We present the results of laboratory simulations of irradiation of hydrogenated carbon grains by cosmic rays. Experiments have been performed under diffuse and dense interstellar medium conditions, using 30 keV He⁺ ions. In both conditions, ion irradiation leads to a systematic decrease of the 3.4 . The destruction cross section of C-H bonds by cosmic ray has been obtained from the reduction of the 3.4 band intensity as a function of the ion fluence. The results of this work strengthen the evolutionary view according to which the observed difference of the band between dense and diffuse medium can be interpreted as a transformation of the aliphatic component of the hydrogenated carbon grains produced by cosmic ray, UV photons and H atoms.