Some aspects of safe-life and damage tolerant design of railway axles

Some Aspects of Safe-Life and
Damage Tolerance of Railway
Axles
ESIS TC 24 Workshop on „Improving Lifetime and NDT“, Milan, 30 March 2012
Uwe Zerbst
BAM Berlin

Axle fracture ICE3, 9 July 2008
Press photo
ICE 518, Hohenzollern Bridge
Fracture after 10 9 loading cycles
Expert‘s report (BAM)
Cause: non-metallic inclusions
in the highly stressed volume?
Source: ARD, Monitor
2008-11-06

Design and operation: Safety levels
State-of-the-art

Primary (Basic) Safety Level
Present state: Quasi-static design
- dynamic magnification factors
- synchrone loading
- Safety margins for VHCF etc.
- Corrosion: Reduction of stress level to 70% (EN 13261)

Effect of corrosion
Effect of corrosion medium
(rain water) on S-N curve
Beretta et al.;
Polimi Mailand, 2010
Dramatic Limitation of Service Time
or
Protection
Measures
LURSAK ®

Basic Safety Level
Present state: Quasi-static design
- dynamic magnification factors
- synchrone loading
- Safety margins for VHCF etc.
- Corrosion: Reduction of stress level to 70% (EN 13261)
Proposals: Damage accumulation design (Eisenbahnfahrwerke 1; WIDEM; etc.)
- different approaches
- classified load spectra necessary

Does load accumulation
„cover“ VHCF?
Concepts for Fatigue
Strength Design
VHCF
Beretta et al.

Basic Safety Level
Present state: Quasi-static design
- dynamic magnification factors
- synchrone loading
- Safety margins for VHCF etc.
- Corrosion: Reduction of stress level to 70% (EN 13261)
Proposals: Damage accumulation design (Eisenbahnfahrwerke 1; WIDEM; etc.)
- different approaches
- classified load load spectra necessary
„One million miles axle“ (A. Lawton, 2003; WIDEM)
- Design such, that 10 9 LC realised under most
disadvantageous conditions
- Inclusion of fracture mechanics

Conventional S-N Curve and S-N Curve Based
on Fracture Mechanics
(Initial Crack depth: 2 mm)
Beretta et al.

Factors, which reduce fatigue strength
during service (1)
Service conditions Max. Corrosion pit depth in in mm
Relatively well protected against
corrosion
Corrosion typical or a bit more
severe than typical
Significantly more severe (wagon
carrying corrosion freight
Korrosionseinfluss
* British Rail Safety and Standards Board
TWI
0.043
0.076
0.346
Estimated
maximum
(1 axle)
0.1
0.24
0.82
Beretta et al.
Estijmated
maximum
(fleet)
0.13
0.35
1.02
Corrosion pits
(mainly freight wagons)
T278 Projekt of
BRSSB*
Watson

Factors, which reduce fatigue strength
during service (2)
Failure case Australia 2007
Beretta et al.
T278 Projekt of BRSSB:
Average depth: 0,8 mm
95% Maximum depth: 2 mm
Stress concentration
(Ceril et al. 2009)
for a/c = 1 K t = 2.15
However:
Flying ballast impacts
(above all high speed traffic)
- Complex residual stress field
- Cracks at notch root?
- Combination with corrosion
K
t
1 6.6 a 2c
1 2 a 2c
With respect to corrosion and ballast
impacts protection measures possible

Optimised Geometry:
Berlin 1940s
based on fatigue
life experiments
K which, by its nature, not a stress concentration
but kind of a fatigue notch factor, although reciprocally
proportional to the usual definition, i.e., K > 1
Depends not only on the notch geometry but also on
material
Empirically derived for A1 steel (UIC Leaflet 881-V,
1979); tensile strength between 550 and 650 MPa
K solutions cannot be simply applied to higher
strength railway axle steels
Note further: stress state at the geometrical transitions
not only influenced by applied loading and notch geo-
metry but also by the press fits (additional axial tensile
stresses adjacent to the press seat regions)
More fundamental investigation is necessary
for deviating geometries:
EN 13103 and EN 13104

Loading of an Axle
Static and dynamic axle loads
Bending and axial tension at curved
track sections, crossovers and
switches
Torsion during traction and braking,
Higher frequency loading due to
stick-slip behaviour
Press fit loading
Residual stresses
from manu-
facturing
Hirakawa et al., 1998
Madia et al., 2008

Problem: non-metallic Inclusions
ICE3 axle of Köln
Failure investigations: BAM
C. Klinger et al.
ultrasonic immersion technique

Non-metallic
Inclusions:
Origin in
Manufacturing
Process
Effect on Lifetime:
C-Steel; Ma et al., 2010

Non-metallic inclusions
Allowed maximum depth(EN 13261 referring to ISO 4967)
Verification: metallographic; requirerd test area min. 200 mm 2
no specification on sampling volume
Problem: Inclusions of critical size at critical position
rather seldom.
Statistics of European Railway Agency for the EU 2006 - 2009:
- 1 fracture event per 2 Billions axle kilometers
- also other mechanicsm than fatigue (e.g., hot box failure)
It is certainly inconsistent to look for seldom events by a very limited sample!

Proposal:
Establish as-is state of inclusions
Establish NDT method (e.g. US immersion
technique for throughout detection of inclusions
above a certain size
Realistic (limit) size: 0.5 mm?
(relevant: Inclusion dimensions normal to axle)
Inclusions smaller than those found by NDT assumed as existent
Establish correction factor for fatigue strength

Design and operation: Safety levels

Secondary Safety Level
PoD = probability of detection

Inspektion interval
Aim: Economically justifiable inspektion interval combined with a high probability
for crack detection (PoD)
Non-detection probability (PoND) corresponds with probability that a potential
crack will be found in due time bevor the axle fails
Parameters affecting overall PoD:
PODoverall 1PONDi i
- Inspektion interval (respectively number of inspections)
- Residual lifetime (Fracture mechanics analysis)
- PoD-crack depth-characteristics (NDT)
i = i-th inspection

Potential Margins with
Respect to Inspektion
intervals
Improved Fracture mechanics
analysis, particularly in the threshold
range (S)
Variable amplitude loading
- Both not unproblematic
- Effect rather small (?)

Problem: Threshold region of
Fatigue crack propagation DK th

Potential Margins with
Respect to Inspektion
intervals
Improved Fracture mechanics
analysis, particularly in the threshold
range (S)
Variable amplitude loading
- Both not unproblematic
- Effect rather small (?)
Improvement of POD
Results: WIDEM (manual testing)

Potential for Improvement of the PoD (1)
Interpretation of A-scans instead of gate mode technique
Optimisation of angle of incidence with respect to geometry
(Phased Array Technik)
Advanced methods such as echo tomograpgy, SAFT, etc.
Regions of increased danger: generally „Analyseprüfung“
However: limited time for a test as a second essential goal
Validation and calibration of test equipment for real cracks!

Potential for Improvement of the PoD (2)
Caution: real cracks behalf
different to saw cuts in
ultrasonic testing!

Thanks for your attention
More detailled information:
uwe.zerbst@bam.de