Creep continued

Averaged experimental creep compliance data are represented by dots in Figure 12.12 as a function of log time. The tests show that even after 14 months the creep continued and deformations kept increasing. After tests of 10000 h the compliance of the materials examined exceeds the instantaneous elastic compliance by a factor varying from 3.8 (for pure PP) to 3.2 (for the 80% PP + 20% PLC blend). This shows that the inelastic strain of these materials manifests itself substantially methods of accelerated testing and prediction have to be subjected to very detailed scrutiny. 

This behaviour is termed creep and, as noted previously, J(t) is termed the creep function. If creep ceases eventually, this corresponds to saying that /(oo) is finite. Equation (1.4.2) suggests that /(oo) may be regarded as a natural generalization of the inverse elastic modulus. We will sometimes denote it by J. For certain materials, creep continues indefinitely. Such behaviour is akin to that of a liquid. Certain classes of viscoelastic materials are in fact referred to as viscoelastic liquids. There are materials with the property that the limit... 

As already mentioned, the present code corresponds to the solution of steady-state non-isothennal Navier-Stokes equations in two-dimensional Cartesian domains by the continuous penalty method. As an example, we consider modifications required to extend the program to the solution of creeping (Stokes) non-isothermal flow in axisymmetric domains ... 

We shall follow the same approach as the last section, starting with an examination of the predicted behavior of a Voigt model in a creep experiment. We should not be surprised to discover that the model oversimplifies the behavior of actual polymeric materials. We shall continue to use a shear experiment as the basis for discussion, although a creep experiment could be carried out in either a tension or shear mode. Again we begin by assuming that the Hookean spring in the model is characterized by a modulus G, and the Newtonian dash-pot by a viscosity 77. ... 

We continue the investigation of the contact problem for a plate under creep conditions. In this section the case of both normal and tangential displacements of the plate is considered. 

Another aspect of plasticity is the time dependent progressive deformation under constant load, known as creep. This process occurs when a fiber is loaded above the yield value and continues over several logarithmic decades of time. The extension under fixed load, or creep, is analogous to the relaxation of stress under fixed extension. Stress relaxation is the process whereby the stress that is generated as a result of a deformation is dissipated as a function of time. Both of these time dependent processes are reflections of plastic flow resulting from various molecular motions in the fiber. As a direct consequence of creep and stress relaxation, the shape of a stress—strain curve is in many cases strongly dependent on the rate of deformation, as is illustrated in Figure 6. 

Static friction decreases with an increase in load, and the static coefficient of friction is lower than the dynamic coefficient. The tendency to creep must be considered carefliUy in FEP products designed for service under continuous stresses. Creep can be minimized by suitable fillers. Fillets are also used to improve wear resistance and stiffness. Compositions such as 30% bronze-fiUed FEP, 20% graphite-filled FEP, and 10% glass-fiber-filled FEP offer high PV values ( 400(kPa-m)/s) and are suitable for beatings. 

A more extensive comparison of many potential turbine blade materials is available (67). The refractory metals and a ceramic, sHicon nitride, provide a much higher value of 100 h stress—mpture life, normalised by density, than any of the cobalt- or nickel-base aHoys. Several intermetaHics and intermetaUic matrix composites, eg, aHoyed Nb Al and MoSi —SiC composites, also show very high creep resistance at 1100°C (68). Nevertheless, the superaHoys are expected to continue to dominate high temperature aHoy technology for some time. 

Wrought lead—calcium—tin alloys contain more tin, have higher mechanical strength, exhibit greater stabiUty, and are more creep resistant than the cast alloys. RoUed lead—calcium—tin alloy strip is used to produce automotive battery grids in a continuous process (13). Table 5 Hsts the mechanical properties of roUed lead—calcium—tin alloys, compared with lead—copper and roUed lead—antimony (6 wt %) alloys. 

At high temperature, the behavior is different. A stmcture designed according to the principles employed for room temperature service continues to deform with time after load apphcation, even though the design data may have been based on tension tests at the temperature of interest. This deformation with time is called creep because the design stresses at which it was first recognized occurred at a relatively low rate. 

The low creep resistance of unalloyed zinc sheet and strip is one of its most serious defects and restricts its appHcation. Strip made with Zn—0.6 Cu—0.10 Ti composition has superior creep resistance and is used in stmctural appHcations where high, continuously appHed stresses are encountered in service. 

As the temperature is raised, loads which give no permanent deformation at room temperature cause materials to creep. Creep is slow, continuous deformation with time ... 

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. 

Climb unlocks dislocations from the precipitates which pin them and further slip (or glide ) can then take place (Fig. 19.3). Similar behaviour takes place for pinning by solute, and by other dislocations. After a little glide, of course, the unlocked dislocations bump into the next obstacles, and the whole cycle repeats itself. This explains the progressive, continuous, nature of creep, and the role of diffusion, with diffusion coefficient... 

Figure 20.6 shows how this evolutionary process has resulted in a continual improvement of creep properties of nickel alloys over the last 30 years, and shows how... 

Clearly related to stress relaxation is creep, when a material is kept under constant load and because of molecular slippage the material deforms continuously with time of loading. 

This has a very high resistance to impact damage, even at subzero temperatures. It has good creep strength in dry conditions up to 115°C but degrades by continuous exposures to water hotter than 65°C. It is resistant to aqueous solutions of acids, aliphatic hydrocarbons, paraffins, alcohols (except methanol), animal and vegetable fats and oils, but is attacked by alkalis, ammonia, aromatic and chlorinated hydrocarbons. 

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