Numerical and experimental study of sheet metal hydroforming AIP Conf.Abstract. In order to find a feasible method to evaluate the deformation of tubes during the Tube Hydroforming (THF) process, the hardness and the strain in two selected deformation areas of hydro formed copper tubes (CI 1000) were measured and tested, and an instinct relationship was found between the hardness and the principal strains of the tubes. The major strain of the surface of tubes had the strongest linear relationship with hardness. A regression formula was used to describe the relationship between hardness and the sensitive strain which is defined in the present work as a dependent variable of major strain and thickness strain.