An experimental procedure for an in-depth analysis of the fatigue damage mechanisms in short fibre-reinforced polymer composites: plain and notched specimens

This study presents an in-depth investigation of fatigue damage mechanisms in short glass fibre-reinforced thermoplastic composites, focusing on the distinction between fibre failure and matrix or fibre–matrix interface failure. Three materials were considered: two recycled polypropylene-based compounds filled with calcium carbonate and reinforced with 10 wt.% and 25 wt.% glass fibre and one polyphenylene sulphide composite reinforced with 40 wt.% glass fibres. Fatigue tests were conducted on both plain and notched specimens, produced either by injection moulding or machining, under tension–tension (R = 0.05) and tension–compression (R = -1) loading conditions. Fracture surfaces were examined using scanning electron microscopy (SEM) and an experimental procedure was developed to match corresponding regions on opposing fracture surfaces, enabling the identification and classification of individual fibre failure events. The main finding of this study is that, under the investigated testing conditions, fewer than 10% of the fibres fail due to fatigue. This observation quantitatively confirms that the fatigue damage predominantly initiates and propagates within the matrix or along the fibre–matrix interface.