Implement a Modified Viscoplasticity Based on Overstress Model into Numerical Simulation of the Incremental Sheet Forming Process

This paper presents a numerical procedure to implement a modified Viscoplasticity Based on Overstress model (VBO) into a finite element simulation. This work can be used to predict the geometric accuracy of the final product which is formed by incremental forming technology with a thermoplastic material (polypropylene) at room temperature. The VBO model is implemented into simulation environment by writing a subroutine in FORTRAN programing language which defines the mechanical constitutive behavior of a material. The VBO model is calibrated by the data from mechanical testing such as tensile, relaxation and creep test with different strain rates and previous strain levels. The calibrated model described excellently the mechanical properties of polypropylene. The FEM simulation is performed with the modified VBO model for polypropylene sheet to predict the geometric accuracy of the final product. The simulated results are compared to practical experiment. There is a good agreement between simulated results and practical experiment about the geometric accuracy which is presented in product profile and final thickness of the final product formed by ISF process. It is concluded that the modified VBO model can be applied for predicting the mechanical fracture and failures in single point incremental forming for polypropylene and other polymer materials.