Creep behavior

Most reported creep data on silicon nitride are expressed and analyzed in terms of classical creep mechanisms for which the creep rate, e, is expressed as a power function of the grain size, g, and the applied stress, a, and an Arrhenius function of temperature, T. 

the parameters, m, n, and Q, are constants ofthefit of Eq. (1) to the data, and go and Oo are normalization constants. In the discussion that follows, published data are discussed in terms of the grain size exponent, m, the stress exponent, n, and the activation energy, Q. 

Creep is the long-term continuing deformation due to sustained deviatoric stress (od =Oi - Os) conditions that occurs as a function of time after dissipation of consolidation excess pore pressures. Thus, creep behavior of sediments is a function of the type of sediment, its physical properties, stress-strain history, and time. Mitchell (1976) distinguished between creep and secondary compression by noting that the former is referred 

Distribution of shear strength of surficial sediments. North Atlantic Ocean basin. (After Keller, G.H., Shear strength and other physical properties of sediments from some ocean basins. Proceedings of the Conference on Civil Engineering in the Oceans, ASCE Press, San Francisco, CA, 319-417,1968. Reprinted with permission of ASCE.) 

Isotach behavior observed in clay for (a) creep and relaxation and (b) stepwise change in strain rate. Nonisotach behavior observed in sand for (c) creep and relaxation and (d) stepwise change in strain rate. (From Lade, PY. et al.,/. Geotech. Geoenviron. Eng., 135,941-963,2009. Reprinted with permission of ASTM.) 

The relationship between steady-state creep rate (SSCR) and a number of factors is shown in Table 8.9. A review of this table shows that as plasticity index, % creep, activity 

Factors Affecting the Steady-State Creep Rate 

Chemical action under mechanical load is characterized using creep tests. Compared with other test methods, creep tests have a number of advantages, such as the selection of various media, both internal and external. This is often decisive for aging behavior. Water and oxygen play a decisive role in aging oxygen causes oxidation, water results in hydrolysis or stabilizer extraction [63]. 

The nearly vertical curve shape in stage 3 illustrates the extreme embrittlement of plastics by oxidation. Stage 3 is subdivided into regimes A, B, and C, corresponding 

During regimes A and B, anti-oxidants are completely extracted followed by rapid degradation of the unprotected polymer (regime C) [63]. 

Oxidation begins shortly before the onset of stage 3. In polyethylene, chain cleavage and oxidation reactions have been observed. Oxidation begins on the inside of the pipe and is caused by stabilizer extraction. Stabilizer loss leads to discoloration with a large number of surface cracks, oxidation spots , which will initiate fractures in stage 3 [63]. 

Left Stage 2 failure in a PE-MD pipe at 6.55 N/mrrf, 60 °C, internal and external medium  

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Creep. The phenomenon of creep refers to time-dependent deformation. In practice, at least for most metals and ceramics, the creep behavior becomes important at high temperatures and thus sets a limit on the maximum appHcation temperature. In general, this limit increases with the melting point of a material. An approximate limit can be estimated to He at about half of the Kelvin melting temperature. The basic governing equation of steady-state creep can be written as foUows ... 

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

Fig. 3. Typical creep behavior for rubber-modified styrene polymers.

Fig. 5. Tensile elongation vs time demonstrating creep behavior of ceramics. Section I is primary creep II, secondary or steady-state creep III, tertiary...

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

PVAc is another important type of adhesive, especially in furniture manufacturing and for carpentry. They form the bond line in a physical process by losing their water content to the two wooden adherends. PVAc adhesives are ready to use, have short setting time and give flexible and invisible joints. They are easy to clean and show long storage life. Limitations are their thermoplasticity and the creep behavior. 

When a viscoelastic material is subjected to a constant stress, it undergoes a time-dependent increase in strain. This behavior is called creep. The viscoelastic creep behavior typical of many TPs is illustrated in Figs. 2-22 and 2-23. At time to the material is suddenly subjected to a constant stress that is main-... 

