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

SGER：通过抑制竞争性表面裂纹萌生，大幅提高工程材料的疲劳寿命

• 批准号: 0635269
• 负责人: K. S. Ravi Chandran
• 金额: --
• 依托单位: University of Utah
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-07-15 至 2007-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635269&HistoricalAwards=false
• 关键词: SGER; Achieving; Large; Improvements; Fatigue

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.

技术支持：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，从而以数量级增加疲劳寿命。PI最近的研究表明，当从表面开始的疲劳裂纹与从样品内部开始的裂纹竞争失效时，在工程材料中可以看到S-N疲劳曲线中的“双重性”。表面裂纹疲劳寿命由于环境因素的影响而大大降低，因此疲劳寿命往往存在两条数量级相差很大的S-N曲线。在这种情况下，有一些有限的概率，试样将失败的表面裂纹产生短的疲劳寿命。这也意味着，材料的真正长疲劳寿命，由内部裂纹控制，不能总是完全实现。这意味着故障可能是不可预测的，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失效的竞争是由分布在材料中的极端尺寸的颗粒引起的。这些颗粒中的一些通常是有意引入的，以获得其他机械性能。例如，用于飞机发动机的Rene'95超合金含有在粉末加工期间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，通过提供具有较窄粒度分布的更细的硬质颗粒，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将集中在三个主要任务：（一）识别的极端尺寸的硬颗粒，负责在Rene'95镍基高温合金的疲劳试样的事后分析的表面裂纹萌生。(ii)制造一种新的Rene'95合金，具有改进的粒度分布，消除了极端尺寸的颗粒。(iii)测试新合金的疲劳寿命改善，并确定通过抑制表面裂纹萌生而获得的寿命增益。非技术性：在GE（美国）、SHINKANSEN钢（日本）和SNECMA（法国）进行的低周疲劳试验中，发现在实际应用条件下，表面引发的裂纹和通道引发的裂纹之间的疲劳失效竞争以及随之而来的疲劳的双重性/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端尺寸裂纹引发粒子的空间统计有关。通过识别导致这种行为的极端尺寸的颗粒和适当的材料改性来明确解决这个问题，将对疲劳临界应用的工程材料设计产生巨大影响。如果成功的话，这项工作也可能代表着在解决许多工程材料常见的“二元”疲劳问题方面的突破。一名研究生和一名本科生（夏季）将被雇用来执行这项研究。将努力雇用一名来自代表性不足群体的少数民族研究生或一名妇女候选人来进行这项研究。