When a uniaxial test under tension or compression is performed, the uniformity of the longitudinal stress field in specimens is of major importance. Every imperfection of the testing system as misalignment of grips, or as bad orientation of gripping planes, induces a perturbation of the stress field in the working part of the specimen due to a parasite bending. To be valid, a tensile testing system shall apply on specimens a percent of bending strain (PBS) lower than a critical value. The aim of this work is to study the effects of imperfections of the gripping system on the level of the PBS, and to determine the easiest experimental procedure to measure the PBS on a specimen (number and localisation of strain gauges). This study has been done with finite elements codes in order to simulate the defects applied on the specimens by the gripping system.