Creep-Rupture Life Prediction for 9Cr-1Mo-Nb-V Weld Metal
Keywords: creep-fatigue; creep-rupture; unified equation; fatigue model .. regarded as the best description of stress-time-temperature relation  / kultnet.info (accessed. Creep and Stress Rupture: Creep is the time-dependent plastic strain at constant stress and temperature Relationship in Cu (Feltham and Meakin ). Creep. • It is a time- dependent deformation under a certain applied load. • Generally The number of possible stress-temperature- time Stress Rupture Tests.
When the stress is maintained for a shorter time period, the material undergoes an initial strain until a time t1 at which the stress is relieved, at which time the strain immediately decreases discontinuity then continues decreasing gradually to a residual strain.
Viscoelastic creep data can be presented in one of two ways. Total strain can be plotted as a function of time for a given temperature or temperatures. Below a critical value of applied stress, a material may exhibit linear viscoelasticity.
Above this critical stress, the creep rate grows disproportionately faster. The second way of graphically presenting viscoelastic creep in a material is by plotting the creep modulus constant applied stress divided by total strain at a particular time as a function of time.
A family of curves describing strain versus time response to various applied stress may be represented by a single viscoelastic creep modulus versus time curve if the applied stresses are below the material's critical stress value. Additionally, the molecular weight of the polymer of interest is known to affect its creep behavior. The effect of increasing molecular weight tends to promote secondary bonding between polymer chains and thus make the polymer more creep resistant.
Similarly, aromatic polymers are even more creep resistant due to the added stiffness from the rings. Both molecular weight and aromatic rings add to polymers' thermal stability, increasing the creep resistance of a polymer.
Creep and Stress Rupture Properties
Polymers experience significant creep at temperatures above ca. Creep of concrete[ edit ] Main article: Creep and shrinkage of concrete The creep of concrete, which originates from the calcium silicate hydrates C-S-H in the hardened Portland cement paste which is the binder of mineral aggregatesis fundamentally different from the creep of metals as well as polymers.
Unlike the creep of metals, it occurs at all stress levels and, within the service stress range, is linearly dependent on the stress if the pore water content is constant. Unlike the creep of polymers and metals, it exhibits multi-months aging, caused by chemical hardening due to hydration which stiffens the microstructure, and multi-year aging, caused by long-term relaxation of self-equilibrated micro-stresses in the nano-porous microstructure of the C-S-H. If concrete is fully dried it does not creep, though it is difficult to dry concrete fully without severe cracking.
Applications[ edit ] Though mostly due to the reduced yield strength at higher temperatures, the collapse of the World Trade Center was due in part to creep from increased temperature. Creep in epoxy anchor adhesive was blamed for the Big Dig tunnel ceiling collapse in Boston, Massachusetts that occurred in July Sagging of the filament coil between its supports increases with time due to the weight of the filament itself.
If too much deformation occurs, the adjacent turns of the coil touch one another, causing an electrical short and local overheating, which quickly leads to failure of the filament. The coil geometry and supports are therefore designed to limit the stresses caused by the weight of the filament, and a special tungsten alloy with small amounts of oxygen trapped in the crystallite grain boundaries is used to slow the rate of Coble creep.
Creep can cause gradual cut-through of wire insulation, especially when stress is concentrated by pressing insulated wire against a sharp edge or corner.
Special creep-resistant insulations such as Kynar polyvinylidene fluoride are used in wirewrap applications to resist cut-through due to the sharp corners of wire wrap terminals.
Teflon insulation is resistant to elevated temperatures and has other desirable properties, but is notoriously vulnerable to cold-flow cut-through failures caused by creep. Hence, it is crucial for correct functionality to understand the creep deformation behavior of materials. Creep deformation is important not only in systems where high temperatures are endured such as nuclear power plants, jet engines and heat exchangers, but also in the design of many everyday objects.
It should be noted that the long-term creep and stress-rupture values For example,h are often extrapolated from short-term tests.
Whether these property values are extrapolated or determined directly, both methods will often provide accurate information on the operating life of high-temperature parts. The actual material behavior is often difficult to predict accurately because of the complexity of the service stresses, variation in application relative to the idealized conditions, uni-axial loading conditions in the standard tests, and also because of the attenuating factors such as cyclic loading, temperature fluctuations, and metal loss from corrosion.
The designer and the metallurgist should therefore be prepared to anticipate the synergistic effect of these variables. During creep testing, the tensile specimen is subjected to either a constant load or stress at a constant temperature.
Most creep tests conducted at constant load are concerned with concerned with information relating to specific engineering applications, whereas creep tests at constant stress are necessary for the specific understanding of the mechanism of creep. During the creep test, strain change in length is measured as a function of elapsed time.
Creep test data is presented as a plot between time and strain known as creep curve. A typical creep curve is shown in Figure 1.
The curve may show the instantaneous elastic and plastic strain that occurs as the load is applied, followed by the plastic strain which occurs over time. Three stages to the creep curve may be identified: Secondary Steady State creep: The minimum secondary creep rate is of most interest to design engineers, since failure avoidance is commonly required and in this area some predictability is possible.
Creep and Stress Rupture Properties :: Total Materia Article
In general two Standards are commonly used: The stress to produce a creep rate of 0. The first requirement would be typical of that for gas turbine blades, while the second standard would be most typically seen in applications such as those for steam turbines. A typical creep curve Secondary creep, which occurs as a linear function of time, is heavily dependent on temperature and stress.