Acoustic Emission Analysis for Assessment of Damage Kinetics of Short-Glass Fibre-Reinforced Thermoplastics—ESEM Investigations and Instrumented Charpy Impact Test

Abstract

The acoustic emission (AE) analysis is a structure-sensitive test method of plastic diagnostics, which enables the characterisation of the damage kinetics as well as damage mechanisms under specific conditions. The AE analysis is linked to release of stored elastic energy, which propagates as spherical volume wave in the material. In this work, short-glass fibre-reinforced thermoplastic materials were examined in the quasi-static tensile test in the environmental scanning electron microscope (ESEM) with simultaneously recording of the AE as well as under impact-loading conditions in the instrumented Charpy impact test (ICIT). Therefore, it was possible to couple the mechanical, the acoustic and the micromechanical results to describe the damage kinetic as well as the damage mechanisms. In dependence on the bonding conditions of the glass fibre in the polymer matrix, different mechanisms of damage to be related to typical frequency ranges can be detected: (i) fibre fracture, (ii) matrix deformation with slipping of fibres in the delamination area and friction processes of the fibres in the matrix, (iii) debonding and pull-out with/without matrix yielding. The coupling of the AE analysis with the ICIT allows the assessment of the damage kinetic and therefore, the determination of the damage initiation under impact-loading conditions. However, in dependence on various bonding conditions, different results could be found. For good bonding conditions, the damage initiation takes placed before the material behaviour changes from elastic to elastic–plastic behaviour. This could be found for the short-glass fibre-reinforced high-density polyethylene materials. For the fibre-reinforced polybutene materials, the first AE takes place at the point of elastic–plastic material behaviour. An energetic approach of the damage initiation by the parameter J Si shows an independent behaviour from the polymer matrix as well as from the glass fibre content. For all investigated thermoplastics, a J Si-value of 0.8 N/mm as resistance against stable crack propagation could be determined.