Dynamic creep testing

H3.1 Dynamic or Oscillatory Testing of Complex Fluids H3.2 Measurement of Gel Rheoogy Dynamic Tests H3.3 Creep and Stress Relaxation Step-Change Experiments... 

Real world materials are not simple liquids or solids but are complex systems that can exhibit both liquid-like and solid-like behavior. This mixed response is known as viscoelasticity. Often the apparent dominance of elasticity or viscosity in a sample will be affected by the temperature or the time period of testing. Flow tests can derive viscosity values for complex fluids, but they shed light upon an elastic response only if a measure is made of normal stresses generated during shear. Creep tests can derive the contribution of elasticity in a sample response, and such tests are used in conjunction with dynamic testing to quantity viscoelastic behavior. 

All these tests are in common use to measure the tensile stiffness of polymers. For example, tests at constant extension rate are often carried out on an Instron tensile testing machine. Tensile creep is used in many cases while stress relaxation is not so common. Dynamic testing is commonly performed using the Rheovibron or other commercial equipment32 or home made equipment33,... 

The derivation of fundamental linear viscoelastic properties from experimental data obtained in static and dynamic tests, and the relationships between these properties, are described by Barnes etal. (1989), Gunasekaran and Ak (2002) and Rao (1992). In the linear viscoelastic region, the moduli and viscosity coefficients from creep, stress relaxation and dynamic tests are interconvertible mathematically, and independent of the imposed stress or strain (Harnett, 1989). 

Fig. 3.26. Creep moduli E for different strains e as a function of the loading time t determined in macroscopic tensile tests dynamic tests and in the microregion at the crack tip...

The creep test can be also used for solid foods and as in the dynamic tests, a compressive or elongational stress can be employed (Rao et al., 1995). However, when a compressive stress is imposed, the sample-metal platen interfece should be either lubricated or bonded to minimize frictional effects. 

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. 

Dynamic testing is the most commonly used one in the study of polymer properties. Because of its widespread usage, dynamic data have been analyzed in terms of six different functions, as opposed to a single one in stress relaxation and creep. The experiment involves (1) imposition (on a specimen of the material) of either a shear stress or a shear strain which varies sinusoidally with time, and (2) study of the corresponding response. 

In the above discussion, six functions Go(w), d(w), G (w), G"(w), /(w), and J"(oj) have been defined in terms of an idealized dynamic testing, while earlier we defined shear stress relaxation modulus G t) (see Equation 3.19) and shear creep compliance J(t) (see Equation 3.21) in terms of an idealized stress relaxation experiment and an idealized creep test, respectively. Mathematical relationships relating any one of these eight functions to any other can be derived. Such relationships for interconversion of viscoelastic function are described by Ferry [5], and interested readers are referred to this treatise for the same. 

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. 

By combining static-creep and dynamic tests, a range of stiffness modulus can be obtained. When measurements of stiffness modulus as a function at time, at various temperatures, are carried out and the results are plotted in logarithmic scales, a graph of the type shown in Figure 4.16 is obtained. 

It is necessary, therefore, to test an adhesive by many techniques to simulate the conditions that it may be subjected to in service. The three types of tests to be discussed are tensile, shear, and peel. These tests are the most common and result in information which is useful for reliable joint design. Response to dynamic testing such as fatigue, creep, and impact will also be introduced. 

With dynamic testing, the processed plastic s elastic modulus (relating to energy storage) and loss modulus (relative measure of a damping ability) are determined. Steady testing provides information about creep and recovery, viscosity, rate dependence, etc. 

For a Newtonian low molar mass liquid, knowledge of the viscosity is fully sufficient for the calculation of flow patterns. Is this also true for polymeric liquids The answer is no under all possible circumstances. Simple situations are encountered for example in dynamical tests within the limit of low frequencies or for slow steady state shears and even in these cases, one has to include one more material parameter in the description. This is the recoverable shear compliance , usually denoted and it specifies the amount of recoil observed in a creep recovery experiment subsequent to the unloading. Jg relates to the elastic and anelastic parts in the deformation and has to be accounted for in all calculations. Experiments show that, at first, for M < Me, Jg increases linearly with the molecular weight and then reaches a constant value which essentially agrees with the plateau value of the shear compliance. 

Between the extremes of viscous fluids and elastic solids are materials that seem to exhibit both traits. These are called viscoelastic materials or memory fluids, and their dual nature becomes most evident when we subject them to time-dependent (unsteady) tests. The three major types of unsteady tests are the so-called relaxation, creep and dynamic tests. In the previous sections, we gave definitions and descriptions for stress, strain and deformation rates. These quantities are now used in defining the various unsteady tests. Thus, in a relaxation test the sample is subjected to a sudden, constant, strain. The stress shoots up in response and then gradually decays ( relaxes ). In the creep test, a sudden stress is applied and held constant. Now the strain picks up quickly and then, while continuing to increase, slows down on its rate of increase. We say the material creeps under the constant stress. In dynamic tests, one confining wall is made to move periodically with respect to another. One monitors both the strain and the stress as a function of time. 

The value of t/o can be obtained from a creep compliance test by waiting until the creeping flow reaches steady state. It can also be obtained from the dynamic mechanical data from the following relation... 

Other variations of this extrapolation method use a constant maximum or minimum stress. Similar to dynamic testing methods, the stress ratio is varied, i. e., minimum stress and maximum stress respectively approach the constant stress by reducing the amplitude. While at constant minimum stress, the mean stress is always larger than the creep load for which the deformation is to be predicted at constant maximum stress this load is never reached. At constant minimum stress the given creep strain will be reached even earlier than at constant mean stress, whereas at constant maximum stress it is not guaranteed that the creep strain will be reached. 

Figure 5.232 Dynamic tests for the extrapoiation of creep at 6 N/mm (singie ioadievei procedure (SLLP), f= 0.35Hz, R = 0.2and0.5, poweriaw)...

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