Low-cycle fatigue property and life prediction model of TC18 titanium alloy material

icaf2023 Tracking Number 86

As alpha+ beta type titanium alloy, TC18 is widely applied in the landing gear and flap slides with the advantages of high strength, excellent plasticity and weldability. Yet more service datum show that low-cycle fatigue failure sometimes occurs in the blocking hook of carrier-based aircraft due to its serious servicing environment. Consequently low-cycle fatigue performance plays an important role in the fatigue life evaluation of aircraft load-bearing structural parts. Through the low-cycle fatigue experiments of standard fatigue coupons, the static and low-cycle fatigue behaviours of TC18 titanium alloy were obtained, such as the stress-strain constitutive relationship, the cyclic stress-strain curve and the strain-life curve. In order to meet the engineering fatigue life evaluation for TC18, on the one hand, based on Manson-Coffin and reliability analysis theory, the strain life model was built and the low-cycle fatigue life curve under a certain survival rate was observed to investigate the life at certain confidence and reliability; on the other hand, combined with the damage mechanical theory, the gradual degradation process of the mechanical properties of TC18 titanium alloy with the increase of load times was described by the Lemaitre plastic damage model, and the fatigue damage accumulation parameters were determined based on experimental results. We obtain the conclusion as following:1) the cyclic behaviour of TC18 titanium alloy material is a strain cycle softening relationship under different strain-rate loading;2) its low-cycle fatigue fracture form is described that: the crack initiated in the form of quasi-cleavage fracture, and propagated by quasi-cleavage fracture or mixed-type fracture of quasi-cleavage and dimple, finally sheared at the edge of the specimen to form a shear dimple fracture ;3) Based on the experimental results, the engineering Manson-Coffin strain life model and macro-cyclic plastic damage theory model in the paper can be used for the low-cycle fatigue life evaluation of TC18 titanium alloy material.