Ch 9
9.26 Give reason for the development of ceramic matrix composites. Name some appliance and explain why they should be effective.
o 9.26 Ceramic matrix composites have been developed primarily to overcome the deficiencies of ceramic as a stand alone material. Namely, that fiber reinforcement gives ceramic a much needed boost in toughness. Ceramic was already superior for its high temperature strength and endurance capabilities. :o The major players in the use of ceramic composites to date have been the military and aerospace industries, but if the manufacturing cost were reduced, they could replace metals in some instances where weight reduction or higher overall temperature would enhance a products performance.
9.27 explain how you would go about determining the hardness measurements on these types of materials meaningful? Does the size of the indentation make any difference? Explain.
o 9.27 The hardness of plastic reinforced materials is hard to determine because it is hard to determine how much if any the reinforcement adds to a standard indention test. If the indenter is made larger, the test becomes more representative, but is really of little use even if it is known. o A more meaningful and reliable test for reinforcement plastics might be a durometer.
9.28 How would you go about trying to determine the strength of a fiber?
o 9.28 Finding the strength of an individual fiber would be difficult, and the results may be meaningless. One possible method would be to perform a tensile test on a fiber, but ensuring it failure in the test zone as opposed to at the clamp may be difficult. o Another possible way to test fiber strength would be to bundle a specific number of fibers into one thread, tensile testing the thread, and then dividing by the total number of fibers in the thread to get an average. This may provide a reasonable strength determining if enough tests were run to narrow the two sigma ranges.
9.29 Glass fibers are aisd to be much stronger than bulk glass. Why is this to?
o 9.29 Glass used in glass fiber production is a very high quality glass with fewer impurities. o When any material, especially a brittle material, is drawn into thin cross sections, the likelihood of flaws existing in that cross section are diminished dramatically compared to a part with a bigger cross section. Surface flaws, the normal point of propagation for cracks, are reduced for the same reasons. o When the glass is drawn during processing, it essentially is given a tensile strength test. Weaker fibers are automatically weeded out in the way because they break.
9.3 Name the renforcing fibers generally used to make composites. Which type of fiber is the strongest? Which type is the weakest?
o 9.3 Reinforcing fibers canbe made from a variety of materials. Typically, the fibers are made from glass, graphite, aramids, or boron. Using more than one of these materials forms a composite fiber known as a hybrid. o The strongest fibers are the aramids with a very high specific strength. On the other hand. Glass fibers are the weakest.
9.30 Describe situation in which a glass could be used as a matrix material.
o 9.30 Glass would make a terrible matrix material in a normal composite because it has no toughness, and would not stay in tact to allow the fibers to do what they are intended to do. o Glass, which used in structural applications, usually contains a wire mesh embedded in the glass which provides the tension strength of the finished product, while the glass itself becomes the structural support.
9.21 what limitations or disadvantages doe composite materials have? What suggestions would you make to overcome the limitations?
o 9-21 Composites do not hold up will in humid or wet environments. They absorb water, which affects the structural integrity and strength of the composite over time from de-bonding. This problem could be alleviated by applying a protecting coating to the composite, or carefully selecting the fibers and matrix materials used in the original composite. o Composites do not have consistent properties throughout, which affects stress distribution within the part. This problem can be addressed by randomly dispersing the reinforcing materials.
9.1 Distriguish between composites and metal alloys.
o 9.1 A composite material represents the fusion or combination of two different and insoluble materials. These materials remain distinguishable at the microscopic scale. Although each individual phase has its own properties, the properties and structural durability of the composite material is superior to each acting on its own. o Metallic alloy contains two or more metallic elements. By combination different elements, the properties of the alloy changes. Example alloys such as stainless steel, an alloy of iron, nickel, chromium and gold jewelry, gold and nickel, have vastly different properties.
