This chapter introduces the general concepts and information pertaining to composites. The composition and properties of the various components that make up a composite will be investigated as well as the various types of composites that are commonly used. This will then be followed by a review of the testing techniques and apparatus that can be used in testing the mechanical properties of composites.
2.2. Composite types and background
There is no universally accepted definition for composites but the following explanation will give an indication as to what composites are. Composite materials are the combination of two or more materials in such a way that certain improved or desired properties are achieved [3]. The materials or elements that a composite is made of work together to produce a material property that is different to the properties of the original materials on their own [4]. This combination of properties is called the principle of combined action which states that better property combinations can be achieved by the judicious combination of two or more distinct materials [5]. Composites are mainly composed of two phases, namely the matrix and the dispersed phase, the matrix is also known as the bulk phase and it is continuous and surrounds the dispersed phase. The dispersed phase is a reinforcement which is added to increase the strength and stiffness of the matrix and it is usually in fibre form. Matrix materials have limited use in the manufacture of structures on their own since their mechanical properties are not very high when compared to most metals [4]. They do however have desirable properties such as the ability to be easily formed into complex shapes. The properties of a composite are a function of the pr...
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...t not deform at all under load and as such an elastomer matrix might not be suitable for this application.
Man-made polymers are generally called ‘resins’ and can be classified under two types; thermoplastic and thermoset, according to the effect of heat on their properties. Thermoplastic materials contain polymer molecules that are held together by weak van der Waals forces or hydrogen bonds [3]. Thermoplastics soften when heated and will eventually melt but they can be hardened again by cooling the material. This process of heating and cooling can be performed many times without having an effect on the material properties and this can be desirable for certain applications. Some types of thermoplastics include ABS, nylon and polypropylene and the main type of dispersed phase used in the creation of composites using thermoplastics is short fibres such as glass [4].
Have you ever wondered why the plastic bag that you left on the porch during winter cracks or breaks more easily than when you left it during summer time but a piece of wood which was left just like the plastic bag has no effect whatsoever? This is because of a phenomenon, which only happens to polymers, known as the glass transition. For each polymer, there is a certain temperature at which the amorphous polymers undergo a second order phase transition from a rubbery and viscous amorphous solid to a brittle and glassy amorphous solid called the glass transition temperature, Tg.1 When the polymer, or in this case, the plastic bag, is cooled below their glass transition temperature, it becomes hard and brittle like a glass but when it is used above their glass transition temperatures, it might have a different effect than when used at room temperature or below the glass transition temperature as normally, different types of polymers like clothes, food packaging, insulations for wires, etc. are either used above their glass transition temperatures or
Chemically all polymeric materials comprise of hydro-carbon chains and usually have high heat of combustion. When they are exposed to fire, they burn rapidly with the release of high amount of heat, flame and smoke. Similarly the natural fibers, wood flour, are highly combustible and burn rapidly. So for safely use of the composites materials they must have flame retardant properties as well.
4 A. Paul, D.D. Jayaseelan, S. Venugopal, E. Zapata-Solvas, J. Binner, B. Vaidhyanathan, A. Heaton, P. Brown, W.E. Lee, “UHTC composites for hypersonic applications” American Ceramics Society Bulletin, 91 [1] 22–30 (2012).
How the polymer chains are put together is important, as it improves the properties of the material. The flexibility, strength and stiffness of Kevlar fiber, is dependent on the orientation of the polymer chains. Kevlar fiber is an arrangement of molecules, oriented parallel to each other. This orderly, untangled arrangement of molecules is described as a “Crystalline Structure”. A manufacturing process known as ‘Spinning’ is needed to achieve this crystallinity structure.
Despite the world being full of diverse people with varying accomplishments and skill sets, people oftentimes assume the qualities and traits of an individual based purely on the stereotypes set forth by society. Although these stereotypes are unavoidable, an individual can be liberated, empowered and ultimately overcome these stereotypes by obtaining an advanced education.
Hughes held Prime ministership of Australia for seven and a quarter years, from 1915-1923, one year prior and five years following World War One
It is said that Western civilization had been primarily male dominated and as a result as diminished the feminine. Women’s roles in society have changed drastically over the past decades. While it took much time, progress for women’s rights has blossomed. Influences in civilization have affected view points of the commonly held mores, expectations, and stereotypes which define gender.
