## 1 Introduction
[6] developed a delamination detection system for glass fiber-reinforced epoxy composite beam by receive the ultrasonic lamb wave using a fiber Bragg grating as a sensing transducer. However, the linear ultrasonic guided wave detection method is not efficient for detecting small delamination damage. The nonlinear ultrasonic guided wave enabled detection method is sensitive to small damage and incipient damage relative to a linear ultrasonic method [24]. In solid materials, due to the irregularity of the self-lattice arrangement and the discontinuity of the contact interface, linear and nonlinear features may exist inside the material. With a linear excitation signal, not only linear response but also nonlinear response will occur inside the material [25]. There is a nonlinear response caused by nonlinear features. By measuring the nonlinearity of the material, the relevant damage information can be obtained, and the damage detection of the material can be realized. [PERSON] et al. [26] assessed the quality of composite with delamination damage using a nonlinear variable including second-order nonlinearity coefficient, third-order nonlinearity coefficient and distortion factor. [PERSON] et al. [27] simulated the delamination damage of composite based on the nonlinear ultrasonic method, and the position, shape, depth and orientation of delamination were investigated by a finite element model. [PERSON] et al. [28] combined the finite element analysis and nonlinear ultrasonic technique to detect the delamination damage with a different size for a stiffened aluminum plate. [PERSON] et al. [29] analyzed the effect of debonding damage to the nonlinearity, which resulted in incorrect detection results. [PERSON] et al. [30; 31; 32] studied the feasibility of damage detection of plastic pipe and glass curtain wallsthrough nonlinear ultrasonic techniques. However, there is still little research of delamination detection for lined anti-corrosion pipes. On the other hand, Lead Zirconate Titanate Transducer (PZT) has been extensively used in ultrasonic structural health monitoring and nondestructive testing for their excellent actuation and sensing capacity [33; 34; 35; 36; 37]. [PERSON] et al. [38] developed a reusable PZT transducer for monitoring initial hydration and structural health of concrete. [PERSON] et al. [39] assessed the interfacial strength of concrete filled fiber reinforced polymer tube using PZT transducers. [PERSON] et al. [40] applied the time reversal technique to evaluate loosened bolted joints based on the actuation and sensing capacity of PZT transducers. [PERSON] et al. [41] utilized the PZT transducers as the actuator and sensor to detect the pre-load status of rock bolts. [PERSON] et al. [42] presented a rapid detection and location method for pipeline leakage utilizing PZT as transducers. In addition, PZT-transducer enabled ultrasonic/stress wave methods have been used to detect debonding and a separation type of damage in various structures [43; 44; 45; 46; 47]. [PERSON] et al. [48] developed a PZT enabled active sensing method to detect the steel and concrete debonding in Concrete Filled Steel Tubes (CFST). [PERSON] et al. [49] investigated the detection of debonding between fiber reinforced polymer bar and concrete structure using piezoceramic transducers and wavelet packet analysis. For composite structures, PZTs are also actively studied to detect the interlayer delamination [50; 51; 52]. Therefore, PZT were utilized as the transducer to excite and to sense the ultrasonic wave in the paper. Delamination damage is a common failure form of lined anti-corrosion pipes. With the expansion of delamination damage, the anti-corrosive liner will rupture and the outer steel layer will corrode, which results in accidents. Therefore, the detection of delamination damage for anti-corrosion pipes is very necessary. In this paper, a nonlinear ultrasonic testing method is developed for delamination detection of lined anti-corrosion pipes. This paper is organized as follows.