冲击频率对低应力多冲碰撞塑性变形的影响研究及建模

发布时间：2019-02-09 11:05

【摘要】：工程中许多关键零部件在应力远小于材料屈服极限的多次冲击碰撞工况下仍会发生宏观累积塑性变形,进而产生失效,给企业带来安全隐患和经济损失。因此对低应力多冲碰撞塑性变形的研究具有重要的工程意义。低应力多冲碰撞塑性变形的影响因素众多,如材料性能、冲击应力、冲击次数等。加载频率是材料动态力学响应过程中非常重要的影响因素,加载频率不同,零部件的力学响应可能不同。本文重点研究冲击频率对低应力多冲碰撞塑性变形的影响,并建立累积塑性形变量与相关影响因素的数学关系模型,为工程应用中低应力多冲碰撞工况下零构件形变量和寿命的预测、加载条件的设计等提供理论指导。选用纯铁和45钢作为试验材料,对每种材料分别进行两组不同冲击频率下的低应力多冲碰撞试验。每组包含三个试验,对应三种不同的冲击频率(0.25Hz、1Hz、4Hz),三个试验的冲击载荷初速度、冲击应力和冲击次数均一致。各组试验结果均表明,冲击频率越大,总累积塑性形变越大;由单层形变率及硬化程度与距冲击表层深度的关系曲线分析,总体上单层形变率、硬化程度也随冲击频率的增加而增大;总形变量、单层形变率均随冲击频率的增加而非线性变化,频率越大,总形变及单层形变率的增加量越小。纯铁材料在低应力多冲碰撞后,微观组织中有明显的穿晶裂纹,铁素体晶粒存在碎化现象。频率越大,晶粒碎化现象则越严重,形变率也越大。45钢材料在低应力多冲碰撞后,微观组织中珠光体晶粒碎化严重,未破碎的珠光体晶粒中可观察到明晰的亚晶界,网状铁素体有压缩细化的现象。冲击频率越大,网状铁素体含量则越少,其细化程度越大,珠光体晶粒碎化现象也越严重,最终形变量也越大。分析认为低应力多冲碰撞塑性形变的产生主要是由于冲击载荷的热激活作用促使位错弯结的产生,降低了位错滑移的临界切应力,多次冲击碰撞造成弯结沿位错线传播,产生位错滑移应变。冲击载荷初速度及冲击应力不变时,冲击频率越大,原子易动性越大,越易产生位错滑移,多次冲击碰撞后,最终累积形变量越大。在冲击碰撞试验的基础上,结合国内外冲击碰撞、微形变的研究成果,建立了累积塑性形变量与冲击次数、冲击应力、冲击频率的S-N-σ-f数学关系模型。定义冲击次数N、冲击频率f无穷大时,总累积塑性形变不超过主要屈服区(10mm)0.2%时的最大应力值为应力阈值σc,可表征金属材料抗低应力多冲碰撞塑性变形的能力。由纯铁和45钢对应数学模型的相关参数对比分析表明,45钢材料抗低应力多冲碰撞塑性形变的能力强于纯铁材料;纯铁材料累积塑性形变量随冲击次数的增加而增大的速率大于45钢材料。
[Abstract]:In engineering, many key parts will still have macroscopic cumulative plastic deformation under the condition of multiple impact collisions where the stress is far less than the material yield limit, which will lead to failure and bring safety hidden trouble and economic loss to enterprises. Therefore, the study of low stress multiple impact plastic deformation has important engineering significance. There are many factors influencing the plastic deformation of low stress impact, such as material properties, impact stress, impact times and so on. Loading frequency is a very important factor in the process of dynamic mechanical response of materials. The mechanical response of parts may be different with different loading frequency. In this paper, the effect of impact frequency on plastic deformation of low stress and multiple impact is studied, and a mathematical model of cumulative plastic shape variable and related influencing factors is established. It provides theoretical guidance for the prediction of the shape variable and life of the zero member under the condition of multi-impact and low stress impact, and the design of loading conditions. Pure iron and 45 steel were selected as test materials. Two groups of low stress and multiple impact tests were carried out on each material at different impact frequencies. Each group consists of three tests, corresponding to three different impact frequencies (0.25 Hz / 1 Hz). The initial velocity of the impact load, the impact stress and the impact times of the three tests are the same. The experimental results show that the larger the impact frequency, the greater the total cumulative plastic deformation. From the analysis of the relationship curve between the single layer deformation rate and hardening degree and the depth from the impact surface, it is found that the single layer deformation rate and hardening degree also increase with the increase of impact frequency. The total deformation and monolayer deformation rate are all nonlinear with the increase of shock frequency. The larger the frequency, the smaller the increase of total deformation and monolayer deformation rate. There are obvious transgranular cracks in the microstructure of pure iron materials after low stress and multiple impact, and the ferrite grains are broken. The higher the frequency, the more serious the phenomenon of grain fragmentation and the greater the deformation rate. 45 steel materials in the low stress impact, pearlite grains in the microstructure of serious fragmentation, unbroken pearlite grains can be observed in clear sub-grain boundaries, The reticular ferrite has the phenomenon of compression and refinement. The larger the impact frequency, the less the ferrite content, the greater the degree of refinement, the more serious the grain fragmentation of pearlite and the greater the final shape variable. It is considered that the plastic deformation of low stress and multiple impact is mainly caused by the thermal activation of impact load, which results in the formation of dislocation bend, which reduces the critical shear stress of dislocation slip and propagates along the dislocation line due to multiple impact collisions. The dislocation slip strain is produced. When the initial velocity of impact load and impact stress are constant, the larger the impact frequency is, the greater the mobility of atoms is, the easier it is to produce dislocation slip, and the larger the final cumulative deformation is after multiple impact collisions. On the basis of impact test, combined with the research results of impact and micro-deformation at home and abroad, the S-N- 蟽 -f mathematical model of cumulative plastic shape variable and impact times, impact stress and impact frequency is established. The maximum stress value when the total cumulative plastic deformation does not exceed the main yield zone (10mm) 0.2 is defined as the stress threshold 蟽 _ c when the impact number N and the impact frequency f are infinite, which can be used to characterize the ability of metal materials to resist low stress and multiple impact plastic deformation. The comparison and analysis of relative parameters between pure iron and 45 steel show that 45 steel has better resistance to low stress and multiple impact plastic deformation than pure iron. The accumulative plastic shape variable of pure iron material increases with the increase of impact times and the rate of increase is greater than that of 45 steel material.
【学位授予单位】：苏州大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG115

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