Creep deformation

In section 8.2.1, the time-dependent behaviour of polymers was described using spring-and-dashpot models. 

Sketch a spring-and-dashpot model suitable to describe creep deformation Consider a material with Young s modulus E and the creep law e = Act . Calculate the time-dependence of the strain in a retardation experiment. Due to its low melting temperature, lead creeps already at ambient temperatures. A thin-walled lead tube fixed at its ends bends under its own weight in the course of time. Estimate by how much the centre of the tube is displaced within one year  

The creep law is assumed to be = Aa since diffusion creep is the dominant mechanism. The creep constant A is 4.11 x 10 Pa s .  

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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. 

When a fiber is stressed, the instantaneous elongation that occurs is defined as instantaneous elastic deformation. The subsequent delayed additional elongation that occurs with increasing time is creep deformation. Upon stress removal, the instantaneous recovery that occurs is called instantaneous elastic recovery and is approximately equal to the instantaneous elastic deformation. If the subsequent creep recovery is 100%, ie, equal to the creep deformation, the specimen exhibits primary creep only and is thus completely elastic. In such a case, the specimen has probably not been extended beyond its yield point. If after loading and load removal, the specimen fails to recover to its original length, the portion of creep deformation that is recoverable is still called primary creep the portion that is nonrecoverable is called secondary creep. This nonrecoverable elongation is typically called permanent set. 

In Chapter 17 we showed that, when a material is loaded at a high temperature, it creeps, that is, it deforms, continuously and permanently, at a stress that is less than the stress that would cause any permanent deformation at room temperature. In order to understand how we can make engineering materials more resistant to creep deformation and creep fracture, we must first look at how creep and creep-fracture take place on an atomic level, i.e. we must identify and understand the mechanisms by which they take place. 

Creep stresses used for design purposes are usually determined based on two criteria the stress for a given acceptable creep deformation after a certain number of hours, which ranges from 0.01 to 1% deformation in 1000 hours and the nominal... 

If a plastic moulding fails in the performance of its normal function it is usually caused by one of two factors - excessive deformation or fracture. In the previous sections it was pointed out that, for plastics, more often than not it will be excessive creep deformation which is the limiting factor. However, fracture. 

Creep deformation can be split into three separate parts. The first, transient creep, is a short lived phenomenon which gives a high initial rate of deformation but decays according to the expression ... 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

Failure can be considered as an actual rupture (stress-rupture) or an excessive creep deformation. Correlation of stress relaxation and creep data has been covered as well as a brief treatment of the equivalent elastic problem. The method of the equivalent elastic problem is of major assistance to designers of plastic products since, by knowing the elastic solution to a problem, the viscoelastic solution can be readily deduced by simply replacing elastic physical constants with viscoelastic constants. 

Note that the term y in Eqs. 2-15 and 2-16 has a different significance than that in Eq. 2-14. In the first equation it is based on a concept of relaxation and in the others on the basis of creep. In the literature, these terms are respectively referred to as a relaxation time and a retardation time, leading for infinite elements in the deformation models to complex quantities known as relaxation and retardation functions. One of the principal accomplishments of viscoelastic theory is the correlation of these quantities analytically so that creep deformation can be predicted from relaxation data and relaxation data from creep deformation data. 

In computing ordinary short-term characteristics of plastics, the standard stress analysis formulas may be used. For predicting creep and stress-rupture behavior, the method will vary according to circumstances. In viscoelastic materials, relaxation data can be used in Eqs. 2-16 to 2-20 to predict creep deformations. In other cases the rate theory may be used. 

Creep behavior, determining, 13 474-477 Creep curve, 21 742 analysis of, 13 472 Creep data analysis, 13 477-480 Creep deformation, 13 470, 471—480 effects of temperature and stress on, 13 474... 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

In order to fully appreciate the potential presented by these materials, it is necessary to look at the structure of the polymer in relation to what is presently perceived as desirable qualities for polymers which are to be employed as asymmetric reverse osmosis membranes. The elevated hydrostatic pressures which prevail during reverse osmosis Impose the requirement of pol5mier rigidity or resistance to creep deformation (compaction). 

An alloy is evalnated for potential creep deformation in a short-term laboratory experiment. The creep rate is fonnd to be 1% per honr at 880°C and 5.5 x 10 % per honr at 700°C. (a) Calculate the activation energy for creep in this temperatnre range, (b) Estimate the creep rate to be expected at a service temperatnre of 500°C. (c) What important assnmption nnderlies the validity of yonr answer to part b ... 

Solid state reactions and creep deformation are important processes in the earth s mantle. They occur only if at least two ionic species migrate simultaneously in order... 

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