Photoelastic investigations on initial plastic straining

Photoelasticity has been used for an investigation of the stress distribution in billets forged on flat dies. The flat models of the billet were made of birefringent material which were loaded to a state approaching the elastic limit. The authors' experiments were based on Nadai's hypothesis that in the first stages of plastic straining the stresses and strains are commensurate with the corresponding extremal values in the elastic range. The stresses were analysed for different values of feed ratio, i.e. relation of (actual) width of contact area between the die and the billet to its height. The experiments showed that if the feed ratio is less than 1·3, tensile stresses are present in the central zone of the billet under the die centre. There are no unfavourable tensile stresses in the central zone of the billet if the feed ratio is more than 1·3. However, for all values of feed ratio at points on the billet axis at a distance of 0·2 of the height of the billet from the die edge, tensile stresses are present. These conclusions lead to practical recommendations, concerning the forging of ingots and bars of complex brittle alloys at a maximum feed ratio.