U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Fracture Mechanicsin Railway Application
Uwe Zerbst
GKSS Research Centre Geesthacht GmbHInstitute of Materials Research, Materials Mechanics
Damage Tolerance of Railway Structures
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Fracture Mechanics in RailwayApplication
Railway Axles- Cracks originating in press fits- Cracks originating in geometrical transitions
Railway Wheels- Block-braked vehicles- Disk-braked vehicles
Railway Rails- Typical rail cracks and
failure scenarios- Stages of fatigue crackextension
- Loading components- Effects of Klingel movementand temperature Broken wheel rim (Austria 1875)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Fracture Mechanics in RailwayApplication
Railway Axles- Cracks originating in press fits- Cracks originating in geometrical transitions
Railway Wheels- Block-braked vehicles- Disk-braked vehicles
Railway Rails- Typical rail cracks and
failure scenarios- Stages of fatigue crackextension
- Loading components- Effects of Klingel movementand temperature
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Axle Failure (Italy)
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Damage Tolerance Analysesof Railway Axles
• Design philosophy: Safe Life + Damage Tolerance
• Very few axle failurese.g., Britain: 1.6 failure events/year(average over 25 years)
• However: Severe consequences possible
• Fatigue cracks initiate at the press fits of wheel and gear or at the geometrical transitions
• Regular inspections:
UltrasoncisMagnetic particles, …
Fracture mechanics: Specify inspectioninterval and/or demands on non-destructiveInspection!
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Input Parameters and Steps of a Damage Tolerance Analysis
Material properties
Crack Extension
FractureDeformation
Probability of Detection
Non-Destructive InspectionComponent Geometry & Loading
Primary LoadingSecondary Loading
For a given inspection interval: crack size which has to be detected
Statistically basedInspection Interval
Initial Crack Size
Residual Lifetime Crack Propagation
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Railway Axle: Applied Loading
1 2Y1 Y2
Q1 Q2
StrainGauge
Frequency or Number of Loading Cycles
102 103 1045 105 106 107 108 109 1010
Mixed Track
Station
High Speed Track
Old Track
Ben
ding
Stre
ss A
mpl
itudeY
Forc
esin
kN
Q F
orce
sin
kN
Time in sec.
Applied load:
- Vehicle weight- Speed- Track quality
accord. to Traupe, 2004Example:
Press fit:- effect on R ratio
+
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Initial crack size
Crack propagation
2co
ao
Initial crack size: ao = 2 mm; 2co = 4 mm
based on (in-service) NDI experience as well as on the possibility of the introduction of sharp notches from ballast impacts
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Demands on Non-destructive Inspection
Residual lifetimeminus one inspec-tion interval
Depth of thecrack whichhas to bedetected.
Loading Cycles1
Break-through
Inspection Interval
Crack depth to bereliably detected
Axle
Wheel
Example
Crack depthin mm
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Railway Axle: Effects Investigated
Reverse vs. rotation bending
Load history effects
Press fit
- Crack originating at press fit- Crack originating at geometrical transition
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Reverse vs. Rotation Bending
Minor effect
Example: 20% reduction in residual lifetime due to rotation bending
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Load History Effects
σ– stresst – time
a – crack depth
N – number ofloadingcycles
da/dN – crackpropagationrate
Temporary tensile overloads may cause crack growth retardation.
Temporary pressure overloads may cause crack growth acceleration.
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Load History Effects - Results
No significantloading sequenceeffect found.
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Press Fit Loading
Effect on R-Ratio!
Crack originatingat the press fit
Crack originatingat the geometricaltransition
Crack nucleation
Crack propagation
R = σmin
σmax
KminKmax
R =
Major effect!
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Press Fit Effect
However: Significantreduction of residual lifetime, when crackorigitates in geome-trical transition!
100%
0
200%
no crack closure
as for R=-2f
ΔKth = 0
no press fit
press fit
ResidualLifetime
Model variations:
morerealistic
Variable amplitude loading
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Press fit crack: Lifetime extension
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Fracture Mechanics in RailwayApplication
Railway Axles- Cracks originating in press fits- Cracks originating in geometrical transitions
Railway Wheels- Block-braked vehicles- Disk-braked vehicles
Railway Rails- Typical rail cracks and
failure scenarios- Stages of fatigue crackextension
- Loading components- Effects of Klingel movementand temperature
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Wheel Failure (Freight Car)
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Damage of Wheels
• Spallings at tread or• Radial crack propagation in radial direction• Release of the press fit between wheel and axle
Crack initiation stage:
Major difference: block-braked and disk-braked vehicles!
