The method of measuring short-time mechanical properties of materials of cladding tubes during the process of expansion of circular specimens is conventional in reactor material science. One of the method limitations is procedure errors caused by the specimen bending on the semicylindrical supports at the initial stage of expansion. The bending induces inhomogeneous plastic deformation of the specimen working parts, which results in the inaccurate evaluation of the yield strength and the plastic properties of the material. This paper contains an analysis of the plastic strain distribution at the different stages of the expansion of the circular specimens of a cladding tube made of austenite steel. The results of the comparison between the true relative deformation and the estimated deformation of the working part are presented together with the assessment of the impact of strain hardening of the material on its mechanical properties.