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Session: Session 5: Fatigue crack growth and life prediction methods  II
Session starts: Tuesday 27 June, 10:50
Presentation starts: 12:10
Room: Theatre room: plenary

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Bemin Sheen (Imperial College London)

Abstract:
Bladed disks are subjected to foreign object damage which results in the initiation of cracks. Blisks in aero-engines are affected by a combination of loads which grow a crack in a multiaxial stress field. Depending on the local stress field, a crack may propagate through the blade, or more undesirably grow towards the bore of the disc due to hoop stress. The overall trajectories of cracks initiating near the base of the blade are particularly difficult to predict. Classical crack trajectory criteria, which are often based on the strain energy density or maximum principal stress during a cycle, are insufficient for predicting the growth of cracks under non-proportional and multiaxial loads. Additionally, a combination of inter-related factors including mode-mixity, cyclic plasticity, and crack closure are also influential on non-proportional crack growth. Therefore, an improved crack trajectory criterion is needed. To create this criterion, a series of fatigue tests are being carried out on representative Ti-6Al-4V test samples to build a database of experimental results. The test rig and a cruciform shaped test samples have been designed to capture the key features of crack propagation in a blisk. Specimens may be loaded in three axes using a biaxial machine and hydraulic fixture. This arrangement aims to reproduce the combination of steady and dynamic loads in a crack in a blisk. The ratio of the applied loads has been determined from fatigue simulations in FRANC3D, a software package which simulates crack growth using linear elastic fracture mechanics. The results of the ongoing tests will help to update the crack growth model to gain an agreement between the simulated and experimental results of the test specimen. The final crack growth model will be used to accurately predict the trajectory of cracks initiating in blisks subjected to non-proportional loading. Simulations on blisk geometries in FRANC3D have shown a good agreement with existing data from literature. Results obtained from the completed fatigue tests have been presented and compared to the results from the simulated specimen.