MSC.Fatigue_Training030806

• The fatigue life of a component is governed
by the loading environment to which it is subject, the distribution of stresses and strains arising from that environment, and the response of the material from which it is manufactured.
Advantages of Quasi Static Analysis
Advantages of Transient Analysis
S-N Analysis (contd..)
Rain flow counting
Mean stress Correction
Mean stress Correction…
sRange =100 s
sMean = 50
S
sRange =100
s
100
sMean = 0
5000
s
N
sRange =100
sMean = -50
Mean stress Correction (contd…)
Palmgreen-Miner’s Linear damage rule
This states that damage can be summed by determining the ratio of the number of cycles experienced to the number of cycles to failure for a given stress range or level and then summing all the ratios for every stress range.
• As a result, the major inputs to any fatigue
analysis are component geometry, service loading, and cyclic material properties.
Fatigue – Black Box
Opening the black box….
1
Mean stresses
=
Rainflow Cycle Counting
2
1
4
Irregular sequences
3
2
Palmgren Miner
1
5
Notches, etc, etc, etc..
E N Analysis
E N Analysis
E N Analysis (Contd..)
s1A* L1(t) + s2A* L2(t) + ... = sA(t)
L1=1 L2=1
Stress for Unit Load Cases
Local Stress Histories
sA
Transient Fatigue
Modal Super-Position Method
Modal Super-Position Method
Modal SuperPosition Method
Quasi-Static Fatigue analysis
Quasi-static method (linear superposition)
Load Time Histories
L1
L2
- repeat for each node/element
Relates stress intensity stress intensity to crack propagation rate crack propagation rate
S-N and E-N Fatigue Analysis Process
Load Time Histories
L1
Local Stress Histories
Pseudo-Static Fatigue analysis
Dynamic Fatigue analysis
Vibration Fatigue Analysis
Transient Fatigue Analysis
Modal Super Position Method
Low Cycle Fatigue and High Cycle fatigue
Definition of fatigue
• Fatigue is basically a type of failure…,
• Weakening of metal due to microscopic changes in molecular structures caused by vibration or exposure. • In materials science, fatigue is a process by which a material is weakened by cyclic loading. The resulting stress may be below the ultimate tensile stress, or even the yield stress of the material, yet still can cause catastrophic failure
sMean = -50
Mean stress Correction… s
s
sRange =200
sRange =100
sMean = 50
S
sMean = 0
200 100
sRange =100
s
sMean = 0
500
5000 7000
3000
N
sRange =50
s
s
sMean = 0
sRange =100
Repeat for each node/element
s1A* L1(t) = sA(t)
L1=1
sA
Fatigue life of Node
Stress for Unit Load Case
S-N Analysis
S-N Analysis (contd..)
S-N Analysis (contd..)
2. E-N analysis (Crack Initiation Method) Relates local strain local strain to crack initiation life crack initiation life
3. LEFM (Crack Propagation Method)
History
Expert’s Opinion
Wohler
How does Fatigue occur?
• Occurs when a material experiences
lengthy periods of cyclic or repeated loads • Fluctuating Loads • Multi-axial Loading • Vibrations / Excitations
Basic Methods for Fatigue analysis
Several methods may be used, but according to the problem considered: S-N analysis (Total Life Method) Relates nominal or local elastic stress local elastic stress to total life
When to use which (Contd..)
Opening the black box….
S-N (Total Life)
Basic Fatigue methodologies
E-N (Crack Initiation)
Crack Growth (LEFM)
Fatigue technology in MSC.Fatigue
MSC.Fatigue / FE-Fatigue
Deepak Deshpande
OUTLINE
• • • • •
Definition of Fatigue How does Fatigue occur? Basic Methods of Fatigue analysis Inputs for Fatigue analysis Fatigue analysis process
S-N (Total Life)
Basic Fatigue methodologies
E-N (Crack Initiation)
Crack Growth (LEFM)
Fatigue technology in MSC.Fatigue
Deterministic Load History In-Deterministic Load History
Neuber’s correction
When to use which
S-N (Total Life) • Long life fatigue problems where there is little plasticity since the S-N method is based on nominal stress • Components where crack initiation or crack growth modeling is not appropriate, e.g., composites, welds, plastics, and other non-ferrous materials • Situations where large amounts of pre-existing S-N data exist • Components which are required by a control body to be designed for fatigue using standard data such as MIL handbook data. • Spot weld analysis and random vibration induced fatigue problems Crack Initiation • Mostly defect free, metallic structures or components • Components where crack initiation is the important failure criterion - safety critical components • Locating the point(s) where cracks may initiate, and hence the growth of a crack should be considered • Evaluating the effect of alternative materials and different surface conditions • Components which are made from metallic, isotropic ductile materials which have symmetric cyclic stress-strain behavior • Components that experience short lives - low cycle fatigue - where plasticity is dominant