Study On Ductile Fracture And The Engineering Application

In this thesis the study was focused on ductile fracture theory, including fracture criterion, void growth during fracture, and the application of fracture mechanics. The ductile fracture was viewed as the result of three stages:void nucleation, growth and coalescence; the experimental observations in three stages were reviewed literally, and the effect factors in every stage on ductile fracture were summarized. Microscopically, the growth of void was modeled via unit cell method under nonaxisymmetric loading, and the results were compared with the ones from Gologanu model; Macroscopically, the sensitivity of ductile fracture criterion on stress triaxiality and initial porosity were studied via unit cell model method with constant value fracture criterion. The study of the dependency of ductile fracture on high temperature and high strain rate was carried out experimentally and numerically. In the end numerical simulation was made to investigate the distribution and development of Lode parameter and stress triaxiality in target-specimen for the preparation work of exploring the3D fracture criterion, which needs to be depicted in the space of εf(fracture strain),θ(Lode parameter), and T (stress triaxiality).In the Introduction (section0), the ductile fracture and its cost in engineering fields were introduced; the physical observation were shown in the manner of electronic microscope photographs from the ductile fracture experiments, and the development of the study of the inner source that leads to ductile fracture was introduced; related engineering background was summarized simply, including aiming materials, its components and its heat treatment. In the end, the content of the thesis was introduced.In the first section "Review of void nucleation, growth and coalescence", the texts were edited in three parts named after void nucleation, void growth and void coalescence. Author reviewed the development of research in related fields, and summarized the main achievements, some of which was introduced as the background of next section.In the second section "Investigation of void growth in porous material under nonaxisymmetric loading", periodic unit cell model was introduced to study the void growth in ductile porous material. Different from early work on axisymmetric model with axisymmetric loading, nonaxisymmetric3D model and nonaxisymmetric loading were adopted (simple uniaxial tension, uniaxial tension with plane strain condition and biaxial tension). Initial porosity f0considered was from0.5%to9%. Von-Mises plasticity theory was applied into matrix of unit cell. Through evaluation, macroscopic mechanical behavior of unit cell and void fraction evolution were obtained. After calibration Gologanu model was applied into single element model with the same mechanical conditions as unit cell. The predictions of void growth for two approaches were obtained and compared. Similarity was found for simple uniaxial tension but big differences were found for uniaxial tension with plane strain condition and biaxial tension. An exponential function was used to make fittings for the results of unit cell approach. The parameters of the function were discussed to describe the effect of the void growth on triaxiality.In the section3,"Numerical investigation on the effect of stress triaxiality and initial porosity on failure criterion of ductile material", unit cell models were employed, which is a cubical (matrix) within spherical void in the centre; a constant failure criteria was prescribed in the matrix, and then the fracture progress of unit cell models were investigated and evaluated under7stress triaxiality states, including biaxial compression (T=-0.667), uniaxial compression under plane strain constraints (T=-0.577), simple uniaxial compression (T=-0.333), simple shear (T=0), simple uniaxial tension (T=0.333), uniaxial tension under plane strain constraints (T=0.577), and biaxial tension (T=0.667). Meanwhile initial porosities, from0.5%to25%, was considered to investigate the effect of initial porosity on fracture features. The effect of stress triaxiality on material fracture matched well with the fracture2D locus of modified Mohr-Coulomb criteria proposed by Wierzbicki in2010; the general trend of the effect of initial porosity on fracture of material was summarized and fitted.In Section4," Prediction of failure of cast iron with dynamic loading and high temperature", the fracture of compressor wheel in turbocharger leads wheel pieces impacting into the container, and a design method is needed to assure full containment of turbocharger failures within the cast iron housings that operate at high temperatures. During the impacting the container of turbocharger is under critical condition:dynamic loading from wheel pieces and high temperature (in some cases it can reach1000℃). In this project the material of container is cast iron GGG400SiMo. To investigate the damage behavior of the material, several groups of experiments were undertaken under specific conditions:quasi-static loading and room temperature, high strain rate dynamic loading and high temperature (up to600℃). The tests took a wide range of triaxiality (σm/σe; σm andσe are mean stress and equivalent stress) value into account. Extended Cast Iron model was adopted to simulate the elastoplastic property of the cast iron; Vohringen model was used to consider the effect of strain rate; and proper failure criteria (including maximum and minimum limits) were proposed and applied into all the tests. The simulations with failure prediction mostly matched the experiment very well except for the ones under temperature of200℃and400℃with quasi static loading. In the end the finite element model of turbocharger was used to simulate the damage behavior of the structure with proposed failure criteria.In section5,"Numerical investigation on stress triaxiality and Lode parameter evolution during mechanical history in flat grooved plate and flat notched plate", because of the ability of well controlling on value of stress triaxiality T and Lode parameter θ, flat grooved plate and flat notched plate were chosen to investigate the distribution and development of the two factors, which are significant in affecting the damage behavior of ductile material. The configurations near the area of groove and the notch of the specimens varied so that different values of T and θ were captured. Numerical simulations were conducted on specimens considering various configurations, and assumed initial elements were defined according to the equivalent plastic strainε. Finally the evolutions of T and θ versus ε at defined initial failure elements were depicted and compared. The conclusions are:with flat notched plate the value of T and θ developed in a stable manner, which is quite helpful for the calibration of failure criteria; with flat grooved plate θ was kept at zero, but the values of T can’t be as stable as ones from another specimen; the increasing size of the diameter of the notched area will decrease the value of T expected, and meanwhileθ will be closing to-1, the θ vs. ε curve will be more unstable.