Static loading Essay
Creep is time dependent flow of material at static loading. This leads to plastic or permanent deformation of material. It is very slow and relevant at elevated temperature. What I mean by elevated temperature is a temperature beyond 0. 4Tm, where Tm is melting point of the metal/ alloy/ material on absolute temperature scale. Because, for many materials like Lead (Pb), glass etc. room temperature itself is more than 0. 4Tm and therefore, these materials undergo creep at room temperature itself.
In fact these materials undergo creep even under their own weight. As creep is a very slow process, therefore, many times the effect is visible after a very long period of time. One classical example of creep is thickening of bottom of window glass in very old buildings. One very important contemporary example of creep from nuclear industry is creep of zircaloy pressure tubes or coolant tubes in Pressurized Heavy Water Reactors (PHWRs). In these reactors, the pressure tube is at temperatures in 250 oC to 300 oC temperature range.
Creep leads to elongation of the pressure tube, which is 5-6 meters long horizontal tube supported at two ends. The elongation due to creep, leads to sagging of the pressure tube,
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This is just one example, there are many. Today, for a variety of reasons – one being improvement of conversion efficiency of thermal energy into electrical energy – there is a strong interest in high temperature engineering materials and one of the most important challenges in developing high temperature engineering materials is creep. It is not so difficult to design a high strength material for room temperature applications. But when the material is put in use at elevated temperature, its strength and creep properties drop like anything.
Due to these reasons, it is very important to understand creep, different stages of creep, different mechanisms causing creep of materials and ways to improve creep resistance of materials under different operating conditions and beneficial applications. In stage I or primary creep, creep rate or strain rate decreases with time until it reaches a minimum steady value in Secondary or Stage II of Creep. In secondary or steady state creep, the creep rate remains constant with time. Any component spends most of its service life in the secondary or steady state creep. The creep rate in stage II is referred to as creep rate of the material.
This value is of engineering importance as design of a component from creep consideration is done using secondary stage creep rate of the material. Tertiary Creep or Stage III of creep is when creep rate increases very rapidly with time and the materials fracture in no time. Mechanisms of Creep: There are different mechanisms that contribute to creep. These are Dislocation Glide, Dislocation Creep, Diffusion Creep and Grain-Boundary Sliding. At a given stress and temperature condition one or more of these mechanisms are operative and contribute to deformation by creep.
Different variable contributing to creep deformation are – Stress (? ) and Temperature (T) from operating variable side and grain size (d) and activation energy for diffusion (Q) from materials side. A general equation giving creep rate as a function of these variables is: (http://en. wikipedia. org/wiki/Creep_(deformation)) Here, C is a material dependent constant, k is Boltzmann’s constant and m and b are exponents that depend on the operating mechanism. In case of dislocation glide, the glide of dislocation is assisted by thermal activation. Dislocation creep: This mechanism is operative at relatively high stress level.
For this mechanism the steady state strain rate is directly proportional to the normalized stress value (? /G) raised to power n i. e. ; where n is from 3 to 8 (Dieter 1988). This equation is also known as power law equation of creep. At lower stress level i. e. ?/G ~ 5×10-6, n is 1 and this is known as Harper – Dorn Creep (Dieter 1988). In case of Diffusion Creep, the normalized stress level is low ? /G <10-4and temperature is high. The creep is caused by diffusion of atoms. The atoms migrate from the grain boundaries under tension to those under compression.
At higher temperature, the diffusion is through lattice and the creep rate is directly proportional to (Dv/d2), where Dv is the diffusion coefficient for bulk diffusion and d is the grain size. This mechanism was proposed by Nabarro – Herring and therefore, this creep mechanism is also attributed to him. At lower temperature, diffusion through grain boundary is the mechanism of creep. This was proposed by Coble and the Coble creep rate is proportional to (Dgb/d3). What we can notice that the diffusion creep rate can be lowered by increasing the grain size.
Thus large grained materials are better from creep considerations. Thus grain size requirement for low creep is just opposite to room temperature design requirements, where fine grain size offers higher strength and ductility. This is because at lower temperature, grain boundaries are stronger than the grain and the opposite is true at elevated temperatures. Besides, there is another very important creep mechanism, which is grain boundary sliding. At elevated temperatures, grain boundaries slide over each other and this helps in preventing the inter-granular porosity developing during creep deformation.
After discussing the mechanisms of creep we must look at one useful manifestations of creep. If we have fine grained (sub micron sized) two phase material, then we can deform it to great extent say 500%, 1000%, without necking and fracture. This is very useful for fabrication of intricate components. Even difficult to process materials, like some ceramics can be converted into useful intricate shapes by exploiting superplasticity. The requirements for superplasticity are – fine grains (sub micron ~0. 1 ? m), two phase microstructure, low strain rate and high temperature (> 0.
5 Tm). Conclusions: Creep is very important life limiting mechanism for high temperature applications of materials like those in power production, nuclear reactors, aerospace application etc. to name a few. At a time , when we are going for high temperature applications of materials, understanding creep is very important to design materials with lower creep rate for prolonged life of components at elevated temperatures. References: Dieter G. E. (1988), Mechanical Metallurgy. Mc-Hill series in Science and Engineering (UK). http://en. wikipedia. org/wiki/Creep_(deformation)