Thermal Responses of FRP Composites

Different temperature-dependent thermophysical property models were introduced by Henderson et al. in 1985 [8,9]. The concept of an effective material property was once again discussed, although not used, because the various phenomena were 

High Temperature Performance of Polymer Composites, First Edition. Yu Bai and Thomas Keller. 

Different thermophysical property models were developed and introduced in Chapter 4. Furthermore, full-scale experimental comparative studies were conducted on cellular beams and columns of glass fiber-reinforced polymer (GFRP) composites, especially for civil engineering apphcations [21, 22]. The experimental procedures and results will be introduced in this chapter and the thermophysical property models from Chapter 4 will be assembled in the final governing equation to predict the thermal responses. The results obtained from the mathematical models will be compared to experimental results in this chapter. 

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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. 

When exposed to high temperatures and fire, FRP composites experience complex changes in material states involving the interaction of thermal, chemical, physical, mechanical, and structural phenomena. Modeling and predicting aU the coupled responses of FRP stmctures is therefore a complex task. By treating independently only one or two of these phenomena in each model, however, the task becomes more reasonable. The thermal phenomena (heat transfer, temperature distribution, etc.) are mainly determined by the thermophysical properties of the material and the thermal boundary conditions, while the mechanical and stmctural phenomena... 

The mechanical responses (stress, strain, displacement, and strength) of fiber-reinforced polymer (FRP) composites under elevated and high temperatures are affected significantly by their thermal exposure. On the other hand, mechanical responses have almost no influence on the thermal responses of these materials. As a result, the mechanical and thermal responses can be decoupled. This can be done by, in a first step, estimating the thermal responses (as introduced in Chapter 6) and then, based on the modeHng of temperature-dependent mechanical properties, predicting the mechanical responses of the FRP composites. 

In this chapter, the post-fire behavior of FRP composites was evaluated and modeled on the stmctural level. Results from the models compared well with results from fuU-scale post-fire experiments on cellular GFRP beam and column specimens that had been subjected to mechanical and thermal loading up to 120 min with inclusion of different thermal boundary conditions. On the basis of the previously proposed thermal and mechanical response models, existing approaches for post-fire evaluation can be applied. Predicted temperature profiles and the conversion degrees of decomposition can be used to estimate the post-fire stiHhess from existing two- and three-layer models. The borders between different layers can be determined either by a temperature criterion or a RRC criterion. 

Temperature-dependant material property models were implemented into stmc-tural theory to establish a mechanical response model for FRP composites under elevated temperatures and fire in this chapter. On the basis of the finite difference method, the modeling mechanical responses were calculated and further vaUdated through experimental results obtained from the exposure of full-scale FRP beam and column elements to mechanical loading and fire for up to 2 h. Because of the revealed vulnerabihty of thermal exposed FRP components in compression, compact and slender specimens were further examined and their mechanical responses and time-to-failure were well predicted by the proposed models. 

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