Ultrasonic in situ investigation of the initiation of Polyethylene's plastic deformation during tensile tests

Among the micro-mechanisms and damage that can simultaneously occur during the plastic deformation of Polyethylene one can quote shear crystallites, fragmentation of crystalline lamellae, cavitation or martensitic transformation. Distinctly, the determination of the initiation of plasticity and damage within this kind of polymers remains questionable. The aim of this study is to characterize the plasticity and damage of several Polyethylene (PE) during tensile tests using the ultrasonic (US) monitoring technique. The proposed methodology uses both guided and longitudinal waves in the ultrasonic frequency range and enables to separate the geometrical effects from those of the material. It is shown that the US attenuation increases when the degree of crystallinity decreases. Besides, the US attenuation appears to be higher in the amorphous phase than in the crystal. During a tensile test, a strong decrease of the transmitted energy is observed once the yield point is reached, due to the formation of the fibrillar microstructure, which breaks the crystalline percolation. Finally, the results evidence that the chain alignment during a tensile test favors the wave propagation; in addition, cavitation induces a significant attenuation, which is strongly anisotropic as the voids are aligned with the fibrils. (C) 2017 Elsevier Ltd. All rights reserved.