|Alternative Title||Observation of characteristic and analysis of mechanism in very high cycle fatigue regime of high-strength steel|
|Thesis Advisor||高福平 ; 孙成奇|
|Place of Conferral||北京|
疲劳是指材料在循环载荷的作用下性能发生永久性的改变，并且经过一定循环周次后萌生裂纹甚至发生完全断裂的现象，其中疲劳寿命大于107周次的研究领域称为超高周疲劳（Very high cycle fatigue, VHCF）。超高周疲劳有其独特的现象与机理，不同于低周疲劳（Low cycle fatigue, LCF）与高周疲劳（High cycle fatigue, HCF）。本文采用超声频率（f=20 kHz）的轴向疲劳实验，研究高碳铬轴承钢GCr15与马氏体不锈钢AISI630的超高周疲劳行为，得到以下主要研究结果。
两种高强钢经历107个周次的循环载荷以后仍然会发生疲劳断裂，不存在传统的疲劳极限。采用扫描电子显微镜（Scanning electron microscope, SEM）对所有的疲劳断口进行观察，发现超高周疲劳断口的裂纹源通常位于试样内部，呈现鱼眼形貌特征，并且在鱼眼内部存在粗糙区，即裂纹的萌生区（Fine granular area, FGA）。随着疲劳寿命的增加，FGA的尺寸增大，然而其应力强度因子幅值ΔKFGA比较稳定。部分马氏体不锈钢AISI630的疲劳断口呈现内部多点起源特征，裂纹倾向于在大尺寸夹杂物处萌生。
采用恒应力幅值的载荷形式，对上述两种高强钢进行超声频率的疲劳实验。采用聚焦离子束（Focused ion beam, FIB）技术制备样品，电子背散射衍射（Electron backscatter diffraction, EBSD）技术和透射电子显微镜（Transmission electron microscope, TEM）观察裂纹萌生区的微结构，发现其为不连续区域的细晶结构。采用变应力幅值的载荷形式，对上述两种高强钢进行超声频率的疲劳实验，在裂纹萌生区留下清晰的年轮状形貌。在年轮状形貌中的光滑区域，发现细化的晶粒。这说明在低应力的作用下材料局部发生晶粒细化，之后的高应力驱动裂纹扩展形成光滑形貌。上述结果说明FGA形貌与晶粒是否发生细化没有必然联系。结合年轮的宽度与所加载荷的周次，计算得到这一区域的裂纹扩展速率，其量级为10-12-10-11 m/cyc，远低于伯氏矢量的量级。
Fatigue refers to the weakening of materials under cyclic loading, resulting in crack initiation or even facture after a number of loading cycles, in which the fatigue life larger than 107 loading cycles belongs to very high cycle fatigue (VHCF) regime. The phenomenon and mechanism of VHCF are different from low cycle fatigue (LCF) and high cycle fatigue (HCF). Ultrasonic fatigue tests were conducted on a high carbon chromium bearing steel, GCr15, and a martensitic stainless steel, AISI630, to investigate the fatigue behavior in VHCF regime. Some main results are listed as below.
The two kinds of high strength steels may fracture after 107 loading cycles, which indicates that there does not exist a conventional fatigue limit in VHCF regime. Observing all the fracture surfaces by scanning electron microscope (SEM), finding the crack origin usually locates in the interior of the specimen presenting fish-eye morphology in which fine granular area (FGA), crack initiation region, reveals. The size of FGA becomes larger with the increase of fatigue life, whereas the stress intensity factor range for FGA, ΔKFGA, remains almost a constant. For the interior crack initiation mode, multi-site crack origins are observed for some of the martensitic stainless steel specimens, which indicates that the crack tends to initiate from the larger inclusion.
The two kinds of high strength steels are tested by ultrasonic fatigue experiment with constant stress amplitude. Preparing samples with the aid of focused ion beam (FIB) technique, the microstructure of FGA is examined by transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique. The FGA is a discontinuous layer composed of ultrafine grains and coarse grains. Ultrasonic fatigue tests with variable stress amplitude were also conducted on the two kinds of high strength steels, leaving “tree ring” patterns in the FGA. It is found that the grain refinement could exist beneath the smooth area in some local regions under the high stress amplitude. This phenomenon indicates that the grain refinement occurs in some local regions under the low stress amplitude for a number of cyclic loadings before the micro crack formation or crack extension. Then, the relative larger crack driving force due to the following high stress amplitude promotes the crack growth and results in the smooth morphology. These results indicate that the refined grain does not always exist beneath FGA, i.e., there is no inevitable relation between the FGA morphology and the grain refinement beneath FGA. The crack growth rate is of the magnitude 10-12-10-11 m/cyc by calculating from the “tree ring” patterns, which is much smaller than the magnitude of Burger’s vector.
An intermittent loading path, a pause duration follows after a loading pulse, is broadly employed in ultrasonic fatigue test to minimize the effect of self-heating in specimens. The stress amplitude gradually damps during the pause duration after a loading pulse. Therefore, the variable low stress amplitude is introduced into the intermittent loading type. Although the loading cycles of the variable low stress amplitude are not counted for the fatigue life, a significant fatigue life improvement of the bearing steel in VHCF regime subjected to intermittent loading is found, in contrast to that without the pause duration. All the specimens are cooled down by the cold air during the fatigue test. A thermocouple is used to measure the surface temperature of the specimens. The temperatures of the specimens under two loading types close to room temperature, and the difference is very small. Therefore the effect of self-heating is negligible. The intermittent loading type does not change the crack initiation mechanism by observing and analyzing all the fracture surfaces under the two loading types. This phenomenon is attributed to the strain aging by the numerous low stress amplitude cycles under intermittent loading, which strengthens the local region ahead of the initiated crack tip by the former high stress amplitude cycles and improves the fatigue life.
Based on the present results, the paper proposes a new mechanism of the crack initiation and early growth of high strength steels in VHCF regime: the crack initiation and early growth is attributed to the grain refinement caused by the dislocation interaction over a number of cyclic loadings followed by micro cracks along with the formation of micro cracks irrespective of the grain refinement during the cyclic loading.
|宋清源. 高强钢超高周疲劳特征观测与机理分析[D]. 北京. 中国科学院大学,2021.|
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