Local Facture Methodolody Based On Probability Model Investigates Constraint Effects Of Crack Tip

This work mainly investigates how to apply local fracture methodology to predicting fracture toughness.At first, physics aspect of fracture mechanism and fracture process is discussed. The prevenient observations on fracture process and corresponding investigation results are summarized in the paper. These results indicate that cleavage fracture includes three steps: (1) slip-induced cracking of a brittle particle; (2) propagation of the microcrack on a cleavage plane of the neighbouring matrix grain across the particle/matrix interface under the local stress state; (3) propagation of the grain-sized crack to neighbouring grains across the grain boundary. The induced condition of these steps are different each other and one step is dominant at a given temperature. Temperature not only gives an effect to fracture toughness but also determines which step is dominant.The local mechanism of fracture must be investigated by local fracture methodology. The main models are Bermin model and multi-barrier model. Bermin model basing on weakest link theory is valid in describing the statistics behavior of fracture.The toughness increases due to in-plane constraint loss for different structures made of ferritic steels. In this paper, the Master Curve and 5% and 95% failure curve are determined by using the methodology apposed by ASTM E1921-05．Bermin model is used to describe the evolution of failure probability with crack tip stress fields. The relation between Weibull stress and J-integral is analyzed according to FEA results. The conclusions are: (1) this relationship deflect from linear when J is greater than 60N/mm for deep crack specimens; (2) the relationship deflect from linear even J is smaller than 10N/mm for shallow crack specimens and tends to another linear relationship; The Master Curve method is not suitable for shallow specimens anymore. A new method is proposed in our paper and the prediction is consistent with experiment data. This gives a confirmation to the approach advanced in our paper.Also, the out-plane geometry has evident effects on crack-tip stress fields and specimen's fracture toughness. Popularly the relationship between thickness and toughness can be dealt with by the assumption that B (K_IC)⁴ is a constant. The analysis of crack tip stress field and Weibull stress are performed by 3D FEA. The results indicate that (1) the stress field in plane-strain grades into stress field in plane-stress, (2) J-integral in centre plane is largest and decrease as near free plane, (3) Weibull stress in centre plane is largest and decreases as near free plane, (4) the relationship between Weibull stress and J-integral is linear and the coefficient evolves with thickness, B. The prediction is consistent with experiment data.Compared with tradition methodology used to character constrain effect, the method advanced in our paper can be applied not only to fracture specimens but also to any flaw structure . The application of fracture mechanic in engineering becomes possible.