Abstract

Part of the lecture will focus on the behaviour of polymer tubes under growing internal pressure. The polymer is characterized by a finite strain elastic-viscoplastic constitutive relation and the calculations are carried out using a dynamic finite element programme. During the tube expansion the onset of necking and subsequent neck propagation is studied. First the idealised assumption of plane strain conditions is considered. Subsequently, the behaviour of a long thin pressurized tube is analyzed, where bulge formation interacts with the necking. Two types of imposed loading are prescribed, either a pressure that increases linearly with time, or an enclosed volume that increases linearly with time. The necks propagate in both the circumferential and the axial directions.

Another part of the lecture will consider the behaviour of voids in a ductile metal subject to pure shear or to a shear dominated stress state. Here the stress triaxiality is so low that instead of void volume growth to coalescence there is void closure leading to micro- cracks that rotate in the shear field. At some stage of the deformation the void surfaces will come in contact so that sliding with or without friction will start to occur. If a maximum overall shear stress is reached, localization of plastic flow will occur, leading to final failure of the material. In the micromechanical model to be presented, an internal pressure is used to approximately represent void surfaces pressed together in frictionless sliding.