## 1 Introduction  
Thin-walled structures are common in engineering design, often seen in the form of domes, shells, plates, and membranes. They provide strong and flexible but relatively light-weight architectures, most commonly automotive components, boats, pressure vessels, tanks, piping, machine components, buildings, and aircraft structures [1; 2; 3; 4; 5; 6]. There are numerous advantages to using these structures, but there are also trade-offs; the mass and strength must be balanced, making the thin structures optimal relative to both but neither individually [7; 8]. This has a significant impact on their performance, especially on buckling behavior since thin-wall buckling failure often occurs long before the yield point of the material [9; 10; 11]. A thin-walled structure is often defined as one that has its smallest internal hollow section at least 20 times the thickness of the wall; the most simple and controllable geometry for exploring this is a thin cylinder [12].  
As additive manufacturing (AM) becomes more widely accepted and used, it is increasingly important to understand the behavior of additively-fabricated structures. AM materials are highly anisotropic [13; 14] and have a well-defined mesostructure that greatly affects the mechanical properties [15]. One of the most common and refined AM processes for polymeric materials is the Fused Deposition Modeling (FDM) process, which works by selectively extruding material from a heated die. It traces out a calculated path for each layer of the part build, making it a scanning-type AM (ST-AM) process. The selective deposition provides the source of raw material, while a polymer melt reaction between each of the elements (and with the previous layer) provides the fusion to join each layer [16; 17]. Under careful tuning, FDM can fabricate very precise geometry using a wide variety of materials with good properties and at a low cost without any special tooling; it can easily fabricate thin-walled and delicate structures, as demonstrated in several previous studies [18; 19; 20; 21; 22; 23; 24; 25]. The mechanical behavior of FDM-processed material has been well-studied, but the buckling behavior of thin-walled FDM structures is still an open question and little is known about their behavior. Due to the structure of AM materials, it is likely that the structures will undergo local plastic yielding before buckling [26; 27; 28].  
To explore this question, this short technical note presents the results of an exploratory experimental study done on the critical buckling load of FDM-processed ABS and PLA cylindrical shells. Some interesting phenomena were observed, which are discussed along with notes taken during the study. Several factors were studied, including wall thickness, material choice, and the use of reinforcement (3% infill and polyurethane foam) within the shell. Critical load tests were completed twice (two replications) on the selected cases, with a total of 32 tests conducted. This study was intended to collect initial data on the problem and provide this information to the mechanical engineering, mechanics, and design communities to further highlight the problem and encourage more attention in this area. It should be fully noted that it was a screening study to demonstrate the importance of research in this area and to collect some useful observations to help guide future work. In the conduct of the project, a number of useful conclusions and observations were made, which will prove of value to the research community when initiating projects in this area. Section 2 presents a brief overview of the essential problems with FDM-processed thermoplastic thin-walled shells and Section 3 provides a detailed overview of the experimental approach taken to explore the described problem. The collected results are given in Section 4, followed by some essential observations about the collected data in Section 5. Finally, Section 6 provides closing remarks, which include lessons-learned from this study and a discussion of the needed research in this area.