Stress-type Fracture Criterion Of Ductile Metals And Three Dimensional Constrained Fracture Theory And Application

The ductile fracture is the main mode to lead the failure of structure in engineering structures.To develop the fracture strength theory of ductile material and solve the fracture problems of cracked specimens,it is very important to obtain precise ductile fracture criterion and realize theoretical prediction of fracture toughness.In the paper,the ductile fracture criterion,the elastic-plastic fracture theory of mode I cracks and the test methods of fracture toughness are systematically studied for the three-dimensional constrained fracture problem.The main works are as follows:1.The large deformation stress-strain relations of six ductile metals,including T225titanium alloy,6061 aluminum alloy,A508-3 steel,30Cr steel,3Cr13 steel and Q345B steel,were obtained based on the experimental load-displacement relations of funnel specimens by using the finite-element-analysis aided testing（FAT）method.The large deformation stress-strain relations was used as the conditional constitutive relations for the finite element analysis of 28 specimens with 6 types of materials and 10 types of specimen configurations,and the simulated load-displacement curve of each specimen coincides well with the experimental curves.Moreover,the microstructure analysis of the root profile of the notched round bar specimens at different tensile deformation stages shows that the large deformation constitutive relation obtained by the FAT method could simulate the load-displacement relation of the funnel specimens accurately which are in high damage.2.Based on the finite element analysis of 10 types of specimens with the large deformation constitutive relations and the method proposed to estimate the critical fracture states of material’s representative volume element（RVE）,the critical failure behaviors were discussed in detail by the analysis of the local stress and strain distributions on the fracture cross-sections of specimens and fractographic observation of specimens.The results show that the first principal stress is the dominant factor of ductile fracture failure.Three novel critical fracture criteria of ductile materials represented by the critical first principal stress and critical stress triaxiality is proposed named as MPS criterion,E-MPS criterion and H-MPS criterion,and the validity of the proposed criteria has been successfully examined.In addition,the data points of the first principal stress and stress triaxial triaxiality of various specimens at different deformation stages are all located on one side of the criterion curves,which indicates that the proposed criterion equations are valid as critical fracture criteria for ductile materials.3.Based on the energy density equivalence and dimensionless analysis,a semi-analytical model（CS-J model）is proposed for solving the J-integral of three-dimensional constrained mode I cracked specimens with the characteristic J-integral and the characteristic load normalized,and a semi-analytical model for describing the dimensionless relations between the load and displacement（CS-Pv model）is also obtained.For a specific configuration of mode I cracked specimens,when the constitutive relations of the materials are known,the model parameters could be calibrated by finite element calculation.Then,the semi-analytical equations of the dimensionless relations between the J-integral and load,load and displacement under linear elastic,pure plastic and elastoplastic conditions are ontained.The CS-J and CS-Pv models were validated by 97（CT specimen）,104（SEB specimen）and 123（CIET specimen）working conditions of materials and geometry sizes of specimens,respectively.The results show that,the J-integral-load relations predicted by the CS-J model and load-displacement relations predicted by the CS-Pv model are in good agreement with the results of finite element analysis.4.For mode I cracked specimens,a descriptive model(CS-σ_1max^T model)was proposed to characterize the dimensionless relations between the maximum first principal stress at the crack tip and the load,and another descriptive model(CS-σ^*Tmodel)is proposed to representate the relation between the stress triaxiality at the crack tip and the dimensionless load.The parameters of the two descriptive models can be obtained by fitting calculations.For the CT specimens,it is concluded that the maximum first principal stress-load relations predicted by the CS-σ_1max^T model and the stress triaxiality-load relations predicted by the CS-σ^*Tmodel are in good agreement with the finite element analysis results under 53 types of working conditions.5.A novel method to test the J-Δa_（J）、Presistance curve of mode I cracked speciemns was proposed based on the CS-J,CS-Pv models and the experimental load-displacement curves.For the CT specimens of A508-3 steel and Q345B steel,the J-resistance curves obtained by the new test method match well with the results obtained by the load separation method recommended in the national standard.Moreover,based on the H-MPS criterion,CS-J,CS-σ_1max^T and CS-σ^*Tmodels,a theoretical comprehensive evaluation method for ductile fracture toughness of mode I cracks under three-dimensional constraints was proposed.The critical fracture toughness-thickness curves of the CT specimens with different crack lengths were obtained,and the effects of thickness and crack length were studied in detail.The novel fracture toughness evaluation method can achieve the equivalent evaluation of fracture toughness between the smaller and larger specimens,and the key of the novel method is to complete a small number of quasi-static fracture tests of highly constrained specimens to obtain the H-MPS criterion.