Mechanical Properties of FRP Composites

The mechanical properties of FRP composites depend on the type of fibre nsed in their production and the fibre content in the final product. These aspects are likely to vary between competing composite products since there is currently no agreed standard specification for their production. Therefore, all design must be based on the actnal properties supplied by the manufacturer and laboratory test results. 

In a typical FRP composite material used in structural applications, approximately 30 to 50% of the cross-section is the resin binder. However, the strength of the FRP reinforcement is mainly determined by the fibre content since the resin matrix contributes very little to the overall strength. Since the fibres make up only a portion of the composite section, the FRP strength is determined by the fibre content present in the section. 

The most relevant mechanical properties of FRP for structural applications are the tensile strength, modulus of elasticity and the ultimate elongation at failure. Some representative values compiled from the literature are given in Table 5.2 for high performance long fibres suitable for structural applications. 

Classification of fibre Tensile strength (MPa) Modulus of elasticity (GPa) Ultimate elongation (%) 

This brittle behaviour of FRP composites has some important consequences in structural applications. First, it may limit the desirable ductile behaviour of RC members strengthened with FRP composites. Secondly, the redistribution of stresses is restricted due to this lack of ductility. Therefore, the design of structures bonded with FRP composites cannot follow the existing methods for RC structures by simply considering 

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The literature published on fracture mechanics testing of FRPs in the last 40 years comprises a large database on delamination resistance or fracture toughness of different types of FRPs. An early review [51] compiled the data available at that time. Selected data from quasi-static mode I and mode II tests on FRPs were compared by O Brien [52], and quasi-static mode I test data from carbon—fibre epoxy and poly-ether-ether-ketone (PEEK) by Brunner [53]. Mechanical properties of FRP composites are compiled in the Composite Materials Handbook version F (2002) [9—11], but this does not comprise fracture mechanics data. Hence, there is no comprehensive and up-to-date database on the available data or literature. 

This concept is applied in Chapters 4 and 5 that describe the temperature-dependent thermophysical and mechanical properties of FRP composite materials subjected to elevated temperature and fire. In Chapter 3, however, the estimation of the effective properties of a material mixture through a distribution function of its individual components (in different material states) is introduced first. 

FRP composites under elevated temperature and fire mostly correspond to a mixture of two material states, because - although three material states can be found in the full temperature range - only two states exist at a certain time. The thermophysical and mechanical properties of FRP composites under elevated temperature and fire estimated by the rule and inverse rule of mixture will be presented in Chapters 4 and 5 respectively. 

In this chapter, existing models for describing the change of mechanical properties of FRP composites under elevated temperatures and fire have been reviewed. On the basis of a kinetic description of the involved physical and chemical processes, the modeling approach developed in Chapters 2-4 has been further extended to predict the degradation of mechanical properties. Those mechanical properties... 

Table 6.6 Mechanical properties of FRP composites based on difunctional and multifunctional epoxies [40, 65, 71] ...

The mechanical properties of FRP composites are dependent upon the ratio of fibre and matrix material, the mechanical properties of the constituent materials, the fibre orientation in the matrix, and ultimately the processing and methods of fabrication, which are the subject of Part II. Chapters discuss prepreg processing, liquid composite moulding (LCM), filament winding processes and pultrusion of advanced fibre-reinforced polymer (FRP) composites. 

In this book, it is intended to provide the reader with useful and comprehensive experimental data and models for the design and application of FRP composites at elevated temperatures and fire conditions. The progressive changes that occur in material states and the corresponding progressive changes in the thermophysical and thermomechanical properties of FRP composites due to thermal exposure will be discussed. It will be demonstrated how thermophysical and thermomechanical properties can be incorporated into heat transfer theory and structural theory. The thermal and mechanical responses of FRP composites and structures subjected to hours of reahstic fire conditions will be described and validated on the full-scale structural level. Concepts and methods to determine the time-to-failure of polymer composites and structures in fire will be presented, as well as the post-fire behavior and fire protection techniques. 

An further alternative approach being developed worldwide is to replace the steel completely by fibre-reinforced plastics (FRP), which consist of continuous fibres as carbon, glass or aramid, set in a suitable resin to form a composite rod or grid. These materials have high tensile strength, low density and are non-magnetic they can be used both for new structures and for repair of existing ones. The mechanical properties of FRP are determined by the amount and type of fibre, while the durability will be a function of both the resin and the fibre. 

