All ETDs from UAB

Advisory Committee Chair

Selvum Pillay

Advisory Committee Members

Vinoy Thomas

Christopher Waldron

Ruigang Wang

Document Type

Dissertation

Date of Award

2021

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

Fiber Reinforced Polymer (FRP) composite have found increasing use in several vital industries and applications, such as transportation, construction, sporting goods, and aircraft. The quality and mechanical properties of FRP composites are significantly influenced by many factors including the manufacturing technique. Introducing the technique of fiber prestressing during the manufacturing process of FRP composites can effectively improve their performance without a considerable additional cost. This study contributes to further investigation and understanding about utilizing the elastic method of fiber prestressing to produce Glass Fiber Reinforced Polymer (GFRP) composites, in the following ways:First, a novel and effective method to manufacture unidirectional prestressed FRP composites was developed. In this method, Vacuum Assisted Resin Transfer Molding (VARTM) process was used to impregnate the glass fiber mats with the epoxy matrix, and a hydraulic tensile machine was used to tension the fiber during matrix curing. Five sets of prestressed composites were prepared using five different fiber prestressing levels 20, 40, 60, 80, and 100 MPa. The microstructure and mechanical properties of the prestressed composites were characterized and evaluated, then, the results were compared with the non-prestressed counterparts. The findings showed that as the fiber prestressing level increases, the void content decreases, and the fiber volume fraction increases marginally. Due to the slight change in fiber volume fraction, all mechanical properties were normalized to a 40% fiber volume fraction. The microstructure of the non-prestressed samples showed a high degree of fiber waviness, while the prestressed samples showed a high degree of fiber straightness. Fiber prestressing level of 60 MPa was found to be the optimum fiber prestressing level corresponding to the highest tensile and compressive strength. While the maximum improvement in flexural properties was obtained at a fiber prestressing level of 40 MPa. Second, a simple theoretical analysis was performed to calculate the residual stresses within the constituents of the prestressed composites. In addition, a mathematical relationship to predict the optimum Fiber Prestressing Level (FPL) as a function in different fiber volume fractions and curing temperature was established. This relationship was verified experimentally by preparing and testing prestressed composites with fiber volume fractions of 25% and 30%. The residual stresses, calculated from the presented theoretical analysis, showed an accurate agreement with the findings from the literature. The results of tensile testing showed that when the theoretically estimated optimum fiber prestressing level was applied to the fibers during curing of the matrix, the maximum improvement in tensile strength was reached. Third, the effect of time on the mechanical properties of the prestressed composites was investigated. Tensile and flexural properties of prestressed composites fabricated with three different fiber volume fractions i.e., 25%, 30%, and 40% were monitored and evaluated over twelve months after fabrication. Tensile and flexural strength data indicated that the loss increases at a decreasing rate as time proceeds. The total loss ratio of the gained improvement showed a negative correlation with fiber volume fraction, where the prestressed composites prepared with high fiber volume fraction showed less loss over time and vice versa. Tensile modulus insignificant reduction over time, while a slight reduction was observed in flexural modulus. Despite the initial improvement in the mechanical properties of the prestressed composites declined over time, some improvements can be maintained for long-term performance. Finally, composite rebar was proposed as an industrial application in which the technique of fiber prestressing can be applied with fewer challenges. A lab-scale prestressed composite rebars were successfully manufactured with a fiber weight fraction of 75% (~57% by volume). The influence of fiber prestressing on the performance of rebars was studied and the optimum level of fiber prestressing was determined. Tensioning the fiber during matrix curing resulted in improved fiber distribution within the matrix, reduced void content, and improved fiber alignment compared to the non-prestressed rebar. The maximum increase in tensile strength was achieved at a fiber prestressing level of 30 MPa. Durability study was conducted by immersing the rebars in an alkaline solution at 60 ° C for 90 days to simulate the concrete environment. After conditioning, the prestressed rebars exhibited relatively less surface corrosion than the non-prestressed counterparts. Compared to the non-prestressed rebars, the prestressed rebars fabricated using the optimum fiber prestressing level showed an increase in the guaranteed tensile strength and average tensile modulus by 7.5% and 2.6% respectively before conditioning, and by 5.6% and 5.4% after conditioning.

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