3. DISCUSSION A pragmatic, engineering approach to assess the fracture integrity of cracked or notched structures requires that constraints in the test specimen approximate that of the structure to provide an “effective” toughness for use in a structural integrity assessment. Constraint effect can be represented by stress triaxiality, Q parameter [10, 11] or T-stress [12]. Here constraint is measured by T-stress which is defined by: ( ) 0, 0 = = = − θ σ σ yy y xx T (10) Term T represents a tension (or compression) stress. Positive T-stress strengthens the level of crack tip stress triaxiality and leads to high crack tip constraint while negative T-stress leads to a loss of constraint. The volumetric method [2] is employed to calculate the notch fracture toughness and to estimate the effective T-stress ahead of the notch tip. It is assumed, according to the mesofracture principle, that the fracture process requires a physical volume. This assumption is supported by the fact that fracture resistance is affected by the loading mode, specimen geometry and scale effect. To explain these effects, it is necessary to take into account the stress value and the stress gradient in all neighbouring points within the fracture process volume. This volume is assumed to be quasi-cylindrical with a fracture process zone of similar shape. The diameter of this cylinder is called the “effective distance“. This procedure leads to a local fracture criterion based on two parameters, namely, the effective distance Xef and the effective stress σef. Due to the fact that T-stress in the fracture process zone is not constant, it is also assumed to be determined by averaging the T-stress distribution over the effective distance according to a line method (Eq. 11): ( ) ( ) T r r dr X T ef X xx ef ef . 1 0 Φ = ∫ (11) where ( ) ( ) ( ) ( ) 0= − = θ σ σ r r T r yy xx xx Figure 4. Τxx and Tzz distributions ahead crack tip ; CT specimen. Stress distribution ahead of crack tip has been computed for CT specimens by finite element method using ABAQUS code. In addition, T-stress is determined by the stress difference
RkJQdWJsaXNoZXIy MjM0NDE=