The effect of nanosecond laser cleaning on steel components of Sydney Harbour Bridge

Abstract

In order to maintain the heritage-listed Sydney Harbour Bridge, the steel surface must be regularly protected with a paint coating. Before applying a new coating, the steel needs to be cleaned of the previous paint layer and any contaminants, such as rust that may have formed. Laser ablation offers a more efficient method for cleaning steel compared with traditional techniques, provided that the underlying steel does not experience changes in microstructure or mechanical properties. This study investigated the effects of nanosecond laser cleaning on the steel components of this iconic bridge. To provide a comprehensive evaluation, the steel samples subjected to nanosecond laser ablation at 120 Hz were characterized by detailed residual stress analysis (including subsurface measurements), surface profiling, microstructural characterization, and hardness testing. The results show that increasing the number of laser passes promotes recrystallisation at the ablated surface and leads to an increase in surface hardness, accompanied by tensile residual stresses confined to the top surface layer. In addition, residual stress measurements indicate that subsurface and through-thickness regions beyond a depth of 0.5 mm are largely unaffected. These findings indicate that while nanosecond laser ablation does not influence residual stresses beyond a depth of 0.5 mm in the steel components, the substantial tensile residual stresses introduced at the steel surface may have implications for fatigue performance. This should be carefully considered in engineering assessments and is likely to limit the applicability of this technology for the conservation of steel structures.