9.11 what are the most common types of glass fibers.
o 9.11 as the name suggests, glass fibers is material comprised of fine fibers of glass. There are number of different types of glass fibers commonly used in industrial and commercial applications. The most common type of glass fiber is known as fiberglass, originally known as glass wool. Fiberglass is actually a fiber-reinforced plastic. o Other glass fibers include E-glass, used in electrical applications and glass reinforced concretes.
9.13 How can a graphite fiber be made electrically and thermally conductively?
o 9.13 Graphite fibers can be made thermally and electrically conductive through the composition of the graphite with other materials. This composite usually combines the graphite with a conductive material such as silicon carbide.
9.14 what is the whisker? What Is the difference between whisker and a fiber?
o 9.14 Whiskers are small single structure crystals between 1 and 10 micrometers in length. This small size makes whiskers less likely to contain significant imperfection that would impact their strength. o Fibrs, on the other hand, are often much longer than whiskers. This length makes any imperfection more likely to affect the mechanical and physical properties of the fibers.
9.15 Explain the composition of boron fibers, why are they heavy?
o 9.15 As their name suggests, boron fiber consist of boron. The boron elements are combined with tungsten or carbon fibers using a chemical vapor deposition technique. o Because tungsten is a high density element, boron are heavy. They additional processing required to produce them also negatively impacts their price.
9.16 how do you think the use of straw mixed with clay originally came about in making brick for dwellings?
o 9.16 Like many innovations, it was probably discovered by accident when straw and clay get mixed. Rather than throw the batch away, they were made into bricks, and masons soon after discovered their improved resistance to breaking apart.
9.17 what products have you personally seen that are made of reinforced plastics? How can you tell?
o 9.17 Visual observation is one easy way to tell if plastic has reinforcement or not. Visible strands or woven mesh usually visible. Also, reinforced plastic will weight more than regular plastic. o Garbage bags, rain tarps, Visqueen, Fiberglass used in boat hulls, and outdoor furniture are all example of reinforced plastic.
9.18 Describe applications that are not well suited for composite materials.
o 9.18 Because of the cost to manufacture composite, they are not well suited to replace any existing product that can be made of thermoplastic or even thin-gauge metal) unless weight is a consideration or the metal is exotic). o Composites are not practical in applications that require the formation of bulky parts.
9.19 Is there a difference between a composite material and a coated material? Explain.
o 9.19 Coating are used for aesthetic and protection reason on parts, and do not lend much in the way of additional strength or support to the finished part. o Composites can be not only aesthetic or protective, but are very good at supporting loads, along with many other abilities.
9.20 identify metals and alloys that have strengths comparable to those of reinforced plastics.
o 9.20 Many reinforced plastics are comparable in strength to aluminum including ABS, Nylon, Polyester and Acetal. Alloys of copper can be compared reasonably to reinforced nylon and polyester. Also, reinforced nylon and polyester also equal magnesium in strength. o In addition, reinforced epoxy is very strong, and compares very well with steel, nickel and titanium. Iron and reinforced nylon also have similar strengths.
9.22 Give an example of composite materials other than those stated in this chapter.
o 9.22 There are many natural and manufactured products that fit the category of composites. Naturally occurring materials include wood, aluminum, and silicon carbide which exists naturally and as a manufactured material. o Other manufactured materials that can be identified as composites are steel-belted radial tires and particle board o One product that is specifically manufactured as a composite is powdered metal parts that have been impregnated prior to processing with copper or molybdenum particles that act as embedded lubricant.
9.24 Explain why fibers are so capable of supporting a major portion of the load in composite materials.
o 9.24 fibers in composites are stiffer than the matrix in which they reside. Subject to the same load, the matrix yields, forcing the non-yielding fibers to carry most of the load.
9.25 Do metal matrix composites have any advantage over reinforced plastics? Explain.
o 9.25 The biggest advantage metal matrix composites have over their reinforced plastic counterparts is that they can withstand a much wider and higher range of temperature without degradation or loss of strength.
