In-situ Automated Fiber Placement Manufacturing for Complex Geometries

Kurzfassung

Thermoplastic Automated Fiber Placement (AFP) has great potential for the time- and cost-efficient manufacturing of large-scale aerospace components by means of in-situ consolidation, omitting subsequent time-consuming and costly autoclave consolidation. While flat laminates have been manufactured using the in-situ AFP process, achieving properties similar to autoclave- or press-consolidated references, complex double-curved geometries present an additional challenge to the already ambitious process. Defects, which inevitably arise when applying two-dimensional tape material to three-dimensional (double-curved) geometries, are not mitigated by subsequent bulk consolidation when opting for in-situ manufacturing. The extent to which these defects can be tolerated in the laminate and what influence they have on the mechanical properties is the subject of this dissertation. The work presented here addresses the combination of complications arising from the process and from the part geometry. To this end three, complex reference geometries were employed as relevant applications of the in-situ AFP process: A constant curvature Variable Stiffness Panel, which is a two-dimensional academic representation of a stiffness-optimized structure; a rotationally symmetrical hydrogen tank geometry; and a complex fuselage section, representing free-form geometries. Using tape placement simulation and specially developed Python scripts, layup strategies were developed and resulting steering defects, angle deviation and gap or overlap defects were quantified to determine a correlation with parameters of the layup strategy. The derived steering radii were used as starting values for a comprehensive experimental study on steering defects. The experimental results provided insight into the mechanisms and onset of different defect types following in-plane path curvature. A novel concept of critical arc length was developed and validated expanding the design space of usable steering radii by introducing radius-dependent arc length limits. High-resolution optical 3D scans were used to develop an empirical model to predict the steering-induced geometry changes of the consolidated tape. From the model, coverage-optimized layup paths were developed to avoid gap defects between narrowing steered tapes. The methodology was successfully validated in example manufacturing experiments. Laminates with subcritical steered plies were manufactured and the consolidation quality was validated using mechanical four and five-point bending tests. Similar interlaminar shear strength and bending strength results were found as vin straight-path reference samples. A novel boundary condition allowing for steering was thus successfully established. The second major defect type, gaps and overlaps, was investigated in another comprehensive test campaign. Realistic triangular staggered gap and overlap defect patterns as derived from the simulation results were introduced into test laminates. The microstructure of the defect laminates was analyzed in detail using advanced non-destructive testing and micro imaging methods and pores were identified as a function of the coverage ratio and ply orientation following the defects. Tensile and compressive coupon tests were carried out and analyzed using Digital Image Correlation with regard to strength, stiffness and failure modes. Overall the realistically staggered defects resulted in a subcritical disruption of the laminate composition and the resulting coupon-level mechanical properties showed little if any statistically significant impact of defects. A greater influence was found for the in-situ AFP process window with regard to tool temperature and close adherence to the optimal process parameters. This work presents a comprehensive analysis of correlations between complex geometries, layup strategies and resulting defects for the in-situ AFP process. It provides initial proof that high quality complex in-situ AFP structures can be achieved.