Research On Fracture Behavior Of Stiffened Plates In Consideration Of Constraint Effects

Cracks usually initiate in the stiffened plates which are widely employed in engineering structures,and the safety assessment of cracked stiffened plates is one of the most significant work in structural integrity assessment.By using the fracture toughness under plane strain state,the traditional fracture analyses ignore the constraint effects,and cannot reflect the actual fracture process.In consideration of the influences of in-plane and out-of-plane constraints,three typical kinds of stiffened plates are investigated in this dissertation,such as double symmetrical edge stiffened plate?DSE-SP?,single central stiffened plate?SC-SP?and double symmetrical central stiffened plate?DSC-SP?.The analytical and numerical solutions of stress intensity factors are studied,the prediction models of fracture toughness and corrected fracture criteria are developed,and then the crack arrest effects and fracture behavior are researched.The main work and conclusions are as follows:?1?By using the Westergaard stress function,the theoretical models of normalized stress intensity factor?i.e.GCF?are proposed in the stiffened plates,and then the closed-form solutions of GCF are given in three typical kinds of stiffened plates.By means of three-dimensional?3D?finite element method?FEM?,numerical solutions of GCF are calculated in these stiffened plates,then the crack arrest effects of stiffened plates are derived based on the GCFs.With the closed-form solutions or mathematical models of numerical solutions,the GCFs can be quickly obtained in the stiffened plates.?2?The FEM solutions and their mathematical models are proposed for in-plane and out-of-plane T-stresses(T₁₁ and T₃₃)in the stiffened plates.Based on von Mises yield criterion and T-stresses,a new analytical solution of crack tip plastic zone size(r_pt)is derived for mode I crack under small scale yielding conditions.By using 3D FEM,the new analytical solution(r_pt)is corrected in the elastic perfectly-plastic material and strain-hardening materials,respectively.Then,an improved analytical solution of crack tip plastic zone size(r_ptM)is proposed in the unstiffened and stiffened plates,and it is applicable for biaxial loading.?3?Under small scale yielding conditions,a new constraint theory is developed based on the crack tip plastic zone.Four new constraint parameters are derived theoretically,i.e.?_r,?_rmax,?_A and ?_?,and then new fracture parameters(i.e.r_p0^1/2,r_pmax^1/2,A_pz^1/4 and ?_p)are proposed accordingly.The effectiveness of new constraint parameters and fracture parameters is confirmed by means of theoretical analysis,numerical simulation and three kinds of metallic materials'test data validation.Subsequently,prediction models of fracture toughness are proposed based on the new fracture parameters r_p0^1/2 and ?_p,then the corrected fracture criteria are given,i.e.K_I=K_IC^r and K_I=K_IC^?.?4?In the framework of new constraint theory based on crack tip plastic zone,the mathematical models of allowable applied loads are given in the unstiffened and stiffened plates,the correction methods of failure assessment diagram?FAD?are introduced,and a new parameter(i.e.??_sp^r)is proposed to characterize the crack arrest effect of stiffened plate.The results show that,in the stiffened plate,the GCFs,T-stresses,allowable applied loads and crack arrest effects are dependent on the stiffener's size,location and integrity.Compared with the traditional fracture analyses and the correction model with in-plane T-stress in the R6 assessment procedure,the evaluations based on the models of r_p0^1/2 and ?_p are more reasonable,because the fracture parameters r_p0^1/2 and ?_p can effectively characterize both the in-plane and out-of-plane constraint effects.Additionally,on the basis of allowable applied loads,the crack arrest effect(??_sp^r)can comprehensively reflect the influences of stress intensity factor and fracture toughness on crack arrest behavior.