A Nonlinear Ultrasonic Lamb Wave Method for Characterizing the Bonding Layer of Composite Sheet

A Nonlinear Ultrasonic Lamb Wave Method for Characterizing the Bonding Layer of Composite Sheet

Deng Mingxi

(College of Logistics Engineering, 400016, Chongqing)

Abstract: With the help of the Lamb wave dispersion curve and the guided wave excitation mode expansion analysis method, the generation process of the fundamental frequency Lamb wave time domain signal and the second harmonic time domain signal are intuitively discussed. Combined with the measurement function of the Ritec-SNAP system, the physical meaning of the second-harmonic time-domain pulse envelope integral expression is analyzed in detail; this integral expression can characterize the second-harmonic during the time-domain pulse propagation of the fundamental frequency Lamb wave Occurrence efficiency, and the degree of dispersion between the fundamental frequency and the doubled Lamb wave mode. In the vicinity of frequencies where the fundamental frequency and the double-frequency Lamb wave phase velocity are equal (or approximately equal), the experimental observation of a significant and mode-free second harmonic signal shows that the Lamb wave propagation process is indeed There can be strong nonlinear effects. For the composite plates with three different bonding situations, the experimental results show that the nonlinear Lamb wave stress wave factor introduced in this paper, combined with the frequency value corresponding to the peak value of the second harmonic amplitude-frequency curve, can effectively bond the plates Characterize the layer properties.

PACS number: 43.25, 43.20

Characterization of adhesive joints of composite solid layers using a nonlinear Lamb wave approach

Deng Mingxi

(Logistics Engineering University, 400016, Chongqing)

Abstract: On the basis of the dispersion curves of the Lamb waves propagating in the composite solid layers and the approach of modal expansion analysis for guided wave excitation, the physical process of generation of the primary Lamb wave time-domain pulse and the corresponding second- harmonic time-domain pulse has been discussed. Considering the measurement function of the Ritec-SNAP system, the physical meaning of integral of the envelope of second-harmonic time-domain pulse has been detailedly analyzed. This integral can describe the efficiency of second- harmonic generation accompanying the primary Lamb wave time-domain pulse, as well as the dispersion degree between the primary and the double frequency Lamb waves. Within the specific frequency range where the phase velocities of the primary Lamb waves equal or approximately equal that of the double frequency Lamb waves, the second-harmonic signals without overlapping of multi-modes of the double frequency Lamb waves have been cle arly observed. This phenomenon shows that the strong nonlinearity does occur in the primary Lamb wave propagation process. For the three different adhesive joints, the experiment results show that both the stress wave factors of the nonlinear Lamb waves firstly introduced in the present paper, and the frequency values ​​corresponding to the peaks of the amplitude-frequency curves of the second-harmonic signals can effectively characterize the properties of the adhesive joints.

introduction

Composite plates are widely used in modern industry, especially in the aerospace field. The quality of the bonding layer directly affects the use of composite plate structures. It is a matter of great concern for people to effectively detect them. As an effective detection method, ultrasonic Lamb wave has the ability to accurately and quickly detect large-area samples, and has been used to detect the bonding quality problems of composite sheet structures [1,2]. In view of the complexity of the properties of the bonding layer of the composite plate and the dispersion characteristics of the Lamb wave propagation process, there are generally multiple Lamb wave modes mixed in the detection result, and it is very cumbersome and difficult to analyze and process it [3,4]. The "quasi-spring" model can be used to effectively describe the physical properties of the adhesive layer [5,6]. Theoretical and experimental studies have shown that the change of the stiffness coefficient of the weak interface layer will cause the change of the Lamb wave dispersion characteristics and thus can The nature of the adhesive layer is characterized [7, 8]. If the mechanical properties of the bonding thin layer change only slightly, or the change of the stiffness coefficient is too small to cause a significant change in the dispersion characteristics of the Lamb wave, the ultrasonic Lamb wave method is used to characterize the composite sheet There will be certain limitations in the nature of the adhesive layer.

Non-linear ultrasonic measurement methods have been used to measure the microstructural changes of materials [9,10], and non-linear ultrasonic measurement methods have also made some progress in detecting adhesion problems between layers [11,12]. Due to the geometric nonlinearity and the elasticity of the plate, the propagation of the fundamental frequency Lamb wave will lead to the occurrence of the second harmonic. Due to the dispersive nature of the Lamb wave, the second harmonic generation effect of the Lamb wave is generally very weak, making it difficult to effectively detect the second harmonic signal. With regard to the research on the nonlinear problems of Lamb waves, great progress has been made in recent years. The first is to use the interface nonlinear acoustic reflection technology to obtain the analytical solution of the second harmonic of the Lamb wave with cumulative growth effect in the resonance situation [13]; People [17] have adopted the mode expansion method of guided wave excitation respectively, and under the second-order perturbation approximation, the analysis has obtained the solution of the more general Lamb wave second harmonic sound field. The experimental work carried out according to the theoretical results verifies that the Lamb wave does have a strong nonlinear effect under certain conditions (the second harmonic of the Lamb wave exhibits a cumulative growth property with the propagation distance) [18].

Combining the Lamb wave detection technology and the nonlinear ultrasonic measurement method is expected to explore an effective method to characterize the properties of the bonding layer of the composite sheet. In this paper, based on the results obtained in the study of Lamb wave nonlinearity, the second-harmonic time-domain signal generated during the time-domain pulse propagation of the fundamental-frequency Lamb wave is analyzed intuitively. Under the condition that the Lamb wave has strong nonlinear effect, the amplitude and frequency characteristics of the fundamental wave and second harmonic of the Lamb wave propagating in the composite sheet are experimentally studied. For composite plates with different bonding states, the experimental results show that the amplitude and frequency characteristics of the second harmonic of the Lamb wave are significantly different, indicating that the second harmonic generation effect of the Lamb wave under the condition of strong nonlinear effects can be used to The properties of the bonding layer of the composite sheet are accurately characterized.

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