Investigation On Influence Factors For Damage Evolution And Mechanical Behavior Of Sintered Porous Metal And Multi-axial Fatigue Life Prediction

Sintered powder materials possessed high freedom of material design,low manufacturing costs,stable part quality,high production efficiency and high degree of processing automation have been widely used in the mechanical,automotive,marine and aerospace industries.The studies of the mechanical properties of the material have only stayed at the light-load,linear elastic,and mechanical test accumulation stage.However,up to now,there is yet no any systematic characterization on the inelastic behavior and the low-cycle fatigue(LCF)life subjected to multi-axial load.Therefore,it is significant that quantifying characterization evolution of mechanical behavior and influencing factors under multi-axial load condition.This dissertation focuses on the damage evolution and the influencing factors of mechanical behavior of sintered powder materials.Through the combination of macro-mechanical tests and microstructure analysis,the continuum damage mechanics(CDM)model is developed to the assessment of material damage evolution and influencing factors,including the manufacturing process,material porosity,and alloying elements.Quantitative characterization of effects on mechanical properties,especially the influence on stiffness degradation under multi-axial loading,is studied.The method combined with observation of fatigue fractures and error analysis is applied to determine multi-axial low-cycle fatigue life prediction methods for sintered powder materials.The main research contents and conclusions of this paper are as follows:(1)Various processing methods have different influences on the mechanical properties of sintered powder materials.Based on the premise of processing damage non-uniformity,the multi-axial damage evolution process under the influence of mechanical processing is characterized by the CDM model combined with the microstructure analysis of the materials,and the machining process is clarified.The influencing mechanism of the mechanical machining process on properties and damage evolution of materials has been revealed.There are micro-cracks caused by the processing load on the surface and sub-surface areas of the machined test pieces.Therefore,the specimens under the influence of machining can be divided into the processing damage affected zone(MAZ)and the based material zone in the principal stress/strain direction,and the processing damage-affected zone can be determined according to the micro-crack size.Multi-axial damage evolution is controlled by damage weight,plastic strain and stress triaxiality in the affected zone of the process damage.(2)To seek the influence of the nonlinear correlation between mechanical behavior and the porosity.The interdependence between the porosity and elastoplastic mechanical property as well as damage evolution in sintered metals is investigated experimentally and analytically.A damage characterization is introduced based on stiffness degradation to predict the inelastic behavior of the sintered material and damage process.The proposed damage evolution model is further verified under multi-axial loading conditions.(3)By comparing the mechanical properties and damage evolution process of sintered porous iron and alloy(short named by BMn),as well as the metallographic analysis of the two materials,the effects of alloying elements such as manganese on the mechanical properties and damage evolution of iron-based sintered powders are determined.As the alloying elements lead to the formation of intermetallic compounds during the sintering process,the microstructure of the alloy material is transformed from total ferrite to pearlite and ferrite mixed phases in pure iron,so that the plastic deformation of the sintered powder alloy reduces and the strength increases.The uniaxial elastic damage test results confirm that the damage of the sintered powder alloy is only caused by the plastic damage,because the plastic deformation ability of the matrix particles of the alloy material is reduced,and the lower stress level cannot drive the elastic damage initiation.(4)Combined with error analysis and fatigue fracture analysis,comparing multi-axial fatigue life models based on stress,strain and energy approaches for multi-axial fatigue life,to determine the multi-axial LCF life applicable to sintered materials.The reliability of multiaxial life prediction based on the energy-based approach is better than others.The modified Glinka model,which contains parameters that characterize the difference between shear and the positive strain energy,has a relative error in prediction of materials.Fracture surface analysis shows that the fatigue fracture subjected to multi-axial cyclic loading is a mixed mode of ductile and brittle fracture with dimples and cleavages.