fatigue analysis of a simple shaft under combined bending ... · pdf filefatigue analysis of a...
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Fatigue Analysis of a Simple Shaft under Combined
Bending and Torsion
Material baseado em exemplo da HBM-nCODE e Ansys
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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
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Introduction
Courtesy nCode
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Introduction
Aloha Airlines Flight 243 (1988)
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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.
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• 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).
SAE notched shaft specimen
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Geometry
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Finite Element Model: Mesh
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Boundary Condition: Fixed Support
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Boundary Condition: Force
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Boundary Condition: Moment
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• 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
Linear Static Structural Analysis
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Results: Applied load is unitary force
Von Mises Equivalent
Stress
Maximum
Principal Stress
Displacements
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Results: Applied load is a Moment
Von Mises Equivalent
Stress
Maximum
Principal Stress
Displacements
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Overview: Fatigue Life Estimation
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• 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 Smith-
Water-Thompson Methodologies
6. Damage Accumulation and Fatigue Life Calculation peformed using
Palmgreen-Miner rule.
Select Method for Life Estimation: Crack Initiation
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• 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
Select Method for Life Estimation: Crack Initiation
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Material Properties
Análise de
Fadiga
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Material Properties: Carbon Steel SAE1045_390_QT
Curva E-N
Curva Tensão Deformação Cíclica
Propriedades extraídas do banco de dados do Ansys Design Life
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nCode Material Propoerties Library Available
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Geometry for Life Estimation using FEM
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Geometry for Life Estimation using FEM
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Loading
Análise de
Fadiga
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LOADING : FORCE
Loading History: Force x Time
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LOADING: MOMENT
Loading History: Moment x Time
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Fatigue calculation in Ansys Design Life
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Fatigue calculation in Ansys Design Life
“Gliphs”
Ambiente de cálculo
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Results
Análise de
Fadiga
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Results
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Results: Life
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Results: Damage
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Results: Spreadsheet format
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New Calculation Cycle (optimization)
Análise de
Fadiga
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• 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 ...
New Calculation Cycle (optimization)
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• 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.
Conclusion
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• [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.
References
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1. Ansys Mechanical 14.5
2. Design Life - GlyphWorks V8.0 ISR3 Release Build 125
Used Software
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