Fracture mechanics&amp_Sundar R

Innovating to simplify!
Mumbai
May 10, 2018
Part 1: Engineering Notch Fatigue
Analysis
or, what is inside that black box in ANSYS, ABAQUS, nCode, etc…
R. Sunder, BiSS (P) Ltd
Bangalore, India, +91 9880-432322
rs@biss.in
1

High Performance and Precision – Delivered Worldwide!
BISS OPERATIONS
• Development, manufacture and support of high
precision, high performance digitally controlled
mechanical test systems “N to MN range”
• Testing services: 25 test systems, 16 servo-actuators
serving aerospace, automobile, nuclear, defence and
energy sectors
• Research & development including collaborative
effort
• Training & education in testing and mechanical
property evaluation related to Fatigue, Fracture,
Dynamic Response (shake tables)

High Performance and Precision – Delivered Worldwide!3
MARKET PRESENCE IN PRODUCTS & TESTING SERVICES
RAILWAYS
EDUCATION AEROSPACER&D AUTOMOTIVE
DEFENCE

COLLABORATION

The dilemma posed by Metal Fatigue –
Born out of Industrial Revolution
e
s
su
sy
s-1
Don’t speed!!!
5

Consequence of mechanical failure
Embarrassment - Damage to image
Injury – death - Liabilities - bankruptcy 6

Material inputs for fatigue analysis:
E, K’, n’, b, c determined as per ASTM E606
'/1
'
n
KE







ss
e

Service Load Spectra
8

• Account for the statistics of amplitude
variation – D = ∑(ni/Nfi)
• Account for variation in notch root
local mean stress
Requirements of
Fatigue Analysis
‘The dance in engineering notch fatigue
analysis is around sm…… ’
9

The Stress-Strain Curve (Masing Model)
Comprehending material ‘memory’by linearization
1
1
2
4
3
5
e
s
2
3
4
5
'/1
'
n
KE







ss
e

Simulation Works….
Early work by Jhansale [1978]
1
1
2
4
3
5
e
s
2
3
4
5
'/1
'
n
KE







ss
e

Smooth Specimen Response Upon Unloading
1
2
e
s
1
1
2
4
3
5
e
s
2
3
4
5

1
2
e
s
1
2
e
s
Other consequences:
Stress gradient
Constraint
Notch Root Response Upon Unloading

Consequence of stress-strain hysteresis
at the notch root
Stress
A
B
C
D
E
A
B
C
F
G
H
sC
s
e
sA
sG
sE
A
B
C
D
E
F
G
H
(a)
(b)
(c)
Material memory + hysteresis
14

11
1
2
4
3
5
e
s
2
1
2
1
S
Transformation of Applied Loads to Local
Stress-Strain
'/1
'
n
t
KE
ESK








ss
e
se
'/1
'
n
aa
a
aaat
KE
ESK








ss
e
es
Two equations, two unknowns…..

And, How do you deal with this??
P
The small ones?
May be the BIG one?
Log(N)
sа
s’
16

Rainflow Cycle-Counting per ASTM E1049
17

Application of Rainflow in
Test Optimization
From such curves, one can determine the
consequences of ignoring smaller load
cycles while performing endurance testing
18

Summary: The Four Columns of the Fatigue Analysis Platform
ESKt ... es 
'/1
'2
2
n
KE





 



ss
e
12
3
1 – Material Memory 2 – Neuber transformation 3 – Rainflow cycle counting
4 – Damage computation
4
19

Positives of LSS Approach
1. Compatible with all FEM packages
2. Highly reproducible results (but not
necessarily with experiment!)
3. Has served industry for almost 40 years
4. Expandable to account for strain
hardening/softening, creep-fatigue
interaction, etc
5. Constants determined by standard
testing practice (ASTM E606):
E, K’, n’, b, c, ef’, sf’

Negatives of LSS Approach
1. Fails to ‘recognize’ fatigue as a
process of fatigue crack growth
2. Shaky rationale under fully
elastic conditions – consequence
of item 1….

Mumbai, May 10, 2018
Part 2:
The Impact of Fracture Mechanics on
Structural Integrity
R. Sunder, BiSS (P) Ltd
Bangalore, India, +91 9880-432322
rs@biss.in
22

Engine for Airbus 380
10,000 hrs between inspections….
24

Strength with defects –
Fracture Mechanics
Theory:
Elliptic stress raiser – /Inglis 1915/
Brittle fracture /Griffiths 1935/
Elasto-Plastic fracture /Irwin 1945/
Ship fractures WWII
25

Characterizing how severely a crack is stressed:
Stress Intensity Factor
Introduced by George Irwin – used as ‘industry standard’by the world from the 1960’s
K = Sa .Y
s = K / .r
K – Stress Intensity Factor (K)
Y – crack/specimen geometry
correction factor
r – distance from crack tip
2a
S
s
26

Kc – ‘Toughness’ in the presence of a defect
K
Kc = Sac .Y
Sc = Kc / .ac
Kc – Critical Stress Intensity
Factor (K)
Y – crack/specimen geometry
correction factor
27

The Compact Tension C(T) Specimen
28

Stress Intensity Factor, K = f (stress, crack size, geometry)
• K – the ‘gift’ from Fracture Mechanics helps characterize
residual strength and residual life of materials with defects (cracks)

Fracture Mechanics – the Platform to Switch to
Maintenance on Condition
ao
ac
Inspection
period
Detectable crack
size
Service Life
“Damage is easier to handle
when you can see it grow”

Fracture Mechanics & Risk Management
31

Summary: The Fracture Mechanics “Business Model”
– 1970’s onwards
K
da/dN
мм/Cycle
K – Stress intensity
Factor: Similarity
criterion for FM
1. K as a similarity
criterion – FEM
solutions
2. Test automation
for material data
generation
3. NDT to set initial
crack size
4. Critical crack size
driven by fracture
toughness
(Materials Engg)
5. da/dN curve
defining crack
kinetics
(Modeling)
1
2
3
4
5

Fracture Mechanics Application –
‘Local’ and Other Stories
33

Will it Leak? Or, will it burst??
34

Internal patch
35

Pressure cycling

J1c test result
for pipe
Minimum to
qualify
Safety
margin
First published
results on J1c
testing of pipe with
through subject to
internal pressure
37

Boeing 747: Fruits of
Structural Integrity Assurance
In service for 50 years
150,000 hrs+
service – 15-20 TBIs
40,000 t/o, landings
~240,000 km
on ground
100,000,000 km
2,500 times
around the equator
38

Five Tasks of ASIP
1. Design
Specification
2. Design &
Development
3. Full-Scale
Testing
4. Fleet Management
Data
5. Fleet Management
to Retirement

Load Spectrum Correction for
Composite Structures

46
Southwest Airlines 1360

SUMMARY
• Design is not merely to ensure functionality on Day #1:
It is about long term safe and economical operations
• Life-cycle outlook for nuclear installations extends to
25-40 years. Service loads and environmental factors
change material and structural behavior
• Design and condition monitoring address issues related
to Fatigue, Fracture, Creep and Corrosion
• Rigorous analysis based on reproducible material data,
supported by testing and periodic inspection and
maintenance determine long-term safety and economy
of operations

2017 – Silver Jubilee Year
Thank you! Questions??
rs@biss.in
49

Fracture mechanics&amp_Sundar R

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