FRACTURE TOUGHNESS DEPENDENCE ON THE THICKNESS AND ELASTIC FRACTURE MECHANICS.

ROLE OF PLASTICITY IN FRACTURE :-- It is necessary to follow linear theory of elasticity ,In reality, materials specially metals exhibit plasticity and there is always a region around crack up tip where plastic deformation occurs .This plastic region is commonly called the crack tip plastic zone. A small amount of plasticity at a crack tip does not affect the usefulness of applying elasticity to determine the general redistribution of forces in a body due to a crack .In the linear elastic fracture mechanics (LEFM) can not be applied to materials which exhibit considerable nonlinearity .Corrections for the plastic behaviour of materials are needed in order to predict fracture of such materials. It is suggested that a crack with plastic zone ahead of it may be looked upon as a crack of longer physical size.Due to plasticity the displacement and stiffness are larger than those for elastic case .One may consider the structure to have an effective crack length .In the majority of fractures critical parts particularly in aircraft structures the thickness of the material is generally small and plane strain condition do not prevail and the fracture occured under plane stress or mixed mode conditions.Fracture toghness dependence on the thickness range must be determined.