Semester

Spring

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Hota GangaRao

Committee Co-Chair

Nithi Sivaneri

Committee Member

P. V. Vijay

Committee Member

Ruifeng Liang

Committee Member

Dimitra Pyrialakou

Abstract

The life-cycle performance, durability, and aging characteristics of Fiber Reinforced Polymer (FRP or Structural Composites) have been of keen interest to the engineers engaged in the FRP design, construction, and manufacturing. Unlike conventional construction materials such as steel and concrete, the design guidelines to account for the aging of FRP are somewhat scattered or not available in an approved or consistent format. Loss of strength over time or aging of any structural material should be of concern to engineers as the in-service lifespan of many engineering structures is expected to exceed 100 years. Use of durability strength-reduction factors or factors of safety (aka knock-down factors) is a common way to account for the anticipated in-situ site conditions during the FRP design phase; however, the considerations for FRP service life is often ignored or smeared into knock-down or safety factors. The individual or combined effect of these factors can be arbitrary and can lead to the system’s premature failure (or overdesigns), rendering FRP commercial application unreliable (or cost-prohibitive). Reliability or risk-based approach to the development of strength reduction factors has been successfully applied in modern Load, and Resistance Factor Design codes (e.g., highway bridge design specifications), and an original design framework (i.e., a set of ideas, tools or techniques that forms the basis for filling in the final details) incorporating the time-dependent behavior of FRP composites (e.g., decrease of mechanical strengths with an increase of variability with aging) is proposed.

The research presented herein utilizes available natural and accelerated aged test databases to develop a relationship between the probability of failures (using reliability index and confidence intervals to measure reliability) and the desired service life of FRP members. The proposed framework illustrates how to use time-dependent reliability techniques to account for environmental and physical effects. For environmental effects, developing a direct relationship of reliability index with time-dependent durability works better, and for physical effects, indirect inclusion of probability in projecting the time (or cycles) to failure is more effective. The techniques presented in this research, along with three real-life design examples and a case study (i.e., the basis of design), can be readily used by design professionals to ascertain an adequate life cycle performance of FRP while maintaining a consistent component or system-level reliability. The intent is to allow others to refine this knowledge bank and to further the professional FRP design practice in a consistent, rational manner leading to the adaptation of formal codes and specifications. Although the presented data and associated findings primarily refer to pultruded glass fiber reinforced polymers (GFRP) in Vinylester resin, the presented framework can be easily extended to other structural composites.

The report entails thorough documentation of published analytical and experimental formulations for various modes of FRP failures due to the typical aging process (e.g., moisture, temperature, alkalinity, and sustained loading, and a combination thereof) along with an associated sampling of durability strength reduction factors. Critical reviews of deterministic and stochastic methods are conducted, and gaps in the current approach to determining durability factors for FRP systems have been identified. A Basis of Design (BOD) for vinylester/polyester-based GFRP in a submerged marine condition using an accelerated test database with illustrative design examples has also been included for a better understanding of the proposed time-dependent reliability-durability concept.

Understanding how an FRP system’s reliability changes over its life-span, designers will be able to confidentially choose the most suitable durability strength reduction factors, or factors of safety, that will meet their design’s target service life-span without exceeding strength or service limit states. Since absolute safety is not possible, all FRP members must be designed for a specific acceptable risk of failure. The research illustrates a unique set of techniques for determining FRP composites' durability strength reduction factors, or threshold design values, by integrating durability characteristics developed in the laboratory tests with desired service lives and commonly acceptable risks of failure. Due to the limited availability of complete durability datasets, vast applications, varieties of FRP composites, and the enormity of calibration efforts required, this research proposes additional work to determine the final durability recommendations for the general use of FRP composites. However, this unique research forms a rational tool for designing specific FRP composites that are consistent with other modern design codes, takes into account their target service lives (e.g., 10, 50, 100 years), and bridges the gap between traditional deterministic FRP design methods and state of the art risked-based design philosophies.

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