Quantification of fatigue damage in AISI 316L stainless steel using X-Ray powder diffraction (XRD)

Ramasimong, Duduzile

Quantification of fatigue damage in AISI 316L stainless steel using X-Ray powder diffraction (XRD)

Thesis / Dissertation

2025

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

In an electricity generation utility, there exists an enduring insistence on improving the efficiency, performance, and reliability of mechanical components with respect to economic challenges. Thus, the comprehension of materials and their behaviour in a typical operating environment is key to enabling and aiding capabilities in component life assessments. Fatigue damage is among the major issues in engineering, because it increases with the number of applied loading cycles in a cumulative manner and can lead to fracture and failure of the considered part. This study systematically investigates the microstructural changes in a material due to cyclic stressing using a non-destructive examination Power X-ray diffraction (PXRD). In industry, the evaluation of material deformation post cycling loading is often performed using microscopic techniques such as transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). However, each of these techniques have their own limitations. The PXRD technique is advantageous when applied to crystalline materials, as it is sensitive to changes in the crystalline structure of the metal, thus providing a foundation for the development of a capability to enable the early detection of fatigue damage with the aim of improving asset management. The microstructural baseline undertaken in the study involves the calculation of dislocation density in the material. The evolution of dislocation density in steels is an important aspect of the mechanical response. It could potentially be used as a fingerprint to relate the material state to the life-consumption fraction in materials subject to fatigue conditions. To mimic the fatigue damage mechanism, cyclic loading of material specimen is performed in a laboratory. In view of the exploratory nature of the present study, a stable single-phase austenitic microstructure, namely AISI316L stainless steel, was selected for the relative ease of characterizing dislocation development and comparison with PXRD analysis. The initial step included plotting an S-N curve for the AISI316L material in the material annealed state following fatigue testing in the INSTRON machine. A specific stress level was selected to ensure sufficient data retrieval prior to point of failure for the material specimen. Thereafter candidate specimen for fatigue including as received and annealed specimens formed part of the material states used in the study for microstructure analysis using three evaluation techniques. The EBSD results mostly show good qualitative agreement with the PXRD analysis. TEM analysis was used to qualitatively visualise the individual dislocations but is very time-consuming to perform quantitatively and the results are subject to large scatter. Validation of the PXRD techniques was performed using qualitative and quantitative TEM dislocation density results and semi-qualitative EBSD results. This study presents a consistent approach to determine the dislocation density using a benchtop laboratory based PXRD. The calculated dislocation density results analysed from the vast volumes of experimental data collected throughout the study highlighted important aspects to be undertaken. These include sample preparation, control of instrumentational parameters and the correct selection of instrument modelling standard. The Williamson-Hall plot and the whole pattern fitting methods are the two evaluation techniques utilised for size and micro-strain broadening of PXRD peaks. The results have proven to be repeatable owing to the systematic manner in the number of samples used per specimen as well as the stringent control of parameters used.

Keywords

Stainless steel

X-Ray

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