The effect of laser assisted tape placement processing conditions on microstructural evolution, residual stress and interlaminar shear strength of carbon fibre/PEEK laminates

In the present study, both experiments and thermo-mechanical coupled simulations were conducted to characterise the diverse crystallisation behaviours and the processing parameter-microstructure-mechanical property relationships occurring in laser-assisted tape placement (LATP) manufacturing of carbon fibre (CF)/Polyetheretherketone (PEEK) laminates. Specifically, at various processing temperatures (350 °C or 400 oC), increasing the compaction pressure from 2 to 4 bar causes distinct defect distribution behaviours. However, variations in processing parameters show minimal effect on the morphology and size of crystallised spherulites, which were consistently around 2–3 μm in size, resulting in a final crystallinity of manufactured laminates within 30%–35%. It was found that the cold crystallisation processes occurring in PEEK during LATP play an important role in determining the final degree of crystallinity. Experimental measurements and simulations indicate that changes in processing parameters have a negligible effect on residual stress levels, especially regarding interlaminar residual stresses. A processing temperature of 400 °C was found to generate a diffuse, yet coherent, interphase spanning the fibre/matrix interface with a thickness approximately 70 nm. In contrast, at a processing temperature of 350 °C, a distinct, incoherent interface was confirmed between fibre and matrix. The formation of the interphase, coupled with fewer defects, leading to a relatively high interlaminar shear strength (78 MPa) of manufactured laminates under appropriate processing conditions. Therefore, it is suggested that regulating the degree of cold crystallisation in polymer matrices while ensuring a strong fibre/matrix interfacial bond by the optimisation of processing temperature, will enable the tailoring of microstructure and design of composites to meet specific strength property requirements.