Manufacture of variable angle tow laminates using carbon fibre/thermoplastic pre-preg tapes

Variable Angle Tow (VAT) composites have gained significant interest since the introduction of Automated Tape Placement (ATP) machines in the 1980s. ATP has scope to enhance design space by laying down tapes or tows in curvilinear trajectories, i.e. in variable angles. Until recently, cutting plies, thereby introducing ply-drops, into the same layer was the only means available to change layerwise ply orientation. VAT has potential to reduce stress concentrations around cut-outs and improve buckling performance, without adding weight. Many numerical studies have shown the benefits of VAT, but few researchers have manufactured and tested VAT structures. Where VAT structures were produced, all were processed using thermoset composite material systems. Thermoset composites require two processing steps, the first to lay-up the laminate stacking sequence, while the second involves consolidating (curing) the laminate. To the author’s knowledge, there is little research available on processing VAT structures using thermoplastic composites. Thermoplastics composites allow in-situ consolidation with laser-assisted ATP (LATP). In-situ consolidation eliminates the secondary processing step, saving time, energy and consumables. This work investigates processing of VAT structures from aerospace grade thermoplastic composite, specifically carbon fibre reinforced polyether ether ketone (CF/PEEK). To determine the suitability of CF/PEEK to manufacture VAT structures, an investigation of LATP processing parameters for tape steering was carried out. Defect occurrences were observed and the bond strength of steered tapes tested for varying steering radii. Measurements showed that the width and thickness of the tapes changed as a result of steering. In addition, bond strength, was found to be a function of lay-down speed. It was shown that excellent bond strength could be achieved with suitable processing parameters. Investigation of CF/PEEK VAT structures progressed to representative structural demonstrators. A variable-stiffness CF/PEEK wingbox was manufactured using a LATP winding, in-situ consolidation process. A full-scale structural test using a bespoke testing frame with representative loads was undertaken. The wingbox buckled elastically at a load close to that predicted numerically with a 14% increase in buckling load compared to the analogous, conventional lay-up CF/PEEK wingbox. One disadvantage of the current process for producing VAT structures are gaps and overlaps that occur between neighbouring tape laydown paths. Gaps and overlaps reduce structural efficiency, so eliminating such defects with in-situ consolidation is highly desirable. A tape spreading device was designed that can be integrated into an ATP head. The tape spreading device allows variable tape width to be achieved locally during lay up. Integration of this device in the ATP process would remove gaps and overlaps in VAT laminates. During trials different width tapes were produced using the spreading device. Three different width increases were investigated; 15%, 30% and 45%, and were compared with as-received tape. Preliminary characterization tests show that the spreading process does not adversely affect properties at tape level. Initial trials indicate that it is possible to achieve a tape width increase of 62%. Finally, to investigate the effect of spreading at laminate level, test samples manufactured from as-received tapes and spread tapes were mechanically characterised and compared. Tensile and compression properties of spread tapes experienced minor decreases. However, interlaminar shear strength decreased by 43%, which is most likely caused by damaged fibres being present in the interphase region. Further work is recommended to develop the technique further to eliminate the issue of relatively poor bond strength between layers.