62nd_lecture_slides.pdf

David K. Matlock Advanced Steel Processing and Products Research Center

Colorado School of Mines Golden, Colorado

Learning from the Past? Fatigue Failures in Engineered

Systems

The Hatfield Memorial Lecture December 2, 2014

Why title: “Learning from the Past?”

Image - Original: Joseph Glynn, Paper No 617, Proc. ICE en:1844. commons.wikimedia.org/wiki/File:Tender_fatigued_axle.JPG.

Railroad Axle Failure: circa 1844

• 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.

en.wikipedia.org/wiki/August_W%C3%B6hler

Fracture at change in diameter = stress concentration

Railroad Axle Failure: 2004

Conclusion: “..Fatigue fracture originated at a …surface profile irregularity... likely introduced during axle reconditioning..”

Final Fracture

Railroad Axle Failure: 2010

Conclusion: “... The axle failed in fatigue near the mid-point of the axle body…”

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

Fatigue Potential: Our Daily Lives

inhabitat.com www.netcarshow.com airplanesihaveknown.blogspot.com

Transportation

Recreation

Energy

taflab.berkeley.edu/ME168-FA13/ME168_Applications.htm

www.lusas.com en.wikipedia.org/wiki/Wind_turbine awcwire.wordpress.com/2009/08/10/how-

wind-turbines-work/

sandiegomountainbikeskills.com www.world-insider.com/usa-the-best-amusement-parks/

Truck Wheels/Axles 1 million mile “typical” life ≈ 500 wheel rotations/mile

500 million rotations

Passenger Car Engine 100,000 mile “typical” use

Average 40 mph @ 2000 rpm 300 million revolutions

How many cycles do we “experience”?

Primer on Fatigue

• Types of loading.. • Material property changes due

to cyclic loading…. • How to measure? • How to control?

Reversed Loading

Load

or S

tres

s

Time

Load

or S

tres

s Time

Unidirectional Loading

Examples of Cyclic Loading: Axial

Examples of Cyclic Loading: Bending

Tension

Compression

Tension

Compression

Reversed Bending

Load

or S

tres

s

Time

Reversed Bending Example: Leaf Spring

Rotational Bending Example: Drive shaft out of alignment

Stress at point as shaft rotates

Load

or S

tres

s

Time

Tension

Compression

Rotation

Applied Bending

Examples of Cyclic Loading: Combined

• Cyclic stress-strain behavior • Measure load & displacement in sample cyclically

loaded from tension to compression

Stre

ss

Strain

Pure Copper Fully Annealed Cold Worked

Effects of Cyclic Loading on Strength

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. 45–87.

SAE 4340 Steel

R.W. Landgraf, Achievement of High Fatigue Resistance in Metals and Alloys, ASTM STP-467, 1970, p. 3.

Cyclic σ-ε

Monotonic σ-ε


Cyclic Hardening

Strain Strain

Stre

ss

Stre

ss

R.W. Landgraf, J.D. Morrow, and T. Endo, J. Materials, JMLSA 4(1), ASTM 1969, P. 176.

Aluminum 2024-T6 Steel

SAE 4340

Cyclic Softening


“Big-Picture” Conclusion: Hard (i.e. high strength) materials

cyclically soften --- while soft (i.e. low strength) materials cyclically harden!


Cyclic Softening

Cyclic Hardening

Strain

Stre

ss

• 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: NTotal = NNucleate + NGrowth + 1Overload

Effects of Cyclic Loading on Fracture Three “stages” of fatigue

Fatigue Crack Nucleation and Growth

R.A. Lund, “Fatigue Fracture Appearances,” ASM Handbook, Vol. 11, 2002. p. 627.

10 mm

Effects of Cyclic Loading on Fracture

Effects of Cyclic Loading on Fracture Fatigue Crack Nucleation and Growth

R.A. Lund, ASM Handbook, Vol. 11, 2002

10 mm

10 µm

TEM Replica: Low Cycle Fatigue 7075 Al – T6 Aluminum

R.D. Sloan, Sloan Research Inds. Inc (Circa 1970)

5 mm

Stable Fatigue Crack Growth

Unidirectional Tension-tension 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

Important points…..Effects of Cyclic Loading

Evidence Fatigue is Critical to Our Daily Lives

1951 Starring

James Stewart

www.metacafe.com/watch/7743905/no_highway_in_the_sky_1951/ (accessed Nov 2014)

www.telegraph.co.uk

P.A. Withey, “Fatigue Failure of the De Havilland Comet I,” Engr. Fail. Anal., vol. 4, no. 2, 1997, pp. 147-154.

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 1954 • Royal Aircraft Establishment pressurization tests

confirmed cabin structural failure by fatigue • Required significant redesign

• Opened the way for modern design and testing concepts.

