Phenomenological theory characterization of viscoelastic-plastic response of PA12 under indentation tests

Nanoindentation is widely used to study the local time-dependent mechanical behavior of polymer materials. Usually, it can be considered that the indentation depth is the sum of elastic displacement, plastic displacement and time-dependent viscous displacement in the indentation process. In this paper, according to the mechanical model composed of a series of secondary mechanical elements, the viscoelastic-plastic characteristics of PA12 under indentation were extracted for different loading conditions. The results indicate that the model demonstrates a high degree of efficacy in predicting the loading segment of the experimental curve, irrespective of the variation in loading conditions. However, as the loading rate decreases, the model's predictive capability appears to diminish. However, it has been demonstrated that the viscous displacement component, caused by the continuous decrease in load during the unloading process, recovers with the increase of unloading time. The utilisation of the VEP model for the simulation of the indentation loading and unloading process is appropriate for ideal elastic-plastic materials or materials exhibiting comparatively minor viscoelastic characteristics. The attainment of a high viscosity coefficient under conditions of Continuous Stiffness Measurement signifies that the influence of viscosity release on the loading curve during the loading process is diminished in comparison to the constant load rate loading method. The impact of viscosity is amplified under conditions ranging from high to low , owing to an augmentation in loading time. Therefore, the discrepancy between the experimental and predicted curves exhibits an escalation. • The viscoelastic-plastic characteristics of PA12 under indentation were extracted according to the VEP model. • The indentation depth is the sum of elastic, plastic and time-dependent viscous displacement in the indentation process. • The VEP model of indentation loading and unloading is appropriate for ideal elastic-plastic materials or materials exhibiting minor viscoelastic characteristics.