On the effect of pre-strain and pre-fatigue on the monotonic behaviour of ultra-high strength steels.

The mechanical behaviour of three ultra-high strength steels has been assessed; AerMet100, 300M and the recently developed corrosion resistant high strength steel, MLX-17. Material heat treatment profiles have been utilised to provide performance optimised for the aerospace industry and specimens have been tested to explore tensile and fatigue properties, in particular when combined with pre-strain to simulate the effects of overload. Testing of this kind has not been reported within the literature, particularly amongst ultra-high strength and corrosion-resistant steels. Baseline mechanical performance for all three materials in their heat-treated conditions has been established and properties such as yield strength and ultimate tensile strength have been assessed following a 75% and 95% pre-strain as well as fatigue in combination with a 75% and 95% pre-strain. Under all loading conditions, resultant tensile mechanical properties are not seen to witness a substantial degradation in performance, but an improvement in terms of yield strength and UTS, due to the role of work hardening. An alloy comparison has been carried out and responses are seen to vary slightly as a result of material microstructure. Correlation of pre-strain and pre-fatigue results with respect to baseline properties and microstructure has contributed to advancing the understanding of the mechanical behaviour of the aforementioned ultra-high strength steels.