Karbhari Durability of Composites for Civil Structural Applications

1. Auflage 2007
ISBN: 978-1-84569-356-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 384 Seiten

Reihe: Woodhead Publishing Series in Civil and Structural Engineering

Given the increasing use of fibre-reinforced polymer (FRP) composites in structural civil engineering, there is a vital need for critical information related to the overall durability and performance of these new materials under harsh and changing conditions. Durability of composites for civil and structural applications provides a thorough overview of key aspects of the durability of FRP composites for designers and practising engineers.Part one discusses general aspects of composite durability. Chapters examine mechanisms of degradation such as moisture, aqueous solutions, UV radiation, temperature, fatigue and wear. Part two then discusses ways of using FRP composites, including strengthening and rehabilitating existing structures with FRP composites, and monitoring techniques such as structural health monitoring.Durability of composites for civil and structural applications provides practising engineers, decision makers and students with a useful and fundamental guide to the use of FRP composites within civil and structural engineering. - Provides a thorough overview of key aspects of the durability of composites - Examines mechanisms of degradation such as aqueous solutions, moisture, fatigue and wear - Discusses ways of using FRP composites, including strengthening and rehabilitating existing structures

Autoren/Hrsg.

Karbhari, Vistasp M.

Leseproben

2 Fabrication, quality and service-life issues for composites in civil engineering
V.M. Karbhari    University of California San Diego, USA 2.1 Introduction
In the context of the present discussion, a composite material is defined as a macroscopic combination of two or more distinct materials having a finite interface between them. One of the constituents is the reinforcement, or reinforcing phase, while the other is the matrix phase. The major, or at least clearly apparent, difference between a material such as a plastic and a composite is thus that the composite consists of both reinforcement (fibers for example) and a matrix (which could be the polymer used to form the plastic itself). Composite materials are generically classified at two different levels. The first, and more generic, is related to the matrix phase. It is noted that the matrix serves a number of functions besides being the binder to hold the reinforcing phase together. It provides environmental and damage protection to the reinforcing phase, toughness and multi-functional non-mechanical properties to the composite and enables the material to be formed into shapes. The common types of composites are ceramic matrix composites (CMC), metal matrix composites (MMC) and polymer matrix composites (PMC). In the context of this book the latter class will be discussed and will be referred to as fiber-reinforced polymers (FRPs) following the commonly used terminology in civil construction. The second method of classification, shown schematically in Fig. 2.1, is based on the form of reinforcement used. For the reinforcing phase to provide a useful enhancement in the properties provided by the matrix phase alone a minimum fiber volume fraction, generally not lower than 10%, is required. This can, however, be in a variety of forms. Particulate reinforcements are those whose dimensions are all roughly equal. These are used for non-structural applications, and are often termed as ‘fillers’, such as for the enhancement of fire resistance, electro-magnetic shielding, thermal conductivity, fracture toughness, etc. In contrast, fiber reinforcement is a term used to denote a phase having one dimension substantially larger than the others. Discontinuous fiber reinforcements have low aspect ratios (ratio of length to diameter). Whiskers are extremely short, generally in the form of single crystals with almost no crystalline defects; their diameters usually fall in the range of 1–25 µm, and have aspect ratios less than 100. Short fibers are fibers with aspect ratios between 100 and 250 and are of the same material as used in continuous reinforcement. Continuous fiber-reinforced composites contain reinforcements having lengths much greater than their cross-sectional dimensions. Although the fiber length does not necessarily have to be comparable in dimension to the part being fabricated it is essential that the length is such that any further increase in length will not change properties such as modulus or strength. These reinforcements are usually used in the form of bundles called rovings and tows, and in the form of fabrics wherein a number of bundles are woven, knitted or braided in specific patterns. In some cases the reinforcement is specially formed using textile processes into a three-dimensional formwork. This allows the entire skeleton of reinforcement to be formed prior to the introduction of the resin and could be considered, albeit on a much smaller dimensional scale, as analogous to the steel reinforcement cages that are tied prior to pouring of concrete. 