Elongation mechanisms measurements

There are two further related sets of tests that can be used to give information on the mechanical properties of viscoelastic polymers, namely creep and stress relaxation. In a creep test, a constant load is applied to the specimen and the elongation is measured as a function of time. In a stress relaxation test, the specimen is strained quickly to a fixed amount and the stress needed to maintain this strain is also measured as a function of time. 

Before the initial mechanical measurements on NbsSn conductors were made, a series of copper and aluminium dogbone-shape tensile specimens were tested at 300 and 4.2 K. Data were obtained on the Young s modulus, yield strength, tensile strength, elongation, and reduction in area for several samples of each material at each temperature. These results gave a reproducibility of better than 3% for each material and an agreement with published results on similar material to within 5%. 

Qian et al. conducted a study on bamboo particles (BP) that were treated with low-concentrations of alkali solution for various times and used as reinforcements in PLA based composites [35]. Characteristics of BP by composition analysis, scaiming electron microscopy, Brunauer-Emmett Teller test, and Fourier transform infrared spectroscopy, showed that low-concentration alkali treatment had a significant influence on the microstructure, specific surface area, and chemical groups of BP. PLA/treated-BP and PLA/untreated-BP composites were both produced with 30 wt% BP content. Mechanical measurements showed that tensile strength, tensile modulus, and elongation at break of PLA/BP composites increased when the alkali treatment time reached... 

In this context rheo-optical FTIR spectroscopy has proved a valuable technique to study the phenomenon of strain-induced crystallization on-line to the deformation process of the elastomer under investigation. Whith the aid of an appropriate absorption band which is characteristic of the threedimensional order in the crystalline phase the onset and progress of strain-induced crystallization during elongation and its disappearance upon recovery can be unambigously monitored simultaneously to the mechanical measurements. Representative for several rubber-like materials which have been investigated by this technique in our laboratory the results obtained with sulfur-crosslinked (1.8 % S) natural rubber (100% 1,4-ds-polyisoprene) and a radiation-crosslinked synthetic polyisoprene (93% 1,4-ds-isomer) lall be discussed in some detail here. 

Mechanical evaluation of the tensile properties was performed on a Tinius Olsen HI OKS bench top tensile tester at a speed of 5 mm min". A stretching force was applied to one pneumatic clamp. Clamp displacement was used to determine elongation ratio and strain. Tensile stresses were calculated from the initial cross-sectional area of the sample and applied load. Young s modulus, E, were determined from the initial slope at A = 1.025 (least squares fit, s = 2.5%). Three to five samples per mechanical measurement were used, with estimated error/uncertainty not exceeding 10% in the data. The membrane samples were cut into a rectangle shape 50 mm X 4 mm (total) and 25 mm x 4 mm (test area). 

However, the method requires elongational measurements of gum rubber and its unvulcanised compound. The experiments as well as the calculation are somewhat involved. If only a relative measure of the filler-rubber interaction is needed, dynamic mechanical measurements and subsequent comparison of modulus of the compound against that of the gum rubber are much simpler. Such comparisons with N550 carbon black were made with the samples listed in Table 9.2 and the moduli data of Figures 9.1 and 9.2 [12]. The modulus ratio is found to be independent of the frequency of the dynamic measurements but varies considerably among different gum rubbers. 

Loss of elongation is the most sensitive criterion for aging measurement regardless of mechanism, and it is favored over measurement of tensile loss for cured compounds. In synthetic mbber production (SBR, in particular), viscosity increases with aging and can affect processing if not prevented. 

As with other plastics materials, temperature has a considerable effect on mechanical properties. This is clearly illustrated in Figure 13.5 in the case of stress to break and elongation at break. Even at 20°C unfilled PTFE has a measurable creep with compression loads as low as 3001bf/in (2.1 MPa). 

Thermal-mechanical analysis (TMA) has proven a more reproducible measure of melt integrity [20]. The TMA test involves measuring the shape change of a separator under load while the temperature is linearly increased. Typically, separators show some shrinkage, then start to elongate, and finally break (see Fig. 5). 

One of the most informative properties of any material is their mechanical behavior specifically the determination of its stress-strain curve in tension (ASTM D 638). This is usually accomplished in a testing machine by measuring continuously the elongation (strain) in a test sample as it is stretched by an... 

The attenuations resulting from this mechanical elongation are not significant and difficult to be accurately measured. They are lower than 1% (Fig. 13). 

The samples were tested at a deformation rate of 1 in./min. for the simple tension experiments. In the case of stress-relaxation measurements, the samples were prestrained to 7% elongation at e = 5 in./min. then allowed to stress relax over a 20 minute period. All mechanical testing were carried out at room temperature. 

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