Study On The Elastic-plastic Fracture Criteria And A Unified Fracture Criterion For Elastic Components With Defects

Fracture is very common in engineering and our life.Some unexpected fracture may lead to disastrous accidents,which need mechanical researchers to avoid by using the knowledge of the fracture mechanics.The fracture mechanics has been developed for more than one century,many theories about the elastic fracture and the elastic-plastic fracture have been put forward.However,there still exist some unsolved key problems in the area of fracture.This paper focuses on two of them:The elastic-plastic fracture criterion and the unified fracture criterion for elastic components with defects.Main conclusions are summarized as follows:Once a crack in the elastic-plastic materials propagates,the plastic unloading appears,and then the strain energy density in the J-integral cannot be defined unambiguously,leading that the J-integral based criterion is not applicable.Up to now,a reasonable and widely acceptable fracture criterion for crack propagation in elastic-plastic materials still lacks.This work aims to rigorously derive a more applicable criterion which is in integral form as J integral.We identifies two ways to extract the energy(or power)flowing into a crack tip during propagation based on the power balance of areas enclosed by a stationary contour and a co-moving contour.Two corresponding energy release rates,a surface-forming energy release rate sG and a local energy release rate LG are proposed.Noting the inconsistency of the two kinds of energy release rate,we deduce that the stress singularity at the crack tip may lead to an accompanying kinetic energy.Besides,combining the cohesive fracture model and the surface-forming ERR based fracture criterion,we give an interpretation of Rice paradox for elastic-perfectly plastic materials which has existed for more that forty years.After discussion on several fracture criteria,we recommend using the surface-forming energy release rate based fracture criterion to predict crack propagation in elastic-plastic materials.The strength criterion and the fracture criterion have their respective application conditions.An appropriate criterion for the common failure problems resulted by defects such as V notches,corners and elliptical holes still lacks.This work aims to develop a unified fracture criterion that applicable to various kinds of defects in components.A basic failure law of components containing defects which called the zeroth law of failure is first proposed.Then,we illustrate the respective limitations of the strength criterion and the fracture criterion.A unified fracture criterion is put forward and a series of experiments are carried out to verify its correctness.It is a two-parameter function,and then we discuss several fracture criteria based on a single critical constant,which proved to be questionable in some cases.The unified criterion is more reasonable and has a wider range of applicability.