Fatigue Analysis of a Simple Shaft under Combined Bending

Fatigue Analysis of a Simple Shaft under Combined
Bending and Torsion
Material baseado em exemplo da HBM-nCODE
e Ansys
PRESENTATION TOPICS
•
•
•
•
•
•
•
•
Introduction
SAE notched shaft Specimen
Geometry and FEM model
Boundary Conditions
Linear Static Analysis Results
Overview: Fatigue Life Estimation
Conclusion
References and Used Software
Introduction
Courtesy nCode
Introduction
Aloha Airlines Flight 243 (1988)
Introduction
• Fatigue life is estimated based on crack
initiation method.
• A notched shaft under combined bending and
torsion loads is analised.
• Two loading are combined , each load with its
own loading history.
• Linear stresses used for the calculation is
obtained from bending and torsion loading
independently.
SAE notched shaft specimen
• Standard SAE notched shaft [1] is modeled.
• This kind of specimen was used [1] to produce experimental
results for multiaxial fatigue ,and later, these results were
compared to 5 different theories to estimate life to multiaxial
fatigue (not the scope of this example).
Geometry
Finite Element Model: Mesh
Boundary Condition: Fixed Support
Boundary Condition: Force
Boundary Condition: Moment
Linear Static Structural Analysis
• Two static analysis were performed
– Bending due to unitary force
– Torsion with a moment of 1000 N.mm
– Following slides present stress distribution and
dispacements
Results: Applied load is unitary force
Von Mises Equivalent
Stress
Displacements
Maximum
Principal Stress
Results: Applied load is a Moment
Von Mises Equivalent
Stress
Displacements
Maximum
Principal Stress
Overview: Fatigue Life Estimation
Select Method for Life Estimation: Crack Initiation
• Crack Initiation(E-N) is the selected method
1. Stresses and strains calculated froma linear static analysis
2. Material Properties: E-N Curve and Cyclic Stress-Strain Curve
3. Variable Amplitude Loading: cycles counting performed through
Rainflow counting method
4. Neuber Correction to consider local plastic strain
5. Mean Stress Correction performed using either Morrow or SmithWater-Thompson Methodologies
6. Damage Accumulation and Fatigue Life Calculation peformed using
Palmgreen-Miner rule.
Select Method for Life Estimation: Crack Initiation
• Note:
• Ansys Design Life is not limited to crack initiation.
• One may perform:
– S-N analysis
– da/dN (Crack Propagation based of LEFM)
– Vibration Fatigue (frequency domain fatigue life
estimation)
– Among other capabilities
Material Properties
Análise de
Fadiga
Material Properties: Carbon Steel SAE1045_390_QT
Curva E-N
Propriedades extraídas do banco de dados do Ansys Design Life
Curva Tensão Deformação Cíclica
nCode Material Propoerties Library Available
Geometry for Life Estimation using FEM
Geometry for Life Estimation using FEM
Loading
Análise de
Fadiga
LOADING : FORCE
Loading History: Force x Time
LOADING: MOMENT
Loading History: Moment x Time
Fatigue calculation in Ansys Design Life
Fatigue calculation in Ansys Design Life
“Gliphs”
Ambiente de cálculo
Results
Análise de
Fadiga
Results
Results: Life
Results: Damage
Results: Spreadsheet format
New Calculation Cycle (optimization)
Análise de
Fadiga
New Calculation Cycle (optimization)
• One may change and try different values without running
linear static analysis again:
–
–
–
–
–
Material
Mean Stress Correction
Loading Value, Scale and Offset
Certainty of Survival
Other parameters ...
Conclusion
• A typical example of Fatigue Life Estimation was presented
• Finite Element Method (Ansys WB) and Ansys Design Life
were used to perform the analysis
• Selected Method for Fatigue Life Estimation: E-N
• Remember: Good Estimation is expected if you have good
imput data ( material properties, geometry and loading)
• We expect to demonstrate the easy of useness and flexibility
of these calculation tools.
References
• [1] Bannantine, J.A. Comer, J.J., Handcrock, J.L.
Fundamentals of Metal Fatigue Analysis Prentice-Hall
1990
• [2] nCode - DesignLife Worked Examples:
3 AN-NC-DL-WE 14.50.083 © 2012 HBM UK Ltd.
Used Software
1. Ansys Mechanical 14.5
2. Design Life - GlyphWorks V8.0 ISR3 Release Build 125
THANK YOU !!
OBRIGADO!!