Examples (according to Edel, 1995)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Railway Wheels: Block-braked vehicles (1)
Initiation of surface cracks:
Trigger: Cyclic thermo stresses duringbrake application- Peak temperature:540oC and more,
- Concentration in „hot spots“- Thermo stressesup to 465 MPa
Particularly whenbrake applicationinterrupted:
local phase transfor-mation from perliticto martensitic structureCrack nucleation in martensic phase
Residual stresses remain17/46
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Railway Wheels: Block-braked vehicles (2)
Left: Typical crack initiation sites
Right: Residual stress field due to braking
(acc. to Edel, 1995)
(acc. to Diener et al., 1992)
in MPa
Tread
Wheel flang
Chamfer
Clampingrim
Marking
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Railway Wheels: Disk-braked vehicles
Surface cracks:
High speed traffic: Traction at tread causes high shear stresses, Intense plastic deformation in the contact zone
• Originating from the pre-damaged surface fatigue cracks grow at a flatangle along plastically deformed grains
• Below (typically) 1.5 to 2 mm crack deviation into circumferentialdirection, Crack branching towards surface Spalling
Sub-surface cracks:
• Nucleation at imperfections, e.g., non-metallic inclusions below tread. • High loads necessary, e.g., caused by impacts; typically 3 to 5 mm
below tread today rather seldom (purity of material)• Crack propagation similar to that of surface cracks (starting at a flat
angle to the surface; subsequently branching towars surface or in axial direction
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Fracture Mechanics in RailwayApplication
Railway Axles- Cracks originating in press fits- Cracks originating in geometrical transitions
Railway Wheels- Block-braked vehicles- Disk-braked vehicles
Railway Rails- Typical rail cracks and
failure scenarios- Stages of fatigue crackextension
- Loading components- Effects of Klingel movementand temperature
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Hatfield 2003
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Typical Rail Cracks - Examples
• Squats: Small surface cracks whichpredominantly occur at straight tracks; in conjunction with dark spots
• Head Checks: Groups of fine surfacecracks (distance 0.5 to 7 mm)
predominantly at curved tracks; occur at the gauge corner of the outer rail
• Squats and Head Checks candevelop into transverse crackscausing spalling as well as railfracture
Head Check
Transverse crack (“kidney shaped”)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Life Cycle of a Rail Crack
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Loading of a Rail
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rails: Loading Components
Contact Loading
Thermal Loading
Residual Stresses
Axle Loads
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rails: Loading Components
Contact Loading
Thermal Loading
Residual Stresses
Axle Loads
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Loading: Contact Stresses
Essential at the early stagesof crack development
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(R.Smith, Imperial College)
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Loading: Bending & Shear
-40 -30 -20 -10-4
10 20
4
8
12
16
x [mm]
Roll over
KI
KII
KIII
y
x
Surface crack
observedpoint
K [M
Pa¦m
]
(acc. to Bogdanski et al., 1996)
I – normal loadingII – in-plane shear loadingIII – out-of-plane shear loading
Role of lubricant (grease + debris):
- Hydraulic pressure
- Reduced crack face friction
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Loading: Effect of Fluid Entrappment
Donzella et al., 2005
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rails: Loading Components
Contact Loading
Thermal Loading
Residual Stresses
Axle Loads
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Pueblo (Colorado)
Factors affecting the rail neutral temperature:
- replacement or repair of a rail segment- track disturbance by tamping at track installation- roadbed freeze-thaw cycles - cumulative vehicle breaking at certain track sections
Rail Loading: Rail Neutral Temperature
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Sluz et al, 1999
U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rails: Loading Components
Contact Loading
Thermal Loading
Residual Stresses
Axle Loads
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Loading: Residual Stresses (Longitudinal)
Roller straightening Butt welding(Webster et al. 1991) (Skyttebol & Josefson, 2004)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Residual Stresses (Roller Straightening)
Neutron scattering(data provided byCORUS Rail)
axial direction
In-service redistribution of residual stresses at surface
tension
compression
tension
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rails: Loading Components
Contact Loading
Thermal Loading
Residual Stresses
Axle Loads
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Loading: Dynamic Effects
Data:
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Loading: Factors Enhancing Dynamic Effects
corrugation flat spots
Example: Nielsen & Oscarsson (2004)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Simulation of Fatigue Crack Propagation
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Investigations on Stage 2 Crack Propagation
70-80o10-20o
70o
c. 1mm
5mm>
RealityPresentmodel
Aims:
- Development of a damagetolerance procedure for rails
- Modelling effects of parame-ters such as the „Klingel movement“ and ambient temperature on residual lifetime
Crack
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Contact Statistics of New and Worn Rails
1) Rail worn – wheel new2) Rail new – wheel new3) Rail worn – wheel worn4) Rail new – wheel worn
cent
relin
e
not in scale
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Rail Toughness – Scatter and Distribution
3 Parameter Weibull Distribution (Master Curve)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Residual Lifetime:Effect of Rail-Wheel Combination (1)
1) Rail worn – wheel new2) Rail new – wheel new3) Rail worn – wheel worn4) Rail new – wheel worn
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Residual Lifetime:Effect or Rail-Wheel Combination (2)
Almost one order differencein residual lifetime
Reality even more complicated
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Effect of Ambient Temperature (1)
Statistics:
Deutsche Reichsbahn
acc. to Edel
Key parameter:
Temperaturedifferenceto rail neutral tempeature
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Effect of Ambient Temperature (2)
Controls Fatigue Crack Extension Controls Fracture
Mean Minimum
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Effect of Ambient Temperature (3)
Simulation:
Left: Inspection in November
Maximum Failure probabilityabout 20% in winter time
Then: Lower risk untilthe next winter
Right: Inspection in April
Low failure probability in spring, summer and autumn
Drastically increased failureprobability in winter time(almost 100%)
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U. Zerbst: Fracture Mechanics in Railway Application Porto, July 7-8, 2008
Thanks for listening!46/46