Methodology for evaluating new high volume composite manufacturing technologies

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.

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

Quantification of microdamage phenomena during tensile straining of high volume fraction particle reinforced aluminum

Marianna Kouzeli, Andreas Mortensen, Ludger Weber

Particle reinforced composites are produced by infiltrating ceramic particle beds with 99.99% Al. Resulting materials feature a relatively high volume fraction (40-55 vol. pet) of homogeneously distributed reinforcement. The evolution of damage during tensile straining of these composites is monitored using two indirect methods; namely by tracking changes in density and in Young's modulus. Identification and quantification of the active damage mechanisms is conducted on polished sections of failed tensile specimens: particle fracture and void formation in the matrix are the predominant damage micromechanisms in these materials. The damage parameter derived from the change in density at a given strain is found to be one to two orders of magnitude smaller than the parameter based on changes in Young's modulus. A simple micromechanical analysis inspired by the observed damage micromechanisms is used to correlate the two indirect measurements of damage. The predictions of this analysis are in good agreement with experiment. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. AII rights reserved.

2001

Fire performance evaluation of GFRP-balsa sandwich bridge decks

Niloufar Vahedi

Bridges are a critical component of national infrastructures and economies. A significant percentage of bridges built all around the world are made of reinforced concrete which according to building codes have a life span between 80-100 years. However, structural inspections demonstrate that deterioration can begin in as little as 10 years due to the limited durability of steel-reinforced concrete. Exposure to humidity, freeze-thaw cycles and de-icing salts during winter result in corrosion of the steel reinforcement and spalling of the surface concrete which subsequently lead to partial or full bridge closure for required repair. Maintenance, upgrading and replacement of bridges may have significant economic and social impact due to disturbances in traffic flow. Mentioned reasons motivated scientists and engineers to develop and use more durable alternative materials with lower life-cycle cost. Composite sandwich bridge decks composed of glass fiber-reinforced (GFRP) face sheets and a balsa wood core, which are subject of this projects, provide the possibility of a more durable, lightweight and easily installed alternative. Balsa wood is a lightweight wood species exhibiting high strength- and stiffness-to-weight ratios, and therefore it is a favorable core material in bridge decks. However, this new type of sandwich bridge decks has to meet the same requirements as traditional concrete decks do. In particular, their behavior during a fire incident has to be known and predictable. Although fire is an accidental action according to Eurocodes, it has to be taken into account in the design of bridges and should not lead to a premature collapse. The knowledge about the combined thermo-mechanical behavior during a fire of these new sandwich decks is therefore crucial, in particular since the polymer matrix of the GFRP material and the balsa core are sensitive to high temperatures and combustible.GFRP sandwich structures are also used in other high-risk fire applications, e.g. in the automotive, naval and aerospace fields and numerous works about their thermo-mechanical behavior have been performed. However, these results cannot be simply transferred to GFRP-balsa sandwich bridge decks which are subjected to much more severe loads and their life span is much longer. The aim of this project is thus to investigate the thermo-mechanical behavior of GFRP-Balsa sandwich bridge decks in order to overcome this barrier of insufficient knowledge.Three level of investigations are planned, a) on the constituent material level, b) on the sandwich panel level, and c) on the bridge deck and superstructure level. Experimental investigations and modeling are conducted on each level from small- to full-scale. Since the performance of GFRP laminates under elevated temperatures have been investigated by many researchers, only thermo-physical and thermo-mechanical properties of balsa wood are investigated by proper experiments. At the second level, thermo-mechanical sandwich response models are developed and validated by experiments. The final level is conducted in order to investigate the thermo-mechanical behavior of the bridge deck at the structural system level which includes adhesively-bonded joints between the individual balsa panels of the deck and between the deck and the bridge girders. Furthermore, post-fire recovery models are established in order to assess the remaining structural capacity after a fire incident.

EPFL2022

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

EPFL2009

Fire performance evaluation of GFRP-balsa sandwich bridge decks

Niloufar Vahedi

EPFL2022