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Influence of Pit Morphology on Crack Propagation

Hashim, Muntasir

[Thesis]. Manchester, UK: The University of Manchester; 2020.

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Abstract

In relation to lifetime assessment of a material undergoing corrosion fatigue, the ‘pit-to-crack’ transition stage presents significant challenges as the process is not well understood. Knowledge gaps exist concerning the early stages of crack development, leading to uncertainties in the temporal and spatial evolution of emergent cracks, including the location of crack nucleation at a corrosion pit and the material and environmental conditions leading to crack nucleation. The main objective of this research is to conduct a time-lapse X-ray computed tomography (XCT) study of crack initiation and evolution from corrosion pits on 316L Stainless Steel specimens undergoing corrosion fatigue. The data collected provided insights into some of the fundamental aspects of pit-to-crack transitions during corrosion fatigue processes, including how anodic dissolution and strain accumulation compete with each other to determine the site of crack initiation. Specimens with single corrosion pits of various morphologies undergoing corrosion fatigue showed that for a shallow corrosion pit, aspect ratio (AR) (depth to half-width ratio) less than 1 (AR< 1), crack initiation took place at the base of the corrosion pit but as the pit-gets deeper (AR~1, or AR>1), the crack initiation site shifted towards the shoulder and mouth. Finite Element Analysis (FEA) of various pit geometries undergoing cyclic loading, indicated that strain near the mouth of the pit increases as AR of the pit is increased. Digital Image Correlation (DIC) was applied on pre-pitted specimens undergoing fatigue in air to show that 316L specimens exhibited a threshold strain of 15±2% at which point crack initiation took place. For all fatigue test conducted in air, no crack initiation occurred at the base of the pit. By contrast, fatigue tests conducted in saline environment did produce cracks where initiation occurred at the base of the pits supporting the hypothesis that corrosion fatigue crack initiation is largely determined by processes whose outcome is the result of competition between strain accumulation and anodic dissolution (with possible involvement of hydrogen embrittlement). Finally, the pit-to-crack transition of specimen undergoing corrosion fatigue has been visualized in-situ by XCT. Direct measurement of sub-surface crack growth showed that multiple cracks initiate and initially, have fluctuating crack growth rate but as the cracks link to form a single half-penny shape crack, the crack growth rate becomes similar for each of the individual crack fronts. Measurement of crack tortuosity (ratio of actual crack length to measured length) has highlighted and quantified the interaction of shorts cracks with microstructural barriers. Crack Tip Opening Displacement (CTOD) profiles of crack initiation and their development before final failure by corrosion fatigue has been directly plotted as a function of time (number of applied cycles).

Bibliographic metadata

Type of resource:
Content type:
Administered thesis
Form of thesis:
Alternative format
Type of submission:
Doctoral level ETD - final
Thesis title:
Influence of Pit Morphology on Crack Propagation
Degree type:
Doctor of Philosophy
Degree programme:
Split-Site PhD Materials
Publication date:
2020-04-07T11:31:48
Institution:
The University of Manchester
Location:
Manchester, UK
Total pages:
197
Abstract:
In relation to lifetime assessment of a material undergoing corrosion fatigue, the ‘pit-to-crack’ transition stage presents significant challenges as the process is not well understood. Knowledge gaps exist concerning the early stages of crack development, leading to uncertainties in the temporal and spatial evolution of emergent cracks, including the location of crack nucleation at a corrosion pit and the material and environmental conditions leading to crack nucleation. The main objective of this research is to conduct a time-lapse X-ray computed tomography (XCT) study of crack initiation and evolution from corrosion pits on 316L Stainless Steel specimens undergoing corrosion fatigue. The data collected provided insights into some of the fundamental aspects of pit-to-crack transitions during corrosion fatigue processes, including how anodic dissolution and strain accumulation compete with each other to determine the site of crack initiation. Specimens with single corrosion pits of various morphologies undergoing corrosion fatigue showed that for a shallow corrosion pit, aspect ratio (AR) (depth to half-width ratio) less than 1 (AR< 1), crack initiation took place at the base of the corrosion pit but as the pit-gets deeper (AR~1, or AR>1), the crack initiation site shifted towards the shoulder and mouth. Finite Element Analysis (FEA) of various pit geometries undergoing cyclic loading, indicated that strain near the mouth of the pit increases as AR of the pit is increased. Digital Image Correlation (DIC) was applied on pre-pitted specimens undergoing fatigue in air to show that 316L specimens exhibited a threshold strain of 15±2% at which point crack initiation took place. For all fatigue test conducted in air, no crack initiation occurred at the base of the pit. By contrast, fatigue tests conducted in saline environment did produce cracks where initiation occurred at the base of the pits supporting the hypothesis that corrosion fatigue crack initiation is largely determined by processes whose outcome is the result of competition between strain accumulation and anodic dissolution (with possible involvement of hydrogen embrittlement). Finally, the pit-to-crack transition of specimen undergoing corrosion fatigue has been visualized in-situ by XCT. Direct measurement of sub-surface crack growth showed that multiple cracks initiate and initially, have fluctuating crack growth rate but as the cracks link to form a single half-penny shape crack, the crack growth rate becomes similar for each of the individual crack fronts. Measurement of crack tortuosity (ratio of actual crack length to measured length) has highlighted and quantified the interaction of shorts cracks with microstructural barriers. Crack Tip Opening Displacement (CTOD) profiles of crack initiation and their development before final failure by corrosion fatigue has been directly plotted as a function of time (number of applied cycles).
Keyword(s):
Thesis main supervisor(s):
Thesis co-supervisor(s):
Funder(s):
Degree grantor:
Language:
en

Institutional metadata

University researcher(s):
Academic department(s):

Record metadata

Manchester eScholar ID:
uk-ac-man-scw:324419
Created by:
Hashim, Muntasir
Created:
7th April, 2020, 10:31:48
Last modified by:
Hashim, Muntasir
Last modified:
8th April, 2020, 09:25:34

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