The Effect Of Continuous Low Stress Impacts On The Deformation Behavior Of Materials

Commonly, materials subjected to stress within their yield limits will not show plastic deformation. However, multiple components suffering from continuous low stress impacts can cause cumulative plastic deformation which is permanent even when the stress is much lower than the yield limit of the material. Yield limit can be obtained through the traditional quasi-static loading experiment. But a dynamic loading is applied repeatedly with impact energy. The present study focused on the effect of the impact energy on the plastic behavior of material subjected to continuous low stress impacts. And the mechanism of plastic deformation caused by continuous low stress impacts was investigated. Two plastic deformation mathematical models were successfully established.In the present study, 65 Mn and YT01 materials were first tested under different levels of energy in continuous low stress impacts experiment. Grid method was employed to measure the deformation of the samples at different time point during the experiment and a regular pattern can be obtained. Deformation hardening phenomenon was analyzed in grid area before and after the impact experiment respectively. After that, metallographic observation and X- ray diffraction were carried out to investigate the microstructure changes under continuous low stress impacts as well.According to the data obtained, the results can be summarized as follows: 1) Plastic deformation and hardening of the two kinds of materials subjected to different levels of impact energy in the continuous low stress impacts test all met "skin effect". The degree of plastic deformation caused by each impact decreased when the times of impacts applied increased. 2) When certain level of stress was applied, higher impact energy can lead to more serious plastic deformation, hardening and micro deformation. According to the results of our experiment, at the same level of stress, which was 31.7 MPa, the cumulative plastic deformation of 65 Mn was increased by 29.3 % and YT01 was 24.4% due to an increase of impact energy from 0.86 J to 1.67 J.Moreover, two mathematical models were successfully established to depict the relationship between plastic deformation and the level of continuous impact energy, which provide valuable guidance for engineering prediction in future research. L-Q-σ-N model was about plastic deformation, energy, stress and times. η-Q-σ-h model was about deformation degree, energy, stress and depth from the Collision surface. In conclusion, the plastic behavior was significantly affected by level of impact energy under continuous low stress impacts. The proportional relationship between plastic deformation and the level of impact energy was nonlinear. The effect of the level of impact energy on plastic deformation decreased with the increase of the stress.To sum up, there are three possible reasons which may cause this: 1) The impact energy might transfer to atomic energy so that the atom was moved to another balance position, which could end up with plastic slip of atomic distance 2) The impact energy excited the generation of twin crystal phase. Therefore, there will be more slip in the material. 3) The impact energy caused shock waves and the energy of the waves cumulated in the material. Thus, when the accumulative energy reached the yield limit of the material, the plastic deformation would occur as a consequence.