Which of the following is a common failure mode in metals under cyclic loading?

• A. Ductile fracture with significant plastic deformation at higher temperatures and slow strain rates
• B. Brittle fracture with little plastic deformation at low temperatures/high strain rates
• C. Creep fracture under long-term high temperature
• D. Fatigue failure under cyclic loading

Answer: (D)

Fatigue failure under cyclic loading is the common failure mode because repeated stress cycles exploit tiny flaws that act as crack initiation sites. Each load cycle subjects these flaws to stress, causing microcracks to form and slowly grow with every cycle. Even if the peak stress is below the metal’s yield strength, the crack can propagate until it reaches a critical size and the part fails catastrophically. This is why components that experience fluctuating loads—like gears, shafts, and aircraft structures—are designed with fatigue life in mind and described by fatigue concepts such as crack initiation, crack growth, and S–N behavior. Other modes describe failures under different conditions: ductile fracture requires substantial plastic deformation under overload, brittle fracture occurs with little plasticity at low temperatures or high strain rates, and creep fracture happens from long-term exposure to high temperature under sustained load. Under cyclic loading, fatigue captures the typical progressive nature of damage leading to failure.