Learning from the Past? Fatigue Failures in Engineered Systems

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1 Learning from the Past? Fatigue Failures in Engineered Systems David K. Matlock Advanced Steel Processing and Products Research Center Colorado School of Mines Golden, Colorado The Hatfield Memorial Lecture December 2, 2014

2 Why title: Learning from the Past?

3 Railroad Axle Failure: circa 1844 Fracture at change in diameter = stress concentration Mid-1800 s Wöhler (and others) showed that fatigue occurs by crack growth from surface defects Developed apparatus for repeated loading of railway axles Contributions led to the S-N or Wöhler curve Result: improved understanding of fatigue. Image - Original: Joseph Glynn, Paper No 617, Proc. ICE en:1844. commons.wikimedia.org/wiki/file:tender_fatigued_axle.jpg. en.wikipedia.org/wiki/august_w%c3%b6hler

4 Railroad Axle Failure: 2004 Conclusion:..Fatigue fracture originated at a surface profile irregularity... likely introduced during axle reconditioning.. Final Fracture

5 Railroad Axle Failure: 2010 Conclusion:... The axle failed in fatigue near the mid-point of the axle body

6 Presentation Overview Introduction: what is fatigue? Modification of material strength and fracture characteristics by the cyclic application of load or stress, often leading to fracture without prior component shape change Present a primer on fatigue Case Studies Fatigue enhancement via metallurgy Design and application

7 Fatigue Potential: Our Daily Lives Transportation airplanesihaveknown.blogspot.com Recreation inhabitat.com sandiegomountainbikeskills.com Energy en.wikipedia.org/wiki/ Wind_turbine awcwire.wordpress.com/2009/08/10/howwind-turbines-work/ taflab.berkeley.edu/me168-fa13/me168_applications.htm

8 How many cycles do we experience? Passenger Car Engine 100,000 mile typical use Average rpm 300 million revolutions Truck Wheels/Axles 1 million mile typical life 500 wheel rotations/mile 500 million rotations

9 Primer on Fatigue Types of loading.. Material property changes due to cyclic loading. How to measure? How to control?

10 Examples of Cyclic Loading: Axial Unidirectional Loading Load or Stress Time Reversed Loading Load or Stress Time

11 Examples of Cyclic Loading: Bending Reversed Bending Example: Leaf Spring Tension Reversed Bending Compression Compression Load or Stress Time Tension

12 Examples of Cyclic Loading: Combined Rotational Bending Example: Drive shaft out of alignment Tension Rotation Compression Applied Bending Load or Stress Stress at point as shaft rotates Time

13 Effects of Cyclic Loading on Strength Cyclic stress-strain behavior Measure load & displacement in sample cyclically loaded from tension to compression Pure Copper Fully Annealed Cold Worked Stress Strain J.D. Morrow, Cyclic Plastic Strain Energy and Fatigue of Metals. In: American Society for Testing and Materials - ASTM STP 378. Internal Friction, Damping and Cyclic Plasticity 1965; p

14 Effects of Cyclic Loading on Strength SAE 4340 Steel Monotonic σ-ε Cyclic σ-ε R.W. Landgraf, Achievement of High Fatigue Resistance in Metals and Alloys, ASTM STP-467, 1970, p. 3.

15 Effects of Cyclic Loading on Strength Cyclic Hardening Cyclic Softening Aluminum 2024-T6 Steel SAE 4340 Stress Stress Strain Strain R.W. Landgraf, J.D. Morrow, and T. Endo, J. Materials, JMLSA 4(1), ASTM 1969, P. 176.

16 Effects of Cyclic Loading on Strength Big-Picture Conclusion: Hard (i.e. high strength) materials cyclically soften --- while soft (i.e. low strength) materials cyclically harden! Cyclic Softening Cyclic Hardening Stress Strain

17 Effects of Cyclic Loading on Fracture Three stages of fatigue Crack nucleation - at point of high applied stress results from local plastic deformation after multiple cycles Stable crack growth - on plane perpendicular to the maximum tensile stress Final fracture - after crack grows to critical length -- i.e. remaining material can no longer support applied cyclic loads Total Fatigue Life: N Total = N Nucleate + N Growth + 1 Overload

18 Effects of Cyclic Loading on Fracture Fatigue Crack Nucleation and Growth R.A. Lund, Fatigue Fracture Appearances, ASM Handbook, Vol. 11, p mm

19 Effects of Cyclic Loading on Fracture Fatigue Crack Nucleation and Growth 10 µm TEM Replica: Low Cycle Fatigue 7075 Al T6 Aluminum R.D. Sloan, Sloan Research Inds. Inc (Circa 1970) R.A. Lund, ASM Handbook, Vol. 11, mm

