Earlier it was considered, that both in basic material and in welded joint cracks existence is not acceptable, and their origination during product service signifies its almost instantaneous destruction.
However, in practical situation it was found, that it is almost impossible to manufacture the product without cracks and quite often the constructions have to work with the existence of developing cracks for a long time.
Destruction of these constructions depends on the stress, which materials with cracks can sustain. The stress index, which materialwith crack can sustain, is so-called fracture toughness that characterizes material resistance for cracks spreading.
Knowing fracture toughness one can evaluate critical length of the crack that causes brittle failure when stressing or choose work stresses, under which there will be no brittle failure.
The science that studies behavior of bodies with cracks during stressing is called fracture mechanics.
The beginning of cracks development studying dated from 1913 – works of Inglis, Mucrelishvili – the task about body equilibrium balance with ellipsoidal cavity in the context of theory of elasticity.
Grifits (1920) – the beginning of new stage in fracture mechanics, the stage of detailed studying of the destruction process itself. It was considered that cracks already exists in body, which cause brittle fracture. He took into account internal forces – forces of surface tension – and obtained critical stresses for infinitely brittle body with I halfwidth rectilinear crack, reaching which the crack spreads spontaneously without supplying the energy from outside.
Important stage in development of cracks spreading theory connected with names of Irvin and Orovan. The developed the theory of quasibrittle destruction by their works – for viscous materials, which destruct brittle under certain conditions. Quasibrittle mechanism – when plastic deformation is concentrated in very narrow layer near the surface of destruction.
One can use Grifits equation for these materials, inputting the work of plastic deformation near crack surface instead of surface energy.
as p >> γ ; P – the work of plastic deformation in subsurface layer of the crack.
By marking 2P value through Gc we will obtain
where Gc – characterizes work necessary for new crack surface of single square origination in the moment of the beginning of its spontaneous growth.
This value depends on the type of chosen state, developing in the period of crack growth. Gc – is energetic criterion of fracture toughness.
It is known that linear theory of elasticity gives univalent relations between stress (σ), deformation and energy (Gc). That is why the energetic criterion of fracture toughness Gc (GIc) has equivalent criterion given through stress.
Irvin found the connection of destruction energetic criterion with stressed state at the tip of the crack.