Plastic deformation creep

FEA is applicable in several types of analyses. The most common one is static analysis to solve for deflections, strains, and stresses in a structure that is under a constant set of applied loads. In FEA material is generally assumed to be linear elastic, but nonlinear behavior such as plastic deformation, creep, and large deflections also are capable of being analyzed. The designer must be aware that as the degree of anisotropy increases the number of constants or moduli required to describe the material increases. 

The present chapter is divided into four parts. After a review of elasticity theory, we shall discuss simple elastic behaviour of single crystals of ice, then examine the relaxation processes which take place for periodically varying stresses and finally take up a brief treatment of plastic deformation, creep and related topics. 

STRESS RAPTURE - A general type of damage referring to carbon steel tubing, when heated above 450°C. Material will plastically deform (creep) and then rapture. 

Similar to time-independent plastic deformation, creep deformation in metals is dominated by dislocation movement, especially at higher stresses. Mechanisms that impede dislocation movement are thus also important in producing creep>-resistant materials. However, these mechanisms have to be temperature resistant. 

The plastic deformation, the creep deformation, and the bonding process on the bonding interface can be presumed from the height of the echo. 

On the other hand, the reliability of the product improves, too, if each state of the plasticity deformation, the creep deformation, and the diffusion joint in the solid phase diffusion bonding as the bonding process, is accurately understood, and the bonding process is controlled properly. 

Frost, H.J. and Ashby, M.F. (1982) Deformation-Mechanism Maps The Plasticity and Creep of Metals and Ceramics (Pergamon Press, Oxford). 

The first section involves a general description of the mechanics and geometry of indentation with regard to prevailing mechanisms. The experimental details of the hardness measurement are outlined. The tendency of polymers to creep under the indenter during hardness measurement is commented. Hardness predicitions of model polymer lattices are discussed. The deformation mechanism of lamellar structures are discussed in the light of current models of plastic deformation. Calculations... 

An important aspect concerning the surface indentation mechanism is the creep effect shown by polymeric materials i.e. the time dependent part of the plastic deformation of the polymer surface under the stress of the indenter14-16. The creep curves are characterized by a decreasing strain rate, which can be described by a time law of the form... 

Mechanical properties per se concerns with the qualities which determine the behaviour of a material towards applied forces. The ability to support weight without rupture or permanent deformation, to withstand impact without breaking, to be mechanically formed into different shapes - all these depend upon a combination of mechanical properties characteristic of metals. Four types of behaviour of a material under stress are very important linear or elastic behaviour, plastic behaviour, creep behaviour and fatigue behaviour. 

Identification of hot spots. Localized overheating of a tube causes localized high-temperature creep. This leads to the plastic deformation of a tube, and hence thin tube walls. Such hot spots are indicated by the color of the tube, as shown in Table 21.1. This chart is not a function of... 

Mass diffusion between grain boundaries in a polycrystal can be driven by an applied shear stress. The result of the mass transfer is a high-temperature permanent (plastic) deformation called diffusional creep. If the mass flux between grain boundaries occurs via the crystalline matrix (as in Section 16.1.3), the process is called Nabarro-Herring creep. If the mass flux is along the grain boundaries themselves via triple and quadjunctions (as in Sections 16.1.1 and 16.1.2), the process is called Coble creep. 

PLASTIC DEFORMATION. When a metal or other solid is plastically deformed it suffers a permanent change of shape. The theory of plastic deformation in crystalline solids such as metals is complicated but well advanced. Metals are unique among solids in their ability to undergo severe plastic deformation. The observed yield stresses of single crystals are often 10 4 times smaller than the theoretical strengths of perfect crystals. The fact that actual metal crystals are so easily deformed has been attributed to the presence of lattice defects inside the crystals. The most important type of defect is the dislocation. See also Creep (Metals) Crystal and Hot Working. 

Values from tables of friction coefficients always have to be used with caution, since the experimental results not only depend on the materials but also on surface preparation, which is often not well characterized. In the case of plastic deformation, the static coefficient of friction may depend on contact time. Creeping motion due to thermally activated processes leads to an increase in the true contact area and hence the friction coefficient with time. This can often be described by a logarithmic time dependence... 

Creep, yielding, and post-yielding plastic deformation (drawing) as well as flow are brought about by the stress-biased deformation and displacement (jumps) of molecular groups and chain segments. Creep is defined as the time-... 

However, at lower constant loads the rate of crystal plastic deformation decreases and (at 80 °C) disentanglement becomes competitive leading to the development of isolated planar craze-like defects extending perpendicular to the tensile axis (Fig. 15). The ensuing concentration of stress will further localize most of the sample deformation in such creep crazes and lead to a macroscopic ductile-brittle transition—in this material observed at 20 MPa (Fig. 14 [67]). 

The plastic deformation characteristics yield stress, ay, plastic flow stress, crpf, and strain softening, have been studied under uniaxial compression at a strain rate of 2 x 10-3 s-1 [53] in a temperature range from - 110 °C to typically Ta - 20 K. Indeed, for temperatures closer to Ta, the experimental results are less reliable, some creep behaviour occurring. 

If pressure is applied at the sintering temperature, plastic deformation by creep increases the rate of sintering. This is referred to by the term HIPping. It is used on castings as well as powder compacts. 

It is well established that the plastic deformation of crystalline solids occurs by the movement of lattice dislocations and/or diffusional creep. The rate of diffusion is expressed as... 

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