This paper deals with multiaxial low cycle fatigue (LCF) crack behavior of maraging steel under non-proportional loading. Strain controlled LCF tests under proportional and non-proportional loading with 90° out-of-phase shift between the axial and shear strains were carried out on tube specimens at room temperature. In the correlation with Mises strain, LCF lives under non-proportional loading were underestimated by a factor of 10 from those under proportional loading. Observation of the specimen surface during fatigue tests showed that a large number of cracks were initiated in shear loading and non-proportional loading compared with uniaxial loading. Finite element analyses for estimating the stress concentration around inclusions revealed that the inclusion under shear loading and non-proportional loading had more locations to initiate cracks than the inclusion under uniaxial loading. The crack initiation life under proportional loading did not significantly differ from that under non-proportional loading. Fatigue cracks under non-proportional loading, however, propagated faster than those under proportional loading. The faster fatigue crack propagation under nonproportional loading presumably results from higher strain intensity at crack tips than that under proportional loading. The fatigue lives were predicted from the crack propagation analysis. Most calculated lives were within a factor of 2 scatter band so that the correlation of the test results appears satisfactory. Therefore, the main cause of shorter fatigue life under non-proportional loading resulted from the faster crack growth due to the higher strain intensity.

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