Unusual Fatigue Crack Growth Behavior of Long Cracks at Low Stress Intensity Factor Ranges

Area of Science:

• Materials Science
• Mechanical Engineering
• Fracture Mechanics

Background:

• Conventional fatigue crack growth (FCG) analysis assumes a monotonic decrease in crack growth rate (da/dN) with decreasing stress intensity factor range (ΔK), reaching a threshold (ΔKth).
• This behavior is typically characterized by da/dN ≤ 10⁻⁷ mm/cycle for a given R-ratio or Kmax.
• This study reviews an unusual FCG behavior observed in some alloys.

Purpose of the Study:

• To characterize and review the unusual lack of a fatigue threshold in certain alloys.
• To discuss the implications of this behavior on damage-tolerant design approaches.
• To highlight the challenge this poses for fatigue life prediction models.

Main Methods:

• Review of experimental observations of fatigue crack growth (FCG) behavior.
• Characterization of the (da/dN)-ΔK relationship under varying conditions.
• Analysis of implications for fracture mechanics and design.

Main Results:

• Some alloys exhibit a plateau or acceleration in da/dN with decreasing ΔK, contrary to conventional threshold behavior.
• This unusual behavior is observed for long cracks under constant R-ratio and Kmax conditions.
• The phenomenon was first observed by Marci in 1996 in the IMI 834 alloy.

Conclusions:

• The observed lack of a fatigue threshold in some alloys challenges established fracture mechanics principles.
• This phenomenon requires further investigation to understand its underlying mechanisms.
• Accurate modeling of this behavior is crucial for reliable fatigue life prediction in structures subjected to low-amplitude vibrations.