# Internal Damage Analysis of Braided Composites Embedded in Carbon Nanotube Yarn
## 1 Introduction  
Using the method of multiaxial orientation, integral braiding endows 3D braided composites a spatial network interlocking structure. This structure overcomes the shortcomings of traditional compound-material laminates, namely, poor interlayer performance and impact resistance [1]. The process can directly braid special specimens in accordance with the parts of shapes and dimensions, such as truncated cone shape, dumbbell, and ribbed cylinder, without seaming and processing. It creates conditions for achieving complex shapes from the entire design and manufacturing. The technology has been successfully applied to the manufacturing of components in aeronautics, space industry, sporting goods, ships, and biological materials [2]. Bearing materials are affected by aging, high pressure, high temperature, and corrosion at high altitudes and tend to exhibit fatigue and damage under the combined action of various complex loads [3]. Current methods of damage detection of composite materials include visual detection, X-ray detection, acoustic emission detection, ultrasonic testing, eddy current, ray, and fiber Bragg grating [4]. These methods usually require complex equipment and online real-time monitoring, and the structure and location of the damage need to be determined initially. Thus, these methods are difficult to use and have serious limitations [5]. A recent study of the application of carbon nanotube (CNT) yarn as a resistance sensor resulted in the development of CNT yarn sensors based on different sensing principles [6]. [PERSON], [PERSON], and [PERSON] analyzed the piezoresistive properties of CNT yarn sensors that can be integrated into polymers and composites without increasing or altering the weight. With regard to the integrity of the main material, the composite strain was analyzed by measuring CNT yarn resistance, and the structure and piezoresistive response of the new strain sensor were investigated. Studies have shown that strain gauges composed of CNT yarn are more sensitive than metal foil strain gauges and thus suitable for various strain measurements [7]. [PERSON], [PERSON], and other researchers achieved significant de-bundling of the network by operating the laser below the ablation threshold. More importantly, they used linearly polarized laser pulses to selectively ablate a single-walled carbon nanotube (SWCNT) that is parallel to the direction of polarizationof the incident laser, creating aligned anisotropic conductivity networks. This research has created opportunities for the application of SWCNT in the field of electronics [8]. [PERSON], [PERSON], and other researchers proposed ink preparation and post-treatment solutions, which can be used as a simple and effective platform for the future preparation of high-quality and residue-free SWCNT films. In order to demonstrate the feasibility of the study, they prepared and characterized the gas sensor with CNT films as sensor material, the screen-printed silver-based films as the interdigital electrode (IDE) structure, and the polyimide as a flexible and solid substrate [9]. [PERSON], [PERSON], and other researchers embedded polyvinyl alcohol (PVA) CNT yarn in glass-fiber-reinforced plastic composites for the strain monitoring of composite materials. They analyzed a specimen's tensile and internal damage under a three-point bending load by measuring the embedded CNT yarn resistance [10]. [PERSON], [PERSON], and other researchers used single-walled CNT yarn to fabricate different structural multifunctional sensors that produce large deformations (90% stretch shrink) and unique features. Compared with multi-walled carbon nanotube (MWCNT) yarn, SWCNT yarn can be used as structural specimens of large deformation monitoring sensors [11]. [PERSON], [PERSON], and other researchers evaluated a CNT yarn sensor and studied its mechanical (toughness and failure strain) and electrical (resistivity and specification factor) properties under a uniaxial tensile load.