Abstract:The physical characteristics of deformation of the material at the pole of the hydraulic bulge specimen was analysed. Based on this analysis and Hill's hypothesis about the moving track of the material particle of bulge specimen, a method was established to determine the ratio of the two principal stresses at the pole of the orthotropic sheet sample directly from the experimental data, thus to determine the states of stress and strain of the pole. The theoretical value of the ratio stated relates to the yield function used. The application proves that the Hosford type yield function can give better results and is convenient for use.

Key words: orthotropic sheet; hydraulic bulge specimen; pole; stress state; strain state