Mingxi Deng

Second Harmonic Generation of Lamb Wave in Numerical Perspective

The influences of phase and group velocity matching on cumulative second harmonic generation of Lamb waves are investigated in numerical perspective. Finite element simulations of nonlinear Lamb wave propagation are performed for Lamb wave mode pairs with exact and approximate phase velocity matching, with and without group velocity matching, respectively. The evolution of time-domain second harmonic Lamb waves is analyzed with the propagation distance. The amplitudes of primary and second harmonic waves are calculated to characterize the acoustic nonlinearity. The results verify that phase velocity matching is necessary for generation of the cumulative second harmonic Lamb wave in numerical perspective, while group velocity matching is demonstrated to not be a necessary condition.

Numerical Perspective of Second-Harmonic Generation of Circumferential Guided Wave Propagation in a Circular Tube*

The effect of second-harmonic generation (SHG) by primary (fundamental) circumferential guided wave (CGW) propagation is investigated from a numerical standpoint. To enable that the second harmonic of the primary CGW mode can accumulate along the circumferential direction, an appropriate mode pair of primary and double frequency CGWs is chosen. Finite element simulations and evaluations of nonlinear CGW propagation are analyzed for the selected CGW mode pair. The numerical simulations performed directly demonstrate that the response of SHG is completely generated by the desired primary CGW mode that satisfies the condition of phase velocity matching at a specific driving frequency, and that the second harmonic of the primary CGW mode does have a cumulative effect with circumferential angles. The numerical perspective obtained yields an insight into the complicated physical process of SHG of primary CGW propagation unavailable previously.

Assessment of accumulated damage in circular tubes using nonlinear circumferential guided wave approach: A feasibility study

HighlightsThe model is presented for analyzing the effect of SHG of primary CGW propagation.The acoustic nonlinearity parameter &Dgr;&bgr; for CGW along one circumference is proposed.Feasibility of using &Dgr;&bgr; to assess accumulated damage in circular tubes is validated. ABSTRACT The feasibility of using the nonlinear effect of primary Circumferential Guided Wave (CGW) propagation for assessing accumulated damage in circular tubes has been investigated. For a given circular tube, an appropriate mode pair of fundamental and double frequency CGWs is chosen to enable that the second harmonic of the primary wave mode can accumulate along the circumferential direction. After the given circular tube is subjected to compression‐compression repeated loading for different numbers of loading cycles, the corresponding ultrasonic measurements are conducted. It is found that there is a direct correlation between the acoustic nonlinearity parameter measured with CGWs propagating through one full circumference and the level of accumulated damage in the circular tube. The experimental result obtained validates the feasibility for quantitative assessment of the accumulated damage in circular tubes using the effect of second‐harmonic generation by CGW propagation.

Selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers

In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers (EMATs). With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb waves mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMATs meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Lamb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMATs geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated (strengthened or restrained) by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs.

Mode pair selection of circumferential guided waves for cumulative second-harmonic generation in a circular tube

HIGHLIGHTSSelection criteria of mode pairs suitable for assessment of damage/degradation are first proposed.A model is presented for analyzing the SHG effect of CGW and its response sensitivity to TOE constants.Mode pairs are found with larger values both of &bgr; and change rate of normalized &bgr; versus TOE constants. ABSTRACT The appropriate mode pairs of primary and double‐frequency circumferential guided waves (CGWs) have been investigated and selected for generation of the cumulative second harmonics, which are applicable for quantitative assessment of damage/degradation in a circular tube. The selection criteria follow the requirements: the higher efficiency of cumulative second‐harmonic generation (SHG) of primary CGW propagation, and the larger response sensitivity of cumulative SHG to material damage/degradation [characterized by variation in the third‐order elastic (TOE) constants]. The acoustic nonlinearity parameter &bgr; of CGW propagation and the change rate of normalized &bgr; versus the TOE constants of tube material are, respectively, used to describe the efficiency of SHG and its response sensitivity to damage/degradation. Based on the selection criteria proposed, all the possible mode pairs of primary and double‐frequency CGWs satisfying the phase velocity matching have been numerically examined. It has been found that there are indeed some mode pairs of CGW propagation with the larger values both of &bgr; and the change rate of normalized &bgr; versus the TOE constants. The CGW mode pairs found in this paper are of practical significance for quantitative assessment of damage/degradation in the circular tube.

