Mechanical modeling of polyamide 6 obtained from different thermal histories

In this study, we propose a mechanical model for Polyamide 6 (PA6) having different crystalline structures. Our latest experimental study on the tensile tests of PAs obtained from different thermal histories clarified that the mechanical properties of PAs significantly changed even for those with similar degrees of crystallinity (Yoshida et al., Polymer, Vol. 282 (2023), 126190). Wide-angle X-ray scattering (WAXS) diffractograms of the PA6s prepared in this study were measured to evaluate the microscopic crystalline structures. Broad distributions were observed around the peaks in the quenched PA6, and the sharpness of the peaks increased with temperature during the annealing and isothermal heating processes. The parameter representing the perfection of the crystalline phase was quantified by distinguishing the distribution patterns of the crystalline and amorphous phases around all peaks in the WAXS patterns. The constitutive model based on the transient network viscoelastic-viscoplastic theory was extended to represent the mechanical behavior of PA6. In the proposed method, the initial properties of the molecular chain networks in the crystalline and amorphous phases are described as a function of the crystal perfection index, and the second yield is modeled to occur when the remaining stress transmitter density reaches its critical value. Two-dimensional finite element method (FEM) simulations of the tensile tests were performed using the proposed constitutive model. The FEM simulation results revealed that the proposed model could represent increases in the slope prior to the first yielding, the magnitude of the first yielding stress, and the strain where the second yielding occurs with increasing crystal perfection index, even for PAs with similar crystallinities. The development of the strain rate distribution during the tensile tests accompanying the onset and propagation of necking predicted by the FEM simulation agreed reasonably well with the experimental results. However, a nonlocal expression of the plastic strain should be investigated to accurately predict the strain field independent of the mesh division of the FEM model. [Display omitted]