Fatigue Life and Structure Integrity
Phenomenological Approach (Based on S-N curves)
Fatigue Crack Growth (Based on Fracture Mechanics)
Estimation of Fatigue Limit in Practice (R=-1)
1) From ultimat
Fatigue Crack Growth (2)
Fatigue crack growth behavior (initiation and propagation)
Residual strength and damage tolerance
-structure is damage/cracked, but has tolerance
to failure
Paris equation
Tough Materials?
Stiff materials?
Strong Materials?
Cheap Materials?
Hard materials?
How to estimate ductility, toughness and hardness of materials?
I. Ductility
f = % EL =
L f Lo
Lo
x100
Adapted fro
Fatigue Failure of Materials
What is fatigue?
What is its implication?
De Havilland Comet 1
Fatigue: a process of
damage and failure due
to cyclic loading.
80% failure is relevant to
Fatigue.
Annual c
Characteristics of Fracture Toughness
At fracture
K = KC
At yield
Is Kc constant?
= c
Dependent on
load, geometry
of structure, and
defect size
Kc is the critical stress
intensity factor at
fracture.
Fatigue of Notched Members
Effects of Notches
Fatigue Notch Sensitivity
How to describe it
Interpretation of S-N Curve
Fatigue Strength Sa
(at a certain life time)
Fatigue Limit
(or Endurance Limit
Youngs Modulus and Yielding Stress
For a particular metallic alloy
can we increase its Youngs modulus?
can we increase its yielding stress?
How do sand particles stay together?
How do individual ato
Effect of Mean Stress
Fatigue Limit (Endurance Limit)
Effect of Mean Stress (Stress Ratio)
Variable Amplitude Loading
Fatigue Life Predication
S
a2
a1
Nf2
Nf1
N
S-N curves
(Stress amplitude (sa) ve
Creep and Creep Failure
How Long a Material Can Sustain at
Elevated Temperatures Before Rupture?
Power-law Creep
Effect of Combined Fatigue and Creep Loads
CREEP
Characteristics
It is stress-dependent
Failure Criteria
How does a material fail under the multiple stresses?
ISSUES TO ADDRESS.
failure criteria in terms of yielding and brittle failure
how to select failure criteria
Uniaxial Stress
Duc
Yielding Stress/Strength
For a particular metallic alloy
can we increase its Youngs modulus?
can we increase its yielding stress?
Elastic-Perfectly Plastic
Most commonly used model in design and ana
Case Studies in Modulus-limited Design
Will discuss two examples in the text book
Flow of Stress/Load
How to define the amount in individual branches?
Load is distributed upon stiffness
How is the loa
A brief review on
Fundamentals of Materials and
Structural Mechanics (2)
Beams
A beam is a structural element that
is capable of withstanding load
primarily by resisting bending. The
bending force in
Fracture Criterion
STRESS CONCENTRATION DUE TO DEFECTS
Elliptical hole in
a plate:
Stress distrib. in front of a hole:
Stress conc. factor:
Large Kt promotes failure:
Kt-stress concentration
facto
1-,2-,and 3-Dimensional (3-D) Stress and Strain
States
Thermal Stresses /Thermal Shock
Three-dimensional Stress State
xy=yx
xz=zx
yz=zy
Why?
Only Six Independent Stress Components!
Free Surface:
z
y
x
Basic Mechanical Property
of Materials
A Tensile Test
What did you measure for a tensile test?
A Load-Displacement Curve
=F/A
The stress
always has a
unit, normally
MPa
Strain:
(Engineering Strain)
or
MECH5362 Materials II
Guideline for Report
Every two students work as a group (see the separated group list) to
submit a report (of five A4 pages maximum, exclusive of cover page) on a
particular engi
Why does the thing get loose?
Stress level is below o
A Tensile Test
Loading Rate?
Displacement Rate?
1 mm/mm
5 mm/min
1 m/min
Normally both the elastic modulus and yielding strength
increase with the
Rules of Mixtures
E X = EmVm + E f V f
Parallel Model
EY =
E f Em
Vm E f + V f Em
G XY =
Series Model
G f Gm
VmG f + V f Gm
v X = v f V f + vmVm
Void content = 0!
Vm + V f = 1
1) Elastic modulus-Longi
Some Useful Points in Design and Analysis
Specific Stiffness and Specific Strength
Specific Stiffness Elastic modulus to weight ratio,
i.e. /
Specific strength strength to weight ratio, i.e. f /
Dyn
Fracture Failure
Why Fracture (Not Yield)?
An event involving propagation of a crack (or cracks)
July 27, 2009
AIRCRAFT engineers have
demanded Virgin Blue
inspect its entire plane fleet
after reports
Composites Materials
How do we estimate properties of composite?
Compounds?
Blends?
Alloys?
Any material made of two or more than two constituents
in macroscopic (or most recently microscopic or nano
MECH 3362 Materials II
(Introduction)
Learning Commitments
There are three lectures per week
Prof Lin Ye
Tutorials
Each student enrolled goes to one of the 3 classes, following
the individual timetabl
Stress-Strain Relationship
Youngs modulus
Poissons ratio
Yielding stress (strength)
Ultimate (tensile) strength
Plasticity
What is the most important mechanical property?
Yielding
strength
.
UTS
Slope
Leak Before Break
Leak Before Break
*Pressure vessels should be designed to leak before they fracture,
i.e. Cc > t or t < Cc (if KIC and stress are known).
Yielding against Brittle Fracture
Sc =
Kc
a
Three-dimensional Stress State
The arrows indicate the positive direction!
Principal Stresses? What are they and why?
How to define them?
3
Why is it 45-degree
x
x
1 = x cos 2
2 = x sin 2
12 = x