Webinar on Metal Fatigue Analysis Using ANSYS

Introduction to Metal Fatigue

Fatigue: Term borrowed from human reaction to repetitive work, leading to tiredness/boredom.
Material Fatigue: When materials are subjected to repeated, fluctuating, or cyclic loads, they develop behavior that differs from static loading.
Fatigue Behavior: Characteristic behavior due to cyclic loading is fundamentally different from usual behavior under static loading.

Repeated Load: Same load repeated, e.g., tensile load from 0 to 50 N back to 0.
Fluctuating Load: Load alternates between tensile and compressive, e.g., 20 N tensile to 10 N compressive.
Alternating Cyclic Load: Load varies in magnitude but alternates in sense, e.g., 20 N tensile to 20 N compressive.

Fatigue Strength: Stress required to cause failure by fatigue in a given number of cycles.
Fatigue Limit (Endurance Limit): Stress level below which a material can withstand an infinite number of cycles without failing.
Fatigue Life: Number of cycles a material can sustain at a specified stress level before failure occurs.
Stress Representations: Normal stress (σ), shear stress (τ), and in fatigue, stress (S) with mean stress (Sigma_mean) and stress amplitude (S_a).

Crack Initiation: Localized atomic structure changes begin within the first few cycles.
Crack Growth: Microscopic cracks grow as cyclic loading continues.
Final Fracture: Rapid failure when remaining material cross-section cannot resist the load.

Intrusions and Extrusions: Weakest planes near the surface initiate cracks due to slip planes.
Stages: Nucleation, crack growth, and final fracture.

Endurance Curve (S-N Curve): Stress amplitude vs. number of cycles to failure.
Rotating Bending Test: Common method to generate S-N data using a standard specimen.
Behavior Below Stress Limit: Material can have infinite life if stress is below the endurance limit.

High Cycle Fatigue: When stress levels are within elastic limits, and the number of cycles is >10^3 and <10^6.
Low Cycle Fatigue: When stress levels are above elastic limits, number of cycles is <10^3.
Endurance Strength Calculation: Derived using straight-line equation in log-log scale.

Loading and Cyclic Characteristics: Maximum tensile stress, stress variation, and number of cycles.
Surface Condition: Surface roughness, residual stresses (compressive/tensile), and heat treatments.
Design Considerations: Material selection, geometric design, avoiding sharp corners/notches.
Environmental Effects: Corrosion, temperature variations, and thermal fatigue.

Surface Treatments: Improving surface finish and inducing beneficial residual stresses.
Design Optimization: Avoiding stress concentrations and using appropriate fillets.
Material Selection: Choosing materials with better fatigue properties and suitable for operating conditions.

Paris Law: Used in fracture mechanics for predicting crack growth rates under cyclic loading.
Stress and Strain-Based Approaches: Representing fatigue life based on elastic and plastic strain-life relationships.
Empirical Relations: Various equations and approaches to estimate crack growth and fatigue life.

ANSYS Workbench: Using static, transient, and coupled field analyses to evaluate fatigue life.
Specimen Preparation and Testing: Standard rotating bending specimen and evaluation under closely controlled conditions.
Fatigue Sensitivity Analysis: Assessing how variations in load affect fatigue life.

The webinar emphasized understanding material fatigue, types of cyclic loads, methods for evaluating fatigue life, and factors affecting fatigue. Advanced methods and practical examples using tools like ANSYS Workbench and ansys encode design life were also discussed.
Recommended Tools:
- ANSYS for basic fatigue calculations.
- ENCODE Design Life for specialized and advanced fatigue assessments.
- Implementing optimization and automation for comprehensive evaluation.