Direct digital manufacture of continuous fiber reinforced thermoplastic high aspect ratio composite grid stiffeners and grid stiffener intersections with radically reduced tooling
Date
2024
Journal Title
Journal ISSN
Volume Title
Abstract
Grid stiffened structures are widely used in the aerospace industry due to their high strength and stiffness to weight ratio and impact damage tolerance. These structures consist of a lattice pattern of stiffening ribs bonded to a thin shell structure, where the stiffening ribs commonly act as the main load bearing members, and the shell acts to cover the ribs and transfer loads through membrane action. These structures offer a variety of beneficial structural properties including high specific strength and stiffness, high impact resistance, high compressive resistance, and high energy absorption. However, the complexity of a grid pattern can lead to excessive manufacturing times, especially for simple constructions such as flat plates. A more promising alternative for manufacturing grid stiffened structures is the use of automated manufacturing methods including ATL, AFP, and filament winding. Because composite grid stiffened structures can be composed entirely of the same composite material, the manufacturing process with these methods can be almost entirely automated, saving time and money. However, the traditional and automated methods of producing composite grid stiffened structures require the fabrication of complex tooling to develop the geometry of stiffening ribs. In addition, all composite grid stiffened structures suffer from the same manufacturing difficulty: for all of the fibers to be continuous through an intersection node, there must be twice as much material at each intersection than in each rib, making intersection compaction extremely difficult. A more recently developed composite manufacturing method is additive manufacturing (AM) in the form of composite 3D printing, which offers a much higher degree of geometric freedom than other autonomous manufacturing methods and does not require tooling. However, composite 3D printing is generally limited to low fiber volume fractions. A manufacturing method with the ability to make high quality, high fiber volume fraction continuous fiber grid stiffened structures without the need for tooling could significantly increase the efficiency and decrease the cost to produce these structures. The current study proposes the use of a novel additive manufacturing method which uses a commingled feedstock and features in situ consolidation to produce grid stiffened structures without the need for tooling. Several stiffener ribs and stiffener rib intersections were produced and tested for composite quality. The fiber volume fraction and void volume fraction through the height and length of printed stiffener ribs and intersections was analyzed to determine if the quality was consistent. A micrograph evaluation was performed on the high aspect ratio stiffener rib and intersection composites to qualitatively evaluate the reinforcement distribution, determine the void locations, and to support the constituent material concentration measurements. The consolidation force was measured during the manufacturing of the samples to better understand the forces experienced during printing and to form a relationship between the consolidation force experienced and the constituent volume fraction of the samples. The results of this study suggest that the application of direct digital manufacture to the placement and consolidation of commingled tow for the fabrication of high aspect ratio grid stiffeners and intersections, without the need for tooling, can readily achieve fiber volume fractions greater than 50% and void fractions as low as 5%. Volume fraction analysis results show that manufactured stiffener ribs and stiffener grid intersections exhibit high fiber volume fractions and low void volume fractions which remain consistent through the height of the samples. Consolidation force measurement results show that a significant decrease in force is experienced between print layers. Microscopic analysis results show that the majority of voids collect at the edges of print layers leading to an increase in void content at the intersection node and potentially masking any quality gradient through the height of samples that may exist. The results of this study show the high potential for the manufacturing of high quality high aspect ratio continuous fiber composite grid stiffener structures through direct digital manufacturing technologies without the need for tooling.
Description
Rights Access
Subject
composite
reinforced
additive
thermoplastic
manufacturing