Abstract: According to the demand of strength design standard and experimental results, the present paper indicates that it is more favourable to properly reduce yield to ultimate tensile strength ratio σ0.2 / σh of ultra-high strength steel for aircraft landing gear under the conditions of σ0.2/σb >0.67. The study on 40CrMnSiMoVA steel verifies that σ0.2value and σ0.2/σb ratio of bainitic microstructure are lower than those of martensitic microstructure in monotonic tensile test. While after a small number of pre-cyclic loads, the σ0.2 and σ0.2 / σb the former become higher than those of the latter. That is , σ0.2 value and σ0.2/σb ratio determined by monotonic tensile test are unconstant, but ultimate tensile strength σb, fracture strength Sk, elongation percentage δ and reduction of cross-sectional area ψ are constant. The lower original σ0.2 / σb ratio of bainitic microstructure causes it cyclic hardening,which could homogenizemicro-deformation and lighten notch sensitivity. As a result, strain-controlled and impact fatigue crack initiation of the steel with bainitic microstructure is delayed in longer life range. Meanwhile, because of the lower σ0.2 / σb ratio, overload effect of the bainitic microstructure is more distinct to prolong fatigue life. So, the lower σ0.2 / σ0.2 ratio characteristic of bainitic microstructure in ultra-high strength steel is an advantage among mechanical properties.

Key words: landing gear, ultra-high strength steel, yield to ultimate tensile strength ratio, bainitic microstructure, fatigue properties