Experimental characterisation of low-cycle fatigue behaviour of thin-walled and solid WAAM-fabricated high-strength steel under variable mean strain
Hagar El Bamby (TU Delft - Civil Engineering & Geosciences)
Vera Popovich (TU Delft - Mechanical Engineering)
Ramesh Babu (Det Norske Veritas)
Milan Veljkovic (TU Delft - Civil Engineering & Geosciences)
Trayana Tankova (TU Delft - Civil Engineering & Geosciences)
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Abstract
Wire Arc Additive Manufacturing enables efficient fabrication of large-scale steel components with high geometric freedom. However, reliable structural use of WAAM high-strength steels requires an in-depth understanding of their cyclic behaviour, particularly in the low-cycle fatigue regime, which is dominated by plastic deformation. This study investigates the LCF behaviour of WAAM-fabricated ER100S-G high-strength steel using specimens extracted from thin-walled (WAAM-W) and solid deposits (WAAM-S). Strain-controlled fatigue tests were conducted at room temperature under strain ranges Δε, of 0.35% to 2.00%. WAAM-W was tested at strain ratios Rε= −1, 0 and 0.5 while specimens from the WAAM-S configuration were evaluated at Rε = 0. Although WAAM-S exhibited higher monotonic and cyclic strength, it underwent more pronounced cyclic softening, with a reduction of approximately 35% in the 0.2% offset cyclic yield stress, compared with 17% for WAAM-W. WAAM-S showed a steeper elastic strain-life slope, indicating that its strength advantage diminished with increasing fatigue life, while WAAM-W exhibited greater fatigue ductility and plastic-strain accommodation. This strength stability trade-off is attributed to the heterogeneous and banded bainitic morphology of WAAM-S, which provides cyclic strain localisation. In contrast, the more homogeneous WAAM-W microstructure and strain-induced transformation of retained austenite contribute to distributed plasticity and improved cyclic stability. For WAAM-W, positive mean strain reduced fatigue life at lower strain amplitudes due to a persistent tensile mean stress, whereas increased plasticity at higher amplitudes contributed to stress relaxation. The SWT formulation captured the mean-strain dependence of fatigue life, while the energy-based analysis linked fatigue life to the elastic and plastic strain-energy contributions.
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