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How can repeated loading cause a crack in a bridge or airplane wing to grow?

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Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).
mechanics of solids Figure 1: The position vector x and the velocity vector v of a material point, the body force fdV acting on an element dV of volume, and the surface force TdS acting on an element dS of surface in a Cartesian coordinate system 1, 2, 3 (see text).

In the context of the mechanics of solids, the growth of a crack in structures like bridges or airplane wings due to repeated loading is known as fatigue.

This process occurs in three distinct phases:

  • Initiation: Repeated cycles of loading and unloading cause microscopic damage to accumulate within the material, eventually forming a minuscule crack.1
  • Growth: With each continued stress cycle, the crack gradually propagates. This is often driven by stress concentrations, where the geometry of the crack tip intensifies the local stress, causing the crack to extend further into the material.12
  • Fracture: As the crack grows, the remaining unaffected portion of the material becomes too small to support the applied load. Once the structure's integrity is sufficiently compromised, a sudden, complete fracture occurs.1

Engineers use the stress intensity factor to predict this growth and ensure that components are designed to withstand cyclic service loads safely.12