Extensometers

Ausdehnungs-arbeit,/. work done in expanding, -koeflizient, m. expansion coefficient, -messer, m. dilatometer extensometer. -vermOgen, n. expansibility extensibility dilatability. -zahl, /. expansion coefficient, ausdenken, v.t. think out. devise, conceive, ausdeuten, v.t. interpret, explain, ausdorren, v.t. dry up, desiccate season (timber). 

Delmung, /, extension stretching elongation tension expansion dilation, delmungsfahig, a. extensible expansible, DelmungS kraft, /. = Dehnkraft. messer, m. extensometer dilatometer. -warme,/. heat of extension. zahl, /. coefficient of extension or expansion. 

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. 

After having performed such tests the new user should be able to assess whether it appears reasonable to study the scientific problem at a synchrotron beamline. Sometimes one will simply be able to use devices (furnace, extensometer, sample recipient) provided at the beamline. Sometimes the researcher will have to adapt some own devices to fit in the beamline, to control it remotely and to record its output signals together with the scattering patterns. Sometimes special equipment will have to be constructed. 

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. 

In order to have better accuracy, technologist Sarah Kushon of the Dow Chemical Company utilizes an extensometer during a tensile test. With permission. 

In the next paper 67), the same extensometer was used with a rod of porous glass 11 cm. in length. The surface area was 117 m.Vg- calculated from water isotherms. Length changes were measured with a sensitivity Al/l of 2 X 10 and were measured for water adsorbed at 11.8, 18.7, and... 

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... 

Extensometer for Testing High Explosive Materials. Proposed is a design for an extensometer to measure unit length changes upon compression or tension in i4x l-inch... 

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... 

Figure 3. JANAF specimen with plastic film extensometer in place...

Figure 4. JANAF specimen with Farris (27) extensometer...

A more delicate and precise instrument is the Searle s Extensometer, described in Ref 2, p 68 (Compare with Compressibility of Explosives and of Propellants in Vol 3 of Encycl, p C491)... 

Extensometer for testing high expl materials 6E497... 

Automation frequently aids accuracy and/or reproducibility by being more consistent than humans. Non-contact extensometers ensure no unwanted stresses on the test piece and any automatic extensometer will be less subjective than a technician with a ruler. Digital thermometers, load cell balances and many other apparatus introductions have made measurements easier and less prone to operator error. 

Methods have not been standardised for measuring Poisson s ratio for rubber but the most obvious approach is to use a second "extensometer" to measure the change in the width of the test piece during a normal tensile test. 

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. 

Alternative procedures are to use a dilatometer to measure volume change or to calculate Poisson s ratio from measurement of two moduli. Laufer et al5 concluded that for soft rubbers the dilatometer is the best approach and describe a suitable apparatus. Kugler et al6 have given a review of attempts at measurement and describe an optical system which they employed on a series of filled rubbers. It would seem reasonable that modem instrumentation, such as a video extensometer, could be used but this does not appear to have been reported. 

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. 

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. 

One grip is fixed and the other attached to a crosshead which is motor driven so as to extend the test piece whilst the force is monitored by the load cell. The output of the load cell, and an extensometer (or crosshead movement), is fed to a computer. 

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. 

The first commercial non-contact extensometers were optical extensometers which used either visible or infrared light to illuminate targets on the test piece. The essential difference between the optical and contact types is in the method of following the extension as illustrated in Figure 8.12, thereafter they both use some form of transducer to measure the movement which largely dictates the range and sensitivity. 

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... 

---