Volume 40, Issue 6 (November 2012)
A New Method for Strain Measurement in the Tensile Testing of Filamentous Materials
A new method (common mode subtraction) for strain measurement in the tensile testing of filamentous materials is presented. In this method, crosshead displacements were divided into the specimen extension and the specimen extension error, the latter is associated with the testing system (machine frame, load-cell, load-string, and grips) and force, but assumed to be independent of the gage length. Common mode tension tests for steel and nylon tire cords at different gage lengths with two types of grips were performed at first. The test results showed an obvious existence of the specimen extension error, which leads to a dependence of the engineering strain on the gage length. The specimen extension error was found to be indeed independent of the gage length and increase with force nonlinearly. Then subtractions between any two of the obtained force–crosshead displacement curves were carried out to remove the specimen extension error, and then the actual force–strain curves (linear for steel cord and nonlinear for nylon cord) were obtained. Finally, comparisons between the common mode subtraction method and other methods, which were reported to be able to correct the specimen extension error, were made. The comparison revealed the relative stiffness is the key, that is, when the specimen is “weak” in relation to the testing system, the grip is the main source of error, but if the specimen is much “stronger,” the influence of the system compliance will become significant. The comparison also showed the common mode subtraction method is clearer and more general in theory, as well as more applicable and efficient in practice.