Fatigue Fracture

While both fatigue and creep are failure mechanisms in metals, they occur under very different service conditions and stress behaviors.

• Creep Fracture: This occurs when a material is exposed to continuous stress at elevated temperatures for a prolonged time. The material slowly deforms (creeps) and eventually fractures. It’s commonly seen in high-heat environments like turbine blades, boilers, and steam piping.
• Fatigue Fracture: Fatigue happens under repeated cyclic loading — even at stress levels well below the material’s yield strength. Over thousands or millions of cycles, microscopic cracks develop and gradually grow until the component fails. Aircraft structures, bridges, and rotating shafts are frequent fatigue-risk components.

Key distinction: Creep is driven by time, temperature, and constant stress, while fatigue is driven by fluctuating stresses and load cycles over time, usually at ambient or moderately elevated temperatures.

Fretting wear and fatigue wear are both forms of mechanical surface degradation, but they occur under very different conditions. Understanding their differences helps in diagnosing damage and selecting effective prevention strategies.

Fretting Wear:

• What causes it: Arises from small, repetitive oscillations between two surfaces in contact — often under load. This stick-slip motion removes material at the contact zone.
• Surface appearance: Typically leads to pitting, fine grooves, debris, and often reddish-brown discoloration from oxidation — a sign of fretting corrosion.
• Where it occurs: Common in press fits, bolted joints, splines, and bearings — especially where slight movement is unavoidable.
• Prevention: Use lubricants, increase surface hardness with coatings, and design to limit micro-movement at joints.

Fatigue Wear:

• What causes it: Develops from repeated cyclic loading and unloading. Over time, microscopic cracks initiate beneath the surface and propagate outward.
• Surface appearance: Manifests as cracking, spalling, or flaking — often progressing to full material separation after many cycles.
• Where it occurs: Found in rotating shafts, springs, gears, and any component under fluctuating stress.
• Prevention: Select fatigue-resistant materials, avoid sharp transitions or stress risers, and manage applied loads carefully.

Quick Comparison:

• Motion: Fretting involves micro-movement at contact surfaces; fatigue involves internal stress cycling.
• Failure origin: Fretting is surface-initiated; fatigue begins internally and grows outward.