Three new approaches to acoustic characterization of polymers and composites have been developed and studied experimentally. The acoustic birefringence methodology is analysed for uniaxial short glass fiber-reinforced polypropylene composites. It is preceeded by the detailed calculations of acoustic wave anisotropy characteristics based on experimental determination of elastic moduli for the material. The results are applied to non-destructive evaluation of the reinforcement direction in the composite. A new methodology of the focused slanted transmission mode (FSTM) of air-coupled ultrasound is developed and applied to local stiffness measurements of materials. The FSTM is based on the “resonance” transmission of an acoustic wave through the sample due to flexural plate mode excitation and re-radiation. The flexural modes generated in the air-coupled FSTM make it sensitive to in-plane elastic anisotropy and presence of internal defects (delaminations, disbonds, etc). The FSTM-imaging demonstrates superior performance to the conventional normal transmission mode in scanning and imaging of the cracked defects and delaminations in polymers and composites. The FSTM is also adapted for an experimental study of elastic nonlinearity of polymers and glass fibre-reinforced composites in a wide range of tensile stress applied (up to the fracture limit). Local non-contact measurements of flexural wave velocity as a function of static strain are used to calculate the second-order nonlinearity parameters β₂ and to study their behaviour through a loading cycle. Molecular untangling and crazing phenomena are identified, respectively, with maxima of positive and negative β₂ in thermoplastics. In composites, the nonlinear approach enables to monitor mechanics of fibre-matrix interaction for brittle and plastic fractures. Hysteresis in velocity variation during loading-unloading cycle is used as an indicator of residual defect accumulation.

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