表面强化技术使零件表层结构呈梯度分布并产生较高的残余压应力，可有效提高零件的使用寿命。近年来，表层梯度结构与残余应力对零件疲劳寿命的影响机理成为研究热点。综述了梯度结构与残余应力及其松弛对材料疲劳性能影响的新进展。材料经表面强化后，其表层晶粒明显细化，尺寸沿深度方向呈梯度分布，促使裂纹源转移至硬化层内部。残余应力与外力叠加，降低了零件的实际受力，从而影响零件的疲劳寿命，然而目前尚不能从机理层面对其进行揭示。对于残余应力松弛，目前的主要问题在于松弛模型的建立尚未完善。此外，在裂纹萌生阶段，梯度结构与残余应力均对零件疲劳强度有重要的影响，二者中哪个因素在疲劳过程中起到主导作用还未可知。

Surface hardening technology can make the parts form a surface gradient structure and generate high compressive residual stress, thus effectively improving the service life of parts. In recent years, the influence mechanism of gradient structure and residual stress on the fatigue life of parts has attracted great concern. The new development of the effects of gradient structure, residual stress and relaxation on the fatigue properties of parts was summarized. After the surface hardening, the grains on the material surface were fined obviously and distributed in a gradient form along the depth direction, thus promoting the crack sources to shift to the hardening layer. The superposition of residual stress and external force effectively reduced the actual stress of parts, thus affecting the fatigue life of parts. However, it cannot be revealed from the mechanism level. The main problem of residual stress relaxation is that the establishment of relaxation model is imperfect. In addition, during crack initiating, both gradient structure and residual stress have important influence on the fatigue strength of parts. However, which factor plays a leading role in this stage has not been assigned.