1.0Introduction So far we have studied mechanical properties of metals at room temperature and we assumed rightly so that they are independent of time. If we apply constant elastic stress on a metal specimen at room temperature, the elastic deformation is calculated as;e= /Where E is the elastic modulus, is the applied stress, and e is the elastic stress. Since the elastic modulus is constant, the elastic strain is a function only of the stress. If we repeat the same test for a metal at a high temperature the metal will immediately deform elastically and then continue to deform at a constant slow rate for a period of time before it increases rapidly until fracture. The time dependent deformation under constant load at high temperatures is called creep and the resulting strain is a function of the applied stress, temperature, and time. The temperature at which a material starts to creep depends on its melting point. It is found that creep in metals starts when the temperature is > 0.3 to 0.4 Tm (the melting temperature in Kelvin). Most metals have high melting points and hence they start to creep only at temperatures much above room temperature. This is the reason why creep is less familiar phenomena than elastic or plastic deformation.1.1Ceramic MaterialCeramic materialsareinorganic,non-metallicmaterials made from compounds of a metal and a non-metal. Ceramic materials may becrystallineor partly crystalline. They are formed by the action of heat and subsequent cooling.Claywas one of the earliest materials used to produceceramics, aspottery, but many different ceramic materials are now used in domestic, industrial and building products. Ceramic materials tend to be strong, stiff, brittle, chemically inert, and non-conductors of heat and electricity, but their properties vary widely.1.2Type of Ceramic MaterialAceramicmaterial may be defined as any inorganic crystalline material, compounded of a metal and a non-metal. It is solid and inert. Ceramic materials are brittle, hard, and strong in compression, weak in shearing and tension. They withstand chemical erosion that occurs in an acidic or caustic environment. In many cases withstanding erosion from the acid and bases applied to it. Ceramics generally can withstand very high temperatures such as temperatures that range from 1,000 C to 1,600 C. Exceptions include inorganic materials that do not have oxygen such assilicon carbide. Glass by definition is not a ceramic because it is an amorphous solid or non-crystalline. However, glass involves several steps of the ceramic process and its mechanical properties behave similarly to ceramic materials.1.2.1Crystalline CeramicCrystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories either to makes the ceramic in the desired shape, by reaction in situ or by "forming" powders into the desired shape, and thensinteringto form a solid body.Ceramic forming techniquesinclude shaping by hand known as throwing,slip casting,tape casting, injection molding, dry pressing, and other variations.1.2.2Non-Crystalline CeramicNon-crystalline ceramics are being glasses and tend to be formed from melts. The glass is shaped when either fully molten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing to a mold. If later heat-treatments cause this glass to become partly crystalline, the resulting material is known as aglass-ceramic.1.3Properties of CeramicThe physical properties of any ceramic substance are a direct result of its crystalline structure and chemical composition.Solid state chemistryreveals the fundamental connection between microstructure and properties such as localized density variations, grain size distribution, type of porosity and second-phase content, which can all be correlated with ceramic properties such as mechanical stress strength by the Hall-Petch equation,hardness,toughness,dielectric constant, and theopticalproperties exhibited by transparent materials.Physical properties of chemical compounds which provide evidence of chemical composition include odor, color, volume, density, melting point, boiling point, heat capacity, physical form at room temperature, hardness, porosity, and index of refraction.