Fig. 2-22 Viscoelastic creep behavior typical of many TPs under long-term stress to rupture (a) input stress vs. time profile and (b) output strain vs. time profile.

Although the creep behavior of a material could be measured in any mode, such experiments are most often run in tension or flexure. In the first, a test specimen is subjected to a constant tensile load and its elongation is measured as a function of time. After a sufficiently long period of time, the specimen will fracture that is a phenomenon called tensile creep failure. In general, the higher the applied tensile stress, the shorter the time and the greater the total strain to specimen failure. Furthermore, as the stress level decreases, the fracture mode changes from ductile to brittle. With flexural, a test specimen... 

Viscoelastic creep data are usually presented in one of two ways. In the first, the total strain experienced by the material under the applied stress is plotted as a function of time. Families of such curves may be presented at each temperature of interest, each curve representing the creep behavior of the material at a different level of applied stress. Below a critical stress, viscoelastic materials may exhibit linear viscoelasticity that is, the total strain at a given time is proportional to the applied stress. Above this critical stress, the creep rate becomes disproportionately faster. In the second, the apparent creep modulus is plotted as a function of time. 

Different viscoelastic materials may have considerably different creep behavior at the same temperature. A given viscoelastic material may have considerably different creep behavior at different temperatures. Viscoelastic creep data are necessary and extremely important in designing products that must bear long-term loads. It is inappropriate to use an instantaneous (short load) modulus of elasticity to design such structures because they do not reflect the effects of creep. Viscoelastic creep modulus, on the other hand, allows one to estimate the total material strain that will result from a given applied stress acting for a given time at the anticipated use temperature of the structure. 

Intermittent loading. The creep behavior of plastics that has been considered so far has assumed that the level of the applied stress will be constant. However, in service the material may be subjected to a complex pattern of loading and unloading cycles (Fig. 2-33). This variability can cause design... 

The load or stress has another effect on the creep behavior of most plastics. The volume of isotropic or amorphous plastic increases as it is stretched unless it has a Poisson ratio of 0.50. At least part of this increase in volume manifests itself as an increase in free volume and a simultaneous decrease in viscosity. This decrease in turn shifts the retardation times to being shorter. 

Fig. 2-39 Tensile-creep behavior of PP top on semilog scale and bottom on log-log scale.

Predictions can be made on creep behavior based on creep and relaxation data. 

Creep behavior Creep is the deformation that occurs over a long period of time in a material subjected to a continuous load, and stress relaxation is the reduction in stress with time that occurs in a material when it is de-... 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

Fig. 5 Comparison of creep behavior of three different Nextel fibers note superior creep resistance of Nextel 720 fiber (data from 3 M Co.)...

Fio. 23. Area-creep behaviors and bright field images of stearic acid crystalline monolayer prepared by multi-step creep method and continuous compression method. 

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... 

Creep behavior is similar to viscous flow. The behavior in Equation 14.17 shows that compliance and strain are linearly related and inversely related to stress. This linear behavior is typical for most amorphous polymers for small strains over short periods of time. Further, the overall effect of a number of such imposed stresses is additive. Non-creep-related recovery... 

Figure 5.44 Influence of applied stress and temperature on creep behavior. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 227. Copyright 2000 by John Wiley Sons, Inc.

The concentration of hydrogen in the polymer during irradiation is low, on the order of 10"6 mole per cc. This is far lower than the concentrations of plasticizers required to cause any significant changes in polymer creep behavior. 

Einaga,Y., Osaki,K., Kurata,M, Tamura,M. Creep behavior of polymer solutions. II. Steady-shear compliance of concentrated polystyrene solutions. Macromolecules 4, 87-92 (1971). 

Creep Behavior of Propellant Material. Subject of a report by D.A. George, NBS Rept 5688r 8th Prog Rept (1 Sept to 31 Oct 1947), Proj No TU2-2Q... 

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