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)

Flight 232 - 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)

¾ inch (19 mm) diameter bolts

“…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

Adopted October 1, 1991

Methods to Assess Fatigue Fracture Properties

Fatigue Life Tests (S-N) Fatigue Crack Growth

• Multiple standardized tests available • Specialized tests designed to simulate

in-service conditions

Fatigue Life Curves

ASPPRC, Colorado School of Mines, Golden, CO USA

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

Failure life = 2750 cycles

L.M. Rothleutner and D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2014

Failure life = 2750 cycles

5 mm L.M. Rothleutner and D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2014

Typical “S-N” Data

Baseline(3)

103 104 105 106 107 108

Cycles

200

300

400

500

600

700

800

900N

omin

al R

ever

sed

Ben

ding

Stre

ss (M

Pa)

30

40

50

60

70

80

90

100

110

120

130

Nom

inal

Rev

erse

d Be

ndin

g St

ress

(ksi

)

NTB

(3)

Direct cooled “Non-traditional” Bainitic Steel

0.34 C, 1.21 Mn, 0.66 Si, 0.09 V 25HRC; 15% retained austenite

27

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. 270-285.

2750 cycles

Krouse-type Bending Fatigue

5 cm

• Displacement controlled; constant frequency • Large constant stress region • Variable R

–1 to 1 • Flat samples


Bending Fatigue of Spring Steel

WQ and AC indicate cooling after tempering

N. Merlano, Effect of Tempering Conditions On The Fatigue and Toughness of 5160H Steel, MS Thesis, Colorado School of Mines, 1989

426 oC Temper

500 oC Temper

As-Quenched

• 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)

Fatigue Crack Growth Analysis

Fatigue Crack Growth Analysis

www.fracturemechanics.net (accessed Nov 2014)

a da

ΔP

T.L. Anderson, Fracture Mechanics: Fundamentals and Applications, CRC Press, Boca Raton, Florida, 1991, p. 603.

Adapted from: J.M. Barsom and S.T. Rolfe, Fracture and Fatigue Control in Structures, 2nd Edition (1987), Prentice-Hall, Englewood Cliffs, New Jersey, p. 287

Tempered Martensitic Steels

Applicability of data: Yield = 560 to 2070 MPa

Ambient temperature Dry air

Potential to Alter Stable Crack Growth

Single Function!

( )mKA

dNda

∆=

1 10 100∆K (MPa√m)

da/d

N (

m/c

ycle

)

12 Ni STEEL10 Ni STEELHY-130 STEELHY-80 STEEL

10-9

10-8

10-7

10-6

10-5

da/dN = 1.36 x 10-10 (∆K)2.25

5 mm

Stable Fatigue Crack Growth

Plastic Zone


A

Interpretation of Single da/dN Function

D.K. Matlock, ASPPRC, Colorado School of Mines, Golden, CO USA, 2009.

A 2

y

Ip

K61r

σπ=

Apply stress = plastic zone



A

dadN

Apply cyclic stress = plastic zone advances

2

y

Ip

K61r

σπ=

Crack advances



A

dadN

2000to10dN

darp =

Growth controlled by “cyclic stress strain” “Hard materials cyclically soften” “Soft materials cyclically harden”


2

y

Ip

K61r

σπ=



A

dadN

2000to10dN

darp =

Growth controlled by “cyclic stress strain” “Hard materials cyclically soften” “Soft materials cyclically harden”


2

y

Ip

K61r

σπ=

Conclusion: Limited opportunity to influence

fatigue life through control of fatigue crack growth rates via metallurgy modifications -- must address crack

nucleation! Or crack growth by design!



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.

Examples: Metallurgical Modifications to Control Crack

Nucleation

• Process Control • Deep Rolling - Shafts

• Alloy Control • Steel Cleanliness – Bearings • Microalloying - Gears

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

en.wikipedia.org/wiki/Crankshaft www.driving-test-success.com/how-cars-work.htm

Connecting Rod

Deep Rolling: Crankshafts M.D. Richards, PhD Thesis, Colorado School of Mines, USA, 2008.

A. Fatemi, et al., “Fatigue Performance Evaluation of Forged Steel Vs. Ductile Cast Iron Crankshafts: A comparative Study,” U. of Toledo, 2007, www.autosteel.org.

Single Cylinder Crankshaft

A. Fatemi, et al., “Fatigue Performance Evaluation of Forged Steel Vs. Ductile Cast Iron Crankshafts: A comparative Study,” U. of Toledo, 2007, www.autosteel.org.

Deep Rolling Laboratory Sample

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. 270-285

Sample Diameter = 25 mm

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.