1.3.1Mechanical PropertiesCeramic materials are usuallyionicorcovalentbonded materials, and can be crystalline or amorphous. A material held together by either type of bond will tend tofracturebefore anyplastic deformationtakes place, which results in poortoughnessin these materials. Additionally, because these materials tend to be porous, theporesand other microscopic imperfections act asstress concentrators, decreasing the toughness further, and reducing thetensile strength. These combine to givecatastrophic failures, as opposed to the normally much more gentlefailure modesof metals.These materials do showplastic deformation. However, due to the rigid structure of the crystalline materials, there are very few available slip systemsfordislocationsto move, and so they deform very slowly. With the non-crystalline (glassy) materials,viscousflow is the dominant source of plastic deformation, and is also very slow. It is therefore neglected in many applications of ceramic materials.1.3.2Electrical PropertiesSome ceramics aresemiconductors. Most of these aretransition metal oxidesthat are II-VI semiconductors, such aszinc oxide. While there are prospects of mass-producing blueLEDsfrom zinc oxide, ceramicists are most interested in the electrical properties that showgrain boundaryeffects. The best demonstration of their ability can be found inelectrical substations, where they are employed to protect the infrastructure fromlightningstrikes. They have rapid response, are low maintenance, and do not appreciably degrade from use, making them virtually ideal devices for this application.Under some conditions, such as extremely low temperature, some ceramics exhibithigh temperature superconductivity. The exact reason for this is not known, but there are two major families of superconducting ceramics.1.3.3Optical PropertiesOptically transparent materialsfocus on the response of a material to incoming light waves of a range of wavelengths.Frequency selective optical filterscan be utilized to alter or enhance the brightness and contrast of a digital image. Guided light wave transmission via frequency selective waveguidesinvolves the emerging field of fiberopticsand the ability of certain glassy compositions as atransmission mediumfor a range of frequencies simultaneously with little or nointerferencebetween competingwavelengthsor frequencies. Thisresonant modeofenergyanddata transmissionvia electromagneticwave propagation, though low powered, is virtually lossless. Optical waveguides are used as components inintegrated optical circuitsor as the transmission medium in local and long hauloptical communicationsystems. Also of value to the emerging materials scientist is the sensitivity of materials to radiation in the thermalinfrared(IR) portion of the electromagnetic spectrum. This heat-seeking ability is responsible for such diverse optical phenomena asNight-visionand IRluminescence.2.0Introduction to CreepCreep is the permanent elongation of a component under a static load maintained for a period of time. This phenomenon occurs in metals and certain nonmetallic materials, such as thermoplastics, rubbers and ceramic, and it can occur at any temperature. Advanced engineering ceramics have a number of material properties that have made them one of the most important classes of engineering materials. Ceramics have an extremely high elastic modulus, maintain consistent performance at elevated temperatures, and have great resistance to wear and corrosion, which has contributed to their widespread use as bearing surfaces, heat resistance, and insulation applications. The ability for ceramics to perform at high temperature has made them the go to material for high end automobile brake rotors and pads, space re-entry vehicle heat shields, and ball bearings in high speed and high temperature applications. The use of fiber reinforcement with a ceramic matrix provides an increase in tensile strength and fracture resistance, making ceramics a viable material for structural applications. The system shown is designed to produce controlled temperatures to 3100F and the ability to perform creep and modulus of rupture tests on ceramic materials.2.1Creep TestingCreep testing aims to investigate plastic deformation of a material when subjected to a constant load or stress at a high temperature. High temperature allows metal to deform more easily since atoms can move more readily. Generally, metals creep at a temperature above approximately 0.4 Tm (Tm is the absolute temperature of the metal). Therefore, low melting point metals will creep at lower temperature in comparison to high melting point metals. Hence, greater movement of dislocations or slips can happen. New slip systems and grain-boundary movement are also possible at higher temperatures. Therefore, engineering alloys utilized at high temperatures is susceptible to creep as well as recrystallization and grain coarsening. In the case of age-hardened metals, over-ageing is feasible, which results in reduced hardness and strength due to the coarsening of the second phase precipitates. 2.2Creep-Testing MachineAcreep-testing machinemeasures the tendency of a material after being subjected to high levels of stress such as high temperatures, to change its form in relation to time or known as creep of an object. It is a device that measures the alteration of a material after it has been put through different forms of stress.

Figure 1.0: Creep testing configuration showing specimen fitted in the testing machine coupled with a high temperature furnace.Creep machines are important to see how much strain or load an object can handle under pressure, so engineers and researchers are able to determine what materials to use. The device generates a creep time-dependent curve by calculating the steady rate of creep in reference to the time it takes for the material to change. Creep machines are primarily used by engineers to determine the stability of a material and its behavior when it is put through ordinary stresses.2.3Standard of Creep Testing usedThe ASTM and ISO have developed standard test methods to aid in the proper testing of the wide variety ceramic materials. These tests address the various applications of ceramic materials and environments in which they will be used. Popular standards for testing ceramic materials at high temperatures are: ASTM C1291 for tensile creep of monolithic ceramics ASTM C1337 for tensile creep of continuous fiber reinforced ceramics ASTM C1359 for rectangular shaped continuous fiber reinforced ceramics ASTM C1366 for monolithic ceramics ISO 22215 for tensile creep of monolithic and particulate reinforced ceramics.2.4Creep Testing Material Creep specimens made from ceramic

Micrometer or vernia caliper

Permanent pen

Creep Testing Machine

Hot and cold bags

Thermometer

2.5Creep Testing Procedure1. Remove any load from the arm of creep machine.

2. Measure and record the specimen dimensions for the calculation of stress and strain from the creep test.

3. Fit a specimen on a creep test machine as shown in Figure 2.1 with a dial gauge positioned in a mid-range of the specimen gauge length for the calculation of specimen extension.

4. Hung the weights of known values at the end of the sample to determine the applied stress. Specimen extension will be read on the dial gauge and time is recorded using stopped watch.

