Date of Award

8-2010

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Roberto Lopez-Anido

Second Committee Member

Eric N. Landis

Third Committee Member

Vincent Caccese

Abstract

Structural composite materials are commonly used in large scale marine construction. Composite materials offer significant material advantages over more traditional materials such as steel; however, the design and production of high strength structural joints in composite structures is particularly difficult. Due to various limitations, many joints must be fabricated using secondary bonds, where additional material is cured onto an existing laminate. Secondary bonds result in planes of weakness, and as a result, composite structures commonly fail at secondary bonded joints due to crack propagation at the bond line of the joint. Two types of joints are examined in this thesis, the doubler plate joint and the tee joint. The fatigue performance is investigated for the double joint, and the static fracture response is investigated for the tee joint. The crack propagation response of secondary bonded doubler plate joints in fiber-reinforced polymer (FRP) composite panels was investigated due to variable amplitude fatigue produced by vessel design spectra loads. The doubler plate joints were analyzed with respect to lifespan and failure criteria typically used for marine composites. The goal of the study is to characterize crack propagation in secondary bonded doubler plate joints under variable amplitude fatigue produced by design spectra loads for seaframes. The main contribution of the study to the marine industry is to improve current design methods for doubler plate joints in vessels under service conditions. Furthermore, the study serves to gain a better understanding of fatigue life prediction in secondary bonded joints for marine composites. Results have yielded insight into how crack propagation in secondary bonded doubler plate joints progresses under service conditions. A typical marine composite tee joint is investigated for fracture toughness. A 2D plane strain finite element model is used to predict failure using the virtual crack closure technique and fracture coupon data from a previous study. The numerical results of the model are considered with respect to the highly variable nature of the fracture toughness of woven fabric composite. Additionally, a detailed sensitivity study is conducted to determine the effect of nine geometric and material parameters on the strain energy release rate (SERR) and mixity of an assumed disbond at two likely crack locations. An innovative test method is used, such that the SERR and mixity can be set to the desired level by changing geometric parameters. The validity of the model is evaluated for deflections, strains, and SERR at failure. Additionally, the behavior of composite materials fabricated using woven fabrics are characterized for fracture toughness. Crack propagation behavior in woven fabric composites is investigated with respect to the periodic pattern produced as a result of the weave. Additionally, experimental methods for determining fracture toughness are investigated for woven fabric composites and a numerical technique to predict the location of crack onset is proposed. The ability to determine crack onset in any fracture test is critical to obtaining consistent and accurate results. Modifications to fracture toughness test methods are discussed. A case study encompassing is presented for a typical marine-grade E-glass fiber reinforced composite with a toughened vinyl ester resin matrix.

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