20 Unidirectional Tension-tension Loading Stable Fatigue Crack Growth 5 mm

21 Important points..effects of Cyclic Loading Strength altered Crack nucleation and growth leads to failure at low stress (e.g. often less than yield stress) Stable crack growth exists prior to fracture Occurs without macroscopic geometry change Grows on plane perpendicular to maximum tensile stress Presence offers the opportunity to utilize non-destructive testing techniques to identify prior catastrophic failure

22 Evidence Fatigue is Critical to Our Daily Lives

23 1951 Starring James Stewart (accessed Nov 2014)

24 Life Imitates Movie De Havilland Comet 1 Innovative airplane Commercial service Initiated 1952 Operated at 40,000 feet Cabin pressurized, 8000 ft equivalent Two catastrophic accidents Royal Aircraft Establishment pressurization tests confirmed cabin structural failure by fatigue Required significant redesign Opened the way for modern design and testing concepts. P.A. Withey, Fatigue Failure of the De Havilland Comet I, Engr. Fail. Anal., vol. 4, no. 2, 1997, pp

25 Aloha Airlines, Flight 243 April 28, 1988 The National Transportation Safety Board (NTSB) determined that the probable cause of the accident was fatigue damage of the fuselage skin lap splice. lessonslearned.faa.gov (accessed Nov 2014)

26 Flight Sioux City, Iowa July 19, 1989 Turbofan engines - fan disk failure Ti alloy. Undetected defect formed during initial manufacture (Dec. 1971). Defect caused the initiation of a fatigue crack Crack grew to a critical size catastrophic failure Disk parts damaged hydraulic control systems Total service time = 41,009 hours and 15,503 cycles (i.e. flights) lessonslearned.faa.gov (accessed Nov2014)

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29 ¾ inch (19 mm) diameter bolts

30 Adopted October 1, 1991 fracture surfaces of three bolts indicated fatigue cracks initiating at multiple sites along the thread roots on diametrically opposite sides of the bolts 10 mm

31 Methods to Assess Fatigue Fracture Properties Fatigue Life Tests (S-N) Fatigue Crack Growth

32 Fatigue Life Curves Multiple standardized tests available Specialized tests designed to simulate in-service conditions ASPPRC, Colorado School of Mines, Golden, CO USA

33 Fully Reversed Test; Frequency = 30 Hz Video starts after 2280 cycles L.M. Rothleutner and D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2014

34 Failure life = 2750 cycles L.M. Rothleutner and D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2014

35 Failure life = 2750 cycles L.M. Rothleutner and D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, mm

36 Typical S-N Data Nominal Reversed Bending Stress (MPa) cycles Direct cooled Non-traditional NTB Bainitic Steel 0.34 C, 1.21 Mn, 0.66 Si, 0.09 V 25HRC; 15% retained austenite Baseline (3) Cycles Nominal Reversed Bending Stress (ksi) Fatigue Limit Or Endurance Limit M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Metall. and Mat. Trans. A, 2013, vol. 441, pp

37 Krouse-type Bending Fatigue Displacement controlled; constant frequency Large constant stress region Variable R 1 to 1 Flat samples 5 cm ASPPRC, Colorado School of Mines, Golden, CO USA

38 Bending Fatigue of Spring Steel 426 o C Temper 500 o C Temper WQ and AC indicate cooling after tempering As-Quenched N. Merlano, Effect of Tempering Conditions On The Fatigue and Toughness of 5160H Steel, MS Thesis, Colorado School of Mines, 1989

39 Fatigue Crack Growth Analysis Fracture mechanics based approach Assume material contains a crack (flaw, notch,..) Machine standard sample Impose cyclic tensile load Measure change in crack length (da) with each cycle (dn) Correlate: da/dn = f(δp) = f (Δσ) = f (ΔK) Where: P = load σ = stress = (load/area) K = stress intensity factor σ g(crack geometry)

40 Fatigue Crack Growth Analysis ΔP da a (accessed Nov 2014) T.L. Anderson, Fracture Mechanics: Fundamentals and Applications, CRC Press, Boca Raton, Florida, 1991, p. 603.

41 Potential to Alter Stable Crack Growth 10-5 da/dn = 1.36 x ( K) 2.25 Single Function! da/dn (m/cycle) Ni STEEL 10 Ni STEEL HY-130 STEEL HY-80 STEEL da dn = A ( K) m Tempered Martensitic Steels Applicability of data: Yield = 560 to 2070 MPa Ambient temperature Dry air K (MPa m) Adapted from: J.M. Barsom and S.T. Rolfe, Fracture and Fatigue Control in Structures, 2 nd Edition (1987), Prentice-Hall, Englewood Cliffs, New Jersey, p. 287

42 Stable Fatigue Crack Growth Plastic Zone 5 mm ASPPRC, Colorado School of Mines, Golden, CO USA

43 Interpretation of Single da/dn Function A D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2009.

44 Interpretation of Single da/dn Function Apply stress = plastic zone A 2 1 K r I p = 6π σ y D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2009.

