Recovery response, creep test

In a creep test a sample is placed under a constant stress, and strain is recorded as a function of time. If an ideal elastic solid is subjected to a creep test, it will exhibit an immediate elastic strain in accordance with Hooke s law, but the strain will remain constant thereafter until the stress is removed, when the sample will return elastically to zero strain. An ideal liquid responds to a creep test quite differently. There is no initial elastic response, and there will be a continuously increasing strain with a slope inversely proportional to the viscosity the strain rate wUl remain constant. When the stress is removed, there is no elastic recovery— the liquid simply stops flowing in other words, the strain rate returns to zero. 

Cfeep Testing Let us examine the response of the Maxwell element in two mechanical tests commonly applied to polymers. First consider a creep test, in which a constant shear stress is instantaneously (or at least very rapidly) applied to the material and the resulting strain is followed as a function of time. Deformation after removal of the stress is known as creep recovery. 

The creep test probes the time-dependent nature of a sample. Creep and recovery tests allow the differentiation between viscous and elastic responses when the viscoelastic material is subjected to a step constant stress (creep) and then the applied stress is removed (recovery). A standard creep experiment provides critical parameters such as zero shear viscosity (qo) and equilibrium compliance (Jeo), which measures the elastic recoil of a material. 

During the creep test, the stress causes a transient response, including the elastic and the viscous contributions. By following the recovery phase after the release of the applied stress, one can separate the total strain into the instantaneous elastic part, the recovered elastic part, and the permanently viscous part. 

Figure 10 shows the curves that represent the viscoelastic response at an applied stress of 5 Pa for the three hydrogels obtained at 37 °C, in a creep test followed by recovery. The creep curves comprise three parts the instantaneous strain, the retardation strain, and the viscous strain. When the applied stress is removed, the recovery process starts, and first the instantaneous strain is recovered, then the retardation one, and finally remains the viscous part. The high elasticity of the hydrogels can be observed, where the reached strain after the stress of 5 Pa was applied for 60 s is very high, and the recovered strain represents 52 % from the maximum value reached by the strain in the creep test. 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

Recovery is the strain response that occurs upon the removal of a stress or strain. The mechanics of the recovery process are illustrated in Fig. 2-34, using an idealized viscoelastic model. The extent of recovery is a function of the load s duration and time after load or strain release. In the example of recovery behavior shown in Fig. 2-34 for a polycarbonate at 23°C (73°F), samples were held under sustained stress for 1,000 hours, and then the stress was removed for the same amount of time. The creep and recovery strain measured for the duration of the test provided several significant points. 

In the present work the effect of temperature on the rheological behaviour of wheat gluten in D20 is compared to that in water. The viscoelastic response was studied in shear by combining dynamical measurements and creep and recovery tests, in order to encompass a large timescale. 

The viscoelastic behavior is evaluated by means of two types of methods static tests and dynamic tests. In the first calegtuy a step change of stress or strain is applied and the stress or strain response is recorded as a function of time. Stress relaxation, creep compliance, and creep recovery are static methods. The dynamic tests involve the imposition of an oscillatory strain or stress. Every technique is described in the following sections. 

A further set of tests was conducted in order to evaluate the accuracy of the finite-element code for the case where creep is followed by creep recovery. A qualitative depiction of the loading and the resulting creep strain is given in Figure 11. Rochefort and Brinson O) presented experimental data and analytical predictions on the creep and creep recovery characteristics of FM-73 adhesive at constant temperature. The Schapery parameters necessary to characterize the viscoelastic response of FM-73 at a fixed temperature of 30 °C are obtained by applying a least-squares curve fit to the data presented in Reference 50. The resulting analytical expressions for the creep compliance function D(i ), the shift function and the nonlinear parameters go> 82 presented in Table 4. 

Viscoelasticity, Figure 1 (a) Creep and recovery tests (1) stress step (2) response of an ideal viscous liquid (3) response of an ideal elastic solid... 

Fig. 3.13 Creep and creep recovery tests stress input (above) and qualitative material strain response (below).

The response of a four parameter fluid in a creep and creep recovery test is given in Fig. 3.26 and is recognized as the response of a thermoplastic type polymer as given earlier in Fig. 3.13. 

Sketch the response of a Maxwell fluid to a creep and a creep recovery tests. 

The problem with the semi-solid, self-bodied emulsion systems has been to measure their consistency. Measurements from continuous shear measurements, as they are the result of structural breakdown in the systems under study, have to be treated with some degree of caution. In order to obtain a true measure of consistency or body the systems should be tested in their native state, this requiring a method of measurement which does not disrupt the structures in the emulsion. Thus so-called creep measurements may be applied in which the emulsions are subject to only relatively minor deformities. In creep a shear is quickly imposed on the sample and maintained at a constant level the time-dependent strain or compliance response to this steady stress provides the creep curve. A recovery curve is obtained on removal of the stress, a typical diagram showing the profile for creep and recovery is given in Fig. 8.38. 

Compressive stress/strain tests were performed to determine the relative softness of these foams. Compressive creep were run to determine the response of the foams to constant apphed force over time to simulate a person lying on mattresses made with these foams. The effect of temperature and humidity on the eompressive stress/strain and creep response of the foams was also measmed. A sequence of compressive ramps to inereasing load levels was applied to the foam 1 with an unload and 20 second hold after each ramp to measure the immediate resilience and rate and degree of recovery before the next ramp. Finally, a comparison of the uniformity of compression between the two foams was made. 

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