| 摘要: |
前文《追溯历史评表面形变纳米化》指出变形的细化结构早在20世纪七八十年代已有研究，21世纪初的纳米晶仅冠以新潮名称。文中从测定方法进一步说明一种结构两个名称的原因。位错胞以错角为参数，纳米晶只强调大小未涉及错角，两者的尺度属同一数量级。近年用滚压加大错角，虽实现纳米晶，却是将30年前的磨损表面当作强化。比较大小和错角对性能的影响，表面纳米化以H-P关系为准则，认为细化晶粒可以提高强度。这个关系是统计工厂中日用材料所得，属经验规律，并无理论依据。近年试验表明形变层位错胞大小对力学性能影响不大，错角却起关键作用。H-P关系不能用于形变层结构，表面纳米化的理论依据不成立。
喷丸的主要贡献是提高疲劳强度，公认残余压应力起主导作用。表面纳米化认为结构细化，屈服强度提高，相应延缓疲劳裂纹萌生，提高了疲劳强度。却没有试验证实此机理的正确性。实际上，获得纳米化的强喷丸表面伴随有裂纹，不存在裂纹萌生阶段。文中展示了喷丸强度和出现微裂纹几率的关系。明确强变形表面属裂纹体，应该用断裂力学而非经典力学的连续体处理。介绍表面形变各影响因素对疲劳裂纹扩展作用的试验，结果表明残余压应力仍是提高疲劳性能的主导因素。追求纳米化的加工硬化，牺牲塑性换取强度，导致裂纹扩展加速，对疲劳有害无益。 |
| 关键词: 裂纹萌生 裂纹扩展 H-P关系 喷丸 |
| DOI：10.11933/j.issn.1007-9289.20190306002 |
| 分类号:TG668 |
| 基金项目: |
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| Reevaluation on Nano-treatment of Surface Attrition for Fatigue |
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HE Jiawen
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State Key Laboratory of Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
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| Abstract: |
The released paper "Comments on Nano-treatment of Surface Attrition Via Historical View" indicates the deformation induced fine structure has been studied since 1970s to 1980s. The so-called nano structure in recent years is just renamed with new fashion. This paper attributes such "two names for one structure" dilemma to their measuring methods. Dislocation cell emphasizes the misorientation angle while nano-treatment is characterized in grain size, without any restriction on angle. The dimensions held by the two names are the same. Surface rolling is applied to increase the misorientation angle and nano sized grain is realized. However, this so-called strengthened surface is damaged worn surface 30 years ago. Regarding the effects of size and angle on mechanical properties, Hall-Petch relation has taken as the theoretical base of surface attrition and grain size is the key factor. In fact, H-P relation is only an empirical rule summarized from conventional materials without theoretical support. A current study indicates that the nano grain size shows little effect on mechanical property, while the misorientation angle plays a critical rule. Since Hall-Petch relation is not valid for deformed structure with small misorientation angles, the base of nano-treatment by surface attrition collapses.
The main contribution of shot peening is to improve fatigue strength and is widely accepted that the compressive residual stress playing a dominant role. Surface attrition claims the yield strength increased by fine structure retards crack initiation, so the fatigue strength increases. However, no experiment confirms this mechanism. In fact, the nano structure, produced by high intensity surface attrition creates micro cracks, thus the fatigue crack initiation stage is skipped. The dependence of micro crack occurrence and peening intensity is exhibited. The highly deformed surface layer is characterized in cracked body and thus should be treated with fracture mechanics, instead of continuum body by classical mechanics. Some experiments separating the effects of different factors on fatigue crack growth are introduced. The results show that the compressive residual stress is the key factor improving fatigue strength. The factor of strain hardening by surface attrition achieves strength by scarifying ductility leading to crack growth rate accelerating. Therefore, the work hardening of surface attrition is detrimental to fatigue. |
| Key words: crack initiation crack growth H-P relation shot peening |
