Study Of The Surface Roughness Evolution In Fatigued 316L Stainless Steel

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Study Of The Surface Roughness Evolution In Fatigued 316L Stainless Steel

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Title: Study Of The Surface Roughness Evolution In Fatigued 316L Stainless Steel
Author: Wang, Ye
Abstract: Fatigue is a critical design consideration in many aerospace structures. As improvements are sought in overall performance, maintenance requirements, and total service life of modern aircrafts, the demands for fatigue control techniques, such as material design for fatigue resistance, reliable fatigue life prediction, and continuous material damage monitoring, have increased. The most common practices to ensure fatigue safety include: fracture mechanics based fatigue life prediction, periodic non-destructive inspections, and material processing control. Reasonable successes have been achieved by applying these three techniques to ensure the safe operation of aerospace structures. It is worth mentioning that most of the present fatigue control techniques are only applicable when there is a dominant crack in the service components, i.e. the crack propagation stage, while the fatigue initiation stage (the evolution of pronounced surface relief, such as Persistent Slip Bands (PSBs) formation, crack nucleation and micro-crack propagation etc), could constitute 90% of the fatigue life for some components. Developing a quantitative understanding of crack initiation processes, therefore, should be considered as one of the most important tasks in fatigue study. Because crack initiation is strongly controlled by the local micro-structural features, systematic study of crack initiation could reveal fundamental information about the underlying mechanisms of fatigue damages which in turn can be incorporated into life prediction modeling to evaluate future damage development. One major challenge of fatigue study during crack initiation is that no physical measurement has been identified to characterize material damage since it is dominated by randomly distributed plastic deformation and micro-crack clusters.In this thesis, we study the feasibility of exploiting the plastic deformation-induced surface roughness as a diagnosis tool to assess the material damage at early fatigue life. Scanning Whitelight Interferometric Microscope (SWLI) was applied to quantitatively study the surface roughness evolution of polycrystalline 316L stainless steel fatigue specimen. We demonstrated that the surface roughness increased with fatigue cycles during the entire fatigue process. In addition, we discovered that surface roughness increases at early fatigue stage were contributed by slip band formation while surface roughness increases at later-stage fatigue were due to the out-of-plane displacement of adjacent grains. Crack initiation and development has also been identified.
Date: 2012-07-25

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