Failure Analysis On Cracked Turbocharger Impeller And Finite Element Numerical Calculation

Two failed turbocharger impellers were investigated. The fracture surfaces of the fractured blades were observed visually and scanning electron microscopy (SEM). The microstructure in various zones of fractured blades were observed by optical microscopy (OPM) and SEM. Chemical composition of the failed impeller materials were determined using optical emission spectrometers. And chemical composition on the fracture surfaces were evaluated by energy-dispersive X-ray analyses (EDA); The hardness and tensile properties of the impeller materials are measured. The results of analysis allowed to assess the failure mechanisms and the main causes of two failed impeller.A finite element numerical calculation was performed on the impellers based on the ANSYS13.0finite element analysis software Static calculations were carried on the impellers by fluid-structure interaction (FSI). Modal analysis was conducted on the single long blade and the entire impeller in consideration of pre-stress. And fracture analysis was performed on the blade assumed the presence of cracks on the blades. The major mechanical factors responsible for the failure of the turbocharger impellers were assessed through the calculations and analysis above.Based on the results of the observation and analysis on the failed impeller and the finite element calculations, the main conclusions can be presented as the followings:1. For I type integrated impeller, fracture occurred at the middle location of blade length and fatigue fracture is its main failure mechanism. The microstructure of blade matrix consists of a-Al and particles of Al9FeNi and Al6Cu3Ni aligning with the circumferential of forging stock. The grain size of matrix. Improper metallurgical structure is mainly responsible for the failure of the impeller.2. For II type separated impeller, fracture occurred at the root of blade and fatigue fracture is its main failure mechanism. The four main reasons leading to fracture of blade are presented as followings:(1) Presence of intergranular network-like heterogeneous phases in the located matrix;(2) Low elongation of the materials;(3) Presence of sharp corners and circumferential machining marks on the leading edge of the blade;(4) Thin root thickness of blade3. ANSYS fluid-structure interaction (FSI) calculations indicate that pneumatic pressure has less influence on the impeller failure, while the centrifugal force should be the major mechanical factor leading to fracture of blade. Under the action of two forces the root of the blades would bear the maximum tensile stress and crack origin is initiated from the root. However, two forces do not play a major role in failure case of blade initiating crack at the middle location of outer edge of the blade.4. The reasonable explanations for three cracking locations present on the long blades were given through the modal analysis of the blades and the impellers. Besides, the rotation speed of impeller which can bring about resonance easily has calculated based on the Campbell figure,5. Depth of the crack propagation and crack propagation direction were determined by structural fracture analysis when instantaneous fracture occurred on the blade. Formation of L-type fracture is caused by instantaneous initial cracking and fatigue propagation.