Life LPB Eliminated Fretting Failures In Tapered Region Metaphyseal Failures Baseline - Neck Failure 5 Baseline - Runout 2 LPB - Runout LPB - Metaphyseal Failure LPB - Radius Failure Outside of LPB E7 Cycles

35 Fatigue Life 14 lbs LPB TREATED BASELINE 5X Life Improvement CYCLES TO FAILURE 35

36 Metaphyseal Fatigue Failures Crack in Metaphyseal 36

37 Dedicated CNC Lathe Based LPB System for Processing Hip Neck Stems CNC Control LPB Control LPB Hydraulics LPB Tool Mounted in Cat 4 Tool Holder 37

38 Production Quality Assurance LPB process at extremes and test Real time monitoring of pressure Software written to verify pressures LPB process cycle time ~ 1 min. 38

39 Number of Failures FDA FAILURE HISTORY - EXACTECH MODULAR HIP SYSTEM LPB Treatment Started Here failures from hips installed before LPB processing 2 Year No Failures (After LPB Treatment) 39

40 Summary Manufacturing Processes Produce RS (- and +) RS Can Dramatically Effect Fatigue Properties Surface Treatments Can Be Used To Introduce Compressive RS Without Costly Changes in Alloy or Geometry Surface Treatment Depends On Many Factors (Part Geometry, Damage Mechanism, Production Requirements) LPB Increased Fatigue Strength by ~33% and Life by ~5X. Compressive RS Completely Eliminated Fatigue Failures From Fretting. 4

41 QUESTIONS? 41