Television has affected every aspect of life in society, radically changing the way individuals live and interact with the world. However, change is not always for the better, especially the influence of television on political campaigns towards presidency. Since the 1960s, presidential elections in the United States were greatly impacted by television, yet the impact has not been positive. Television allowed the public to have more access to information and gained reassurance to which candidate they chose to vote for. However, the media failed to recognize the importance of elections. Candidates became image based rather than issue based using a “celebrity system” to concern the public with subjects regarding debates (Hart and Trice). Due to “hyperfamiliarity” television turned numerous people away from being interested in debates between candidates (Hart and Trice). Although television had the ability to reach a greater number of people than it did before the Nixon/Kennedy debate, it shortened the attention span of the public, which made the overall process of elections unfair, due to the emphasis on image rather than issue.
...an be seen that composite teeth form a high stability bond than PMMA teeth. This is due to the filler content allowing for low shrinkage, increased wear resistance and better cross-linkage with the base. In overall consensus the technique of heat-curing is believed to achieve significantly more polymer cross-linkage than that of self-curing the acrylic resin PMMA base - giving us a stronger base to teeth interface. It should be noted however that both techniques can be used for denture fabrication to achieve a desired result and it is up to the dentist and the technician to determine which one they prefer however, composite teeth bonded to a heat-cured PMMA base works best. Though the tooth and base by themselves may be strong, if the interface between them is not strong, this will result in the overall denture produced being weak independent of material selection.
People have all different kinds of values, wants, needs, long-term goals, and short-term goals. Their differences come from many places such as religion or age. These differences are what makes each individuals values, wants, and needs make them unique to each person individually. My values, wants, needs, long-term, and short-term goals are all unique to myself.
In their articles, Maltzman and Sigelman, as well as Hill and Hurley, explored the connection between a representative speech giving in Washington as a means of representing their constituency. The advent and permeation of mass media and connectivity to political events into American society raises the question of whether or not representatives can use their media presence to represent their constituency? Social media and news access provide representatives with an outlet to express empathy, demographic identification, and policy platform. Has the ability of a representative to broadcast their symbolic representation and policy through mass media significantly change the representational strategies of modern representatives from the strategies of their predecessors? The opportunity to connect with a constituency through mass media could provide a representative with the privilege of representing from Washington without the need to connect at home to the same degree as their predecessors.
Dental composites are widely used for crown preparation and in-lays formation. These materials are similar to those used in direct fillings and are tooth-colored. Their strength and durability is not as high as porcelain or metal restorations and they are more
Polymers are large molecules that are formed of many smaller molecules joined together as sub units, known as monomers. They portray a huge role in society as they tend to make up most plastics used ranging from plastic shopping bags to styrofoam. Polymers can be found naturally an example being DNA the building blocks of life but most usable polymers are man made (American Chemistry Council, n.d). The polymers in plastics widely used by society today are known as homopolymers, as they are the product of two or more identical monomers that have undergone polymerisation. However, here is an alternative form of polymer that is not as widely used called copolymers. Copolymers are constructed when two or more
There are many attributes that affect the properties of a composite material, based on their intrinsic properties of the constituents. The properties that are harnessed to reinforce a composite material are essentially governed by the similar factors that affect other fiber composite materials, namely the fiber architecture and the fiber–matrix interface.
Since the Wright Flyer first took off from Kitty Hawk, North Carolina, in 1903, aircraft designers have been searching for ever better materials to build aircraft with. Over the years, we have seen construction materials progress from simple wood frames covered with fabric to advanced structures built entirely out of metal. As aircraft designs became more advanced, the need arose for materials which offered both higher strength and lighter weight. Since the beginning of World War II, aircraft construction consisted mainly of structures built from aluminum. Beginning in the 1960’s, NASA and the United States military began experimenting with the use of composite materials in aircraft. This revolutionary material seemed to be the answer the aviation world was looking for. It promised both gains in strength and weight reduction. However, with more and more composites being added to aircraft over the years, problems arose which ultimately may negate the overall benefits of the composites.