The Influence Of Laser Shock Peening On Rolling Contact Fatigue Properities Of ER8 Steel

Laser shock peening(LSP)is a new surface strengthening technology.It uses laser-induced,high-energy shock wave to enhance the impacted area of metal.LSP is different from the traditional surface modification process.It is green and simple.While the good enhancement performance and the little effects on the surface roughness of the material,make LSP have been widely used to improve the fatigue resistance,wear resistance and corrosion resistance of metals.Rolling contact fatigue(RCF)is controlled by cyclic multi-axle loads.It is not only the main reason of failure of gears,bearings and other mechanical components,but also the main form of wheel damage.However,few people have studied how to improve the resistance of rolling contact fatigue performance by using LSP.How to describe the influence of combined surface strengthening with residual stress on resistance of RCF is the focus of this dissertation.Based on the experiment,the main contents of this dissertation are as follows.The mechanical properties of ER8 steel after LSP with different laser parameters were be characterized by optical microscopy,XRD analysis,and hardness test.It has been found through experiments that the depth of surface layer affected by laser shock hardening is above 1mm.The maximum residual compressive stress generated on specimen surface is around 1GPa.When the surface coating material is a combination of glass and aluminum foil,if the impact power density is more than 2.8GW/cm2,the surface hardness will not change significantly with the increase of laser power density.At the same time,the surface hardness value is increased about 20%.Although the LSP can strengthen the material and introduces the residual compressive stress,it does not change the microstructure of the material.The twin-disc test showed that laser shock treated steel disc presents better rolling contact fatigue resistance than untreated one.LSP induced strain hardening and compressive residual stress.After RCF test,obvious plastic flow,grain refinement and the thick fibrous structure has been observed.LSP alter crack inclination angle and propagation behavior which results in a longer fatigue life.Taking residual stress as variables,the propagation rates of cracks at different angles are analyzed by using the multi axis and non-proportional loading conditions of crack expansion angle criterion.When the maximum value of the distributed residual compressive stress reaches 1 GPa,the crack propagation is completely controlled by K11.It is also found that,the distribution of residual compressive stress field has greater influence on K field than the uniform distribution of residual compressive stress field.Residual compressive stress in the material can change the crack growth rate and effectively reduce the probability of crack propagation to the surface.The dissertation includes a total of 52 figures,3 tables and 67 references.