Fatigue Failure of a Distortion Screen during Testing

A distress was observed in a distortion screen during testing. This screen is positioned in front of an aero engine during testing to check the engine performance with a distorted air intake. To ensure a distorted air intake, the screen contains dense meshes in some areas whereas in other areas they are less dense. Non-uniformity in the mesh density results in distorted air flow into the engine thereby facilitating evaluation of the engine performance with disturbed aerodynamic inlet. The component, i.e. the distortion screen was under development and was subjected to aerodynamic tests on the test bench. It was found to have fractured from a rib connecting two meshes. Loose end of the mesh had detached and resulted in permanent deformation of the adjoining ribs and meshes. The fractured surface exhibited glossy crystalline appearance. Scanning electron microscope revealed striations at higher magnifications (>500X) as was observed in the case of fatigue failure. Two crack fronts were adjacent and near to one corner of the rib, and the third crack front had originated from the other corner placed diagonally opposite to the other two crack fronts. This observation indicated failure of the screen under reverse bending fatigue condition. Design and material condition were reviewed. After modification it was found that the newly modified distortion screen did not show any sign of deformation after testing.