Strain extensometer

Exploratory creep tests were performed on tensile specimens of NT-154 silicon nitride at 1300 and 1370 C, These specimens were made from the same lot of the material, designated as CP-serles, used In the cyclic fatigue tests discussed In the last progress report, Creep strain was measured using the laser diffraction strain extensometer described elsewhere,2 Figure 7 shows the creep curves of specimens CP-28 and CP-10 tested at 1300 C under applied stress of 160 and 180 MPa, respectively. Both tests were shutdown Inadvertently due to equipment adjustments. The problems have been corrected since then. Therefore, the results of the tests do not represent the creep life of the specimens. However, there were sufficient data points to delineate the essential features of the creep behavior up to the steady-state phase which was clearly definable. Both curves showed a reversed Inflection at the transition between the primary and secondary creep phases, resulting In forming a bump In the otherwise smooth creep curves. 

Strain extensometer A device for determining elongation of a test specimen as it is strained when conducting tests. 

Tensile tests were carried out, following the 150527-2 1997 standard, in order to measure the yield stress, the stress at break and the strain at break. Specimens with 10x115x4 mm in the narrow section were tested on an electromechanical testing machine (MTS Alliance RT/ 5) under displacement control at a cross-head sp>eed of 50 mm/min. Strain values were measured with a high strain extensometer (model MTS DX2000) attached to the sample. 

Extensometer An extensometer is an instrument to monitor strain in the linear dimension of a test specimen while a load or force is applied to it. The automatic plotting of load with strain produces stress-strain curves. 

In one current closure site in California, a waste transfer facility with an 18-ft wall is being built within a 30-ft trench on top of a 130-ft high landfill. The waste transfer facility will settle faster than the adjacent area, causing tension at the edge of the trench. Electronic extensometers are proposed at the tension points to check cracking strains in the clay cap and FMC. 

Physical characterization of polymers is a common activity that research and development technologists at the Dow Chemical Company perform. A material property evaluation that is critical for most polymer systems is a tensile test. Many instruments such as an Instron test frame can perform a tensile test and, by using specialized software, can acquire and process data. Use of an extensometer eliminates calibration errors and allows the console to display strain and deformation in engineering units. Some common results from a tensile test are modulus, percent elongation, stress at break, and strain at yield. These data are then used to better understand the capabilities of the polymer system and in what end-use applications it may be used. 

The raw data from a tensile test are the load versus elongation measurements made by the load cell and the extensometer, respectively. To eliminate sample geometry effects, the extension is divided by the initial length to obtain the dimensionless strain (which is occasionally multiplied by 100 and reported as % elongation for samples... 

Other techniques for measuring strain more satisfactorily in the gage section of the JANAF specimen have been developed. One inexpensive and simple method is the use of a clear plastic film extensometer which is attached to the gage section of the specimen. A mark on the face of the specimen is pulled past evenly spaced lines on the clear film, and the... 

Since the lateral contraction is half the tensile strain and the width of dumbbells is much smaller than the gauge length, a very high performance is needed from the "extensometer" to achieve sensible accuracy. Not surprisingly, accurate measurements have proved very difficult to obtain. 

Various considerations when producing data have been discussed by Kim et al20. An experimental scheme for efficient characterization has been proposed21 and an intermediate approach between using simple uniaxial tension and two independent strains to obtain input data given22. A novel technique based on use of a speckle extensometer to give the whole displacement field in two dimensions has also been described23. 

Whilst it is generally held that an extensometer is necessary, it would be rather less expensive if elongation of dumbbells could be obtained from crosshead movement. Tay and Teoh76 devised a numerical scheme whereby the stress strain characteristics could be derived from measured load versus total elongation data from a finite element analysis of the dumb-bell shape. Their method was shown to work to within 10% of values measured with an infra red extensometer for two fairly soft plastics and a silicone rubber. To be effective, the tensile test must be carried out with grips which essentially prevent any slippage and it is, of course, necessary to have the computing facility set up to carry out the analysis. 

Laser extensometers are relatively cheap to produce compared to the optical type and can be used through the window of an oven but the problems of diffraction in the glass are more severe. There may also be difficulties with cord specimens or if the marks distort. Accuracy at low strains is limited by the measurement of angle but a useful advantage is that the gauge length need not be known. 

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. 

Pieces of the compression-moulded sheets were pressed into dogbone-shaped specimens of thickness 1.7 mm and a gauge width of 15 mm for the tensile tests. Stress-strain curves were obtained at room temperature with a Zwick tensile tester at a strain rate of 0.0001 s. The samples were strained up to 3.6% strain and the strain was measured with an extensometer with a gauge length of 50 mm. The tensile tests were done only on the composites with 50 vol% glass beads, with different amounts of the silane. 

Additive Extensometer tensile modulus, E (GPa) DMA" storage modulus, E (GPa) Tensile strength, a (MPa) Failure strain, (e) (%) Toughness (MPa) Poisson ratio (v)... 

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. 

CED is determined by calculation, using the Van Krevelen increment values (Van Krevelen, 1990). There is no apparent effect of the crosslink density on K, which seems to depend (as CED), only on the molecular scale structure (polarity, hydrogen bonding). The following results were obtained with quasi-static tensile measurements (10-3-10 4 s-1 strain rate) in a temperature range 200 K-Tg, using a bidimensional extensometer to determine E and v (from which K and G could be determined) (Verdu and Tcharkhtchi, 1996) ... 

But true stress (load/actual section) and true strain, calculated as the natural logarithm of the relative height, n(L/L0), must be used to obtain better information on the material. The accuracy of strain measurements may be improved by the use of extensometers or strain gauges and actual cross section determination needs the use of double extensometers or stiffness correction of the machine and rigs (Cook et al., 1998). 

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. 

Fig. 10. Strain-to-break measured by axial extensometer as a function of sub-Tg annealing time...

Videometric extensometer made it possible to evaluate the volume change during deformation as it gives access to the strains in the three directions of the space. 

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