9.31 When the American plains states were settled, no trees existed for the construction of housing. Pioneers cut bricks from sod—basically, prairie sold as a matrix and grass and its root system as reinforcement.
o 9.31 Growing grass would exhibit the same characteristics as straw blended in brick. Which had been a successful practice thousands of years earlier by the Egyptians. o The orientation of the grass would strictly depend on the application. If the structural support was load in pure tension or compression, the stands would point in the same direction as the applied load. If the support was designed to carry wind load, the fibers would need to be oriented in the same direction as the bending moment.
9.32 By incorporating small amounts of blowing agent, it is possible to manufacture hollow polymer fibers with gas cores. List possible applications for such fibers.
o 9.32 When used in a composite, fibers with hollow cores would make the overall material lighter without significantly affecting its strength. o Since the hollow fibers would most likely be filled with air, the finished composite would act as an insulator since air is a poor conductor of heat. The composite could be made in thin sheets with good thermal isolation properties.
9.4 what is the range in length and diameter of typical reinforced fibers?
o 9.4 Fibers can be categorized by length. Short, or continuous, fibers are one in which the mechanical properties improve as the average length of the fibers increases. These fibers will range between 20 and 60. Long, or discontinuous, fibers will range between 200 and 500. o Regardless of length, the average diameter of fibers used within reinforced plastics is less than .01 mm.
9.5 List the important factors that determine the properties of reinforced plastics.
o 9.5 The properties of reinforced plastics are determined by those of the fiber reinforcing them. Special surface treatments consisting of coating and coupling agents improves adhesion at the interface. Silane can be used to improve between the fiber and the matrix. o In order to determine specific properties, i.e. permeability and dimensional stability, polymer plastics can be used.
9.6 comments on the advantages and linitations of metal matrix composites, reinforced plastics, and ceramic matrix composites.
o 9.6 Metal matrix composites usually have a higher density and good toughness than other composites. This contributes to a greater difficulty when processing parts made from this material. o Reinforced plastics are susceptible to flaws, making inspection and quality control a priority. This necessity often increases the cost of the overall product by as much as one fourth its manufacturing cost. o Ceramic matrix composites are formed using ceramics, which are often both strong and un-malleable. Despite this, they lack the toughness of other composite materials.
9.7 what are the most commonly used matrix materials? Why?
o 9.7 There are four main matrix materials. Polyester often contains polycarbonate. Polyimidies are composites of imide monomers. Often common matrix materals are expoy, or poly epoxide and silicon. o Similarly, some thermoplastics, such as poly ether ketone are also used as matrix materials.
9.9 what material properties are improved by the addition of reinforcing fibers?
o Reinforcing fibers can be made up a number of materials which improve the properties of materials. These fibers are often made up of compounds such as graphite, boron, aluminum oxide and silicon carbide. o The addition of these fibers to materials greatly improve the strength of the bonds, as well as the tensile strength of the material. The material also often has higher stiffness.
9.8 describe the advantages of hybrid composites over other composites.
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9.2 describe the functions of the matrix and the reinforcing fibers. What fundamental differences are there in the characteristics of the two materials.
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9.12 explain the difference between a carbon fiber and a graphite fiber.
o The principal difference between a carbon and a graphite fiber is in its composition. A carbon fiber will contain between 80 and 90 percent carbon, A graphite fiber, on the other hand are usually 99 percent carbon or higher. o The temperature of pyrolysis and material purity will play a significant role in determining which fiver has better performance.
9.10 Describe the purpose of the matrix material.
o There are three important purposes for matrix materials. First and foremost, the matrix materla is used to support the fibers and transfer some of the stresses off them. o The second important purpose of the matrix material is to prevent the change and spread of cracks in the composite material. This is possible due to the higher ductility and toughness of the plastic reinforcing matrix. o In general, a matrix material will have a high toughness. Despite this, their thermal resistance will be lower.