Process Distortions in Composite Manufacturing - From an Experimental Characterization to a Prediction Approach for the Global Scale, Dissertation

Kurzfassung

The increasing application of carbon fiber reinforced plastics as a structural material pursues the aim to realize high-performance structures with improved efficiency and reduced ecological impact. Increasing interest of the automotive industry and significantly increasing quantities for modern aircrafts in aerospace, targeting at more than 50 aircrafts/month, generates steadily increasing challenges for composite-specific manufacturing strategies and related processes. The compliance of narrow geometrical tolerances is one essential challenge in order to achieve a simple and structure-friendly assembly process. This is of particular importance for large CFRP structures as they are used in aerospace applications. Currently, manufacturing-induced distortions are an inherent, cost-driving issue within composite manufacturing. These undesired distortions are induced by the composite’s anisotropic material properties in combination with the commonly used high-temperature processes. On the one hand this anisotropy enables the superior specific mechanical properties, while on the other hand these properties induce the complex shape-changing effects during manufacturing. Within the assembly, these shape deviations result in inadmissible large gaps between mating parts which must be compensated with soft- or hardshim. For some structures, assembly can become impossible which results in increasing scrap rates. Manual shimming as well as tool re-work, often referred to as non-added value operations, are related to significant time and cost efforts. The present thesis pursues the aim to provide a methodology to predict the unavoidable, manufacturing-induced distortions in order to use this data within an improved part development chain. This enables the consideration of manufacturing effects within the structural sizing and in particular the tool design as appropriate compensation measures can be derived on a virtual level prior the first part or tool is fabricated. A comprehensive experimental investigation of the distortion-inducing effects warpage, spring-in (-back) and forced-interaction represents the first essential part of this thesis. The main aim of the experimental studies is to elaborate the main drivers of the single effects. Furthermore, a phenomena-specific characterization is performed which focuses on relevant part and process parameters such as layup or tool material. The development of a prediction methodology for expectable process-induced distortion represents the second essential part of this thesis. The major novelty of this approach is the integration of a multitude of process-relevant parameters within one L-shaped specimen. Measured manufacturing-induced distortion of this small specimen is analytically transferred to an equivalent scale-independent simulation parameter while the structural FE model uses conventional shell elements. Thus, model-size limitations and the extraordinary large parameter requirements of phenomenologically- based state of the art process analysis tools are bypassed which lead to a very quick and efficient simulation strategy. The thesis concludes with a set of case studies which verify the suitability of the developed approach numerically and experimentally.