Research On The Impact Resistance Of Casing Bolted Flange Structures And Failure Mechanisms Of Bolts

Damages of the casing bolted flange area often occur in uncontainment accidents of aero-engines,the impact resistance of the casing bolted flange structure plays an important role in the containment capacity of the casing.In this dissertation,the impact resistance of the casing bolted flange structure of aero-engines is systematically studied for the first time.The failure modes and failure mechanisms of the casing bolted flange structure are revealed by theoretical analyses,ballistic tests and numerical simulations.The structural failure criterion for the casing bolted flange structure is proposed,and the impact resistance optimization design of the casing bolted flange structure is carried out,the the failure mechanism of the bolts under multi-axial loading and the bolt failure prediction model under multi-axial loading are investigated.The main research and achievement are as follows:The structural failure of the casing bolted flange structure is studied by theoretical analyses,yield modes and failure modes of the structural failure are obtained,and the structural failure criterion is proposed.Impact resistance tests are conducted using the smoothbore powder gun.Three tests are conducted for each of the six different casing bolted flange structures.The failure evaluation criterion is validated by the impact resistance tests.FE models of the casing bolted flange structures under impact loading are created.Simulations are conducted by using LS-DYNA based on the impact conditions of the impact resistance tests.The accuracy of the numerical simulations are verified by comparing impact processes,structural failure and lateral deformations of the casing bolted flange structures.Simulation results show the failure mode of bolt fracture and flange fracture are both caused by the stretch of the structure under impact loading,and the structural failure mode is determined by the relative strength of the bolt and flange.Then simulations of ring casing bolted flange structures under the impact of rotating blades are conducted,and failure modes of ring casing bolted flange structures are the same to plain casing bolted flange structures.Thus,the failure modes evaluation criterion in plain casing bolted flange structures is also available for ring casing bolted flange structures.Parameter studies are conducted by using numerical simulations to study the influences of various factors on the impact resistance of the structure.The influences of the casing thickness,flange thickness and bolt-hole’s location are simulated,and the simulations further verify the accuracy of the failure modes evaluation criterion.The suggestion for improving the impact resistance of the structure is proposed.Based on NSGA-II algorithm,the multi-objective optimization design of the casing bolted flange structure is carried out.The numerical simulation results show that the optimization design can reduce the weight of the structure and improve the impact resistance of the structure.From the study of the influences of impact position,impact angle,sleeve and rabbet,one side of the flanges is known to be the weakest position under impact.The results show that the design of sleeve and rabbet can compensate for this weakness.Numerical simulations are conducted to investigate the mechanical response of the bolt under different loading angles.The accuracy of the FE model and the feasibility of the numerical simulations are verified by comparing with the experimental data in the literature.The FE model which can predict the mechanical response of the bolt under different loading angles is obtained.The original cause of the difference of bolt yield strength between shear dominated and tensile dominated is studied,and the mechanism of the influence of loading angle on bolt yield strength was revealed that the stress concentration at the contact tip increased with the increase of loading angle.Based on the semi-infinite notch theory,the calculation formula of the stress around the bolt contact tip is deduced,and the stress distribution of the bolt is obtained by stress superposition.By comparing the stress distribution in the bolt calculated by theory with that calculated by finite element method,and analyzing the contact closure condition of the tip under different loading angles,it is verified that the mechanism of the bolt yield strength changing with the loading angle is that the stress concentration increases with the increase of the loading angle,The original reason for the increase of stress concentration is the increase of the small-scale-yielding at the contact tip.A theoretical model to predict the yield force of the bolt under 45° to 90° loading angles is proposed based on the investigation.Based on BP neural network,a model is established to predict the mechanical response of bolts under different loading angles.The BP neural network optimized by genetic algorithm can accurately predict the curve within the displacement range of the sample data,and also can predict the curve extension trend outside the displacement range.