Research On Modeling Of Plastic Accumulation Of Coating Strengthening Material Under Low Stress Repeated Impact

In the engineering field, many of the parts fail due to repeated impacting with the conduct of the collision-separating movements and the stress on these parts are usually very low (about 1/5～1/2 of materials'yield strength). It is found that the failures of parts which work under repeated impact load can be attributed to macroscopic cumulate deformation. According to these phenomena, this thesis did the repeated impact test about the parts of 1Cr18Ni9Ti with laser coating and the parts without laser coating under the same low stress condition and then built the math model of macroscopic cumulate deformation.This thesis improved the first generation of the repeated impact testing machine. (1) In the part of hydraulic circuit, designed a new reversal valve used one electromagnetic directional valve and two cartridge valves; (2) in the part of electric control, designed a new inhibiting device and a counting system; (3) in the part of test system, chose a piezoelectric transducer and designed a new jig and operating floor. Also designed two kinds of spring and calibrated them.1Cr18Ni9Ti and nickel-base alloy Ni-2 always used in the chemical pumps and valves are selected as test materials. Meshed the parts and put forward the new way of layered plastic distortion response. Repeated impact test are conducted both of the parts with laser coating and the parts without laser coating under the same condition (impact stress ? =100MPa, impact stroke h=20mm, impact frequency p=4Hz). The thesis researched on the fatigue strain of materials with time changing and cumulative laws. Put forward the concept of skin effect and get the conclusions: (1) during the initial thousands of times the deformation is obvious and the deformation becomes decrease with impact times increasing. At last it will be steady after twenty-five thousand times;(2) the deformation, the strain ratio and the hardness change of the surface are maximum, with the layer depth increasing the deformation, the strain ratio and the hardness change become smaller and smaller;(3) both coating and base of coating strengthening parts have better capacity of resist repeated impact deformation.The relationship among cumulate strain, impact times and coordinate has been built. The models about cumulate strain of 1Cr18Ni9Ti parts and coating strengthening parts have been made. According to the models, the cumulate strain? will be solve if substitution of its impact times N and coordinate y under the stress? .The experimental results show: there will also be macroscopic cumulate deformation when the impact stress is very low which is 1/5～1/2 of materials'yield strength. All above note that there is a lower threshold value ?K th in the repeated impact test. The low stress repeated-impact plastic deformation, different from the once impact deformation, the general fatigue failure, the ratchet effect and creep can not be measured by AK. The laser coating has a better capacity of resist repeated impact deformation.