Efficient simulation and process mechanics of incremental sheet forming

Single Point Incremental Forming (SPIF) is a displacement controlled process performed on a CNC machine. A clamped blank is incrementally deformed by the movement of a small-sized tool that follows a prescribed lengthy tool path. The strain achieved by the SPIF process is higher than the strain achieved by classical forming processes e.g. deep drawing. This motivated many researchers for the last two decades studying the process mechanics and still a definite explanation is missing. The finite element method is a powerful tool in studying the forming processes. Compared to e.g. deep drawing, the FE model for SPIF is very simple. However, simulation of the process is a challenging task because of the enormous computing time as a result of performing thousands of load increments on a relatively fine FE model. This limits the use of the finite element method to simple academic cases that already require weeks of computing time. The focus of this thesis is to efficiently use the implicit time integrationmethod in order to drastically reduce the required computing time for incremental forming simulation. In addition, this thesis presents a fundamental study on a particular aspect of the process mechanics involved in the SPIF process. The study is carried out on the continuous bending under tension (CBT) process. It has the advantage of reducing the 3-dimensional complex bending in the SPIF process to a merely 2-dimensional case. It is shown that combined bending and tension can stabilize the deformation of a strip to a high level of strain. An increase of the force is required to introduce additional stable deformation. This condition requires that the averaged tangent stiffness has to be larger than the averaged stress. The presence of compressive stress reduces the average stress while the elastic fibers increase the average tangent stiffness of the cross section. Bending introduces both the compressed fibers and the elastically loaded fibers. A further analysis is carried out on the achieved cyclic force–displacement curve of the CBT test. The cycle consists of two parts: steady and transient. The part having a steady level of force represents the deformation of the strip governed by significant curvature change of the strip because of bending. The transient increase of the force results fromthe deformation of the strip by increasing the tension force with no significant change in strip curvature.