On mitigation of hydrogen embrittlement and theoretical modeling of hydrogen accelerated fatigue crack growth

Hydrogen embrittlement and fatigue are two modes of structural materials failure for which mechanistic understanding is still lacking. In addition, it has been found experimentally that concurrent cyclic loading and hydrogen gas exposure can lead to enhanced fatigue crack growth rates. This work aims to propose a methodology to mitigate failure of steels by hydrogen embrittlement under monotonic loading and develop a mechanistic model for fatigue crack growth in ductile materials. It has been suggested that hydrogen embrittlement of steels may be mitigated by introduction of hydrogen traps in the steel microstructure to suppress the accumulation of the hydrogen necessary for crack nucleation at fracture initiation sites. It has also been found experimentally that nano-scale vanadium carbides (VC) significantly improve the hydrogen embrittlement resistance of bainitic steel. To understand and quantify the effect of VC precipitates on hydrogen embrittlement, we analyze the re-partitioning of hydrogen among various trap states during cooling from high temperature and then during loading of a cracked test-piece at room temperature. For the case of hydrogen-accelerated fatigue crack growth, we first attempt to theoretically model fatigue crack propagation induced by alternating crack tip plastic blunting and re-sharpening in the mid-range of growth rates. The model is so designed that it relies on inputs from experiments that measure macroscopic material behavior, such as the uniaxial cyclic straining, and it can successfully reproduce the response in the Paris regime. The model is also used to investigate the dependence of fatigue crack growth on the hydrostatic constraint. We then explore the hydrogen effect on crack propagation under equilibrium conditions of hydrogen with material straining. The hydrogen effect is accounted for through the modification of the hardening response of the material according to the experimentally observed acceleration of the motion and generation of dislocations in the presence of hydrogen. Lastly, we investigate the constitutive response of materials in the presence of hydrogen under cyclic loading. To accomplish this, we expand on the Chaboche constitutive model which we calibrate using a sequence of experimental data from uniaxial strain-controlled cyclic loading tests and uniaxial stress-controlled ratcheting tests of a low carbon steel, JIS SM490YB, in the absence and presence of hydrogen. From the combined experimental data and calibrated Chaboche model, we may conclude that hydrogen decreases the yield stress and the amount of cyclic hardening. On the other hand, hydrogen increases ratcheting, the rate of cyclic hardening, and advances stronger recovery.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2020

Author dc:creator

• Hosseinisarani, Seyedehzahra

Contributors dc:contributor

• Sofronis, Petros
• Sehitoglu, Huseyin
• Ertekin, Elif
• Krogstad, Jessica A

• Fatigue crack growth, Hydrogen embrittlement

Statement dc:rights

• Copyright 2020 Seyedehzahra Hosseinisarani

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/107950

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/107950