Fracture Mechanics

The study of how fractures spread across materials is the focus of the mechanics discipline known as fracture mechanics. The driving force on a crack is calculated using analytical solid mechanics techniques, while the material's resistance to fracture is described using experimental solid mechanics techniques. The stress becomes infinite in theory ahead of a sharp crack tip, hence it cannot be utilised to represent the situation surrounding a fracture. Using a single parameter to represent the whole loading condition at the crack tip, fracture mechanics is used to characterise the stresses on a crack. The stress state at the crack tip is caused by elastic forces within the material when the plastic zone at the crack tip is tiny in comparison to the crack length. This stress state is known as linear elastic fracture mechanics LEFM and may be described using the stress intensity factor K. Although the force acting on a fracture may vary at random, in 1957 G. According to Irwin, each condition may be boiled down to a mixture of three separate stress intensity factors:

Mode I - Opening mode a tensile stress normal to the plane of the crack,

Mode II - Sliding mode, it is characterised by a shear stress operating perpendicular to the fracture front and parallel to the crack plane.

Mode III - Tearing mode, A shear stress operating parallel to both the fracture front and the crack plane.