## 1 Introduction  
Nomex honeycomb sandwich structures have been widely used in the aerospace and rail transportation fields because of their excellent characteristics of high specific strength and stiffness, superior impact resistance, fatigue resistance, wave transmission, and thermal insulation [1, 2, 3]. The Nomex sandwich structures are manufactured by bonding honeycomb cores and the upper and lower skins together, exhibiting diverse shapes, such as the application of the flaps, elevators, rudders, nacelles, and wing-body fairings of large aircraft [4, 5, 6]. The bonding surfaces of honeycomb cores need to be processed into specific geometric shapes according to the assembly requirements. Nomex honeycomb cores are typical difficult-to-process materials due to the characteristics of thin-walled porosity, weak stiffness, and anisotropy. The traditional processing method of high-speed milling is prone to generating defects such as tearing and burr, in which processing quality is not easily guaranteed [7]. Currently, ultrasonic cutting has become the advanced processing method of Nomex honeycomb cores, thanks to its significant performance of reducing cutting force, improving surface quality, and decreasing dust harm [8, 9, 10]. During ultrasonic cutting of Nomex honeycomb core, the disc cutter is usually used as the precision processing tool, deciding the ultimate bonding surface quality. In recent years, scholars have carried out related research in the ultrasonic cutting disc cutter machining process. [PERSON] et al. [11] analyzed the factors influencing the cutting force during the ultrasonic cutting of the Nomex honeycomb core with the disc cutter and carried out an ultrasonic cutting experiment of the Nomex honeycomb core to obtain the influence laws of disc cutter machining parameters on cutting force and surface quality. [PERSON] et al. [12; 13] designed a new ultrasonic circular saw blade cutter based on the original structure of the disc cutter and studied the variation law of cutting force and surface quality of the circular saw blade cutter and disc cutter under different process parameters. The results showed that the ultrasonic cutting of the Nomex honeycomb core by the circular saw blade cutter and disc cutter had good results. Moreover, [PERSON] et al. [14; 15] investigated the cutting force and the surface quality of aluminum honeycomb during ultrasonic cutting with a disc cutter utilizing finite element simulation and experimental validation, showing that ultrasonic cutting using a disc cutter can achieve high quality and high efficiency in the machining of aluminum honeycomb. Due to the long machining path caused by the large size of honeycomb workpieces in actual machining, the disc cutter inevitably generates tool wear and breakage, thus affecting the machining accuracy, surface quality, and machining efficiency of honeycomb materials. So, it is critical to research disc cutter tool wear and breakage. On the research methods of tool wear and breakage in processing honeycomb material, [PERSON] et al. [16] used an optical microscope to study the chipping law during ultrasonic cutting of wave-absorbing honeycomb material with the disc cutter and showed that the number of chipping edges and notches increased with the increase in cutting length. [PERSON] et al. [17] conducted tool wear analysis of the entire high-speed milling of the Nomex honeycomb core using a high-speed camera and clearly observed that the tool underwent a morphological change process such as significant wear of the lowest teeth on the tool and missing multiple cutting edges. [PERSON] et al. [18] studied the state of tool wear and breakage for machining. Nomex honeycomb core material using a scanning electron microscope, and the adhesion of phenolic resin was clearly observed by the scanning electron microscope. The results indicated that the tool wear and breakage were mainly divided into the adhesion of the phenolic resin to the tool, flank wear, and loss of edge sharpness.