Creep curve sample

A linear output is associated with a liquidlike or viscous response. The retardation response of a Newtonian liquid is a straight line through the origin, where the slope of the line is inversely proportional to the viscosity. A stress relaxation response for a Newtonian liquid is a narrow spike, as energy is dissipated very quickly. Typically the recoil part of a creep curve will be almost horizontal if there is little elasticity in the sample. 

Figure 7.8. Typical creep curve for a plastic fat. A instantaneous deformation upon loading B instantaneous sample recovery upon unloading C time-dependent recovery of sample D permanent sample deformation (adapted from deMan and Beers, 1987).

The typical creep curve for a plastic fat is shown in Figure 7.8 and demonstrates the effect of working on the structure of butter. The softening that occurs in plastic fats is dependent on both the amount of force or deformation applied and also on the testing time (deMan and Beers, 1987). When a force is applied (i.e., when the sample is compressed), there is an initial elastic response (A), which can be represented by a Hookean spring (deMan et al., 1985 deMan and Beers, 1987). If the yield stress is exceeded,... 

FIG. 13.47 Small strain tensile creep curves of rigid PVC quenched from 90 °C (i.e. about 10 °C above Tg) to 40 °C and further kept at 40 0.1 °C for a period of 4 years. The different curves were measured for various values of time te elapsed after the quench. The master curve gives the result of a superposition by shifts that were almost horizontal the arrow indicates the shifting direction. The crosses refer to another sample quenched in the same way, but only measured for creep at a te of 1 day. From Struik (1977,1978). Courtesy of the author and of Elsevier Science Publishers. 

The creep curves of Fig. 4 show the three regions described above. The instantaneous compliance can be used to calculate the instantaneous modulus only for cases where is fairly large(to give a measurable Gq) This is the case with the sample containing 30 g dm bentonite which gives J = 1.1 X 10 m N corresponding to a G of 91 N m. With the sample containing 45 g dm bentonite,... 

Creep is a measure of sample length as a function of time when the sample is subjected to a constant load. Normally, creep curves are obtained by plotting data in terms of sample length, strain, or compliance versus log (time), as illustrated in Figure 2. The basic nature of creep curves is explained at the molecular level as a distortion of bond... 

Creep deformation. The deformation of single crystals of wet synthetic quartz under a constant compressive load at atmospheric pressure has been studied by McCormick (1977), Kirby and McCormick (1979), and Linker et al. (1984). The creep curves for all samples loaded almost immediately after the test temperature had been reached are characterized by an incubation period that decreased rapidly with increasing axial stress and increasing temperature. A typical creep curve is shown in Figure 9.5. 

Figure 5 Flow curve for a 6 wt% solution of diblock Ej/sB/j (E = oxyethylene, B = oxybu-tylene, subscripts = number of repeats) in water at 25 C. Every point was measured from an individual creep curve on fresh samples, to eliminate orientation effects. The plateau at around a-50 Pa indicates inhomogeneous flow, probably shear bandingf ...

In practice, viscoelastic properties can be determined by static and dynamic tests. The typical static test procedure is the creep test. Here, a constant shear stress is applied to the sample over a defined length of time and then removed. The shear strain is monitored as a function of time. The level of stress employed should be high enough to cause sample deformation, but should not result in the destruction of any internal structure present. A typical creep curve is illustrated in Fig. 13A together with the four-element mechanical model that can be used to explain the observations. The creep compliance represents the ratio between shear strain rate and constant stress at any time t. 

To determine if either of these answers were correct, the properties of a series of polyurethane networks which contained photo labile disulfide bonds were studied (7). The creep curve of the s imple in the initial crosslink density state (comparable to Vq) was determined in the usual way. Then, after complete recovery, another creep experiment was started but, at time t during this run, the sample was subjected to ultraviolet irradiation which photochemlcally broke some of the network chains. 

Two series of RubCon samples of type A and type B were tested. The creep curves are shown in Figures 2.39 and 2.40. 

Two sets of special experiments were run that involved temperature variation during creep (Fig. 3). In the first set, a sample was cooled to a specified temperature Ti. Then, under a stress, r, equal for all temperatures, the creep curve was... 

The value of the simplified technique for producing isochronous stress-strain curves for non-linear isotropic materials by successive loading and unloading of a single sample have been amply demonstrated over many years and fully described elsewhere.These techniques become even more valuable in studies of anisotropy, where samples may be difficult to obtain in large numbers and where the scope of the problem is much larger. A considerable proportion of work on oriented materials reported in the literature is essentially confined to this measurement and does not include studies of time dependence of behaviour. Detailed work has been carried out validating this procedure for oriented materials by comparison of the isochronous stress-strain data with isochronous sections from families of creep curves. ... 

The creep data were represented as creep compliance D(t) = e t)/(7, where D t) is the compliance, t is time, and s and (T are the engineering strain and stress, respectively. To minimize scattering, three to five creep curves were measured on different samples at each temperature level. Standard deviations were less than 10%. Averaged experimental creep compliance data are shown by the dots in Figure 12.1. As may... 

The experiment we introduced at the beginning of the previous subsection is also called the creep experiment. A small stress of Gq is imposed on a solid sample for a time period of to at a constant temperature after the stop of stress, the strain of changing with the time period of t monitors the relaxatirMi curve. There are four typical responses separately corresponding to viscous, elastic, anelastic and viscoelastic responses, as illustrated in Fig. 6.8. The creep curve of polymer viscoelasticity exhibits both instant and retarded elastic responses upon imposing and removal of the stress, and eventually reaches the permanent deformation. 

Fig. 6.2 (a) A sample set of creep curves for various temperatures T, and (b) their time-temperature shift factors for reference temperature T = 100°C [63] (b), for SMP3 based on DBDI/PTHFeso [63]... 

The curve J(t,Ag) is the hypothetical creep curve that would be obtained if the age of the sample could be fixed immediately after the beginning of the creep experiment, i.e., A is equal to the preconditioning time, t, and Equation 7 can be written as ... 

The overall microstructure of the bulk sample following a full creep run at 1620 K was found to have changed little from that observed following the 4-h anneal. No cavitation was observed, In order to determine the reason for the break observed in the creep curve at 1620 K, a careful TEM analysis was conducted on samples taken both before the break in cuve and after, and these were compared to a sample taken from the lower temperature creep run, 1470 K, where no break was observed. The microstructure representing the material before the break was obtained by allowing the material to deform at stresses of 50,100, and 150 MPa with a final true strain of -0.9%. The sample used to represent the microstructure after the break was deformed at stresses of 50,100,150,200,250,300 and 350 MPa with a final true strain of -1.5%. High resolution electron microscopy was used to examine the grain boundaries in each sample. 

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