fracture mechanics in railway application

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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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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- Loading components- Effects of Klingel movementand temperature

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Axle Failure (Italy)


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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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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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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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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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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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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Reverse vs. Rotation Bending

Minor effect

Example: 20% reduction in residual lifetime due to rotation bending

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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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Load History Effects - Results

No significantloading sequenceeffect found.

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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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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








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Wheel Failure (Freight Car)


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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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


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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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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Hatfield 2003


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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Life Cycle of a Rail Crack

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Loading of a Rail

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Rails: Loading Components

Contact Loading

Thermal Loading

Residual Stresses

Axle Loads

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Contact Loading

Thermal Loading

Residual Stresses

Axle Loads

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Rail Loading: Contact Stresses

Essential at the early stagesof crack development

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(R.Smith, Imperial College)


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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Rail Loading: Effect of Fluid Entrappment

Donzella et al., 2005

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Contact Loading

Thermal Loading

Residual Stresses

Axle Loads

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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



Contact Loading

Thermal Loading

Residual Stresses

Axle Loads

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Rail Loading: Residual Stresses (Longitudinal)

Roller straightening Butt welding(Webster et al. 1991) (Skyttebol & Josefson, 2004)

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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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Contact Loading

Thermal Loading

Residual Stresses

Axle Loads

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Rail Loading: Dynamic Effects

Data:

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Rail Loading: Factors Enhancing Dynamic Effects

corrugation flat spots

Example: Nielsen & Oscarsson (2004)

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Simulation of Fatigue Crack Propagation

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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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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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Rail Toughness – Scatter and Distribution

3 Parameter Weibull Distribution (Master Curve)

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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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Residual Lifetime:Effect or Rail-Wheel Combination (2)

Almost one order differencein residual lifetime

Reality even more complicated

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Effect of Ambient Temperature (1)

Statistics:

Deutsche Reichsbahn

acc. to Edel

Key parameter:

Temperaturedifferenceto rail neutral tempeature

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Controls Fatigue Crack Extension Controls Fracture

Mean Minimum

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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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Thanks for listening!46/46

fracture mechanics in railway application

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