PhD Oral Exam - Islam Osama Mohamed Nabil Elsayed Nagy, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Structural components are susceptible to different types of loading, such as monotonic and cyclic loadings, etc. Fatigue loading is cyclic in nature and falls into two general categories, namely, low-cyclic fatigue and high-cyclic fatigue. Low-cyclic fatigue is observed when high-stress levels are applied to structures for a relatively low number of cycles, for example, in structures subjected to seismic and storm loads. High-cyclic fatigue occurs when a high number of cycles at low to moderate stress levels are applied to the structures. Bridges, parking garages, commercial buildings, stadiums, hospitals and airport pavements are examples of structures subjected to high-cyclic fatigue loading. Continuous application of cyclic loading on the elements can cause fatigue rupture, resulting in catastrophic structural failure.

Reinforced concrete (RC) beams are widely used to transfer loads to other structural components. Steel rebars in the tension side of the RC beam have been utilized as internal reinforcement in most design cases due to their ductile behaviour. However, steel reinforcement corrodes in structures subjected to harsh environmental conditions. Glass fibre-reinforced polymer (GFRP) bars are a suitable replacement for steel reinforcement rebars due to their corrosion-resistant characteristics. This characteristic warrants an extended service life for GFRP RC structures.

The research investigates the fatigue performance of ribbed GFRP bars in concrete, which is crucial due to the growing use of GFRP as a substitute for traditional steel reinforcements. The conflicting results in current research about how the concrete environment affects the fatigue performance of GFRP bars highlight the originality of this study. An experimental program was carefully planned to evaluate the fatigue life of ribbed GFRP bars within concrete beams, including factors such as concrete strength and the degrees of fatigue stress applied. An innovative displacement-controlled testing technique was devised to address the limitations of conventional force-controlled fatigue testing methods. The study also provides deep insights into the interaction between GFRP bars and concrete under fatigue stresses, covering phenomena like cracking behaviour, deflection patterns, and bar slippage. The results demonstrate that ribbed GFRP bars may withstand up to 2 million cycles of fatigue stress, exceeding current code standards and questioning previous empirical data. Additionally, this thesis highlights the influence of the stress ratio on fatigue performance.

This research also includes the fatigue characteristics of ribbed GFRP bars during tension-tension fatigue through a detailed review and innovative experiments. An analysis of how structural components are affected by different types of loads, specifically low-cyclic and high-cyclic fatigue loading. The experimental program included the fatigue, the fatigue life and the behaviour of ribbed GFRP bars, classifying them according to the testing protocol. Low-frequency fatigue testing is between 0.03 and 0.04 Hz, and fatigue tests are under higher frequency fatigue testing at 4 Hz. The modulus of elasticity remained consistent during the fatigue life, although both maximum and lowest stresses increased steadily until failure occurred. The empirical results helped create an S-N curve that correlates fatigue life with maximum stress, providing. The study investigated how different loading frequencies affect fatigue life and compared the results with existing literature to confirm their accuracy and consistency. This research identified an optimal gripping mechanism for conducting fatigue tests on GFRP bars and evaluated the feasibility of using conventional universal testing machines for fatigue life assessment, commonly found in many structural laboratories. In addition, the impact of influential factors such as stress ratio is examined through a testing program.

Finally, the study expands to present a simplified model based on the Sendeckyj model, but it utilizes a normal distribution to forecast the fatigue life of FRP-RC elements under repeated loading situations. This novel method is used to analyze fatigue data for GFRP, CFRP, and BFRP materials, allowing for the creation of S-N-P curves to be simplified with accurate estimations. This model's effectiveness is compared to the classic Sendeckyj model, which uses the Weibull distribution to confirm its suitability in construction engineering.