Internal Damage Analysis of Braided Composites Embedded in Carbon Nanotube Yarn

Carbon nanotube (CNT) yarn sensors were embedded in 3D braided composites in the form of arrays to detect the internal damage of specimens and study the internal damage monitoring of the 3D braided composites. The signals collected by the sensor array of CNT yarn were preprocessed using the dynamic wavelet threshold algorithm. The exact position of the damage was calculated based on the main features of the resistance signal matrix, which was calculated using the quadratic matrix singular value. The results show that the internal damage localization of the specimens was consistent with the actual damage. The localizations in this study can provide a basis for enhancing the structural health monitoring of smart 3D braided composites.

Strain-Sensing Characteristics of Carbon Nanotube Yarns Embedded in Three-Dimensional Braided Composites under Cyclic Loading

Carbon nanotube yarns are embedded in three-dimensional (3D) braided composites with five-axis yarns, which are used as strain sensors to monitor the damage of 3D braided composites. In the cyclic mechanical loading experiment, the strain-sensing characteristics of 3D braided composites were studied by in situ measuring the resistance change of the embedded carbon nanotube yarn. The 3D five-directional braided composite prefabricated part based on carbon nanotube yarns was developed, and the progressive damage accumulation experiments were carried out on carbon nanotube yarns and specimens embedded in carbon nanotube yarns. The research results show that there is a good correlation between the change of relative resistance of the carbon nanotube yarn and the strain of the composite specimen during cyclic loading and unloading. When the tensile degree of the specimen increases beyond a certain range, the carbon nanotube yarn sensor embedded in the specimen shows resistance hysteresis and produces residual resistance. Therefore, the fiber can better monitor the progressive damage accumulation of 3D five-direction braided composites.

A parameterized unit cell model for 3D braided composites considering transverse braiding angle variation

In this paper, a parameterized unit cell model for 3D braided composites considering transverse braiding angle variation is proposed, to assist the mechanical characterization of such materials. According to the geometric characteristics of 3D braided composites, a method for automatically generating textile geometries based on practical braiding parameters, including the main braiding angle, the transverse braiding angle, and the fiber volume fraction, is established and implemented in a CAD software package. In this model, the addition of transverse braiding angle educes a more flexible control of fiber volume fraction distribution, and with the combination of control parameters according to the actual fiber distribution needs of users, it can suggest the appropriate parameters for the unit cell. The generated unit cell models are used in finite element analysis and the results are validated against experiments for a number of 3D braided composites in terms of fiber volume fraction and elastic constants, and good agreement is observed. Based on the parameterized unit cell model, the effects of main braiding parameters on the elastic properties of 3D braided composites are discussed.

Open hole size effects on tensile properties of 3D braided composites

Owing to the excellent integrated structure, notch-insensitivity, delamination-free characteristics, 3D braided composites have a broad range of engineering applications. In this paper, the notch size effects on two types of 3D braided composites were experimentally examined. Style I incorporated 40% of longitudinal lay-in yarns. Style II was the pure braids. The Point Stress Criterion (PSC) was applied to predict the open-hole strength of 3D braided composites. It is found the 3D braided composites can keep higher proportion residual strength after involving the different circular hole sizes compared to plain woven laminates. The open-hole pure braided specimen shows better performance than that the braids with longitudinal yarns, the lay-in longitudinal yarns improve neither specimens’ un-notched strength, nor the modulus. The predicted open-hole strength were compared with experimental results. The traditional analytical method can predict the open-hole strength of 3D braided composite to some extent. Under uniaxial tensile stress, the failure behaviour of two types of 3D braided specimens are different. For un-notched specimen, clear cracks usually show up on the Style II specimen, while it is not true for Style I coupon. For notched specimen, the crack of both notched specimens will propagate along the notch and finally render the specimen to fail

Failure behaviors of 3D braided composites with defects in different locations under low-velocity impact compression

This paper aims to investigate the effects of the impurity defects in different locations on the transverse compressive behaviors of three-dimensional (3D) braided carbon fiber/epoxy composites under low-velocity impact. The composites with defects in different locations were prepared by placing polytetrafluoroethylene particles at the bottom surface, corner and center parts, respectively. The compression test was implemented with the drop-weight impact method. The failure morphologies were characterized using high-speed photography and micro computed tomography (Micro-CT). A mesoscale finite element model considering the defect was developed to enhance the physical understanding of the compression process. It is found that defects located at the corner zone and the central zone have a greater impact on the compression behavior of the 3D braided composite than the defect located at the bottom surface part. The defects at the corner and center zones reduce the compressive strength of the composite, increase the failure area, and cause severe damage to both the yarn and resin. In addition, the defect causes local stress concentration on the surrounding yarns. The defects in different locations cannot change the main shear failure mode of the 3D braided composite. It is shown that great attention should be given during the manufacturing or detection process to avoid such a deterioration effect of defects located in those zones on composite strength.