Study On Modes Of Ductile Fracture Of Metal And Its Application In Pressure Vessel

The problem of ductile fracture of metals under complex stress has been investigated extensively, but has not been resolved as yet. In the present thesis, the mesomechancisms of two different fracture modes of metals has been analyzed bymesomechanics methods. 16MnR and 45# steel are taken as examples. The obtained results have been applied to analyze the safety of operation pressure and rationality of design of a kind of engineering structure-a flange joint of hemispherical head to cylindrical shell. Try to seek a sort of new method to assessment the capability of structure to load beyond the traditional method which regards stress and strain as main parameters.Study two kinds of fracture modes of I -II complex ductile fracture of metal under room temperature and positive triaxiality. One is void-model fracture which is resulted from initial, dilatation and coalescence of voids in material. The fracture criterion of void-model fracture is VGC -criterion or CTODI-criterion, and its fracture angle is able to be determined by M-criterion. Another is shear-model fracture which is thought that it is associated with dislocation motion (shear + rotation) of crystal at microlevel, fragmentation of solid at mesoscale level and large plastic strains at the macroscopic level. There are two fracture modes simultaneously in fracture process and they compete with each another. There is a criteria triaxiality, which is a material constant, which when triaxiality of material is larger than the fracture belongs to void-mode, and in reverse, belongs to shear-mode.The research work of the thesis include mainly:(1)Observe a phenomenon which the two fracture modes are simultaneously found near crack tip of a four-point-bend specimen when the ratio of M/Q is close to 3.0mm, criteria M/Q, when the fracture mode of four-point-bend specimen transits from void-mode to shear-mode. Intensely location plastic distortion is observed in the front of crack tip and the macro-fracture of the specimen with the same M/Q belongs to shear-mode fracture. In the blunt region of the crack tip, initial and extend of void-mode fracture is found, but it is no influence to the macro-fracture-mode of this specimen.(2)Discuss the meso-mechanism of shear-mode fracture. Author thinks that the direction of the maximum plastic distortion along a constant radius around the crack tip may be considered as the direction of shear-mode fracture (named pmax -criterion) and the load when located shear band prongs the ligament in the front of crack tip isregarded as the limited load of shear-mode fracture.(3)The FAD (Fracture Assessment Diagram) of titled crack is proposed based on achievements of mesomechanics about I-II complex ductile fracture of metal, and the titled crack is classified two kinds: void-mode fracture is assessed by fracture assessment curve, and shear-mode fracture by limited load.(4)Using a FE program based on Plantle-Ressue theory to calculate some mesomechanical parameters, a flange joint structure of a high pressure vessel (Design pressure 31.4MPa and the thickness of the hemispherical head is half of that of shell.)is analyzed to determine whether the structure under operation pressure is safe or not, forecast the mode of potential invalidation and argue for the rationality of design of the structure. The following conclusions are gained:(1)The located shear band prongs the ligament near the top crack tip and the structure may occur shear-mode fracture at the angle of 155?to crack when load is 87.92MPa; the mode of fracture of the flange joint structure is not possible to be void-mode;(2)It is proved that loading and then unloading repeatedly can not increase the possibility of invalidation of the structure when the times of loading and unloading are not too more when load is operating pressure;the structure is safe when the vessel is operating;(3)In the FE model of the thesis, not considering the influence of water pressure test which is in the process of fabrication of vessel in FE results in larger devi...