An Investigation On Thickness Effect On Plastic Zone Size At The 3-D Crack Front

Fatigue and fracture are the most common damage modes taking place in ships and marine engineering structures.The elastic mechanics shows that the stress tends to be infinity at the crack tip,and any engineering structure materials will yield to form a plastic zone.The yielded zone size close to the crack front is a factor of fundamental importance in the fracture and fatigue behavior of metallic,and it is very important to evaluate the safety of ship and marine structures during service period.At present,most researches of plastic zone size are confined to two dimensional fracture theories,and the influence of material thickness on the plastic zone is ignored.In this work,the finite thickness plate with central through crack as the research object,based on fracture mechanics,the method of combining theoretical analysis with finite element numerical calculation is used to study the plastic zone size of crack front.Breakthrough the limitations of 2-D fracture theory established the plastic zone size evaluation formula which considering the effect of the thickness,make the calculation of the plastic zone size is no longer confined to the two-dimensional plane theory and more suitable for engineering practice.The major contents of this paper are summarized as follows.At first,by introducing the out-of-plane constraint factor,the different triaxial stress state along the thickness can be divided into a series of thin layer with different stress state.An expression for crack front plastic zone considering the thickness effect is obtained based on linear elastic crack tip stress field and von Mises yield condition.Then,the function relation between the plastic zone size,the out-off-plane constraint factor and the crack maximum opening displacement is obtained by combining the relation between the crack maximum opening displacement and loads.Next,the distribution law of the parameter T_z in the function is studied in depth and determines that the T_z value at R_y/2 can represent the comprehensive influence of T_z in the plastic zone.Subsequently,the distribution rule of T_z?Ry/2?under tensile load is obtained based on 3-D elastoplastic FEM method,combining with theoretical analysis the formula for calculating the plastic zone size under tensile load is presented,and the influence of considering the initial crack length,Yield stress,elastic modulus,strain-hardening characteristics and transverse stress on the formula.For the calculation of plastic zone size in non-mid plane,the formula was modified by introducing plate thickness reduction coefficient.Finally,the cyclic plastic zone size calculation considering thickness effect is studied under cyclic tensile load.The distribution law of T_z?Ry/2?during cyclic unloading phase and cyclic loading phase is obtained respectively based on numerical calculation,and then combined with theoretical analysis,the calculation formulas of cyclic plastic zone in cyclic unloading phase and cyclic loading phase are given.The influence of initial crack length,yield stress,elastic modulus,stress ratio and strain-hardening characteristics on the formula is considered and the influence of overload ratio and loading sequence on the formula is analyzed under variable amplitude cyclic tensile load.In this paper,the calculation formula of the crack front plastic zone size considering the thickness effect is established,which provides a reference for the fatigue life assessment of the crack damage component in the engineering practice.