18.2 Mechanical durability and fatigue resistance

Mechanical durability and fatigue resistance are crucial for energy harvesters. These factors determine how long devices can withstand repeated stress cycles before failing. Understanding fatigue life, cyclic loading, and stress-strain relationships is key to designing robust energy harvesting systems.

Accelerated life testing and fatigue analysis help predict long-term performance. Techniques like Weibull analysis and finite element analysis identify weak points in designs. By applying these methods, engineers can optimize energy harvesters for longer lifespans and better reliability.

Fatigue and Cyclic Loading

Understanding Fatigue Life and Cyclic Loading

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• Fatigue life defines the number of stress cycles a material can withstand before failure
• Cyclic loading involves repetitive application and removal of stress on a material
• Stress-strain relationship characterizes material behavior under cyclic loading conditions
• Elastic deformation occurs when stress remains below yield strength, allowing material to return to original shape
• Plastic deformation happens when stress exceeds yield strength, resulting in permanent shape change
• Stress concentration areas experience higher localized stress, often initiating fatigue cracks
• S-N curves (stress amplitude vs. number of cycles) help predict fatigue life for different stress levels

Accelerated Life Testing and Fatigue Analysis

• Accelerated life testing simulates long-term fatigue effects in shorter time periods
• Involves subjecting materials to higher stress levels or frequencies than normal operating conditions
• Weibull analysis used to interpret accelerated life test data and estimate product lifespan
• Finite element analysis (FEA) helps identify high-stress regions prone to fatigue failure
• Rainflow counting method analyzes irregular loading patterns for fatigue life estimation
• Cumulative damage models (Miner's rule) predict fatigue life under variable amplitude loading
• Fatigue strength improvement techniques include shot peening, surface treatments, and design optimization

Failure Mechanisms

Crack Propagation and Fracture Mechanics

• Crack propagation describes the growth of existing flaws or defects in materials under cyclic loading
• Paris law relates crack growth rate to stress intensity factor range
• Fracture toughness measures a material's resistance to crack propagation
• Critical stress intensity factor (K_IC) determines when unstable crack growth occurs
• Stress intensity factor (K) depends on applied stress, crack size, and component geometry
• Fracture mechanics principles used to predict remaining fatigue life of cracked components
• Crack closure effects can slow down crack growth rates, influencing fatigue life predictions

Material Degradation and Environmental Factors

• Creep involves time-dependent deformation under constant stress, particularly at elevated temperatures
• Primary creep stage shows decreasing strain rate
• Secondary creep maintains constant strain rate
• Tertiary creep exhibits accelerating strain rate leading to failure
• Material degradation occurs due to various environmental factors (corrosion, radiation, temperature)
• Stress corrosion cracking results from combined effects of tensile stress and corrosive environment
• Hydrogen embrittlement causes loss of ductility in metals due to hydrogen absorption
• Thermal fatigue results from cyclic temperature changes, leading to thermal expansion and contraction

Failure Modes and Prevention Strategies

• Failure modes include ductile fracture, brittle fracture, fatigue failure, and creep rupture
• Ductile fracture characterized by significant plastic deformation before failure (cup-and-cone fracture)
• Brittle fracture occurs with little or no plastic deformation (cleavage fracture)
• Fatigue failure typically shows beach marks and striations on fracture surface
• Creep rupture exhibits intergranular cracking and void formation at grain boundaries
• Failure prevention strategies include material selection, design optimization, and regular maintenance
• Non-destructive testing techniques (ultrasonic, radiographic) used to detect early signs of failure
• Redundancy and fail-safe design principles enhance overall system reliability