Investigation On Mechanism Of Containment For Composites Engine Casing

With the increased requirements of thrust-weight ratio and economy for modern large-scale commercial aviation turbine engines,the engine containment system needs to reduce its weight as much as possible on condition that meeting the containment requirements.On the basis of the application requirements of aviation turbine engines,carbon fiber reinforced epoxy resin matrix composites(CFRP)are gradually replacing metal containment system due to their high specific strength,high specific stiffness and designability of mechanical properties,and the extensive applications of CFRP on aviation turbine engines have become an inevitable trend.However,the containment process of engine containment system is a typical nonlinear highspeed impact process,and the high-speed impact response and damage mechanism of carbon fiber reinforced resin matrix composites are much more complex than those of metal materials,which poses an unprecedented challenge to the application of composite materials in the containment system.Therefore,the investigations of some factors(fiber performance,resin performance,ply angle,projectile,contact form et al.)on the impact response of composites can help to clarify the mechanism of containment process for composite casing.The research is of great significance to the optimal design and structural verification of large commercial aviation turbine engine containment system in China.In this paper,ballistic impact test,numerical simulation and theoretical analysis are combined to conduct an in-depth study on the mechanism of composite containment process.The specific work is as follows:(1)The mechanism of containment process for aluminum alloy casing was investigated by conducting the ballistic impact tests and numerical simulations.A new method was developed to solve three important problems(the measurement of failure strain,the selection of stress triaxiality and the strain rate effect of failure strain)in the process of obtaining the J-C failure model.The loading speeds,strain distribution and stress triaxialities of the specimens were analyzed and revised by using the experimental results and finite element models(FEM).A series of quasi-static and dynamic experiments were conducted under different temperatures combined with the help of the digital image correlation(DIC)method and micro speckle to implement the measurement of the strain field on specimens,thus establishing the relationships between the failure strain,revised stress triaxialities,strain rate and temperature,which helps to obtain the failure parameters of 2618 aluminum alloy and TC4 titanium alloy.Based on the numerical simulation method,the influences of the strengthening rib on the impact resistance of aluminum alloy plate were studied.Results show that the strengthening rib can improve the critical penetration velocity and impact resistance of aluminum alloy plate to some extent.(2)The investigation on impact resistance and containment ability of CFRP plates were conducted.A new strain-based damage initiation criterion which not only considers the in-plane stress state but also considers the influence of normal stress on the damage of composite materials under impact load was developed based on the 3D Hou criterion and the CDM damage evolution criterion.The continuous damage evolution is also adopted based on the CDM damage evolution criterion proposed by Ladeveze.A large number of mechanical tests were carried out,and the mechanical parameters and failure parameters of composites were obtained,which laid a foundation for the simulation of impact response on metal and composite materials under high-speed impact load.Moreover,the ballistic impact finite element model of the composite plate was established,and the accuracy of the constitutive model and the damage criterion of the composites and the feasibility of modeling method were verified by comparing the delamination area,in-plane strain history and the maximum normal deflection.Based on the parameters and modeling method,the impact response of the composite thick plate was investigated by establishing the finite element model,and the weight reduction advantage of the composite material over the metal material(aluminum alloy)was studied by taking the critical penetration velocity as the measurement index.Results show that the critical penetration speeds of composites are higher(15.8%?16.7%,19.2%?20% and 6.1%?6.25%,respectively.)than that of aluminum alloy target plates.Moreover,the critical penetration velocity of composites increases with the increase of the thickness,but the higher impact speed and impact contact force may cause shear failure before the fibers transfer the in-plane tensile load in the front of the laminated plates,thus making the improvement of the critical penetration speed is decreasing when increasing the same thickness.The containment ability of the composite halfring casing was investigated.Ballistic impact tests were conducted on composite half-ring casing by using blade projectile.Results showed the composite half-ring casing specimens were all penetrated by the TC4 blade projectile at the speeds of 210m/s and 130m/s.The outer surfaces of the half-ring casing specimen show tensile fracture of fibers,matrix cracking,delamination failure and extensive tearing failure caused by the pulling of blade projectile.An angle of the blade projectile will first contact the inner surface of the half ring casing specimen when the rotation angle is large,thus generates obvious local shear deformation.The inner surface of the half-ring casing specimens appears different failure modes when the rotation angle is different.(3)The effect of 2.5D woven on the containment ability of composites casing was investigated.A breakthrough has been achieved to control the impact attitude of projectile in ballistic impact test by using the reconstructive inner wall of gas gun tube,the advanced coaxial positioning technology and the cutter-broken-shell separation technology,the attitude control of the blade shaped projectile has been well achieved,thus helps to complete the ballistic impact test of three woven types of 2.5D composite materials.The impact response,failure modes and critical penetration velocities of 2.5D woven composites with three woven types(shallow direct linking,shallow crosslinking and shallow crosslinking with surface yarn)under the impact of titanium alloy projectile under different impact velocity were obtained.Results show that,for 2.5D woven composites with crosslinking structure,the warp yarn only interweave with the adjacent weft yarns,and the normal stiffness increases obviously if compared with the 2.5D woven composites with direct linking structure,thus improving the impact resistance of the materials.Results also show that impact resistance of the 2.5D woven composite plate can be effectively improved by adding surface yarns on the upper and lower surfaces of the shallow crosslinking2.5D woven composite at the expense of some fibers along the weft direction.A theoretical model for impact analysis of composite plates is established based on the traditional energy balance,and the critical penetration velocity of 2.5D woven composite plate can be estimated by only a few basic material parameters measured under static conditions.The model considers the actual impact damage modes and the estimated critical penetration velocities are close to the results from the experiments.Thus,the model can greatly save the cost of test and be used to guide the design,material selection and weaving type of composites casing to some extent,also provides a reference for the design of engine containment system.In addition,the critical penetration velocity of composites was compared with those of 2.5D composites.Results show that the critical penetration velocity of shallow direct linking,shallow crosslinking and shallow crosslinking with surface yarn 2.5D composites plates are 2.2%,14.7%,and 22.1% higher than that of composites plates.(4)The effect of strengthening rib structure on the containment ability of composites casing was investigated.The ballistic impact tests of the composite plate with strengthening rib were carried out to investigate the effect of strengthening rib structure on the containment ability of composites casing.Combined with numerical simulation,the impact resistance,damage characteristic and critical penetration velocity of the composite plate with strengthening rib and the uniform thickness composite plate were compared.Results show that the composite plate with strengthening rib has better energy absorption,and the higher critical penetration velocity of composite plate with strengthening rib is 16.7%-17.6% higher than that of the uniform thickness composite plate.