Time dependence of material properties of FRP composites in fire

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

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

The Effect Of Processing Conditions On The Morphology, Mechanical And Electrical Properties Of Batio3-Polymer Composites

Sara Dalle Vacche, Yves Leterrier, Jan-Anders Månson, Véronique Michaud

Ferroelectric ceramic – polymer composites have a unique mix of electrical and mechanical properties. Piezoelectric and pyroelectric activity, a wide range of dielectric constants and high breakdown strength are combined with mechanical flexibility, formability and low acoustic impedance. Furthermore their properties may be tailored by the judicious choice of the polymeric matrix and ceramic filler, of their volume fraction, and of the type of connectivity, making these materials very attractive for applications as embedded capacitors, sensors and actuators. Composites with 0-3 connectivity, i.e. a continuous polymer matrix filled with a ceramic phase discontinuous in all directions, are particularly interesting because of their relative ease of fabrication and processing. The functionality of these composite materials is mostly due to the dielectric, ferroelectric and piezoelectric properties of the ceramic phase, which therefore often needs to constitute more than 50% in volume. On the other hand the possibility of integrating these composites as embedded devices in host structures, which are often polymer based, or using them as efficient actuators depends greatly on their thermomechanical properties. Most of the studies on ferroelectric ceramic-polymer composites, however, focus on their electrical properties, and relatively few systematic studies on their thermal and mechanical behaviour have been published so far. Furthermore the ceramic volume fractions considered are often much lower than those used in most applications [1, 2], studies of highly filled composites mainly being limited to lead-based ferroelectric materials [3]. In the present study 0-3 BaTiO3 – polymer composites were prepared, containing up to 60% volume fraction of ceramic phase. BaTiO3 was chosen as the ceramic filler as it is a common piezoelectric material with high dielectric constant, having the additional benefit from an environmental point of view of being lead-free. For the matrix, a low viscosity epoxy resin and a polyvinylidene fluoride based polymer were chosen. The first is a thermoset resin commonly used for several applications, and the second is a thermoplastic semicrystalline polymer which displays piezoelectric activity. A common route for achieving very high loading of ceramic filler is the use of solvents to reduce the viscosity during processing. However, as the materials prepared via the solvent route were found to have poor mechanical properties, attributed to the presence of residual solvent, most of the effort focused on obtaining high ceramic volume fractions via a solvent free process. The thermomechanical, dielectric, ferroelectric and piezoelectric properties of the obtained composites were investigated, and discussed in the light of the properties of the basic components, the processing route and the resulting morphology. X-ray diffraction was carried out to determine the crystal structure of the BaTiO3 in the pure powder and in the composites, confirming that processing did not alter the crystallographic structure of the ceramic phase. Scanning electron microscope observation of fracture surfaces of the composites evidenced that a satisfactorily homogeneous dispersion of BaTiO3 in the matrix could be obtained, although aggregates with diameter of some micrometers and some porosity were detected. The thermal and mechanical behaviour of the composites was studied by differential scanning calorimetry and dynamic mechanical analysis. The glass transition temperature was found not to significantly depend on the amount of filler, while both the storage and loss moduli increased upon addition of BaTiO3. Furthermore, all the composites displayed a region of linear stress-strain behaviour smaller than the pure matrices. Finally, experimental results also evidenced the possibility to obtain low leakage current and high breakdown voltage.

2010

Durability of structural adhesives in bridges

Omar Moussa

Besides the 50s revolution of the use of adhesives in the new construction in the works of strengthening and repair of different structural elements, adhesives were extensively used in bridges for a long time. Different applications on bridges were carried out in the last few decades. The most common application of adhesives in bridge engineering was in the form of resin mortar for either bridge bearings or expansion joint nosings. Recently more applications of structural, semi-structural and non structural joints could be executed using adhesives in bridges. In reason of its ease of execution and maintenance as well as its long-term durability, resins represent highly challenging materials in last few decades for the means of formulating bridge connections. Although adhesives are recently used on wide ranges in joints in bridge engineering, their complicated behaviour, particularly under low temperatures, could disallow an easier use of such materials in structures, notably in bridges. As the following project targets to provide a better understanding of thermo-mechanical behaviour of resins, two different categories of testing were carried out, in order to investigate the most important and basic characteristics of resins as well as direct relations connecting different properties. The first series of tests was focusing on the physical characterization of resins aiming to investigate the influence of the degree of cure (conversion degree) on different characteristics as well as the behaviour of the material, From previous experiences on the behaviour of resins, it was expected that the more cured the material, the better properties it could present, however this investigation was held in LTC lab using a DSC machine. Different categories of tests were carried out during the physical characterization in order to establish a reliable relation between conversion degree/conversion rate and time. Another effective resin property that was considered during the physical characterization of resins is the glass transition temperature (Tg); Tg which represents the temperature at which the material loses part of its strength and/or stiffness, can also involve another concern which is the degradation of adhesives when exposed to high temperatures. The second series of testing involved in this project is mechanical testing, which was held in Lab of structures. This series was aiming to investigate the mechanical properties of the resin, basically compressive, tensile and shear strengths of these materials. Finally this project was focusing on relating the physical properties including mainly the degree of cure as well as the glass transition temperature to the mechanical properties and their influence on the mechanical behaviour of adhesives.

2008