45 Interpretation of Single da/dn Function Apply cyclic stress = plastic zone advances A r p = 1 6π K σ I y 2 da dn Crack advances D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2009.

46 Interpretation of Single da/dn Function Apply cyclic stress = plastic zone advances A r p = 1 6π K σ I y 2 da dn r p = 10 da dn Growth controlled by cyclic stress strain Hard materials cyclically soften Soft materials cyclically harden D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, to 2000

47 Interpretation of Single da/dn Function Apply cyclic stress = plastic zone advances Conclusion: Limited opportunity A to influence 1 6π fatigue life through control p of fatigue crack growth rates via r metallurgy modifications p = da da must address crack dn dn nucleation! Or crack growth by design! Growth controlled by cyclic stress strain Hard materials cyclically soften Soft materials cyclically harden D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, r = K σ I y to

48 Lessons Learned Lab Tests Summary of approaches to produce structures with enhanced fatigue performance Decrease surface cyclic tensile stress Remove Loads!! Remove Cycles!! Minimize stress concentrations Design Manufacturing Induce residual compressive stress Increase material strength ( EL UTS ) Bulk or surface Maximize material quality i.e. minimize inclusion contents, etc.

49 Examples: Metallurgical Modifications to Control Crack Nucleation Process Control Deep Rolling - Shafts Alloy Control Steel Cleanliness Bearings Microalloying - Gears

50 Drivers: Future Automobile Engines Lighter weight + higher performance = higher stresses High-strength fatigue-resistant materials facilitate designs

51 Deep Rolling: Crankshafts M.D. Richards, PhD Thesis, Colorado School of Mines, USA, Connecting Rod en.wikipedia.org/wiki/crankshaft

52 A. Fatemi, et al., Fatigue Performance Evaluation of Forged Steel Vs. Ductile Cast Iron Crankshafts: A comparative Study, U. of Toledo, 2007, Single Cylinder Crankshaft

53 Deep Rolling Laboratory Sample Sample Diameter = 25 mm M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, Metallurgical and Materials Transactions A, 2013, vol. 441, pp A. Fatemi, et al., Fatigue Performance Evaluation of Forged Steel Vs. Ductile Cast Iron Crankshafts: A comparative Study, U. of Toledo, 2007,

54 Deep Rolling M.D. Richards, The Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, PhD Thesis, Colorado School of Mines, USA, 2008.

55 Deformation during Deep Rolling Roller Geometry Change Due to Deformation Residual Stress Notch Constraint Strain Deformation Volume Radially symmetric, non-uniform strain Increases local strength Mechanically burnishes surface Develops residual stress Residual stress stability depends on response to M.D. Richards, The Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled cyclic loading Medium Carbon Steels, PhD Thesis, Colorado School of Mines, USA, 2008.

56 Test Methodology: R = 1, Freq. = 30 Hz Sample Diameter = 25 mm M.D. Richards, The Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, PhD Thesis, Colorado School of Mines, USA, 2008.

57 Baseline Fatigue Performance Nominal Reversed Bending Stress (MPa) Steel Three Steel Baseline (3) (3) Nominal Reversed Bending Stress (ksi) Alloy Alloys Fatigue Ratio EL/UTS NTB 0.47 C38M Cycles M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, Metallurgical and Materials Transactions A, 2013, vol. 441, pp

58 Deep Rolled Fatigue Performance Nominal Reversed Bending Stress (MPa) (3) Steel Deep Rolled (2) (3) Baseline (3) (3) Cycles Nominal Reversed Bending Stress (ksi) Alloy Nominal Endurance Limit S f-dr (MPa) Fatigue Ratio k t *S f-dr /UTS NTB C38M Deep rolling increases endurance Limit by 50 to 60 %. M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, Metallurgical and Materials Transactions A, 2013, vol. 441, pp

59 Processing to Optimize Fatigue Resistance Hypothesize Fatigue resistance improved by Stabilization of cold worked dislocation structure Stabilization of residual stress distribution Approaches to process modifications Age previously rolled samples Roll at dynamic strain aging temperatures (up to about 350 o C) M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, Metallurgical and Materials Transactions A, 2013, vol. 441, pp

