Stress Concentration Factor vs. Stress Intensity Factor

The elastic stress concentration factor is not applicable to sharp cracks, as the elastic stress concentration approaches infinity (it is a singularity). Instead, we use a different quantity, the stress intensity factor (𝐾) which is actually a quantity related to the gradient of the stress singularity. The stress intensity factor is thus used when analyzing fatigue cracks.

Modes of Crack Opening

The stress intensity factor is often broken up into components related to different modes of crack opening which we call fracture modes:

• Mode I: tensile opening mode of the crack
• Mode II: in-plane shear opening of the crack
• Mode III: transverse shear opening of the crack

A Material Property for Crack Resistance

Using the stress intensity factor approach, we can relate the critical load at which a crack will result in failure of a structure to a material property known as the Fracture Toughness. The fracture toughness is a critical stress intensity factor that defines failure of different loading modes.