The effects of manufacturing defects on the material properties of carbon fiber reinforced low melting point polyaryletherketone (PAEK) composite laminates manufactured by autoclave consolidation

The usage of thermoplastics composites is increasing day by day in aerospace industries. Their advantages and key features like excellent damage tolerances, resistance to low-velocity impact damages, recyclability, and durability to ambient conditions are very convincing to replace traditional thermoset composites by thermoplastics. However, replacing thermoset composite by thermoplastic ones will not be easy due to lack of sufficient knowledge and experience about non-destructive inspections and effects of defects coming from manufacturing processes. The main object of this study is to find the effect of the defects that may occur in advanced thermoplastic composite materials used in aerospace industry by destructive and non-destructive test methods. The defects, primarily foreign object inclusions, voids and porosities, which have the highest probabilities to occur during manufacturing and co-consolidation processes. With the presented experimental methods, characterization of defects and their effects on mechanical and physical characteristics of the carbon fiber reinforced polyaryletherketone composite parts are included in this study. Different autoclave co-consolidation scenarios are performed to form bondline defects like porosity and voids in order to observe negative impacts on mechanical performance of co-consolidated CF/PAEK composites. Furthermore, randomly sized foreign object inclusions are placed at defective bondline to witness any additional changes in mechanical and physical properties of the defective specimens. A series of [+45/−45]5S 2.3 mm thick laminates were used to get reference and discrepant test panels. Defect contents were assessed by varying the vacuum value from -573.45 to 246.9 mmHg and pressure value from 2 to 5 bar during autoclave co-consolidation process. Bondline inclusions are observed with the resulting ultrasonic signal loss in dB units for a range between 0 and 49.4% during phased array and pulse echo inspections. Microscopic evaluations and fiber volume content tests proved the presence of void, porosity and foreign object inclusions. However, it was observed that foreign object inclusions prevent and deflect the ultrasonic signal losses caused by void and porosity contents in a positive manner. Additionally X-ray radiographic evaluations are indicated that defective specimens have distorted bondlines compared to reference specimens. Effects of average ultrasonic signal loss at joint areas on mechanical and physical properties of CF/PAEK composites are evaluated. Results show that shear strengths are decreased by 14% as increasing average ultrasonic signal losses due to void and porosity formations. On the other hand addition of foreign objects to defective specimens having voids and porosities showed negative effect on shear strength by lowering by 17% with respect to defect free specimens. In the light of the acquired information in this study, a relation is set between ultrasonic inspection outputs and lap shear strength for specimens having void and porosities. Moreover, experimental outputs can be used to identify accept/reject criteria of mentioned defects in carbon fiber reinforced PAEK composite parts for now and future.