60 Dynamic Strain Aging (DSA) Changes in Deformation Mechanisms Decrease dislocation mobility pinning Increase dislocation density Change in dislocation structure from cellular to diffuse tangles ELONGATION C.C. Li, and W. C. Leslie, Effects of dynamic strain aging on the subsequent mechanical properties of carbon steels, Metallurgical Transactions A, December 1978, Volume 9, Issue 12, pp

61 Deep 340 o C Nominal Reversed Bending Stress (MPa) Steel 4140 Deep Rolled - HT (3) (3) Deep Rolled - RT (2) (3) Baseline (3) (3) Cycles M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, Metallurgical and Materials Transactions A, 2013, vol. 441, pp

62 Summary: Deep Rolling Fatigue crack nucleation made more difficult Deep rolling at elevated temperatures increases EL by approximately 100% Processing at DSA temperatures proved very cost effective to enhance fatigue performance M.D. Richards, M. Burnett, J.G. Speer, and D.K. Matlock, Effects of Deformation Behavior on the Fatigue Performance of Deep Rolled Medium Carbon Steels, Metallurgical and Materials Transactions A, 2013, vol. 441, pp

63 Examples: Metallurgical Modifications to Control Crack Nucleation Process Control Deep Rolling - Shafts Alloy Control Steel Cleanliness Bearings Microalloying - Gears

64 Fatigue in Gears and Bearings Drive Gear Contact Bending Driven Gear commons.wikimedia.org/wiki/file:spur_gears_animation.gif

65 Rolling Contact Fatigue in Bearings 1000 Today Cleaner Steel 1980 P. Kramer, An Investigation of Rolling-Sliding Contact Fatigue Damage of Carburized Gear Steels, MS Thesis, CSM 2013 Fatigue Life (Millions of Revolutions) Vacuum Arc Remelted Improved Bottom Pour Precipitation Deoxidation + Shrouding Original Bottom Pour Precipitation Deoxidation Vacuum Carbon Deoxidation Stress profile adapted from L.E. Alban, Systematic Analysis of Gear Failures, American Society for Metals, Metals Park, OH (1985), pp Total Length of Inclusion Stringers (mm/cm 3 ) C.V Darragh, Engineered Gear Steels A Review, 2001 Drives and Controls/Power Electronics Conference, pp

66 Bending Fatigue: Gear Steels Utilize higher temperature carburizing more efficient (vacuum, plasma) Issue, need to suppress grain growth & refine austenite grain sizes to increase performance Utilize microalloy (Nb) precipitates to suppress grain growth G. Krauss, D.K. Matlock, and A. Reguly, Microstructural Elements and Fracture of Hardened High-Carbon Steels, Proc. of Thermec 2003, Trans Tech Publications, Inc., Uetikon-Zurich, Switzerland, 2003, pp

67 0.1 Nb Nb-Ti Modified 8620 Steel : Vacuum o C 0.06 Nb 100 µm All Alloys ºC min Nb 100 µm Stress (Mpa) Nb 0.06 Nb 0.02 Nb 600 b 100 µm Cycles R.E. Thompson, D.K. Matlock, and J.G. Speer, "The Fatigue Performance of High Temperature Vacuum Carburized Nb Modified 8620 Steel," SAE Transactions, Journal of Materials and Manufacturing, Vol. 116, Sect. 5 (2007) pp

68 Selected Case Studies to Illustrate Engineering Solutions to Fatigue Failures.. importance of design, manufacturing, and maintenance

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70 Design Example: Fatigue Failure in Bullwheel Axle Shaft

71 Lower Terminal Bullwheel Axle Failure Hub Sheave Main Bullwheel Shaft D.K. Matlock, "Lift Fatigue, Ski Area Management, vol. 23, no. 1, 1984, pp , 80 (

72 Bullwheel Shaft Dia = 5 ¼ inch (13.3 cm) Crack Location D.K. Matlock, "Lift Fatigue, Ski Area Management, vol. 23, no. 1, 1984, pp , 80 (

73 Hub Sheave Main Bullwheel Shaft D.K. Matlock, "Lift Fatigue, Ski Area Management, vol. 23, no. 1, 1984, pp , 80 (

74 Hub Sheave Main Bullwheel Shaft D.K. Matlock, "Lift Fatigue, Ski Area Management, vol. 23, no. 1, 1984, pp , 80 (

75 Have we learned anything from the past? What about the future?

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77 Closing Comments So. Why do fatigue failures continue to occur? Multiple inputs affect fatigue performance Design Material Manufacture Maintenance Application/Use Fatigue fractures will continue to occur!

78 Closing Comments Opportunities exist for continued development of high-performance clean materials Inspection Opportunities for smart NDE technologies to identify cracks before catastrophic failure Continual fatigue education critical All parties involved must appreciate factors which control fatigue life still need good Common Sense Engineering

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