2.1 Classification based on form of reinforcing phase. The ability to combine the phases macroscopically provides immense opportunities for the tailoring of materials. This in fact enables the true creation of ‘materials by design’ since properties and performance can be designed through selection and proportion of constituent materials, orientation of the reinforcing phase and layup of different layers in a laminated structure. Thus, depending on the set of requirements it is possible to create a range of materials from those that are homogeneous and isotropic to those that are heterogenous and anisotropic, as well as all combinations in between. A composite, if conceptualized in the correct fashion is a designer’s dream, whereas in the hands of a novice it could well become a nightmare. Results from the aerospace industry are often used to argue that if FRP composites could be accepted for use in such high-precision applications with very low tolerance for failure, then their use in civil infrastructure (often considered ‘low-tech’ in comparison) should pose absolutely no problem. It should, however, be remembered that the use of FRP composites in aerospace applications has been predicated on extensive materials testing for the purposes of qualification followed by strict adherence to prescribed specifications for autoclave-based fabrication in highly controlled factory environments. These materials and processes are unlikely to find significant application in civil infrastructure due to cost and processing-specific aspects. Civil applications, currently, are more likely to: (a) use processes such as wet layup, pultrusion and resin infusion than autoclave molding; (b) use fiber and resin as separate constituents rather than in the form of preimpregnated material; (c) use resin systems such as polyesters, vinylesters, phenolics and lower temperature cure epoxies rather than the higher temperature curable epoxies and thermoplastics. Furthermore, in cases of rehabilitation, there is likely to be extensive use of processes under ambient conditions in the field, rather than fabrication in factory-controlled environments. Even in the case of prefabricated elements, adhesive bonding to substrates has to be conducted under field conditions with little control, if any, of humidity and/or temperature. Thus, the civil engineering environment not only brings with it new challenges for the control of quality and uniformity of FRP composites, but also makes it difficult (if not impossible) to use the well-established databases generated by DoD (Department of Defense)-sponsored research (such as those for commonly used carbon/epoxy systems designated as AS4/3501–6 or T300/ 5208) for more than comparative baseline and trend analysis purposes. Furthermore, aerospace-grade FRP composites are to a large degree extensively inspected at routine intervals within carefully controlled environments, whereas inspection and maintenance requirements in civil infrastructure are neither as regulated nor as thorough. In considering the durability of FRP materials, manufacturing methods for composite structures need to be considered as important as aspects of materials selection, configuration design and development, since the successful integration of fibrous reinforcement and matrix materials to create a composite is largely dependent on the processing method used. This is especially true with thermoset resin-based composites where the material is itself formed at the same time as the structure. The selection of a manufacturing process is, in general, much more critical for composites than for most conventional engineering materials. This is because each process is limited in the shapes and microstructures that can be created, as well as in the material combinations that can be utilized. As with more traditional materials, manufacturing processes for composites consist of a series of steps or stages, as shown in Fig. 2.2. Within each step there are a number of choices, including in some cases the possibility of skipping a step. Obviously, process economics and reliability are tied to the number of steps needed within a process to move from the raw materials stage to the finished product. 2.2 The materials transformation process. 2.2 Fabrication processes
The successful integration of fibrous reinforcement and matrix materials to create a composite is largely dependent on the choice of processing method. There are a large number of processing methods available and each process has specific attributes. In general, the fabrication scheme for any composite structure can be outlined using eight generic steps as outlined below. 1. Design – stress and geometric envelope. 2. Materials selection. 3. Arrangement (orientation and configuration) of the reinforcment. 4. Assembly of the reinforcement and resin system. 5. Application of heat and pressure as appropriate to cure the composite. 6. Finishing processes. 7. Assembly. 8. Quality control and non-destructive inspection. In an ideal scenario a process should be such that it has the attributes given below. 1. High productivity – i.e. short cycle times, low manpower requirements, low requirements for capital expenditure and minimum permanent use of space. 2. Minimal conversion cost – minimum cost spent on processing stages used to combine the fiber and matrix in order to form the composite. 3. Maximum tailorability – maximum ability to tailor the performance of the composite through materials and configurational choices. 4. Maximum geometrical flexibility – the...