Abstract

The results that will be presented in this seminar are a mixture of old and new. Most of them are related to the techniques developed at the LMS during the last 15 years in the field of dynamic testing. The fundamentals of force and displacement measurements based on the knowledge of wave propagation in a bar will be presented. An evaluation of the various assumptions made and their quality will be proposed in order to optimise the accuracy of the measurements, in particular when considering the classical Split Hopkinson Pressure Bar (SHPB). Various aspects of measurement problems will be emphasised such as force matching, optimising specimen dimensions, precise shifting of the waves (including damping and dispersion when viscoelastic bars are used). Specific problems found when extending the technique to tension or shear testing will also be addressed. A recent method for the measurement of dispersion relations in bars will also be described. A multi-point measurement technique will also be discussed. This may lead to the possibility of using bars for making measurements of unlimited time duration. The technique is based on a wave separation method making use of the maximum likelihood method. This method, applied in the processing of the strain and velocity measurements recorded at various points on a bar, takes wave dispersion into account. It permits the calculation of stress, strain, displacement and velocity at any point on the bar. Since there is no intrinsic limit to the measurement time in an SHPB , the maximum strain in a given specimen calculated using this technique no longer decreases with strain rate, as is the case for the classical processing method. The technique will be illustrated by test results obtained on various materials for a wide range of strain-rates.