Research And Modeling Of The Plastic Deformation Under Low Repeated Impact Stress In Different Impact Frequencies

In the engineering, many key components undertake the repeated impact stress. Although the stress is far less than the material yield strength, it still results in macroscopic cumulative plastic deformation and failure, which brings hidden security danger and economic losses to enterprises. Therefore, the study about plastic deformation under low repeated impact stress is of obvious significance. The plastic deformation is affected by many factors, such as material properties, impact stress and number. Loading frequency is very important in the research of material dynamic mechanical response, and different loading frequencies may cause different mechanical responses. This paper focused on the effect of impact frequency on the plastic deformation under low repeated impact stress, and established mathematical model of the cumulative plastic deformation with related factors, which could provide theoretical guidance for deformation prediction, life prediction and loading design in similar engineering conditions.Pure iron and 45 steel were selected as the test materials, and two groups of low stress impact test were conducted on each material in different impact frequencies. Each group consists of three tests, corresponding to three different impact frequencies of 0.25 Hz, 1Hz and 4Hz. The initial velocity of impact load, the impact stress and number were consistent for the three tests. Test results showed that the higher the frequency was, the greater the total cumulative plastic deformation, monolayer strain and degree of hardening were got. The total deformation and monolayer strain both changed non-linearly along with the increase of impact frequency. The higher the frequency was, the less the total deformation and monolayer strain increased.After low stress repeated impact, the microstructure of pure iron material had obvious transgranular cracks, and fragmentation phenomenon existed in the ferrite grain. The higher the frequency was, the more serious the grain fragmentation phenomenon was resulted in, the strain was also greater. After low stress repeated impact, the pearlite grains of 45 steel were seriously broken, and clear subgrain boundaries could be observed in the unbroken pearlite grains. Reticular ferrite was compressed and refined. The higher the impact frequency was, the less the content and the greater the refinement of the reticular ferrite were, the fragmentation of pearlite was also more serious, and finally the deformation was greater.Analysis showed that heat activation of impact load led to the generation of dislocation kink, and the critical shear stress of dislocation slip was reduced. Under repeated impact load, dislocation slip strain was produced through the spread of kinks along the dislocation lines, and finally the low stress repeated impact plastic deformation was got. When the initial velocity of impact load and the impact stress were constant, with the increase of impact frequency, the atom become more and more easily to move, the dislocation glide was also more easily produced, and eventually the cumulative deformation was greater after repeated impact.Mathematical model of cumulative plastic deformation with impact times, stress and impact frequency was built combined with the existing studies of impact and micro-deformation. When the impact number(N) and frequency(f) were infinite, and the total cumulative plastic deformation was less than 0.2% of the main yield area(10mm), the maximum stress value was defined as stress threshold σc, which could represent the resistance to low stress repeated impact plastic deformation. Comparative analysis of corresponding parameters in the mathematical model showed that the resistance ability to low stress repeated impact plastic deformation of 45 steel was stronger than that of the pure iron, and the cumulative plastic deformation of pure iron increased faster with the impact number increase than that of the 45 steel.