Fatigue fracture analysis of helicopter landing gear cross tube

Abstract

This paper presents the failure analysis results of the helicopter skid landing gear. The helicopter skid landing gear fractured while the helicopter was on the ground at the airport. The aft cross tube of the landing gear assembly was found fractured into two pieces. Observation of crack propagation area at low magnification revealed numerous secondary cracks, inner surface irregularities while fracture surface indicated fatigue process. Macro-fractographic analysis of the fracture surface disclosed features that gave additional indication of fatigue. Examination of mating fracture surfaces under scanning electron microscope (SEM) detected the existence of irregularities in the material. Chemical analysis showed the aft cross tube is made of high-strength aluminum alloy. The stress analysis of skid landing gear, using the finite element method (FEM), has identified the stress-deformation condition of the helicopter landing gear with a detail insight of the stress state in the vicinity of the crack origin. The stress value, in case of maximal takeoff weight, was very high in the crack initiation zone. The results indicate the superposition of manufacturing-in, designing-in as well as operating in defects which led to total failure of the helicopter skid landing gear aft cross tube.