Material behaviourAcoustoelastic effect in polyamide 6: Linear and nonlinear behaviour

The aim of this work is to investigate the acoustoelastic effect in a thermoplastic polymer (polyamide 6) under uniaxial compression using ultrasonic waves (longitudinal and shear). The acoustic velocities varied linearly, as expected, when small strains or stresses were applied, from which Murnaghan coefficients were obtained for the first time for this type of polymer. This relationship changed to be nonlinear above a critical strain at about 0.7%. This nonlinear behaviour has already been observed for polymers under tensile tests but without interpretation. In order to explain the nonlinear part of the curves, an analysis was conducted from a thermodynamic standpoint and ultrasonic measurements were related to the molecular structure of the polymer. The Grüneisen parameter was introduced as an index and its magnitude showed how the system changes from anharmonicity to harmonicity of interatomic (intermolecular and intramolecular) interactions according to the applied strain.