A study on the effects of different residual stresses fields on corrosion fatigue behaviour

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: A study on the effects of different residual stresses fields on corrosion fatigue behaviour

ارائه دهنده: Provider: Ava Khajeian

اساتید راهنما: Supervisors: Dr. Amir-Hossein Mahmoudi

اساتید مشاور: Advisory Professors: Prof. Rahman Seifi

اساتید ممتحن یا داور: Examining professors or referees: Prof. Gholamhossein Majzoobi, Prof. Gholamhossein Farrahi, Dr. Mohammad Chamani

زمان و تاریخ ارائه: Time and date of presentation: 2024.2.6 - 10:30 a.m.

مکان ارائه: Place of presentation: Conference Room

چکیده: Abstract: Corrosion fatigue has been known as one of the main failure mechanisms of the components working under the fatigue loading in the corrosive environments. Therefore, having a good knowledge of the corrosion fatigue phenomenon, and considering its effects while designing components used for the mentioned conditions is of paramount importance. The complexities of conducting experimental corrosion fatigue tests compared to the fatigue ones and meanwhile, the difficulties of studying this phenomenon with numerical methods have restricted the number of researches in this field. Residual stresses that are induced in the components through manufacturing processes or intentionally, are some of the factors affecting the corrosion fatigue phenomenon remarkably. This is due to the fact that the residual stresses may affect both of the fatigue and corrosion processes, and thus, they make the study of this phenomenon more challenging. In the current thesis containing both experimental and numerical parts, attempts have been made to cover some of the missing points in the previous studies. In the experimental part of the research, at first, the performance of two types of accelerated corrosion fatigue tests (using pre-pitted and artificially-pitted specimens) were investigated for the as-received material. At the next stage and for the first time, artificial pits were utilized to conduct corrosion fatigue tests using specimens containing compressive and tensile residual stresses. Furthermore, some other microscopic and experimental studies (such as electrochemical corrosion tests, etc.) were carried out to assess other aspects of the corrosion fatigue behaviour in the absence and presence of residual stresses. In the numerical part, fatigue and corrosion fatigue lives of the specimens (with and without residual stresses) were predicted using the finite element method and an elastic-plastic damage mechanics-based model. Although prediction of corrosion fatigue life in the presence of residual stresses was made for the first time, the main plus point of the current numerical model was its simplicity and the fact that it requires the minimum number of input parameters. The model only requires fatigue test data and brief microscopic observations. Most of the simulations performed to predict the corrosion fatigue life consisted of a sphere geometry for corrosion pits. However, in a part of the numerical studies, pits with a random-generated geometry were also used to investigate the effect of more realistic pit geometries on corrosion fatigue life predictions. In this part of the study, a cellular automaton (CA) model was utilized to generate random-generated goemetries for corrosion pits. It is worth noting that in the current study, tensile and compressive residual stresses were induced by grinding and shot peening, respectively. The experimental results showed that the utilized accelerated corrosion fatigue test methods had sufficient accuracy. Moreover, artificially-pitted specimens can be used as suitable alternatives to the pre-pitted ones in corrosion fatigue tests of as-received samples and those containing tensile residual stresses within the ranges of the study. Artificial pits with the same geometry as in the as-received samples can be used with marginal differences compared to the pre-pitted specimens. In the current study, performing shot peening and grinding processes led to an increase and decrease in the both fatigue and corrosion fatigue lives, respectively and their effects were more obvious in the high-cycle regime. The maximum life extension through shot peening was observed to be roughly 72% and the maximum life reduction for ground specimens was about 20%. Furthermore, according to the hardness measurements and microstructural observations, the results of fatigue and corrosion fatigue tests of the current study were independent of hardness and microstructures in the absence and presence of residual stresses. Numerical results of the study were also in a good agreement with the experimental ones, which means that they could eliminate the necessity of conducting time-consuming corrosion fatigue tests to a great extent. Comparing the numerical results obtained from the models with spherical and random-generated corrosion pits showed that using simple spherical geometries for predicting corrosion fatigue lives in numerical studies has reasonable accuracy, and there would be no needs to use complex geometries

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