An indentation method for evaluating the residual stress of polymeric materials: Equi-biaxial and non-equi-biaxial residual stress states

A method is proposed for evaluating the residual stress of polymeric materials using indentation tests. This method deals with elastic-perfectly plastic materials with in-plane residual stress in the equi-biaxial state and non-equi-biaxial state. The elastoplastic properties (Young's modulus (E)/yield strength (σY)) of the materials have a large range, which covers general engineering polymer materials. Numerical experiments with the finite element method (FEM) were first carried out to simulate the spherical indentation test for various materials experiencing equi-biaxial residual stress. It was found that the indentation curve (force and penetration depth) changes with both yield strength and residual stress. Next, a parametric FEM study was conducted, in which the residual stress and yield strength were changed to deduce the relationship between the indentation curve and the parameters of both residual stress and yield strength. This relationship can be expressed using dimensionless functions with simple formulae. Hence, a simple estimation method using a single spherical indentation was established. To verify the method, indentation tests were conducted for several polymeric materials, including polycarbonate (PC), polymethylmethacrylate (PMMA) and acrylonitrile butadiene styrene (ABS). For PC, residual stress was imposed by bending the fixture, and an indentation test was carried out. It was found that the method evaluates residual stress and yield strength simultaneously. Finally, evaluation of the non-equi-biaxial residual stress state was explored. By using a Knoop indenter, it was found that the indentation curve changes with the axes, for which residual stresses are different. By using these characteristics and the above spherical indentation method, an evaluation method is proposed for yield strength and residual stress for the non equi-biaxial residual stress state.