Fatigue Crack Growth Behavior of WAAM Steel Plates: Experimental Analysis and Comparative Study

Wire arc additive manufacturing (WAAM) combines traditional welding technology with advanced robotic control to fabricate large metal components layer by layer. This process offers significant advantages in terms of speed, cost, and material efficiency, making it an attractive option for constructing large-scale structural components. With its potential for geometric optimization, WAAM not only enhances structural efficiency but also offers new possibilities in architectural design. However, the adoption of WAAM in load-bearing applications necessitates a thorough understanding of the material properties, especially regarding fatigue, given the cyclic loads that structures typically endure. Previous studies have primarily focused on the fatigue properties of WAAM materials in aerospace and automotive applications, with limited research on WAAM carbon steels. The existing literature reveals a gap in understanding the fatigue crack growth behavior of WAAM steel plates, particularly in comparing the effects of different deposition strategies on material performance.

A new study published in International Journal of Fatigue conducted by Dr. Cheng Huang and Professor Leroy Gardner from Imperial College London alongside PhD candidate Yuanpeng Zheng and Professor Tao Chen from Tongji University and also Professor Elyas Ghafoori from Leibniz University Hannover, the researchers investigated the fatigue crack growth (FCG) behavior of Wire Arc Additive Manufacturing (WAAM) steel plates. Their research was aimed at understanding how WAAM steel, both normal- and high-strength, performs under cyclic loading conditions, a critical factor for its application in structural engineering. The team fabricated WAAM plates using a parallel deposition strategy. Compact tension (CT) specimens were then machined from these plates, with specimens extracted in different directions to explore potential anisotropy in material properties. Before conducting FCG tests, the team performed material characterization to determine the tensile properties and hardness of the WAAM steels. This involved tensile testing and Vickers hardness measurements, establishing a proportional relationship between the hardness and ultimate tensile strength of the examined materials. The core of the research involved conducting FCG tests on 13 WAAM steel CT specimens. These tests were designed to measure the rate of crack growth under cyclic loading, allowing for the determination of Paris’ law constants for the materials. The tests were carried out under controlled conditions, with specific attention to load ratios and crack monitoring techniques. Afterward, the authors performed fractography on the tested specimens to analyze the mechanisms of crack growth. This involved examining the fracture surfaces under scanning electron microscopy to identify features like fatigue striations, secondary cracks, and dimples, which provide insights into the material’s behavior during crack propagation. The authors confirmed a proportional relationship between hardness and tensile strength in WAAM steels. This fundamental understanding aids in predicting material behavior under stress. The FCG tests revealed that WAAM steels exhibit similar FCG behavior to conventionally produced steels, with no significant anisotropy detected. This is a promising finding, indicating that WAAM steel can potentially match or exceed the performance of traditional steels in structural applications. Additionally, the authors found that the deposition strategy (parallel vs. oscillatory) influenced the FCG behavior, particularly for high-strength steels. This suggests that optimization of deposition parameters could further enhance the fatigue performance of WAAM materials. They also found that the FCG behavior of WAAM steels to be generally well captured by the FCG laws given in BS 7910. The research provided evidence that existing models and standards for conventional steels can be applied to WAAM materials, albeit with some adjustments for high-strength variants. Fractography revealed predominantly brittle behavior during FCG but somewhat ductile behavior shortly before fracture. This dual-mode failure mechanism underscores the complexity of WAAM steel’s behavior under cyclic loads and highlights the need for detailed microstructural analysis in future studies. The new study has significant implications for the construction industry and other sectors considering the use of WAAM for structural applications. The authors’ work provided a deeper understanding of the fatigue properties of WAAM materials and contributes to the development of guidelines for the design and assessment of WAAM-produced structures. Future research directions include further investigation into the fatigue strength of WAAM steels and the development of fatigue design guidance tailored to WAAM steel structures.