Investigation of 3D moisture diffusion coefficients and damage in a pultruded E-glass/polyester structural composite

As civil engineers seek to improve buildings and bridges, they are turning to composite materials for some structural components. Thus the long-term life of these materials in damp highly acidic conditions (from concrete pore solutions) is critical. This article presents a study of moisture ingression and damage in a pultruded compression element under exposure to pore solution. The goal was to experimentally find the orthotropic diffusion coefficients for pore solution in the composite material and to evaluate the damage in the composite using SEM. A method for calculating 3D diffusion coefficients based on weight measurements by selectively sealing some surfaces against moisture ingression was successfully employed. This research extends the application of the 3D diffusion solution developed by Pierron et al. to selectively sealed specimens of fixed dimension. The Arrhenius equation was then used to model the diffusion coefficients with respect to temperature. For the first time, sequential SEM images were performed in the same location before and after specimen exposure in an attempt to identify damage development separately from initial damage. These images showed little if any change in specimens exposed at room temperature over the first 19 days: however, characteristically different damage was notable at elevated temperature and for a specimen exposed for 650 days.

Methodology for evaluating new high volume composite manufacturing technologies

Simon Jespersen

Composite materials are increasingly used in high volume automotive applications, usually as a replacement for assemblies of multiple metallic parts such as stamped and spot-welded steel sheets. One of the motivations is to reduce tooling and assembly costs, in particular at lower production volumes. Other key driving forces are increased styling freedom, reduced system cost and weight reduction. With soaring fuel prices and environmental focus on CO2 emissions, reducing vehicle mass is presently a major concern for the automotive industry. Nearly all the composites used for high production volume automotive applications today are based on short or medium length random glass fibre technologies. However, there is a great interest in extending the range of materials and processes to advanced composites (continuous aligned fibre and high fibre volume fraction materials) as they would enable superior mass specific properties compared with metals, resulting in substantial weight savings. Advanced structural composites have, however, not seen widespread use in high volume automotive applications due to a series of inhibiting factors. These typically comprise higher cost and risk when compared with standard metal forming solutions. Novel composite technologies target some of the current drawbacks, but need to mature before being widely accepted in the high volume market. It is therefore necessary to not only evaluate and develop these technologies further, but also to propose practical tools to evaluate their performance and risk level, until they reach a state where their mechanical and economic performance clearly outweighs the risks associated with their implementation. The present work aims at developing a methodology to facilitate the evaluation of new composites manufacturing technologies, while at the same time decreasing the risks associated with implementation. A second objective is to identify suitable composite material and process technologies, which will offer competitive economic and performance gains. An evaluation methodology is thus proposed and validated with a practical case study based on the analysis of two novel structural composite processes for high volume automotive components. A spare wheel well was chosen as the demonstrator since it is a structural component and is assembled as a bolt-on part, making integration within the vehicle easier. Weight assumptions, which are used as performance criteria for all examined materials, have been obtained through extensive design and FEA studies for both strength and stiffness based criteria. Cost, discounted part price, investment and other figures of merit are used as economic performance criteria and have been established through novel technical cost evaluation techniques. The presented methodology manages uncertainties and evaluates performance, and is used as a basis for establishing relations between opportunities and risks. To achieve this, tools are introduced which support the evaluation methodology, notably: Risk based costing, including probabilistic distributions for input data to measure the quality of the results, Discounted technical economic modelling, including time value of money in the technical cost assessment to point out commercially focused effects of investments, discount rates and returns, Multi-objective optimisation, providing improved control with trade-off situations. For the case study, established as well as emerging processing technologies have been evaluated to serve as benchmarks for the study of two novel composite technologies. The analysis showed that the economic performance of currently used composite technologies is severely impeded by a high material cost in the final products and in some cases by long cycle times. It is also shown that the lower investment, when compared with metal stamping, can make such technologies favourable, especially for smaller series parts, despite the base material cost being higher. New technologies showed potential for improving the economic performance further; however, high discount rates due to risk impede commercialisation. Composites generally improved the weight performance compared with mild steel, especially for strength based designs. For the spare wheel well, the processing limitations, in particular the minimum achievable part thickness, were found to limit weight reduction substantially, giving unnecessary high properties. Thermoset materials showed more consistent weight reduction between low and high temperatures, followed by PA and then PP based composites. In conclusion, the technology assessment methodology presented here is intended for implementation in an industrial environment, where it will define common objectives facilitating better communication between engineering and management. This will in turn lead to an improvement in the efficient use of resources. In addition, it has been used to identify and develop supporting tools to aid assessment of technologies and to show techno-economic effects that are valuable in an industrial context. Finally, emerging composite technologies, where the base material constituents are combined together in a direct forming process, have been evaluated. The results have shown how the supply chain can be streamlined by avoiding for example, separate weaving and needling steps, improving the overall cost performance and providing the system supplier with improved leverage.

EPFL2008

Experimental investigation on temperature-dependent thermo- physical and mechanical properties of pultruded GFRP composites

Thomas Keller

The temperature-dependent thermo-physical and mechanical properties of a pultruded E-glass fiber-reinforced polyester (GFRP) composite are investigated in this paper. Fitting of theoretical models of the material properties to results of TGA, DSC, hot disk, and DMA experiments demonstrated good agreements. The constants for an Arrhenius representation of the decomposition mass-loss were determined using multi-curves methods. The effective specific heat capacity for the virgin material was found to increase during the decomposition process. A series model based on component volume fraction during decomposition provided an accurate description of the thermal conductivity as a function of temperature as measured by hot disk. Models based on the kinetic theory can describe the material degradation during glass transition as indicated by DMA results, while the parameters still need to be accurately identified. This paper provides a full set of temperature-dependent physical properties of a polyester matrix composite and demonstrates the applicability of theoretical models to represent the experimental results. © 2008 Elsevier B.V. All rights reserved.

2008

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

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

EPFL2009

Methodology for evaluating new high volume composite manufacturing technologies

Simon Jespersen

EPFL2008