In-situ Evaluation of Laminates with Gaps and Overlaps - Uncertainties in Modelling and Analysis

The Automated Fiber Placement (AFP) enables rapid deposition of large quantities of pre-impregnated material (pre-preg), so-called tows. For complex composite structures (complex in geometry or fibre architecture) manufacturing induced deviations are inevitable. Typical deviations in this process are fibre undulations, gaps and overlaps. Due to missing knowledge about their exact effects on the mechanical properties the manufacturing process of the composite part has to be interrupted in order to repair or re-work affected areas or in worst case the part has to be scrapped. Virtual material tests of laminates containing such manufacturing induced deviations provide profound physical understanding of the actual effects of defects, i.e. to assess their influence on the structural behaviour. Herein the modelling and analysis are key aspects to ensure required reliability in characterization of these deviations. Online monitoring systems measure relevant part and process parameters during layup process. These parameters in turn can be used to build up simulation models. However, the actual material firstly forms together with the composite part in the following process steps. However the goal is to qualify occurring deviations based on available measurements during manufacturing (in-situ evaluation). This limited knowledge, only from the first phase of the manufacturing process, poses several uncertainties to the modelling and analysis process. This paper focuses on the methodology of in-situ structural evaluation, exemplarily applied to the fibre deposition process. Prior to manufacturing typical manufacturing features such as locally varying fibre orientations, gaps and overlaps are studied based on given knowledge from previous manufacturing as well as from literature. From this a categorization of these manufacturing features is proposed. Related uncertainties influencing the modelling of gaps and overlaps in laminates manufactured using AFP are examined. Their effects on the structural properties are investigated within the expected parameter ranges. The real detected features are provided by an online monitoring system during the fibre deposition process. Based on these results an in-situ structural evaluation of detected features is performed already during manufacturing in combination with a decision making with respect to required part correction. The developed key technologies and tools for the in-situ evaluation process are presented, and their prototype application is shown. Currently, the in-situ evaluation of gaps and overlaps is neither considered in the design nor performed in the manufacturing of aviation structures. Therefore safety factors and strict tolerance guidelines are used to compensate for the influence of uncertainties on the reliability of the evaluation methods used. This leads to high conservatism in the part design and tolerance guidelines in manufacturing. The proposed methodology provides the opportunity to evaluate gaps and overlaps and identify critical zones from a structural mechanics point of view during manufacturing.