Mechanical extensometers

The mechanical performance of all samples was evaluated by tensile testing with an Instron 5567 machine. As the samples used were too weak to support a conventional mechanical extensometer, an Instron Advanced Video Extensometer model 2663-821 that allows for accurate non-contact strain measurements was used. The tensile testing was performed according to the ASTM standard D882 with a nominal gauge length of 50 mm, a crosshead speed of 6 mm/min, at a temperature of 21.5 °C, and a humidity of 50%. As per the standard, the results of at least five specimens were averaged for each sample. They are summarized in Table 11.2. 

The principle and use of an optical non-contact extensometer available commercially has been described in some detail77. Two photoelectric sensing devices automatically follow, by means of a servo mechanism, contrastingly coloured gauge marks on the test piece. The separation of the auto followers... 

The extensometers described above measure the overall strain. Bilgili81 has argued that standard mechanical tests are inadequate for non-homogeneous rubbers and that measurement of full-field displacement is needed. The application of speckle extensometry to rubbers to obtain the two-dimensional field of in-plane displacements has been demonstrated82. 

Figure 6. A membrane mechanical testing system consists of a MTS horizontal load frame, a home-built environment chamber with temperature and RH control, and a high-precision video-extensometer.

Mechanical tests were carried out with an Instron 1123 mechanical test machine operated at a crosshead speed of 2 mm/min. Moduli were determined using rectangular bar specimens that were pulled in tension using an extensometer to obtain accurate strain measurements. Initial slopes of the stress-strain curves represent the moduli. Strength measurements were made using ASTM Type V tensile bars (cut after the composites were produced) that were pulled in tension, and the maximum tensile stresses attained were taken as the strength values. 

Mechanical properties of the composite materials were tested by a hydraulic-driven MTS tensile tester manufactured by MTS Systems Corporation, Minneapolis, Minnesota. A strain-rate of 5x 10 5 s 1 was used. During deformation, the linear actuactor position was monitored and controlled by a linear variable differential transformer (LVDT), while strain was measured using MTS-brand axial and diametral strain-gauge extensometers. The axial extensometer serves to measure the tensile deformation in the direction of loading while the diametral extensometer serves to measure the compressive deformation at 90° to the loading axis due to Poisson s contraction. All tensile tests were performed at 23 °C and in accordance to ASTM D3518-76. 

Mechanical testing was conducted on samples 25 mm in length, 10 mm in width and 0.1 mm in thickness, Because of the high compliance of the films, it was difficult to mount an extensometer on the samples to measure E. A DMA was therefore used to assess the elastic properties of the composite samples in the tensile mode of loading. A 5 N static tensile load and displacement amplitude of 16 /tm at a frequency of 1 Hz were applied. Nine measurements for each sample were made and the average values are reported here. 

The use of 0.05% strain as the lower limit for determining modulus is a very stringent requirement and needs an extensometer accurate to at least I micrometer. There arc also implications for the test piece in terms of flatness or warping and for the precision of the gripping mechanism which, frankly, are unrealistic in some cases. It may be that some of these points will be addressed at the next revision of the standard. 

The Al-stabilized, 0.91-mm wire containing 121 NbTi filaments was tensile-tested in an Instron machine with a clip-on extensometer. The crosshead speeds were 8 nm/sec until the 0.2% yield strength and 40 nm/sec until wire fracture. The mechanical properties are compared in Table I with those of a Cu-stabilized superconductor tested in a similar manner. Although the yield stress and ultimate strength of the A1 conductor were lower than those of the Cu conductor, the A1 content may not have been causing the weakness. The lower proportion of NbTi in... 

The initial part of the curve, OA in Fig. 1, is the characteristic linear-elastic behavior of the material, i.e., the extension that occurs is fully reversible and the relationship between the force and the extension is linear. At an atomic level the bonds between the atoms of the crystal structure are just flexing. The extension in this region is however very small and can only be measured using special extensometers. This linearity ceases at point A and the material starts to behave irreversibly, i.e., permanent or plastic deformation occurs. This phenomenon is known as yielding. In this region the atoms take up new position relative to each other by the mechanism of dislocation activation. 

ISRM. (1992). Commission on Testing methods. Suggested methods for installation of borehole extensometer. International Society for Rock Mechanics. 

The video-extensometer eliminates the use of an external extensometer, which can cause micro-cracking during handling. Furthermore, small-sized specimens in the micron range may be tested using this method. Here, the video-extensometer was used to evaluate the tensile properties of silicon carbide monofilaments 100 pm in diameter and to determine the change in distance, A1 between the marked targets, caused by the mechanical strain to the specimen. The strain is then calculated in the conventional manner, as indicated in Eq. (1.6), rewritten here as ... 

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