Time sweep tests

The sample is loaded onto the instrument and the time reference is noted by starting a timer or resetting a timer in the software. A dynamic test for viscoelastic structure is then used to monitor changes in the sample that could result from mechanical relaxation, drying, or thixotropy. A time sweep test is usually performed at a constant temperature. The test is also run at a constant frequency that is comparable to real-time observation (typically 1 Hz) or at a constant angular frequency (10 rad/sec or 1.6 Hz). 

Examples of time sweep test results at 2 Hz for an antibacterial hand soap are shown in Figure 4.11 and Figure 4.12. Figure 4.11 summarizes the complex modulus components, G andG", and the complex viscosity, n, while Figure4.12 shows the experimental variables of phase angle and amplitude obtained at 23 to 24°C. 

Figure 46. Storage modulus, C/, of gluten - water mixtures in a time sweep test 10% strain, I Hz frequency). The master curve has been obtained by applying the time-temperature superposition principle. Temperatures considered 40 ( ), 50 ( ), 60 (0), 70 (O), 80 (A), and 90 ( ) °C (modified from [263])...

Figure H3.1.3 For oscillatory (sweep) testing, four control parameters can be varied amplitude, frequency, time, and temperature.

In the frequency sweep test, the idea is to obtain LVE data from the test material over the widest possible (or realistic) range of frequencies. The lower limit of testing is never difficult for a rheometer to achieve physically, but it may be impractical to explore. Typically, the time required to obtain data at frequencies of <0.01 rad/sec or 0.006 Hz is impractical for a laboratory schedule. (At 0.006 Hz, each data point would take 167 sec for a single iteration most rheometers perform at least two or three iterations.) Furthermore, samples may change or degrade in nonsterile conditions over extremely long tests (i.e., hours). If it is desirable to obtain... 

Because the duration for one measurement is very short (e.g., with a 1-Hz input, a cycle is completed in 1 sec), a dynamic test is suitable for gaining information in a short time frame or for monitoring time-dependent changes in gel network properties. When monitoring the gelation process at a fixed frequency, it usually takes a few hours for G to become approximately constant. The constancy can be judged by a constant value of G at a fixed frequency during a subsequent frequency or strain sweep test, which usually takes several minutes. 

The four variables in dynamic oscillatory tests are strain amplitude (or stress amplitude in the case of controlled stress dynamic rheometers), frequency, temperature and time (Gunasekaran and Ak, 2002). Dynamic oscillatory tests can thus take the form of a strain (or stress) amplitude sweep (frequency and temperature held constant), a frequency sweep (strain or stress amplitude and temperature held constant), a temperature sweep (strain or stress amplitude and frequency held constant), or a time sweep (strain or stress amplitude, temperature and frequency held constant). A strain or stress amplitude sweep is normally carried out first to determine the limit of linear viscoelastic behavior. In processing data from both static and dynamic tests it is always necessary to check that measurements were made in the linear region. This is done by calculating viscoelastic properties from the experimental data and determining whether or not they are independent of the magnitude of applied stresses and strains. 

The procedure for performing FTMS measurements using a controUed-stress instrument differs only slightly from conventional test procedures. The test is configured in the same manner as in a time sweep but in this case the selected frequency acts as the fundamental from which further harmonics are selected, with each harmonic frequency being an integer multiple of the fundamental frequency. 

Fig. 22 (a) True stress (deformation ratio (A) of a surfactant-containing HM PDMA hydrogel from cyclic compression (A < 1) and elongation (A > 1) tests. The tests were conducted with increasing strain, with a waiting time of 7 min between cycles, (b) G filled symbols), G" open symbols), and tan S curves) of a HM PDMA hydrogel shown as a function of the strain fro) at ft) = 6.3 rad s . Sweep tests were conducted in up dark red circles) and down directions blue triangles), as indicated by the arrows Co= 15 % (w/v), C17.3M = 2 mol%, SDS = 7 % (w/v), NaCl = 0.5 M. From [41] with permission from Elsevier... 

The modes of operation of a DMA are varied. Using a multifrequency mode, the viscoelastic properties of the sample are studied as a function of frequency, with the oscillation amplitude held constant. These tests can be run at single or multiple frequencies, in time sweep, temperature ramp, or temperature step/hold experiments. In multistress/strain mode, frequency and temperature are held constant and the viscoelastic properties are studied as the stress or strain is varied. This mode is used to identify the LVR of the material. With creep relaxation, the stress is held constant and deformation is monitored as a function of time. In stress relaxation, the strain is held constant and the stress is monitored versus time. In the controlled force/strain rate mode, the tanperature is held constant, while stress or strain is ramped at a constant rate. This mode is used to generate stress/ strain plots to obtain Young s modulus. In isostrain mode, strain is held constant during a tanpera-ture ramp to assess shrinkage force in films and fibers. 

Generally, actuator displacement or time-sweeps are only marginally adequate for detecting specimen stability. Success depends upon the choice of scales, the test system noise levels, and the fracture behavior of the material. Locally placed extensometers and strain gages are usually excellent. Figure 2.2 shows load as a function of various detection methods for a-SiC. 

Rheological tests of epoxy resin were performed using a TA Instruments Discovery Hybrid Rheometer with a peltier plate fixture of 25-mm disposable parallel plates. Similar to DSC tests, batches of approximately 50 g of epoxy resin, with GO contents of 0,0.05,0.1, and 0.2 wt%, and the appropriate amount of hardener were mixed. Samples of approximately 2 mL of mixed resin samples were pipetted onto the rheometer bottom plate. Rheological tests were performed with a 0.5-mm plate gap at a constant shear rate of 2.5 s Temperature sweep tests were completed for the determination of temperature-dependent viscosity evolution and were performed on all samples from 25 to 95 °C. Isothermal rheology tests were conducted at 80 °C to determine the viscosity evolution with respect to time. This temperature was selected after investigating the dynamic DSC scans in which peak temperature was approximately 82 °C. 

Rest Time before test. When a gluten-water mixture was examined in a 40 minute sweep test at 10% strain amplitude and 1 Hz frequency, no significant difference in the rheological response was noticed, whether the text was carried out just after mixing or after 60 minute rest [101]. This suggested that relaxation phenomena, which took place in gluten network as a eonsequence of manual mixing, could evolve too rapidly or too slowly with respect to the experimental time scale. 

Tests preeeding the fi equency measurements were an oseillatory time sweep and an amphtude sweep in order to establish thermal stability and linear viseoelastieity guidelines. 

The strain temperature sweep measurement is conducted with a preselected amplitude for the applied strain (y) and a constant frequency (f). The changing parameter is the temperature T, which is given in a temperature-time profile [T = T(t)]. This test method serves to illuminate the structural build-up, the softening, the melting and the gelation of pectins influenced when the temperature changes. 

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