Micromechanics-based models provide powerful tools to predict initiation of ductile fracture in steels. A new criterion is presented herein to study the process of ductile fracture when the effects of both stress tria...Micromechanics-based models provide powerful tools to predict initiation of ductile fracture in steels. A new criterion is presented herein to study the process of ductile fracture when the effects of both stress triaxiality and shear stress on void growth and coalescence are considered. Finite-element analyses of two different kinds of steel, viz. ASTM A992 and AISI 1045, were carried out to monitor the history of stress and strain states and study the methodology for determining fracture initiation. Both the new model and void growth model (VGM) were calibrated for both kinds of steel and their accuracy for predicting fracture initiation evaluated. The results indicated that both models offer good accuracy for predicting fracture of A992 steel. However, use of the VGM leads to a significant deviation for 1045 steel, while the new model presents good performance for predicting fracture over a wide range of stress triaxiality while capturing the effect of shear stress on fracture initiation.展开更多
In this paper, a new model to describe the dynamic ductile fracture is proposed. The constitutive relation of the elastic-viscoplastic matrix has an overstress form given by previous authors. A dynamic loading surface...In this paper, a new model to describe the dynamic ductile fracture is proposed. The constitutive relation of the elastic-viscoplastic matrix has an overstress form given by previous authors. A dynamic loading surface at constant equivalent strain rate(with the volumetric part also taken into account) is derived and an approximate expression for this dynamic loading surface is suggested. In the case where the porous material element is subjected to spherically symmetric tension, the Carroll-Holt’s model and Johnson’s model are recovered; and in the case where the strain rate sensitivity of the material tends to zero, the Gurson’s model is recovered. Moreover, the normality condition of the plastic strain rate for the dynamic loading surface is discussed, and it is shown that the normality rule is no longer valid in general. Finally, comparisons of this model with the models recently proposed by Pan, Saje and Needleman as well as by Perzyna are also presented.展开更多
Samples of SA508 grade 3 nuclear pressure vessel ferritic steel were subjected to tensile straining whilst being simultaneously imaged in 3D in real time using high resolution,high frame rate time-lapse synchrotron co...Samples of SA508 grade 3 nuclear pressure vessel ferritic steel were subjected to tensile straining whilst being simultaneously imaged in 3D in real time using high resolution,high frame rate time-lapse synchrotron computed tomography(CT).This enabled direct observation of void development from nucleation,through growth to coalescence and final failure validating many inferences made post-mortem or by theoretical models,as well as raising new points.The sparse,large inclusions were found to nucleate voids at essentially zero plastic strain(consistent with zero interfacial strength);these became increasingly elongated with straining.In contrast,a high density of small spherical voids were found to nucleate from the sub-micron cementite particles at larger strains(>200%)only in the centre of the necked(high triaxiality)region.An interfacial strength approaching 2100 MPa was inferred and soon after their nucleation,these small voids coalesce to form internal microcracks that lead to the final failure of the specimen.Perhaps surprisingly,under these conditions of generally low triaxial constraint the large voids are simply cut across and appear to play no significant role in determining the final failure.The implications of these results are discussed in terms of ductile fracture behaviour and the Gurson model for ductile fracture.展开更多
基金the National Science Foundation (Grant 1344592)the National Natural Science Foundation of China (Grant 51778462)the National Key Research and Development Plan (Grants 2017YFC1500700 and 2016YFC0701400).
文摘Micromechanics-based models provide powerful tools to predict initiation of ductile fracture in steels. A new criterion is presented herein to study the process of ductile fracture when the effects of both stress triaxiality and shear stress on void growth and coalescence are considered. Finite-element analyses of two different kinds of steel, viz. ASTM A992 and AISI 1045, were carried out to monitor the history of stress and strain states and study the methodology for determining fracture initiation. Both the new model and void growth model (VGM) were calibrated for both kinds of steel and their accuracy for predicting fracture initiation evaluated. The results indicated that both models offer good accuracy for predicting fracture of A992 steel. However, use of the VGM leads to a significant deviation for 1045 steel, while the new model presents good performance for predicting fracture over a wide range of stress triaxiality while capturing the effect of shear stress on fracture initiation.
基金Project supported by the National Natural Science Foundation of China and Chou Peiyuan's Science Foundation of Peking University.
文摘In this paper, a new model to describe the dynamic ductile fracture is proposed. The constitutive relation of the elastic-viscoplastic matrix has an overstress form given by previous authors. A dynamic loading surface at constant equivalent strain rate(with the volumetric part also taken into account) is derived and an approximate expression for this dynamic loading surface is suggested. In the case where the porous material element is subjected to spherically symmetric tension, the Carroll-Holt’s model and Johnson’s model are recovered; and in the case where the strain rate sensitivity of the material tends to zero, the Gurson’s model is recovered. Moreover, the normality condition of the plastic strain rate for the dynamic loading surface is discussed, and it is shown that the normality rule is no longer valid in general. Finally, comparisons of this model with the models recently proposed by Pan, Saje and Needleman as well as by Perzyna are also presented.
基金funding through a European Research Council Grant(COREL-CT)(No.695638)funded through EPSRC Grants(Nos.EP/R00661X/1,EP/S019367/1,EP/P025021/1 and EP/P025498/1)+1 种基金the Institute of Metal Research,the National Science Fund for Distinguished Young Scholars(No.5172510)the National Science and Technology Major Project(No.J2019-VI-0019–0134)。
文摘Samples of SA508 grade 3 nuclear pressure vessel ferritic steel were subjected to tensile straining whilst being simultaneously imaged in 3D in real time using high resolution,high frame rate time-lapse synchrotron computed tomography(CT).This enabled direct observation of void development from nucleation,through growth to coalescence and final failure validating many inferences made post-mortem or by theoretical models,as well as raising new points.The sparse,large inclusions were found to nucleate voids at essentially zero plastic strain(consistent with zero interfacial strength);these became increasingly elongated with straining.In contrast,a high density of small spherical voids were found to nucleate from the sub-micron cementite particles at larger strains(>200%)only in the centre of the necked(high triaxiality)region.An interfacial strength approaching 2100 MPa was inferred and soon after their nucleation,these small voids coalesce to form internal microcracks that lead to the final failure of the specimen.Perhaps surprisingly,under these conditions of generally low triaxial constraint the large voids are simply cut across and appear to play no significant role in determining the final failure.The implications of these results are discussed in terms of ductile fracture behaviour and the Gurson model for ductile fracture.
