Finite Element Analysis of Elastoplastic Crack Tip Stress State for Assessing Constraint and Thickness Effect

Abstract: Cracks existing in structural members may affect the safety of a structure and may cause a catastrophic failure of the structure.Therefore, strength of the cracked member is important matter of concern when maintaining structures.In recent years,many methods have been performed to study the stress and constraint effect for both two dimensional and three dimensional models.The two-parameter J-A_2 method is adopted for analysing constraint effect at the crack tip fields.Mltiaxiality quotient q and out-of-plane stress constraint factor T_z are employed for analysing constraint effect of three-dimensional cracked models.X80 and X100 steels are the newly used steel in oil and natural gas pipelines.Some test specimens geometry and load configuration for the two kinds of steels are selected in this paper.Mltiaxiality quotient q and out-of-plane stress constraint factor are performed to study three-dimensional stress state and elastic-plastic constraint effect for the side deep crack and side shallow crack for different thickness.Discussed the effect of thickness and crack length on the stress state of the surface crack.Finally,disscussed the effect of thickness on palstic zone of middle plane.The elastic-plastic fracture J-A_2 method is used to study the collinear two cracks in plate under biaxial loading.The fracture driving force J-integral and constraint parameter A_2 of the main crack-front are calculated with different crack sizes and different distances of crack in biaxial loding condition for different load radios.As a comparison, the result under uniaxial loading condition is also presented.And also discussed the size of plastic zone.At last,compared the stress state of slanted cracks with that of straight cracks for discussing the effect of slant and thickness on the stress state…
Key words: constraint effect; biaxial loading; out-of-plane constraint factor; multiaxiality quotient; finite element analysis

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