5. Repeat the tests at the same load used above but at different temperature

2.6Stage of CreepCreep is dependent on time so the curve that the machine generates is a time vs. strain graph. The slope of a creep curve is the creep rate d/dt. The trend of the curve is an upward slope. The graphs are important to learn the trends of the alloys or materials used and by the production of the creep-time graph; it is easier to determine the better material for a specific application.

Figure 2.0: Schematic illustration of a typical creep curve. From the graph in Figure 2.0, we are able to determine the temperature and interval in which an object will be disturbed once exposed to the load. Some materials have a very small secondary creep state and may go straight from the primary creep to the tertiary creep state. This is dependent on the properties of the material that is being test on. This is important to note because going straight to the tertiary state causes the material to break faster from its form.Nevertheless, each metal creeps at different rate and thus require different time to finish the test, ranging from minutes, hours, days, weeks or months. According to the typical creep curve in figure 2.0, it should be noticed that the creep curve can be divided into three main stages; primary, secondary and tertiary creeps. Each stage of creep behavior is influenced from both work hardening and annealing mechanisms occurring at the same time. However, work hardening and annealing will take place at different rates depending on response of metals to applied tensile force with time. The creep rate therefore changes accordingly. There are three stages of creep:1.Primary Creep: The primary creep or transient creep exhibits a decreasing creep rate with time as shown in figure 2.0. A very sharp increase in the initial stage is observed with the original strain, o, taking place before the creep rate starts to decrease. The creep rate then diminishes until reaching the secondary creep region as detailed in figure 2.0. This diminished creep rate in the primary creep region accounts from work hardening mechanism of the metal. Multiplication and interaction of dislocations rule out the annealing effect at this stage, resulting in increasing the creep resistance of the metal. 2.Secondary Creep/Steady State Creep: Beyond the primary stage, the creep rate is reaching a steady state where the creep rate is said to be relatively constant with time and gives the minimum creep rate of all the three regions. This minimum creep rate is used to represent the creep rate of the metal being tested at particular test temperature and load. The constant creep rate is due to balancing of strain hardening and annealing (recovery) processes according to the applied stress and temperature. The amount of dislocations being generated by work hardening is equal to the number of dislocations being annealed out.

3.Tertiary Creep: The tertiary creep region gives a rapid creep rate approaching failure. This is due to the formation of necking. Load bearing capability decreases due to the simultaneous reduction in the cross-sectional area of the specimen, which is related to local stress acting on this area. Furthermore, tertiary creep is associated with microstructural alterations due to increasing temperature such as coarsening of precipitate phases, recrystallization and diffusion of phases. These mechanisms effectively increase the tertiary creep rate, and eventually lead to fracture under creep.

Figure 3.0: Effects of stress levels on the shape of creep curves at constant temperature.However, factors influencing the shape of the creep curve depend on the levels of the stress and temperatures involved. If the temperature is remained constant, the creep curves will shift upward and to the left with increasing applied stresses as shown in figure 3.0. Similarly, if the creep test is carried out at various temperatures but at a constant stress level, the creep rate will increase with increasing temperatures. A linear graph denotes that the material under stress is gradually deforming and this would be harder to track at what level of stress an object can handle. This would also mean that the material would not have distinct stages, which would make object's breaking point would be less predictable.3.0ConclusionAs conclusion, the creep test has the objective of precisely measuring the rate at which secondary or steady state creep occurs. Increasing the stress or temperature has the effect of increasing the slope of the line if the amount of deformation in a given time increases. The results are presented as the amount of strain (deformation), generally expressed as a percentage, produced by applying a specified load for a specified time and temperature. From the creep testing, the designer can calculate how the component will change in shape during service and hence to specify its design creep life. 4.0 References 4.1Book1. Kingery, W. D. (1960). Introduction to ceramics.2. Callister, W. D., & Rethwisch, D. G. (2007).Materials science and engineering: an introduction(Vol. 7, pp. 665-715). New York: Wiley.3. Cannon, W. R., & Langdon, T. G. (1983). Creep of ceramics.Journal of Materials Science,18(1), 1-50.4. Carroll, D. F., & Wiederhorn, S. M. (1989). Creep testing of ceramics. InA Collection of Papers Presented at the 13th Annual Conference on Composites and Advanced Ceramic Materials, Part 2 of 2: Ceramic Engineering and Science Proceedings, Volume 10, Issue 9/10(pp. 1244-1244). John Wiley & Sons, Inc..4.2Website1. http://en.wikipedia.org/wiki/Creep_(deformation)2. http://en.wikipedia.org/wiki/Ceramic_materials3. http://ceramics.org/learn-about-ceramics/structure-and-properties-of-ceramics4. http://www.testresources.net/application/modulus-of-rupture-and-creep-test-equipment-for-ceramics-at-1700c-3100f10