An attempt was made to estimate the value of the deformation energy of a metal specimen and the research system during impact strength tests on pendulum hammers. In the experimental research it was found that the rectangular metal specimens of the same cross-sectional area demonstrate different impact strength, depending on the direction of the load (bending stiffness) whereas the destruction work of such samples exposed to static bending is comparable. The article presents the results of the experimental research, completed with numerical calculations carried out to assess the value of the deformation energy during the impact tests. By performing numerical calculations, the authors estimated the deformation energies of specimens characterised by elastoplastic properties with a reinforcement (bilinear) under destructive loads. The energy of the elastic deformation of the hammer arm was estimated analytically. Based on the research, it was found that in the impact strength test, a large part of the recorded energy is connected with the deformation of the test device, in particular, the pendulum which undergoes bending. Also the recorded impact strength of the material is not proportional to its actual impact strength.