Study On Low-velocity Impact And Residual Compression Properties Of Three-dimensional Four-directions Braided Composites After Damp-heat Aging

Due to its high specific strength,high specific stiffness and low specific density,composite materials have been applied in various fields such as aerospace,ships,fighter jets,civil airliners,medical instruments and fitness equipment.Compared with the traditional composites such as laminate composite and skin reinforced composite,three-dimensional(3D)four-directional braided composite has more stable mechanical property,hardly debonding and delamination,strong interlayer performance,strong impact resistance and high damage tolerance.3D four-directional braided composite materials are inevitably affected by the damp-heat of the external environment in the application environment.Therefore,in this test,the external environment of the test parts is firstly subjected to damp-heat aging treatment.In this paper,3D four-directional braiding specimen with two kinds of resin matrix(thermosetting resin matrix,epoxy resin matrix,H;Thermoplastic resin matrix,bismaleimide resin matrix,S)and two kinds of braiding angles(15 degrees braiding Angle and 30 degrees braiding Angle),H15,H30,S15,S30,were designed.First,the specimen is damp-heat aging for 500 h,1000h and 2000 under room temperature,40℃-Soak,70℃-Soak and 70℃-Humidity 85%,respectively.Secondly,30 J lowspeed impact experiment was carried out on all the damp-heat aged specimen in the drop weight test machine to drawn the impact response curve of the specimen by obtaining test data.The internal damage at the depth of 0.15 mm was scanned on the ultrasonic C-scanning instrument to measure the internal damage area after the impact and investigate the internal damage expansion process.Then,the axial compression test of all the impacted specimen was carried out on the universal test machine,while the digital image correlation(DIC)test was carried out,to obtain the residual compression load and compression damage process of the specimen.Finally,a 3D model was built by UG and imported into ABAQUS for simulation experiment.The fiber bundle adopted 3D Hashin damage criterion and the matrix adopted the first strength theory damage criterion.Combined with the test and simulation results,the failure mode and damage evolution process of the specimen under impact load were studiedThe results show that under the condition of the same matrix and braid angle,the higher the temperature and damp and the longer the damp-heat aging time,the higher the moisture absorption rate,the lower the impact limit load,the larger the internal damage area after impact and the lower the residual compression load.Under the same braiding angle and the condition of damp-heat aging,the moisture absorption rate of the thermosetting resin specimen is higher than that of the thermoplastic specimen,the impact limit load is lower,the internal damage surface is smaller and the residual compression load is lower.Under the same matrix and damp-heat aging conditions,the larger the braided angle,the lower the moisture absorption rate,the lower the impact limit load,the larger the internal damage area after impact,and the lower the residual compression load.The main failure mode of the test article under impact is tensile failure,and the internal damage is mainly the extension damage along the knitting direction of the fiber bundle.The main failure modes of compression of specimen are as follows: the test pieces with 15° braided angle are shear failure,and the specimen are transverse fracture.The test piece at a braided angle of 30° is buckling failure,and the test piece breaks at a direction of the fiber weave.