Steel structures such as bridges, load-bearing frameworks, or industrial installations are exposed to heavy loads over decades. Over time, fatigue cracks can develop that compromise the safety of these components. Since permanently installed elements are often difficult or costly to replace, researchers at Empa are exploring alternatives: metal 3D printing is intended not only to repair damaged steel components in the future, but also to open up new possibilities for their design.
A fine crack runs through the steel plate. As the robotic arm moves over the damaged area, what forms is not an ordinary weld seam, but a three-dimensional metal reinforcement. This is made possible by a process known as "Wire Arc Additive Manufacturing" (WAAM), in which a welding wire is deposited layer by layer onto the damaged area using an electric arc.
Unlike conventional welding, metal 3D printing here is not used to join components, but to provide targeted reinforcement with an individually designed geometry. Damaged areas can thus be repaired locally, without the need to replace the entire component. Empa researchers aim to use this process to repair cracked elements of bridges and load-bearing structures — since permanently installed steel parts are often difficult, or even impossible, to replace without considerable effort.
Geometry Matters More Than Material Volume
"What matters is not depositing as much material as possible," explains Hossein Heydarinouri from Empa's Engineering Structures laboratory. "The shape is far more important: an optimized geometry distributes stresses in a way that stops or significantly slows the propagation of existing cracks."
As part of a master's thesis, researchers from Empa and ETH Zurich succeeded in extending the service life of damaged steel plates by up to four times.
In extensive tests conducted in Empa's testing hall, cracked steel plates were fitted with metal reinforcements of varying shapes and then subjected to repeated loading. The results were clear: all reinforced samples showed significantly higher fatigue life than unrepaired comparison plates. Two-layer, graduated reinforcement geometries proved particularly effective.
However, the study also revealed the limits of the approach: if the geometry is poorly chosen, new stress concentrations can arise, for instance at the transitions between the base material and the printed metal.
"Our results show how important a targeted design of the reinforcement structure is," says Heydarinouri.
Practical Hurdles for On-Site Use
Fatigue cracks are among the most common types of damage in steel structures, and targeted reinforcement is significantly more efficient than fully replacing a damaged component.
"With 3D printing, we can apply metal reinforcements exactly where they are structurally needed," says Heydarinouri. "Repairs save material, energy, and costs."
Despite this potential, the path to practical application still involves challenges. Metal 3D printing currently relies on industrial robotic systems that have only limited transportability.
"Damaged components are usually installed within the structure itself," says Heydarinouri. "Today, they would need to be brought to a workshop for repair, which isn't always realistic in practice."
While initial approaches for mobile or portable robotic systems already exist, further development is needed for broader on-site use. Even so, the research team sees advantages for applications where components are easily accessible or can be removed during maintenance work.
From Repair to Intelligent Structural Design
Beyond repairing damaged components, Heydarinouri's team is also working on more advanced concepts. Combining intelligent geometries, metal 3D printing, and new materials is expected to enable metal structures that deliberately yield under extreme loads, absorb energy in the process, and then return to their original shape — or at least avoid permanent damage. Such structures could eventually serve as metallic damping elements against earthquakes or vibrations, for example in bridges, buildings, or technical installations. The Empa researcher also sees potential in mechanical engineering, for instance for lightweight yet highly stressed components in production machinery. WAAM is particularly advantageous where only a few, geometrically optimized components are needed.
"3D printing gives us enormous geometric freedom," says Heydarinouri. "We can specifically optimize structures — for example, to reduce weight while maintaining, or even improving, load-bearing capacity."
At the same time, Empa materials scientist Maryam Mohri is investigating how shape memory alloys (SMAs) can be further developed to enhance material properties. These materials have the ability to return to their original shape after deformation, for instance through heating. This allows improved material properties to be combined with tailored geometries, opening up new possibilities for material-efficient, adaptive metal components. The corresponding geometries are developed at Empa using numerical simulations and subsequently tested experimentally, ensuring that the printed components meet real-world conditions and are suitable for industrial applications.
Source: Empa
