Low-velocity Impact Performance And Damage Mechanism Of Carbon/epoxy 2.5D Woven Composites With Yarn Reduction

2.5D woven composites are widely used in conical rotating body components such as aerospace engine nozzles,aerospace radomes and aero-engine combustion chamber flame barrels because of their excellent mechanical properties,flexible structural designability and overall near-net forming capability.Among them,the yarn addition or reduction method is the core technology to realize the prefabricated body forming of tapered rotating body.However the introduction of yarn addition and subtraction can cause the local structural variation or even defects.Importantly,these components are inevitably subjected to low velocity shock loads during preparation and service.The initiation and evolution of damage caused by the yarn addition and reduction region has not been clarified,which seriously affects the reliability of the component in subsequent service.This paper takes the practical application of 2.5D woven composite tapered rotary body as a traction.2.5D woven composites without yarn reduction,with half row yarn reduction and with all row yarn reduction were designed and prepared.On this basis,a combination of experimental and numerical calculations was used to focus on the mechanical properties of low velocity impact,progressive damage and damage mechanisms.In order to enhance the structural design and application of conical rotating body to provide data support.The main research contents and conclusions of this paper are as follows.(1)The impact energy was set at 54 J and 72 J.Low velocity impact tests were conducted on 2.5D woven composites without yarn reduction,half row yarn reduction and all row yarn reduction using a drop hammer impact meter.Load-time curves,load-deflection curves,energy-time curves,and key mechanical characteristic values were obtained.The surface and internal damage of the post-impact specimens were analyzed using optical microscopy and Xray computed tomography(Micro-CT)techniques,respectively.The impact damage mechanisms of the three specimens were elucidated.It was shown that yarn reduction slightly reduces the low velocity impact properties of 2.5D woven composites.The mechanical property retention rate of the specimens with half row yarn reduction was 3% higher than that of the specimens with all row yarn reduction.At the same impact energy,the specimens with all row yarn reduction showed the most severe damage.Moreover,the damage of the specimens without yarn reduction was mainly in the form of yarn and substrate debonding.The damage of the specimen with half row yarn reduction was mainly in the form of breakage of the two remaining knotted warp yarns.The damage of the specimens with the all row yarn reduction is mainly in the form of delamination and shear damage over a large area.(2)Based on the "local" damage characteristics of low velocity impact and Micro-CT image scanning results.A parametric modeling method was used to build a 2.5D woven composite macro-meso hybrid finite element model without and with reduced yarns.ABAQUS finite element software was used.The 3D Hashin criterion,the Von-Mises criterion,and the maximum stress criterion and the stiffness discounting damage evolution method were selected.Numerical calculations of low velocity impact of 2.5D woven composites were carried out.The low velocity impact progression process was further clarified.The results show that the developed macro-meso hybrid finite element model can accurately simulate the mechanical response curve,macroscopic damage morphology and meso damage distribution area of 2.5D woven composite materials.Moreover,the matrix in the yarn reduction region is subjected to higher stress levels during the low-velocity impact process.(3)Combined with digital image correlation(DIC)technique.Compression after impact tests were performed on specimens without yarn reduction,with half row yarn reduction and with all row yarn reduction using a universal testing machine.The load-deflection curves,residual strength,full-field strain progression distribution and damage patterns were obtained.It is shown that yarn reduction reduces the compressive strength of the specimens slightly.Because of the difference in damage volume produced by low velocity impact,the residual strengths of specimens containing half row yarn reduction were 11.8% and 4% higher than those of specimens containing all row yarn reduction at 54 J and 72 J impact energy,respectively.In addition,the compression damage of the unimpacted specimens was randomly distributed.In contrast,the compression damage of the after impact specimens was concentrated on both sides of the impact center.The progressive damage process was similar for the specimens without yarn reduction,with half row yarn reduction and with all row yarn reduction.All of them showed the first large stress concentration in the impact damage area and gradually extended linearly to the edge of the specimen.