Fatigue (Aircraft Structure)
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Fatigue in Aircraft Structures refers to the weakening of materials due to repeated stress and strain, leading to crack initiation and propagation, potentially causing structural failure.
Overview
Fatigue is a time-dependent deterioration that affects aircraft structural components due to repetitive cyclic loading during operations. Without detection and mitigation, it can lead to catastrophic failure.
Causes
- Repeated stress from pressurization cycles, G-forces, and turbulence.
- Crack initiation from stress concentrations like fastener holes or surface defects.
- Crack propagation over time with each flight cycle.
- Final failure when the crack reaches a critical size.
Factors Affecting Fatigue
- Material: Aluminum alloys are widely used but fatigue-prone.
- Stress Concentration: Holes, joints, and sharp corners increase risk.
- Loading Conditions: High cyclic loads accelerate fatigue.
- Corrosion: Weakens structures and interacts with fatigue damage.
Crack Growth
- Initiation: Cracks form at stress risers.
- Propagation: Crack length increases with each load cycle.
- Critical Crack Size: Beyond this point, structure can no longer sustain flight loads.
Fail-Safe Design
- Multiple Load Paths: Load redistribution if one path fails.
- Inspectability: Designed for accessible inspection.
- Residual Strength: Withstands damage while remaining functional.
Detection and Prevention
- Regular inspections using non-destructive testing (NDT).
- Adherence to aircraft's fatigue life and Limit of Validity (LOV).
- Repairs and modifications to mitigate damage.
- Maintenance tracking and condition-based actions.
NDT Techniques
- X-ray Radiography
- Computed Radiography (CR)
- Digital Detector Arrays (DDA)
- Ultrasonic Testing
- Liquid Penetrant Testing