Fatigue Damage Evaluation Using Nonlinear Lamb Waves with Quasi Phase-Velocity Matching at Low Frequency

Due to the dispersive and multimode natures, only nonlinear Lamb waves with exact phase-velocity matching were generally used in previous studies to evaluate the evenly distributed microstructural evolution in the incipient stage of material degradation, because of the cumulative generation of second harmonics, which was also found within a significant propagation distance for mode pair S0-s0 with quasi phase-velocity matching at low frequency. To explore the feasibility of fatigue damage evaluation by using this mode pair and fully utilize its unique merits, the cumulative second harmonic analysis was performed on aluminum alloy specimens with various material damage produced by the continuous low cycle fatigue tests. Similar to mode pair S1-s2 with exact phase-velocity matching, a mountain shape curve between the normalized acoustic nonlinearity parameter and the fatigue life was also achieved with the peak point at about 0.65 fatigue life for mode pair S0-s0, even though a relatively higher sensitivity to fatigue damage was observed for mode pair S1-s2. The excited frequency selection was further analyzed in a certain frequency range, where the quasi phase-velocity matching condition was satisfied for mode pair S0-s0 owing to the less dispersive property. Results show that the fatigue damage can be effectively detected using the mode pair S0-s0, and a relatively lower excited frequency was preferred due to its higher sensitivity to microstructural evolution.

Numerical and experimental investigations of nonlinear S0 Lamb mode for detection of fatigue damage

Nonlinear Lamb wave has been widely acknowledged as an efficient nondestructive method for evaluating early stage of fatigue damage. The previous studies focused on the evaluation of fatigue damage using the particular mode pairs, such as S1-S2 mode pair. Recently, the nonlinear S0 Lamb mode with low-frequency was considered for its unique features of less dispersion and easy to be generated, and was indicated may as sensitive to fatigue damage as other mode pairs reported before. However, few researches were performed on experimental measurement. In this work, the S0 mode is demonstrated as an alternative for fatigue damage evaluation.

Characterization of micro-crack evolution using Nonlinear Lamb waves

Recent studies show that nonlinear ultrasonic methods are better than conventional linear ultrasonic methods that are sensitive only to severe defects. Micro-cracks grow at a low rate on the early stage. However, once the surface crack length reaches to a critical size, the cracks would propagate to final fracture quickly. For this reason, it is important to identify the fatigue crack initiation and growth mechanisms of the material. Nonlinear ultrasonic Lamb waves technique have been considered as a positive method for accurately evaluating the early stages of material degradation. So research on micro-cracks detection using nonlinear acoustic character, is both of theoretical significance and practical value. However, there was little research concerning micro-cracks whose lengths are limited to 0.5 mm. Nonlinear Lamb waves were used to detect the growth of micro-cracks in this research. The experimental results could be concluded that the nonlinear effects of ultrasonic Lamb waves might offer a potenti...

Evaluation of accumulated damage in a circular tube using effect of second-harmonic generation of circumferential guided wave propagation

The feasibility of using the nonlinear effect of primary circumferential guided wave (CGW) propagation for evaluating accumulated damage in circular tubes is investigated. For a given circular tube, an appropriate mode pair of fundamental and double frequency CGWs is chosen to enable that the second harmonic of the primary wave mode can accumulate along the circumferential direction. After the given circular tube is subjected to cyclic loading for different cycle numbers, the corresponding ultrasonic measurements are conducted. It is found that there is a direct correlation between the acoustic nonlinearity parameter measured with CGWs propagating along the tube circumference and the level of accumulated damage in the circular tube. The experimental result presents a potential for quantitative evaluation of the accumulated damage in circular tubes using the effect of second-harmonic generation by CGW propagation.