Flexural Buckling Behavior of WAAM-Fabricated Tubular Columns: An Experimental Analysis

Additive manufacturing, in its essence, involves layer-by-layer material deposition, a method contrasting starkly with conventional subtractive and formative manufacturing techniques. Wire Arc Additive Manufacturing (WAAM), a subset of Direct Energy Deposition processes, utilizes wire feedstock to deposit molten material, enabling the creation of complex and large structural components. This process not only enhances manufacturing flexibility but also reduces material wastage, aligning with sustainable manufacturing principles. Despite its burgeoning application across various sectors, the structural engineering field is yet to fully understand the material and geometric variabilities introduced by WAAM, especially concerning their impact on the structural integrity and performance of fabricated elements. To address this, a new study published in the Journal of Structural Engineering and conducted by Dr. Cheng Huang, Dr. Xin Meng, Dr. Craig Buchanan, and Professor Leroy Gardner from the Imperial College London, the authors focused on understanding the flexural buckling response of stainless steel tubular columns manufactured using WAAM. Their experimental work aimed to fill the gap in knowledge regarding the structural performance of WAAM-fabricated elements, specifically at the member level, under axial compression loads.

The research study involved the testing of 18 stainless steel columns, comprising both square hollow section (SHS) and circular hollow section (CHS) profiles, under axial compression. These columns were manufactured using WAAM, a process that deposits material layer by layer using an electric arc as the heat source and metal wire as the feedstock. The selection of regular SHS and CHS profiles allowed for direct comparison with conventionally manufactured sections, thereby isolating the influence of the WAAM process on structural performance. The researchers varied the cross-section sizes and column lengths to cover a broad spectrum of member slenderness.

To accurately characterize the as-built geometry and global geometric imperfections inherent to the WAAM process, the team employed advanced techniques including 3D laser scanning to capture the detailed geometry of each column, facilitating the measurement of specimen geometries and the detection of global imperfections, while digital image correlation technology was applied to monitor surface deformations of the columns during the axial compression tests. This non-contact optical method enabled the researchers to measure strains and displacements across the column surfaces, providing a comprehensive understanding of the deformation behavior. The authors’ experimental data revealed that the WAAM columns exhibited flexural buckling behavior similar to those for equivalent, conventionally manufactured tubular sections, although the geometric undulations and variability inherent to WAAM had a noticeable impact on the structural performance of the columns. The presence of these imperfections necessitated adjustments in the analysis and design approaches to ensure accurate prediction of the columns’ buckling strengths. Furthermore, the study highlighted the significance of considering the as-built material properties of WAAM-fabricated elements in structural design. These properties differ from those of conventionally manufactured materials due to the anisotropy and heterogeneity introduced by the layer-wise manufacturing process. Additionally, the researchers assessed the applicability of current structural design provisions (from EN 1993-1-4 and AISC 370) to WAAM-fabricated stainless steel members. They found that, with appropriate consideration of the as-built geometric and material characteristics, these design provisions could be adapted to predict the buckling strength of WAAM columns reasonably well. However, the study highlighted the need for further research and potential revision of design standards to fully account for the nuances of additive manufacturing processes. In conclusion, the experimental investigation of Huang et al. provided detailed insight into the flexural buckling of WAAM-fabricated tubular columns and generated valuable data for the advancement of structural analysis and design of WAAM elements. It also highlighted the critical role of geometric imperfections and material properties specific to WAAM, and contributed to the development of more accurate and reliable design methodologies for additive manufactured structural elements.