###### Abstract  
The nondestructive testing technique used to evaluate the quality of bolt support by detecting the axial force is suitable for the bolt with a short construction time. For the bolt in the support state for a long time period, it may lead to the detection of small axial force only and ignore the fact that the bolt has entered the necking fracture stage, and thus fail to detect safety hazards in time. Three sets of parallel tests were conducted to solve the misjudment of detection results due to the reduction of axial force when the bolt was in the necking fracture stage. Bolt deformation characteristics and stress wave propagation characteristics in bolts under axial tensile stress were studied by using the bolt tensile and stress wave detection test system which was modified and built independently. The results showed that (1) The diameter of the necking position decreases continuously during the tensioning process of the bolt and the rate of decrease increases abruptly during the necking fracture stage. (2) Due to the bolt stretching, the stress wave propagates in the bolt with different degrees of reflection and transmission, resulting in the attenuation of the stress wave energy; the energy attenuation ratio of the stress wave signal in the necking fracture stage reaches 35%, and the energy attenuation ratio increases exponentially as the necking continues to occur. (3) The frequency distribution of the stress wave signal during the bolt stretching process is from scattered to concentrated, the dominant frequency is gradually prominent and changes from low frequency to high frequency, the high-frequency signal is more sensitive to the cracks and necking of the bolt, and the dominant frequency is between 9500 and 10,000 Hz. (4) The average error of the stress wave method is 1.945% and the maximum and minimum values are 4.12% and 0.51%, respectively. The method is promising and provides a reference for the study of nondestructive testing of bolt stress waves in field support. nondestructive testing; bolt; stress wave; necking; energy attenuation ratio; dominant frequency; detection error +
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0000-0002-0001-9999-22001-9999-22001-9999-22001-999-22001-9999-22001-9999-22001-9999-22001-9999-22001-9999-22001-9 solve safety hazards in a timely manner. However, the bolt project is a concealed project, and it is difficult to identify and investigate the quality problems effectively. The traditional bolt support quality inspection method is destructive and the number of sampling is small; hence, a nondestructive inspection method is required to investigate the characteristics of bolt deformation under tensile stress. When the bolt is stretched, the mechanical properties of the bolt and the deformation inside the material change continuously as the stretching proceeds. Many scholars have analyzed the stress state and the stress-strain curve of the bolt stretching process through the bolt stretching test and concluded that the bolt stretching process can be divided into four stages, which are the elastic stage, yield stage, strengthening stage, and necking fracture stage [7; 8; 9]. These studies are much helpful for a better understanding of the bolt tensioning process and promote the development of bolt nondestructive testing technology. The study of bolt NDT technology is closely related to coal mine safety, and some scholars have been continuously researching and innovating bolt NDT technology for decades [10; 11; 12; 13]. The core NDT techniques used nowadays are ultrasonic, acoustic emission, and stress wave methods. The advantages of the ultrasonic method are its applicability and penetration ability, high sensitivity, and high sensitivity to defects. However, when using ultrasonic waves to detect complex structures and irregularly shaped objects, wave attenuation is severe, so it is difficult to be used to detect composite rock formations and a variety of anchored media. The ultrasonic excitation frequency is high.