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Creep and Fatigue Research Paper

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Added on  2020-04-13

Creep and Fatigue Research Paper

   Added on 2020-04-13

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Creep and Fatigue 1CREEP AND FATIGUEA Research Paper on Creep and Fatigue ByStudent’s NameName of the ProfessorInstitutional AffiliationCity/StateYear/Month/Day
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Creep and Fatigue 2INTRODUCTIONCreep is a term used in referring to a process of progressive deformation of a material at a stress that is constant. Fatigue refers to the process of deformation of a material at repetitive stress. The interaction between creep and stress refers to the deformation of a material under stresses that are repetitive at high temperatures. Fatigue and creep are the phenomenon that leads to deformation and ultimately failure of the material.CREEP1. Explain and discuss the following phenomena creep with practical examples i. Stress relaxationStress relaxation is the decrease in stress in response to the same amount of strain generated in the structure. This is basically due keeping the structure is a strained condition for the samefinite interval of time causing some amount of plastic strain. The figure below shows the stress-strain curve for a material undergoing stress relaxation.A practical example is a rubber band hung from a pin.
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Creep and Fatigue 3It is then stretched out over a second a second pin in B. The 2nd is then removed quickly and rubber returns to its original shape in C. The experiment is repeated and the rubber band is hung at step 1 and stretched out at step 2. It remained stretched and to an elevated temperature for a long time. Step 3 shows the rubber band after the exposure. There is no notable difference between step 2 and step 3. However, the pin is removed at step 4, the permanent set becomes visible. Different materials will show different resistance to stress relaxation[ CITATION Sun14 \l 2057 ]. ii. Creep rupture strengthThis is the stress required to cause the fracture in a creep test within a specified time. It is alsoknown as stress rupture strength or stress which at a given temperature will cause a material to rapture at a given time. Such as a hot air balloon when the temperature is applied beyond which it can sustain, the balloon will rupture due to the stress of expansion. iii. Creep lifeAlloys used in applications like blades experience a wide range of stresses and temperatures for varying times during start-up, take-off, steady flight, landing, and shut-down. Creep strains accumulate during these stages and design could assume worst case scenario, but could lead to a weight penalty in the aero-engine and less payload. Alternatively, an approachsimilar to Miner’s law in fatigue can be adopted, this is called life fraction rule[ CITATION Joh14 \l 2057 ]. Which states that the output occurs when the sum of all the fractions of rupture life at different stress/temperature combination becomes equal to unity as shown in the equation below:
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Creep and Fatigue 4iv. Factors affecting creep (15%)In materials, creep is one mode of mechanical failure of solid materials. Creep is the slow andpermanent deformation of the solid material under the influence of different stresses. The factors affecting creep include:Applied loadExposure timeExposure temperatureMaterial properties[ CITATION Joh121 \l 2057 ]2. A secondary creep rate is given in the table below were obtained from creep tests carried out on Aluminium alloy at 100◦ C.σ, MPa 120 160 210 250 280 300 ̇ ǫs, h−1 6.2 × 10−6 4.2 × 10−5 1.42 × 10−4 2.3 × 10−4 4.7 × 10−4 6.2 × 10−4Determine the parameters of the Norton power law for creep: Where ̇ ǫs is the secondary creep rate, σ is stress, and A & n are constants. The result should include MATLAB polyfit with clear steps.
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