Fatigue behaviour and damage mechanisms of short glass fibre-reinforced recycled polypropylene produced by pellet-based polymer material extrusion under different load ratios

This study investigated the fatigue behaviour of short glass fibre-reinforced polypropylene with a recycled matrix produced via Additive Manufacturing through Pellet-Based Polymer Material Extrusion (AM), and injection moulding (IM). In particular, the focus is on the influence of load ratios (R = −1 and R = 0.05) and infill patterns (0°, 45°, and 90°) on the fatigue performance of materials processed via these two manufacturing processes. Approximately 70 fatigue tests were carried out on the AM and IM specimens; the material with a virgin matrix was also tested by using the latter process for comparison purposes. The fibre distribution was analysed via computed tomography, and macro- and microscopic damage analyses were conducted after fatigue failure. The main experimental outcome is that the fatigue strength for each R value, both for the AM and IM specimens, is proportional to the tensile strength. The normalised Walker equation was calibrated on all of the resulting data, providing a fatigue curve unifying all the processes and parameters considered for the analysed materials. • Fatigue of SGFR-recycled PP specimens by AM and IM is investigated and compared. • μCT reveals that the fibre direction is strictly correlated with the bead direction. • The influence of load ratio on fatigue damage mechanisms is investigated. • Crack paths in AM specimens depend on infill strategy but not on load ratio. • Walker model best correlates the fatigue strength of all data for all load ratios.