【作者】 张龙；

【导师】 袁荒；

【作者基本信息】 北京理工大学 ， 动力机械及工程， 2018， 博士

【摘要】 烧结粉末材料由于具有较高的材料设计自由度、低制造成本、稳定的零件质量、高生产效率以及高度的加工自动化等优势,在机械、汽车、船舶及航空工业已经得到广泛应用。国内外对烧结粉末材料的力学性能研究到目前为止的工作只是停留在低载荷、线弹性和试验积累阶段,材料非弹性行为和多轴低周疲劳寿命的研究并不充分,因此,量化烧结粉末材料高载荷下力学行为演化规律及其影响因素以及研究多轴组合载荷下多轴低周疲劳寿命具有重要意义。本论文针对烧结粉末材料损伤演化及力学行为影响因素进行研究。通过宏观力学试验结合微观组织分析的方法,以连续损伤力学为框架,分析材料损伤演化过程及影响因素,包括加工过程、材料孔隙率及合金元素,定量表征上述因素对材料力学性能,尤其是材料多轴载荷下损伤演化规律的影响。比较不同多轴疲劳寿命预测模型,结合疲劳断口观察,通过误差分析方法,确定适用于烧结粉末材料多轴低周疲劳寿命预测方法。本论文的主要研究内容和结论如下:(1)不同的加工方法对于烧结粉末材料力学性能的影响不同,以加工损伤非均匀性假设为前提,通过损伤力学方法结合材料微观组织分析表征了机械加工影响下多轴载荷损伤演化过程,明确了机械加工过程对于材料力学性能及损伤演化的影响机理。由于机械加工试件表面及亚表面区域存在加工载荷导致的微裂纹,通过微裂纹尺寸确定加工损伤影响区损伤权,使得试件在应力/应变主方向上可以分为加工损伤影响区和母材区,因此试件多轴载荷下的损伤演化由加工损伤影响区损伤权、塑性应变以及应力三轴度三者共同控制。(2)针对烧结粉末材料高孔隙率的特点,通过不同孔隙率材料多轴损伤试验,建立孔隙率与连续介质损伤模型相关参量间关系,表征了孔隙率对于材料损伤演化的影响。建立孔隙率与材料性能参量幂函数关系,结合连续介质损伤力学模型表征不同孔隙率烧结粉末纯铁多轴载荷损伤演化过程;研究表明:孔隙的局部塑性应变会影响材料疲劳损伤机理及累积速率,较大的孔隙率导致材料在较低的应力水平下出现疲劳损伤。(3)通过比较烧结粉末合金及烧结粉末纯铁金相分析、力学性能及损伤演化过程,确定了锰等合金元素对铁基烧结粉末材料力学性能及损伤演化的影响。合金元素导致烧结过程中生成金属间化合物,合金材料显微组织由纯铁时的全铁素体转变为珠光体与铁素体混合相,根据单轴弹性损伤试验结果确定烧结粉末合金损伤仅由塑性损伤组成,合金材料基体颗粒塑性变形能力下降,使得较低的应力水平无法驱动弹性损伤萌生,进而烧结粉末合金塑性强度提高。(4)比较基于应力、应变以及能量准则的多轴疲劳寿命模型对于两种烧结粉末材料多轴疲劳试验寿命预测结果,通过统计学误差分析方法,结合疲劳断口分析,确定了适用于烧结粉末材料的多轴低周疲劳寿命预测方法。基于能量准则模型对于烧结粉末材料多轴寿命预测的可靠性好于基于应变及应力准则的多轴模型,其中含有表征剪应变能和正应变能差异参数的修正Glinka模型对于烧结粉末合金材料预测误差相对最小;疲劳断口分析表明多轴循环载荷下烧结粉末材料疲劳断裂为韧性断裂与脆性断裂的混合断裂模式。更多还原

【Abstract】 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.更多还原