Reference: Abbott, Richard. Analysis and Design of Composite and Metallic Flight Vehicle Structures 3 Edition, 2019

Most common metallic alloys used in aircraft structures can seemingly exceed the ultimate strength of the material.

This illusion depends on stresses derived using an elastic analysis. The material cannot exceed its ultimate stress but as the material is stressed beyond the proportional limit the local load can be redistributed.

This redistribution relies on local material at a lower strain (and therefore stiffer) for the load to locally transfer to.

Local regions of high strain, beyond the elastic limit, are generally caused by two situations: a) Significant bending and b) Geometric features causing stress concentrations.