an introduction of fatigue and fracture
DESCRIPTION
An Introduction of fatigue and fracture mechanics. Mechanical failure modes; Review of load and stress analysis – equilibrium equations, complex stresses, stress transformation, Mohr’s circle, stress-strain relations, stress concentration; Fatigue design methods; Design strategies; Design criteriaTRANSCRIPT
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
A - INTRODUCTION
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Course Content:
A - INTRODUCTION
Mechanical failure modes; Review of load and stress analysis – equilibrium equations, complex stresses, stress transformation, Mohr’s circle, stress-strain relations, stress concentration; Fatigue design methods; Design strategies; Design criteria.
B – MATERIALS ASPECTS OF FATIGUE AND FRACTURE
Static fracture process; Fatigue fracture surfaces; Macroscopic features; Fracture mechanisms; Microscopic features.
C – FATIGUE: STRESS-LIFE APPROACH
Fatigue loading; Fatigue testing; S-N curve; Fatigue limit; Mean stress effects; Factors affecting S-N behavior – microstructure, size effect, surface finish, frequency.
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
Failure Versus Fracture
FailureInability of a component to perform according to its intended function.
FractureSeparation of a component into two or more parts.
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
Some modes of failure• Gross yielding• Ductile failure• Brittle fracture• Creep Rupture and relaxation• Buckling• Stress corrosion cracking• Wear • Fatigue fracture
• Fatigue crack nucleation and growth• Uniaxial and multiaxial fatigue• Creep-fatigue• Corrosion fatigue• Constant and variable amplitude loading
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Load
Equilibrium equation
Complex stresses
Stress transformation
Mohr’s circle
Stress-strain relations
Stress concentration
Mechanics of Materials
A branch of mechanics that studies the relationships between external loads applied to a deformable body and the intensity of internal forces acting within the body.
REVIEW OF LOAD AND STRESS ANALYSIS
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Types of Loading on Structures
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Applications involving combined loading
Typical Engineering Structures
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Equilibrium of a Deformable Body
A body is said to be in equilibrium when the resultant of all forces and moments acting on the body is zero.
0
0
oM
F
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
Equilibrium of a deformable body
Determine the internal load at cross section marked C of each structure.
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Stress Under General Loading Conditions
Stress – intensity of a force acting at a material point
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Simple Stresses
J
Tr
A
P
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Complex Stresses
T
P•A
What is the magnitude of stress and strain on specific plane at A?
Does the stress and strain represent critical / maximum values at A?
If not…
what is the maximum & minimum (principal) stresses and maximum shear stresses?
What is the corresponding strain values?
On which planes do these stresses act?
A
Shear stress
Normal stress
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Stress Transformation Equations
2sin2cos22 xy
yxyxx
2cos2sin2 xy
yxyx
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Mohr’s Circle
Graphical visualization of the stress states at a given material point
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Fracture Planes
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Engineering Stress-strain Curve
Necking
Fractured
Tensile failure in ductile material is associated with large plastic deformation.
Total = el + pl
B
C
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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STRAIN,
0.0 0.1 0.2 0.3 0.4 0.5 0.6
ST
RE
SS
, (
MP
a)
0
200
400
600
800
STRAIN,
0.0000 0.0002 0.0004 0.0006 0.0008 0.0010
ST
RE
SS
, (
MP
a)
0
50
100
150
200
= E
Linear
Non-linear /Power-law
= E
= K(p)n
SS316 steel
Engineering Stress-Strain Curve
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Mechanical Properties of Some Materials
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MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Stress Concentration
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MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Stress Concentration
Stress concentration factor
avgtK
max
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Stress Concentration
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Stress Concentration Factors
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Deterioration of a material by initiation and propagation of crack when subjected to repeated load.
FATIGUE
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Deterioration of a material by initiation and propagation of crack when subjected to repeated load.
FATIGUE
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Fatigue design flow diagram
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MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Fatigue life models
• Nominal stress-life (S-N) model [1870s]
• Local strain-life (-N) model [1960s]
• Fatigue crack growth (da/dN-K) model[1960s]
• 2-stage model, combining -N and da/dN-K to incorporate fatigue crack nucleation and growth
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Fatigue life models
• Nominal stress-life (S-N) model [1870s]
• Local strain-life (-N) model [1960s]
• Fatigue crack growth (da/dN-K) model[1960s]
• 2-stage model, combining -N and da/dN-K to incorporate fatigue crack nucleation and growth
INTRODUCTION M.N.Tamin, CSMLab, UTM
MMJ1133 – FATIGUE AND FRACTURE MECHANICS
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Fatigue design criteria
• Infinite-life design Unlimited safety criterion where local stresses and strains are essentially elastic, and below fatigue limit.
• Safe-life design Designing for finite life with consideration on margin for scatter in fatigue data.
• Fail-safe design Structures are arranged so that cracks will not lead to failure before they are detected and repaired. Requires that if one part fails, the system does not fail.
• Damage-tolerant design Leak-before-burst design. Fracture mechanics analysis and tests are used to ensure that existing cracks will not propagate before they are detected by periodic inspection