Experimental investigation of fatigue crack propagation under non-proportional multiaxial loading

Blisks (bladed disks) are critical components in modern aero-engines that offer significant weight savings compared to conventional blade and disk rotor designs, resulting in improved fuel efficiency. However, due to their integrated design, blisks are susceptible to unique failure modes following foreign object damage (FOD) and crack initiation. Of particular interest is the trajectory of crack propagation from FOD sites, which determines whether failure will occur via a blade-off event or rupture of the blisk. This work presents an experimental test setup which replicates the key features of non-proportional loading in a blisk. A novel feature of the test rig was the ability to apply loads independently in three axes using a biaxial machine equipped with only two hydraulic servo-actuators. A series of multiaxial fatigue tests were completed on notched cruciform specimens and a wide range of crack trajectories were achieved, validating the design of the test rig. Crack trajectories produced by non-proportional load paths are not accurately predicted by conventional criteria, such as the maximum tensile stress criterion (MTS), when cracks are subject to high mean mode-II loads. The results of these experiments underscore the complexity of modelling non-proportionally loaded cracks and the acquired crack trajectory data is a useful tool for validating further models. • Design of experiment to simulate a complex multiaxial stress field. • Reporting of experimental results. • Examination of fracture surface to give information on crack trajectory.