Compressive measurement sample preparation

Sample preparation and measuring apparatus has been discussed and hence this section will deal with the principal item, the tensile machine, together with grips and extensometer. The tensile machine is in fact very often a universal machine in that, apart from tensile tests, it can also be used for flexural, compression, tear and adhesion tests. The basic elements of a tensile machine are shown diagrammatically in Figure 8.10. 

NOTE Preparation of specimens requires that dimensions be measured precisely. It is assumed that the dimensions of the compressed specimen are small with respect to the dimensions of the contacting surfaces (plates) to ensure that the sample remains in contact with the plates. Occasionally, it is desirable to compress a sample in its natural state (i.e., no specimen cutting). 

The time required for one TPA test of a sample is 5 to 10 min, which includes sample positioning and instrument cleaning. It is usually the case that sample preparation (e.g., cooking) requires much more time than TPA measurements. Most uniaxial compression instruments offered to the food industry are computer controlled and have software to immediately calculate the TPA parameters. 

From previous statistical studies on similar systems which Include the combination of errors Involved in sample preparation and measurement of static compressive stress-strain characteristics, the following reproducibility results were obtained (4,5, ) ... 

Specimens used for compressive strength measurements were prepared by a modified pultrusion process. Bundles containing carbon and HP-PE yarns in known proportions were immersed in a bath of epoxy resin and pulled into tubes with PTFE-inserts (6 mm internal diameter). Carbon and HP-PE yarns were intermingled as homogeneous as possible. The samples were cured for 4 hours at 80 C and then post-cured at 110 C for 12 hours. After curing specimens were removed from the mold and cut into 12 mm lengths using a low speed diamond saw. Volume fraction of fibres was held constant at 50% and the proportions of HP-PE fibre in the composite was varied from 0 to 100% with steps of 20%. 

Fibrous materials may be analyzed by a number of different methods, including the standard method for sample preparation—compressed halide pellets. In this latter case, normally the fiber structure is destroyed during sample preparation. Alternatively, one can consider using diffuse reflectance, photoacoustic, or infrared microscopy. With due consideration to the preparation procedures, the fibers may be retained in their original form with these methods. In some cases, such as with IR microscopy, it is possible to study the orientation characterisitics of the fiber material and to correlate this information to certain mechanical properties of the fiber. The use of IR polarizers is imphed with this type of measurement. Diamond compression cells work well for single-filament fibers by both transmission (diamond anvil cell) and ATR. 

In studies performed in our lab, we use a fully automated purification device developed by Lunkenheimer et. al. which ensures a complete removal of these unwanted trace impurities [18]. The aqueous stock solution undergoes numerous of purification cycles consisting of a) compression of the surface layer, b) its removal with the aid of a capillary, c) dilation to an increased surface and d) formation of a new adsorption layer. At the end of each cycle the surface tension cr is measured. The solution is referred to as surface chemically pure grade if ae remains constant in between subsequent cycles. Quite frequently more than 300 cycles and a total time of several days are required to achieve the desired state. The sample preparation is time consuming and tedious but mandatory for the investigation of equilibrium properties of adsorption layers of soluble surfactants at the air-liquid interface. 

In the surface washing method, a sample containing a certain coneentration of additive was kept under controlled temperature and was immersed in time intervals for 5 seconds in diethyl ether. After evaporation of diethyl ether, the amount of removed additive was weighed. Data obtained from this test had large scatter because small amounts of extracted additive cannot be determined with high precision. The major problem of the method, which cannot be verified by data, is in diffusion of solvent into the material, which inadvertly modifies the diffusion process, and most likely its kinetics as measured by the method. It was observed that results depended on the method of sample preparation (extmded or compression molded), and its thickness. ... 

This need for thin films for quantitative analysis usually means that polymer film samples should be prepared in the 10-100-p,m range of thickness. Solvent casting or compression molding of the samples is usually required to prepare such thin films and, unfortunately, these sample preparation techniques transform the sample through melting or dissolution, with the loss of the thermal and process history of the polymer sample. The film samples must be randomly oriented, because nonrandom chain orientation influences the value of the measured absorbances. 

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