Study On Repeated Impact Test And Low Stress Plastic Deformation Of Metallic Materials

In project, macroscopic cumulate deformation often happens to many key parts which work under low stress (about 1/10～1/3 of materials'yield strength) repeated impact load. As to this phenomenon, this paper analysed the plastic deformation rule of parts which work under repeated impact load and discussed the deformation mechanism.According to the requirements of repeated impact test, this paper designed and improved the repeated impact test machine: analysed the stress of reversal valve and got the conclusion: the maxium driving torque of this valve is 153.5N.m;Designed a new reversal valve used one electromagnetic directional valve and two cartridge valves. It works steadily and reliably in practice and meet the demand of this test;Designed a data acquisition system based on high speed Charge Coupled Device(CCD);Test data can be got in the level of pixel using this system, so the higher data acquisition accuracy is got.1Cr18Ni9Ti which is always used in the fields of nuclear power station and chemical industry and nickel-base alloy Ni-2 which is always used as laser cladding material are selected as test materials. The same test condition (repeated impact frequency: 4 Hz,repeated impact stress: 130Mpa) is used in order to study comparely: (1)drew the curves of average strain ratio and cumulate strain following the growth of layer depth and impact times and found these two materials have the same plastic deformation rules: a) strain epistrophe; b) different material has different strain depth and total strain; c) laminar harden; (2) it is found through analyzing the metallic structure that after repeated impact, the structure of superficial coat became blurry and disordered, there are obvious destruct phenomenon of grain boundary and the broken phenomenon of grain and it is lightened following the growing of layer depth until there are not change of metallic structure; the direction of deformation have an angle of 30o～45o with the force axis; (3) According to these two materials, regression analysis of total strain and impact times is made and got the regression curve. Using this curve, the total plastic deformation of different impact times can be got.Based on the test data, associating with the analysing of microstructure, this paper discussed the plastic deformation mechanism under repeated impacted load. (1) seted up the repeated impacted dislocation movement model and analysed it associating the test data.(2) according to the plastic deformation theory, the plastic deformation mechanism under repeated impact load is discussed: a) kinetic theory: under repeated impact load, mass point maybe locate on the hemidome of energy hill of position. After repeated impact, the mass point will be push through the energy peak, then the dislocation will move and got plastic deformation; b) warm-up effect: under repeated impact load, because of the warm-up effect,dislocation will climbing out of the blocking cluster and it will contribute to repeated impact plastic deformation; c) change of structure: under repeated impact load, there are many phenomenons of grain boundary movement,substructure,twin crystal. These phenomenons are propitious to staete and reproduction of dislocation; d) dynamic response theory of repeated impact load: under repeated impact load, stress wave will appear and transmit from surface to internal of the sample. Because of the reflection of stress wave, the nestification stress of several stress waves will exceed the yield strength of material, the plastic deformation happened.