• Radially symmetric, non-uniform strain • Increases local strength • Mechanically burnishes surface • Develops residual stress

• Residual stress stability depends on response to cyclic loading

Deformation during Deep Rolling

Geometry Change Due to Deformation

Deformation Volume

Strain

Notch Constraint

ResidualStress

Roller


Test Methodology: R = 1, Freq. = 30 Hz Sample Diameter = 25 mm


Baseline Fatigue Performance

Baseline(3)

103 104 105 106 107 108

Cycles

200

300

400

500

600

700

800

900

Nom

inal

Rev

erse

d B

endi

ng S

tress

(MP

a)

30

40

50

60

70

80

90

100

110

120

130

Nom

inal

Rev

erse

d Be

ndin

g St

ress

(ksi

)

4140

(3)(3)

Alloy Fatigue Ratio EL/UTS

4140 0.49 NTB 0.47

C38M 0.43

4140 Steel Three Steel Alloys


Deep Rolled Fatigue Performance

Cycles

200

300

400

500

600

700

800

900

1000

1100

Nom

inal

Rev

erse

d Be

ndin

g S

tress

(MP

a)

30405060708090100110120130140150

Nom

inal

Rev

erse

d Be

ndin

g St

ress

(ksi

)104 105 106 107

Deep Rolled

Baseline

(2)

(3)

(3)(3)

(3)

4140

AlloyNominal

Endurance Limit Sf-DR (MPa)

Fatigue Ratio kt*Sf-DR/UTS

4140 469 0.74NTB 448 0.76C38M 386 0.69

Deep rolling increases

endurance Limit by 50 to 60 %.

4140 Steel


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 oC)


Dynamic Strain Aging (DSA)

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 1765-1775.

ELONGATION

• Changes in Deformation Mechanisms • Decrease dislocation mobility – pinning • Increase dislocation density • Change in dislocation structure from cellular to diffuse tangles

103 104 105 106 107 108

Cycles

200

300

400

500

600

700

800

900

Nom

inal

Rev

erse

d B

endi

ng S

tress

(MP

a)

4140

(2)

(3)(3)

(3)

(3)(3)

(3)

(3)

(3)Baseline

Deep Rolled - RT

Deep Rolled - HT(3)

Deep Rolled @ 340 oC

4140 Steel


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


Examples: Metallurgical Modifications to Control Crack

Nucleation

• Process Control • Deep Rolling - Shafts

• Alloy Control • Steel Cleanliness – Bearings • Microalloying - Gears

Bending

commons.wikimedia.org/wiki/File:Spur_gears_animation.gif

Contact

Fatigue in Gears and Bearings

Drive Gear

Driven Gear

0.0001 0.001 0.01 0.1 1 10

Total Length of Inclusion Stringers (mm/cm3)

1

10

100

1000

Fatig

ue L

ife (M

illio

ns o

f Rev

olut

ions

)

Cleaner Steel

Vacuum Carbon Deoxidation

Precipitation Deoxidation

Precipitation Deoxidation+ Shrouding

Original Bottom Pour

Improved Bottom Pour

Vacuum Arc Remelted

1980Today

Rolling Contact Fatigue in Bearings

C.V Darragh, “Engineered Gear Steels – A Review,” 2001 Drives and Controls/Power Electronics Conference, pp. 21-26.

P. Kramer, An Investigation of Rolling-Sliding Contact Fatigue Damage of Carburized Gear Steels, MS Thesis, CSM 2013

Stress profile adapted from L.E. Alban, Systematic Analysis of Gear Failures, American Society for Metals, Metals Park, OH (1985), pp. 94–106

• 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

Bending Fatigue: Gear Steels

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. 835-840

0.06 Nb

0.1 Nb

0.02 Nb

100 µm

100 µm

100 µm

Nb-Ti Modified 8620 Steel : Vacuum Carburized @1050 oC

103 104 105 106 107

Cycles

500

600

700

800

900

1000

1100

1200

1300

1400

Stre

ss (M

pa)

0.1 Nb

0.06 Nb

0.02 Nb

All Alloys - 114 ºC min-1

b

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. 392-407.

Selected Case Studies to Illustrate Engineering Solutions

to Fatigue Failures…….. importance of design,

manufacturing, and maintenance

Design Example: Fatigue Failure in

Bullwheel Axle Shaft

Lower Terminal – Bullwheel Axle Failure

Main Bullwheel Shaft

Hub Sheave

D.K. Matlock, "Lift Fatigue,” Ski Area Management, vol. 23, no. 1, 1984, pp. 62 63, 80 (http://www.saminfo.com/article/lift-fatigue).

Bullwheel Shaft Dia = 5 ¼ inch

(13.3 cm)

Cra

ck

Loca

tion


Main Bullwheel Shaft

Hub Sheave


Have we learned anything from the past?

What about the future?

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!

• 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”…

Closing Comments

http://www.johnsoncontrols.com/publish/us/en.html

http://www.tmk-group.com/ipsco.php

62nd_lecture_slides.pdf

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