## 2 Experimental Methods  
The experimental apparatus for the winding process is shown in Figure 1. A diode laser source was fixed on an external frame as well as a CNC motion table for the sample movement along the x and z direction. A stepper motor was mounted on the motion system so as to provide a rotational speed to a steel tube with the external diameter of 20 mm. The adopted laser was a diode laser (Rofin-Sinar, DL 015) with a 940 nm wavelength and a rectangular spot (3.8 \(\times\) 1.2 mm\({}^{2}\)) due to the superposition of two different rays, each one coming from a 750 W emitter diode. A 63 mm long focus lens was used to maximize the depth of field. A flux of nitrogen gas was used to protect the lens during working. Figure 1: Experimental apparatus for laser-assisted winding of thermoplastic prepregs. Figure 2 shows a detail of the consolidation apparatus regarding the processing area. A narrow prepreg tape was wound around the steel tube (_i.e.,_ the mandrel) and was forced to pass through two polyethylene sheets mechanically clamped together. Due to the friction of the two polyethylene sheets, a consolidation force was applied to the prepreg during winding. In the experimental procedure, the prepreg tape was wound for a round on the mandrel and shaped by means of a heater. That operation was made by hand, and was necessary for the subsequent positioning under the laser source otherwise the tape stiffness made the laser focalization impossible. After clamping the mandrel to the stepper motor, the non-wound part of the tape was placed between the polyethylene sheets that were clamped in turn. In that condition, the laser-assisted winding process could start: the laser spot was focused on the wound prepreg by means of the z-axis of the CNC table. The laser power was fixed at 50 W and 3 complete revolutions were made without changing the distance from the laser focus. After that, the laser beam was focused again on the external surface of the wound prepreg and other 3 revolutions were performed with the same laser power and rotational speed. Different winding tests were made by only changing the winding speed (14.4, 16.2 and 18 deg/s) because of the very high sensitivity to the laser power: three samples were produced for each winding speed. Higher power values easily led to material degradation or excessive melting, whereas lower power values resulted in poor consolidation. In order to evaluate the consolidation force during winding, a simple experimental test was performed. A prepreg tape was clamped to a universal testing machine (Alliance RT/50 by MTS) as well as the tensioner. A constant rate was applied to the prepreg and the friction force was measured. The test rate was fixed equal to the peripheral velocity of the tape during the winding tests, and a small change of the friction force in dependence of the speed rate was observed in the experimented range. For example, Figure 3 shows the friction force at the test rate of 210 mm/min which corresponds to the winding speed of 16.2 deg/s. As expected, a maximum is observed (about 3 N) at low displacements, and a plateau value (about 0.5 N) at higher displacements. On average, a consolidation force about 1 N was observed during the 160 mm long test. Figure 2: A detail of the consolidation apparatus. Unidirectionally reinforced thermoplastic prepregs (by Jonam Composites Ltd, UK) were used to produce the composites. The GPP45 glass/polypropylene prepreg was supplied in the shape of a long tape, 95 mm in width and 0.35 mm in thickness, with the nominal glass content of 45 vol %. Some narrow tapes, 4 mm wide, were cut from the 95 mm tape and used for winding. As the laser spot was focused on the prepreg surface during laser processing, the maximum axis of the spot was aligned with the tape width, so as to cover the entire tape width by the laser spot. This way, a single laser pass was able to consolidate the entire processed tape.