Evaluation of near-threshold fatigue crack propagation in harmonic-structured CP titanium with a bimodal grain size distribution

•CP titanium with a bimodal harmonic structure exhibits low fatigue crack growth resistance.•A bimodal harmonic structure reduces the extent of crack closure.•A bimodal harmonic structure reduces the effective threshold stress intensity range ΔKeff,th.•Effect of grain size on ΔKeff,th of CP titanium becomes more significant with stress ratio. To examine near-threshold fatigue crack propagation in commercially pure (CP) titanium with a bimodal harmonic structure, which is defined as a coarse grained structure surrounded by a network of fine grains, K-decreasing tests were conducted. The crack growth rates for harmonic-structured CP titanium were higher than those for homogeneous material with coarse grains at comparable stress intensity range, while the threshold stress intensity range was lower. This phenomenon was attributed to a reduction in the extent of crack closure and the effective threshold stress intensity range, resulting from the presence of fine grains in the harmonic structure.