SGER: Achieving Large Improvements in Fatigue Life of Engineering Materials by the Suppression of Competing Surface Crack Initiations

技术：将进行探索性研究，以确定抑制工程材料表面裂纹萌生的方法，将疲劳寿命提高数量级。国际疲劳研究所最近的研究表明，在工程材料中，当疲劳裂纹从表面萌生到失效，而裂纹从试件内部萌生时，在S-N疲劳曲线上表现出“二元性”。由于侵蚀性环境的影响，表面裂纹的疲劳寿命较低，因此疲劳寿命往往存在两条相差数量级的S-N曲线。在这种情况下，试件由于表面裂纹产生短疲劳寿命而失效的可能性是有限的。这也意味着，由内部裂纹控制的材料的真正长疲劳寿命并不总是完全实现的。这意味着失败可能是不可预测的，工程设计可能是灾难性的。PI怀疑，由于表面引发的裂纹对疲劳破坏的竞争是由分布在材料中的极大尺寸的颗粒引起的。这些颗粒中的一些经常是故意引入的，以利于其他机械性能。例如，用于飞机发动机的Rene‘95高温合金含有粉末加工过程中夹带的氧化物颗粒和用于蠕变强化的氮化物/碳氮化物颗粒。PI假设，通过提供更细的硬颗粒和更窄的颗粒尺寸分布，可以消除极端尺寸的颗粒，并抑制限制疲劳寿命的表面裂纹萌生。在这项研究中，PI将专注于三个主要任务：(I)通过对疲劳样品的尸检分析，确定导致Rene‘95Ni基高温合金表面裂纹产生的极端尺寸的硬颗粒。(Ii)制造一种新的Rene‘95合金，该合金具有改进的颗粒尺寸分布，以消除极大尺寸的颗粒。(3)测试新合金的疲劳寿命，并确定通过抑制表面裂纹的萌生而增加的寿命。非技术性：在GE(美国)、Shinkansen钢(日本)和斯奈克玛(法国)进行的低周疲劳试验中发现，在实际应用条件下，表面裂纹和内部裂纹之间的疲劳失效竞争以及随之而来的疲劳的二元性/可变性。促进这种不寻常行为的材料和力学方面似乎与材料中极大尺寸的引发裂纹的颗粒的空间统计有关。通过确定导致这一行为的极端尺寸的颗粒并对材料进行适当的改性来明确解决这一问题，将对疲劳关键应用的工程材料设计产生巨大影响。如果成功，这项工作也可能代表着在解决许多工程材料普遍存在的“二元性”疲劳问题方面取得的突破。一名研究生和一名本科生(暑期)将受雇进行这项研究。将努力从代表性不足的群体中雇用一名少数族裔研究生或一名女性候选人来进行这项研究。

TECHNICAL: Exploratory research to determine the means to suppress the surface crack initiations in engineering materials, to increase the fatigue lives by orders of magnitude, will be performed. Recent research by the PI has shown that "duality" in S-N fatigue curves is seen in engineering materials when fatigue cracks initiating from surface compete to failure with the cracks initiating from the interior of the sample. The surface-crack fatigue lives are much lower due to aggressive environmental effect, thus there are often two S-N curves that are widely separated by orders of magnitude in fatigue life. Under this condition, there is some finite probability that the specimen would fail by surface cracks producing short-fatigue-lives. What this also means is that the true long fatigue life of a material, governed by internal cracks, cannot be fully realized always. This means failures can be unpredictable and engineering designs can be disastrous. PI suspects that the competition due to surface-initiated-cracks for fatigue failure is caused by extreme-sized particles that are distributed in the material. Some of these particles are often introduced intentionally, for the benefit of other mechanical properties. For example, Rene'95 superalloy, used in aircraft engines, contains oxides particles entrapped during powder processing and nitride/carbonitride particles for creep strengthening. PI hypothesizes that by providing finer hard particles with a narrower particle size-distribution, one can eliminate the extreme-sized particles and suppress the surface-crack-initiations that limit the fatigue life. In this research PI will focus on three principal tasks: (i) Identification of the extreme-sized hard particles that are responsible for surface crack initiations in a Rene'95Ni-base superalloy by postmortem analysis of fatigue samples. (ii) Making of a new Rene'95 alloy with a modified particle size distributions that eliminates the extreme-sized particles. (iii) Testing the new alloy for fatigue life improvement and determine the gain in life achieved by the suppression of surface crack initiations. NON-TECHNICAL: Competition for fatigue failure between the surface-initiated and the interior-initiated cracks and the consequent duality/variability in fatigue has been found to occur in actual application conditions in LCF tests conducted in GE (USA), SHINKANSEN steels (Japan), and SNECMA (France). The material and mechanics aspects that promote this unusual behavior seem to be connected to the spatial statistics of extreme-sized crack-initiating particles in the material. A clear resolution of this problem by identifying the extreme-sized particles that cause this behavior and an appropriate modification of the material will have a tremendous impact on engineering material design for fatigue critical applications. If successful, the work may as well represent a break-through in solving the "duality" fatigue problem that is common to many engineering materials. One graduate student and one undergraduate student (summer) will be employed to perform this research. Efforts will be made to employ a minority graduate student from an underrepresented group or a women candidate, to perform this research.