The mechanical, thermal, and hygrothermal properties of FRP composites are a function of selected constituent materials, namely fiber and matrix, and the fiber, matrix, and void volume fractions that are a result of the manufacturing process. In this analysis, the influence of fiber volume fraction on the failure probability of FRP-rehabilitated piping components is also examined. Procedures for micromechanical and macromechanical analysis of lamina (i.e., determination of lamina elastic moduli and strength properties) from basic constituent properties are well established and readily available in standard texts (Kaw, 2006). Table 5.2 summarizes the constituent properties of carbon fiber, glass fiber, and epoxy matrix utihzed in the reliability analyses presented in this chapter. These standard properties were obtained from Kaw (2006). 

The U.S. Navy has sponsored research at Virginia Tech to look at structural performance of FRP for the previously mentioned deck applications. This included work on determining the postftre structural properties. This work was reported in detail in the doctorate dissertation of Steven Boyd in 1996.27 Gibson et al.28 did work on modeling the residual mechanical properties of composites after fire. There have been recent studies on the mechanical properties of composites after fire,29-31 some of which have been cited previously in this chapter.1314 The requirements for the Navy applications have been described by Sorathia.19 20 32... 

Tranj-laminar fracture of composites with a certain amount of fibres in the throughthickness direction will lead to fibre breaking with significant effects on delamination resistance (see e.g. Refs [24,37] for details). Woven fibre mats, 3D fibre performs or additional 3D reinforcement (pins and stitching) of FRP composites with fibres ahgned in one plane (see e.g. Refs [84,85]) have been developed and can be tested for their fracture mechanics properties. The typical approach for testing these is to apply a standard test method developed for unidirectionally reinforced FRP composites and to assess the difference in delamination resistance compared to the standard laminate. So far, that approach has yielded (nominal) numbers, but their interpretation is not... 

Kinetic theory was formulated to model the conversion degree of a material from one state to another. At each temperature, a FRP material can be considered as a mixture of materials in different states, with changing mechanical properties. The content of each state varies with temperature, thus the composite material shows temperature-dependent properties. If the quantity of material in each state is known and a probabilistic distribution function accounting the contribution from each material state to the effective properties of the mixture is available, the mechanical properties of the mixture can be estimated over the whole temperature range. 

Mouritz, A.P. (2003) Simple models for determining the mechanical properties of burnt FRP composites. Mater. Sci. Eng., A359, 237-46. 

Where, in addition to blast resistance, the FRP composite must also have an external fire rating, supplementary means of augmenting the blast resistance properties or the fire retardant properties will need to be provided, as the mechanical properties of fire retardant gel coats and lay up resins are lower than those of normal resins because of the inert content required to provide the fire-retardant qualities. 

The use of FRP is convenient also in older metal constmctions, since the mechanical properties of composite materials are well suited to the stmctural features of buildings employing cast iron. The exceptional tensile strength of the fibers makes up for the low resistance of the cast iron and the corrosion resistance of both materials makes the intervention durable. The traditional techniques that rely on welding are unfavorable because fliey require flie complete disassembly of the work, along with an inevitable increase of costs and time consumption. It demonstrated, moreover, the effectiveness of flie application of FRP in improving the brittle behavior of cast iron. 

The performance of FRP pultraded composites greatly depends on the characteristics of their eonstituent materials (particularly the mechanical properties of the fibres and the matrix), on the composition and arrangement of the reinforcing fibres (i.e. their type, length, orientation and architecture) and also on the interaction between the fibres and the matrix, i.e. the fibre-matrix bonding. 

In terms of eonstituent materials, new types and forms of fibre reinforcements are likely to be introduced. In relation to this it is worth mentioning basalt fibres as an emerging alternative to the reinforcement of FRP pultruded composites. Although the manufacturing cost of basalt fibres currently exceeds that of E-glass fibres (Ross, 2006), the mechanical properties of... 

The mechanical properties of both polymeric and hbrous components in FRP strengthening systems play an essential role in determining the load transfer mechanism, and thus the initiation and evolution of the critical stress variants responsible for controlling the failure pattern and its location within the composite joint, as detailed in succeeding sections of this chapter. 

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