Modeling of strength degradation for fiber-reinforced polymer composites in fire

A model for predicting composite material strength degradation under elevated and high temperatures is proposed. This model is based on the morphology of the mixture of materials in different states. The degradation of resin-dominated shear strength can be well described by the rule of mixture while the degradation of nominal compressive strength tends to follow the lower bound of strength defined by the inverse rule of mixture. Composite materials under tension may exhibit fiber- or resin- dominated behavior. In a lower temperature range, strength is dominated by the fiber tensile strength, while at higher temperatures, tensile components may exhibit resin- dominated failure in joint regions. The parameters required in the model can be obtained on the basis of kinetic analysis of dynamic mechanical analysis results. The fitting of experimental curves of material strength degradation is not necessary. The proposed modeling scheme can easily be incorporated into structural theory to predict mechanical responses and time-to-failure. © 2009 SAGE Publications

Material and structural performance of fiber-reinforced polymer composites at elevated and high temperatures

As the range of applications for fiber-reinforced polymer (FRP) composite materials in civil engineering constantly increases, there is more and more concern with regard to their performance in critical environments. The fire behavior of composite materials is especially important since complex physical and chemical processes such as the glass transition and decomposition occur when these materials are subjected to elevated and high temperatures, possibly leading to considerable loss of stiffness and strength. This stiffness and strength degradation in composite materials under elevated and high temperatures is the result of changes in polymer molecular structures. When polyester thermosets are subjected to elevated and high temperatures, they undergo three transitions (glass transition, leathery-to-rubbery transition, and rubbery-to-decomposed transition), corresponding to four different states (glassy, leathery, rubbery and decomposed). At a certain temperature, a composite material can therefore be considered as a mixture of materials that are in different states. As the content of each state varies with temperature, the composite material exhibits temperature-dependent properties. Since these changes in state can be described using kinetic theory, the quantity of material in each state can be estimated and the thermophysical and thermomechanical properties of the mixture can thus be determined. These concepts formed a basis for the development of thermophysical and thermomechanical property sub-models for composites at elevated and high temperatures and even for the description of post-fire status. Incorporating these thermophysical property sub-models into a heat transfer governing equation, thermal responses were calculated using a finite difference method. Integrating the thermomechanical property sub-models within structural theory, the mechanical responses were described using a finite element method and the time-to-failure was also predicted by defining a failure criterion. The modeling results for temperature responses, mechanical responses and post-fire behavior were compared with those obtained from structural endurance experiments on full-scale cellular GFRP (glass fiber-reinforced polymer, in this case polyester resin) panels subjected to a four-point bending configuration and fire from one side. The modeling results for time-to-failure were compared with those from the experiments carried out on GFRP tubes under combined compressive and thermal loadings. In each experimental setup, two different thermal boundary conditions were investigated – with and without water cooling through specimen cells – and good agreement was found. The understanding gained and modeling of the behavior of GFRP composites under elevated and high temperatures carried out in this thesis could be applicable for different composite materials, and also benefit investigations regarding both active and passive fire protection techniques in order to improve the fire resistance of structures made of such materials.

EPFL2009

Time dependence of material properties of FRP composites in fire

Thomas Keller

Temperature dependence of physical and mechanical properties is considered in most thermal and mechanical response models for composite materials. Based on thermogravimetry analysis, differential scanning calorimetry, and dynamic mechanical analysis conducted at different heating rates, it is demonstrated that thermophysical and thermomechanical properties are not univariate functions of temperature, but also functions of time. Temperature- and time-dependent models for physical and mechanical properties at elevated and high temperatures are proposed, which show good accordance with experimental results. Based on a finite difference method, complex realistic thermal loading programs such as the ISO fire curve can be taken into account. © 2009 SAGE Publications

2009

Stochastic analysis of clear timber as a structural material

Alireza Farajzadeh Moshtaghin

Wood/timber has been widely used for house and bridge construction. It is a widely available natural material that necessitates low energy for the production, following simple processes. The environmentally friendliness, together with the low cost of raw material makes it an efficient building material. Moreover, timber possesses attractive mechanical properties such as high specific strength and stiffness. In contrast, timber constructions have, to a large extent, been based on experience and craftsmanship, which prevents taking full advantage of this material. There are several reasons for this. Timber has a complex mechanical behavior being a natural highly anisotropic fiber composite, with properties that are also affected by moisture content. For specific species, geographical location, local growth conditions and moisture content, the material properties depend, among others, on the age, the structural imperfections, the location of timber within the tree, and load history. Consequently, the mechanical properties of timber are, inherently, highly variable. Variability of timber properties includes statistical and spatial variabilities, referred to as random spatial variability (RSV). This entails adopting a probabilistic/stochastic approach to analysis of timer structures. The aim of this research is to understand and model the effect of the RSV on the clear timber mechanical properties, as well as the experimental characterization of RSV for clear timber, and also to develop a stochastic finite element framework for random response assessment of clear timber components. A size effect model was developed which takes into account the RSV in the strength field. The theory of random fields was used for this purpose. Using the spectral representation scheme, realizations of strength field in each specimen were generated. The stochastic response was obtained via the Monte Carlo method. The model results was compared to the existing experimental data in the literature. Also, an analytical expression was provided to facilitate the application of the model. Clear timber specimens of different lengths were fabricated for longitudinal tensile tests. Local deformations along the lengths of the specimens were recorded during the tests in order to characterize the RSV in longitudinal properties. A connection between the mesostructure of the clear wood and its local elastic modulus was observed. Statistics concerning the elastic modulus, strength and strain to failure and the effect of length change on these properties were extracted. The correlations between the strength, the elasticity and the density were obtained. Transverse properties were also investigated which are of particular importance in some applications such as mechanical and adhesively-bonded timber joints. Regularly positioned and randomly positioned specimens were cut from different timber boards. Statistics and size effects concerning the elastic modulus, strength and strain to failure as well as the correlation between the properties were studied. The spatial variability in the transverse elastic modulus, the tensile strength and the failure strain was also experimentally studied. Mesostructural patterns of clear timber were shown to have a direct effect on the local elastic modulus. Finally, a stochastic finite element framework was established by combining the spectral representation scheme for RSV modelling and the finite element software ABAQUS in a non-intrusive manner. This framework can be used for the stochastic structural response assessment of timber structural components made of clear timber. To show the applicability of the model in real applications, the failure of adhesively bonded double-lap timber joints were simulated under tensile loading. The effect of size on the strength was also taken into account. The results were in a fairly well agreement with the available experimental data in the literature.

EPFL2016

Material and structural performance of fiber-reinforced polymer composites at elevated and high temperatures

EPFL2009

Stochastic analysis of clear timber as a structural material

Alireza Farajzadeh Moshtaghin

EPFL2016