## 1 Introduction  
The helicopter hub damper bolt is an essential fastener on the helicopter propeller system and plays a role in connecting, positioning, and so on. During the helicopter service, serious safety accidents may occur from bolt fatigue resulting from the damper bolt shanks being subjected to alternating loads for a long time [1]. Hence, the safe operation of the helicopter needs to undergo regular non-destructive testing and evaluate the service life of fatigue crack initiation and the propagation life of the damper bolt shank [2; 3]. Currently, the common non\(-\)destructive testing method is to remove the bolt and then detect the bolt with magnetic particle testing or penetrant testing [4; 5; 6]. However, these methods have the disadvantages of a heavy workload, long detection cycle, and low efficiency [7]. More importantly, potential safety risks for helicopters may exist due to improper commissioning during bolt disassembly and reassembly. Therefore, in the presence of in\(-\)service bolts, non-destructive testing of fatigue cracks on the bolt shanks of in\(-\)service dampers is of great significance for ensuring the safe operation of helicopters and reducing the working intensity of operators. With strong penetration ability, fast detection speed, and strong detection mobility, ultrasonic testing is a non-destructive method widely used in metal material flaw detection [8]. However, the ultrasonic beam is difficult to shoot into the crack root of the bolt effectively due to its narrow and tall structure. When using the conventional ultrasonic testing method to detect the cracks of bolt shanks, missed detection and misjudgment of cracks are easily caused by the pseudo defects and interference wave from waveform conversion [9]. Compared with the conventional ultrasonic method, phased array ultrasonic testing technology has advantages, including optional deflection angle of the sound beam, adjustable focusing depth, and high detection sensitivity [10; 11]. At present, fatigue cracks of bolt shanks have been detected with ultrasonic testing in the port [12], nuclear power [13], and other industries. For example, defect echoes of cracks can be intuitively distinguished from the structural interference wave in the ultrasonic detection of in\(-\)service bolt shanks, combined with the signal characteristics of phased array S\(-\)scan imaging and A\(-\)scan in the ultrasonic view [14]. Compared with conventional ultrasonic testing, implementation of the phased array ultrasonic S\(-\)scan detection method improves the detection accuracy and reduces the rate of missed detection and misjudgment caused by the detection personnel. Generally, the diameter of port bolts is larger than M30 [12]. It has been shown that ultrasonic detection of cracks in port bolt shanks requires the detection accuracy to reach a 0.5 mm depth [15]. By contrast, compared with the bolts used in ports, nuclear power, and other fields, the service conditions of helicopter damper bolts are more stringent. Hence, to ensure the safe operation of the helicopter hub, it is necessary to detect more minor cracks in damper bolts effectively [16]. Moreover, there is a high demand for the detection of deeper cracks due to the bolt shank size of the helicopter damper M18 \(\times\) 120. However, due to the weak echo energy of cracks with large buried depths, the conventional ultrasonic phased array S\(-\)scan imaging has the problem of a low signal\(-\)to\(-\)noise ratio, resulting in the effective defect echo submerged in the structural wave of the bolts [17]. In terms of improving the signal\(-\)to\(-\)noise ratio, researchers have carried out lots of efforts [18; 19; 20]. Among them, adaptive coherent weighting [21] is a common post\(-\)processing imaging method which calculates the weighting coefficient according to the coherence of the echo signal of each pixel and carries out coherent weighting on the original DAS (delayed and sum) signal.