Fatigue Limit of Metals Under Multi Axial Stress Conditions - Part I Theory
Extreme Value Theory in Fatigue of Clean SteelsExtreme Value Theory in Fatigue of Clean Steels Clive...
Transcript of Extreme Value Theory in Fatigue of Clean SteelsExtreme Value Theory in Fatigue of Clean Steels Clive...
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Extreme Value Theoryin Fatigue of Clean Steels
Clive AndersonUniversity of Sheffield, UK
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Metal Fatigue• repeated stress,
• deterioration, failure
• safety and design issues
The Context
Approaches to Studying FatiguePhenomenological – ie empirical testing and
prediction
Micro-structural, micro-mechanical – theories of crack initiation
and growth
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Outline
1. Background: the Fatigue Limit
2. Inclusions and the Rating Problem
3. Extreme Value Theory & Stereology
4. Design
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1 Background: the fatigue limit
For ,
Constant amplitude cyclic loading 2σ
Fatigue limit σw
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2 Inclusions in Steel & the Rating Problem
inclusions
• propagation of micro-cracks → fatigue failure
• cracks very often originate at inclusions
Rating Problem: classify steel quality in relation to inclusion content
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Murakami’s root area max relationship between inclusion size and fatigue limit:
in plane perpendicular to greatest stress
Rating Problem: classify steel in relation to size of largest inclusion in adesignated volume
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Can measure sizes S of sections cut by a plane surface
butnot routinely observable
Inference problem: how use data on S to estimate extremes of V?
3 Extreme Value Theory & Stereology
Rating problem: classify steels in terms of
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Models for Sizes of Large Inclusions
Initial Model:
• spherical particles
• diameters V distributed as Generalized Pareto above a threshold v0
• centres form a homogeneous Poisson process, mean rate for those with V > v0 equal to λ0
Data: surface diameters S > v0 in knownarea
– a Marked Poisson Process Model
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where GPD
Stereology
For spherical inclusions with centres at points of a Poisson process
Wicksell 1925Thus
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A missing data problem
If V1, …, Vn had been observed, inference would be simple.
Inference: hierarchical model
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MCMCSample repeatedly from completeconditional distributions of unknowns:
expected no. si
a b
n
v1, v2, … , vn
s1, s2, … , sn
prior parameters
unknowns
unknowns
where eg
from Wicksell
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Inferences• posterior dists of parameters• posterior distributions of derived quantities• predictive distributions for further observations
Example: from 112 measurements on clean bearing steel T7341
T7341: posterior pdf of ξ T7341: posterior pdf of σ and ξ
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Given the parameters , the distribution of is Generalized Extreme Value.
Predictive distribution of
10 20 30 40 50
0.00
0.05
0.10
0.15
0.20
m
pred
ictiv
e pr
ob d
ensi
tyT7341: predictive pdfof for C = 100
Predictive distribution of = largest V in volume C
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Sensitivity of Inferences to Sphericity?
Generalized Model:• inclusions of same 3-d shape but different sizes,• random uniform orientation , in principle • sizes Generalized Pareto,• centres in homogeneous Poisson process
ThenE( no. inclusions of size , orientation
intersecting plane in shape of size )
for a function depending on the shape.
E( no. inclusions of size intersectingplane in shape of size )
where
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Titanium Inclusions
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Predictive Distributions for Max Inclusion MC in Volume C = 100
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4. Use in Design
In most metal components internal stresses are non-uniform
-2.5-1.5
-0.50.5
1.52.5
-3-2
-10.0
12
30
100
200
300
400
500
600
700
800
Prin
cipa
l stre
ss, M
Pa
X/hole radius
Y/hole radius
Stress in thin plate with hole, under tension
Component fails if a large inclusionoccurs at a point of high stress amplitude
Failure probability under marked Poisson model?
from stress field inferred from measurements
100mm
5mm
50mm 2mm
Reason for interest in inclusions: design of safe steel components
ie
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• Under the marked Poisson model:
inclusions at which local stress is too great to bear
≡ thinned (inhomogeneous) Poisson process
If no. of such = N, then
Pr( component fails) = Pr( N > 0)
= 1 – exp( - E(N))
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• Expected no., E(N), of inclusions causing failure in acomponent of volume C
mean no. of inclusions in volume C
proportion experiencing unbearable stress
consider inclusions of size :
Over all sizes
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from Generalized Pareto model from stress distribution and size – fatigue limit relationship
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Effect of • modifying the design• improving cleanness of steel