Theoretical Characterization Of Quasi-Static Fracture For Mode ? Crack And New Testing Method Of Ductile Fracture Toughness

The uniaxial stress-strain relations and fracture toughnesses(e.g.,J resistance curve)of ductile materials play an important role in safety assessment of structure.With the extensive use of small component and the urgent requirement of micro-damage or non-destructive testing for in-service structure,it is significant to develop small specimen testing method to obtain the mechanical properties of structures.In this paper,a systematic study has been conducted in three aspects,and they respectively are obtaining the fracture toughnesses of mode I cracks by a novel method,obtaining the uniaxial equivalent stress-strain curves of ductile materials by small disks,obtaining the critical fracture criteria and J resistance curves of ductile materials by small disk specimens.The main works are introduced as follows:(1)Based on energy equivalent method,the semianalytical load vs.displacement(S-LD)model and the semianalytical J integral vs.load(S-JL)model are proposed to describe the dimensionless load-displacement(P-h)relations and J integral-load(J-P)relations of mode I cracks,respectively.And the parameters of the two models are determined by simple finite element analysis(FEA).The P-h relations and J-P relations for compact tension(CT),C-shaped inside edge-notched tension(CIET),C-shaped outside edge-notched compression(COEC)specimens etc.are obtained by FEA.It can be observed that the P-h and J-P relations of specimens with different crack lengths,geometrical configurations and hardening exponents of materials predicted by S-LD and S-JL models,show good agreement with the relations obtained by FEA when the ratio of specimen thickness B to width W keeps constant.Further,the constraint-concerned semianalytical load vs.displacement(CS-LD)model and the constraint-concerned semianalytical J-integral vs.load(CS-JL)model are proposed by introducing a geometrical factor f.The P-h and J-P relations of mode I cracks with different constraints can be well predicted by CS-LD and CS-JL models.(2)According to the CS-LD model,CS-JL model and the experimental P-h curves,a fracture toughness test based on energy equivalent(FT-TEE)method is put forward to determine real-time crack lengths and J resistance curves of mode I cracks.Five materials represented by 26NiCrMoV11-5,Cr2Ni2 MoV,1Cr12Mo,P92 and A508-III and five kinds of mode I cracks are adopted to perform fracture toughness experiments.The corresponding J resistance curves of them are successfully determined by FT-TEE method.For Cr2Ni2 MoV and 26NiCrMoV11-5 steels,the J resistance curves determined by FT-TEE method are in agreement with the experimental results obtained by traditional normalization method.(3)The fracture toughness experiments of COEC specimens are conducted at high temperature.And the corresponding J resistance curves of P92 and A508-III steels are determined by FT-TEE method.Based on CS-LD model,the real-time crack lengths of sharp cracked specimen can be obtained.Meanwhile,the rotational radiuses R of specimens with different crack lengths,thicknesses and hardening exponents of materials can be determined by 3D FEA.Consequently,the crack tip opening displacement test based on energy equivalent(CTOD-TEE)method and the crack tip opening angle test based on energy equivalent(CTOA-TEE)method are proposed to obtain the crack tip opening displacement and crack tip opening angle of mode I cracks by combining the CS-LD model and the rotational radius R.(4)According to energy equivalent method,the theoretical models are proposed to describe the stage III load-displacement curves of small punch testing specimens and the elasto-plastic load-displacement curves of small disk lateral-compression specimens.Further,the small punch testing based on energy equivalent(SPT-EE)method and the small lateral-compression testing based on energy equivalent(SLT-EE)method are put forward to obtain the uniaxial stress-strain relations of ductile materials,respectively.A lot of simulated P-h curves for SPT-EE and SLT-EE specimens are obtained by FEA with imaginary materials.Further,the corresponding stress-strain curves are predicted by SPT-EE and SLT-EE methods.The results indicate that the predicted stress-strain curves agree well with the imaginary stress-strain curves input by FEA.Four materials represented by P92,DP600,A508-III and Q345 B are employed to conduct SPT-EE and SLT-EE experiments at room temperature.And two materials represented by A508-III and Q345 B are used to perform SPT-EE experiments at 300?.The corresponding stress-strain curves and tensile strengths predicted by SPT-EE and SLT-EE methods coincide with the results obtained by traditional uniaxial tension tests.(5)Based on the equivalent stress-strain relations determined by SPT-EE method,a new method is proposed to obtain the critical fracture criteria of ductile materials controlled by first principle stress through small punch testing(SPT)specimen and notched small punch testing(NSPT)specimen.According to the simulation result obtained from finite size specimen and large deformation analysis,a modified first principle stress formula is put forward to describe the first principle stress in front of the crack tip.Further,a novel fracture toughness evaluation based on small punch testing(FTE-SPT)method is proposed to predict the J resistance curves of ductile materials on the basis of the critical fracture criterion and the modified first principle stress field.For P92 and DP600 steels,the critical fracture criteria and J resistance curves are successfully obtained according to the small disk specimens.