Extensometer

Abstract: Miniature tensile specimens, having various sizes and geometries, are often used to measure the mechanical properties of bulk nanostructured materials. However, these samples are generally too small for use with conventional extensometers so that the strains are usually calculated from the crosshead displacements. This study uses experimental results and finite element modeling (FEM) to critically evaluate the influence of the specimen dimensions and strain measurement methods on the tensile curves obtained from miniature specimens. Using coarse-grained Cu as a model material, the results demonstrate that the values of strain obtained from the crosshead displacement are critically influenced by the specimen dimensions such that the uniform elongation and the post-necking elongation both increase with decreasing gauge length and increasing specimen thickness. The results provide guidance on the optimum procedures for the tensile testing of miniature specimens of both coarse-grained and nanostructured materials.

Abstract: Extensive diametral repeated load tests were performed on asphalt concrete specimens. Extensive repeated load triaxial tests were performed on base and subgrade materials. New resilient modulus test methods were developed for these materials and presented in the form of American Association of State Highway and Transportation Official (AASHTO) type test procedures. The repeated load test is not easy to perform and must be carried out on a carefully calibrated testing system including using synthetic specimens. To minimize human errors, a fully automated, closed loop testing and data acquisition system should be used. For asphalt concrete, the Strategic Highway Research Program (SHRP) Load Guide diametral testing device should be employed to minimize rocking. A mountable extensometer provides less variance and hence better repeatability for resilient modulus measurement. Poisson's ratio can be evaluated using a surface mounted linear variable differential transformer (LVDT) to measure vertical deformation and externally mounted LVDTs to determine horizontal deformation. The value of Poisson's ratio used to determine the resilient modulus has an important influence on its value. Poisson's ratio should be measured rather than assumed. To reliably determine resilient modulus, axial deformation of base and subgrade materials should be measured on the specimen using either (1) an optical extensometer, (2) non-contact proximity gages or (3) small LVDTs supported by lightweight clamps. The optical extensometer is highly recommended. For granular materials a vacuum triaxial test offers an excellent approach when an optical extensometer is used. An unconfined repeated load test is recommended for cohesive subgrade soils. For cohesive soils, bedding irregularities at specimen ends, specimen aging and soil structure must all be properly considered to obtain reliable resilient moduli.