scholarly journals Nonlinear Lamb Wave Micro-Crack Direction Identification in Plates with Mixed-Frequency Technique

2020 ◽  
Vol 10 (6) ◽  
pp. 2135 ◽  
Author(s):  
Liqiang Guan ◽  
Mingxia Zou ◽  
Xili Wan ◽  
Yifeng Li
Keyword(s):  
Lamb Wave ◽  
Three Dimensional ◽  
Interaction Mechanism ◽  
Forward Direction ◽  
Forward Scattering ◽  
Inversion Method ◽  
Nonlinearity Parameter ◽  
Acoustic Nonlinearity Parameter ◽  
Micro Cracks ◽  
Frequency Mixing

This paper investigates the direction identification of micro-cracks with nonlinear components generated by Lamb wave with frequency-mixing technique. Three-dimensional finite element simulations were carried out to investigate the interaction mechanism between Lamb wave signals and micro-cracks. Upon re-visiting the conventional Lamb wave excitation signal with two kinds of fundamental frequencies (f1 and f2), it was found to be possible to generate new types of frequencies (f1 ± f2) at the sideband if nonlinear sources existed in the plate. A pulse inversion method was used to extract the sideband frequency for nonlinear ultrasonic detection. By arranging piezoelectric chip arrays around the micro-crack, the acoustic nonlinearity parameter β related to the fundamental frequency and the sideband frequency for different micro-crack directions was calibrated. It was shown that β varied for different crack directions, which provides useful information about the scattering features of the nonlinear Lamb wave interacting with the micro-crack to characterize its directivity. Moreover, the scattering degree defined with the relative nonlinear parameter β′ of the micro-crack in different directions was investigated in detail by changing the size of the micro-crack. The outcomes showed that the forward scattering signal of the crack had a greater amplitude, whereas the backscattering signal had a smaller amplitude compared with the scattering signals in other directions from micro-cracks. In addition, the signal scattering degree in the forward direction from micro-cracks increased with the increasing micro-crack length, but decreased with increasing crack width. Furthermore, for the buried crack, the forward scattering degree of Lamb wave from micro-crack decreased as crack was buried deeper in plate. In summary, the findings of this study can help to further advance the use of nonlinear Lamb wave with the frequency-mixing technique for identifying the direction of micro-cracks.

scholarly journals Wave Propagation Analysis in Composite Laminates Containing a Delamination Using a Three-Dimensional Spectral Element Method

2012 ◽  
Vol 2012 ◽  
pp. 1-19 ◽  
Author(s):  
Fucai Li ◽  
Haikuo Peng ◽  
Xuewei Sun ◽  
Jinfu Wang ◽  
Guang Meng
Keyword(s):  
Wave Propagation ◽  
Lamb Wave ◽  
Composite Laminates ◽  
Composite Structures ◽  
Spectral Element Method ◽  
Three Dimensional ◽  
Spectral Element ◽  
Wave Mode ◽  
Diagonal Form ◽  
Element Method

A three-dimensional spectral element method (SEM) was developed for analysis of Lamb wave propagation in composite laminates containing a delamination. SEM is more efficient in simulating wave propagation in structures than conventional finite element method (FEM) because of its unique diagonal form of the mass matrix. Three types of composite laminates, namely, unidirectional-ply laminates, cross-ply laminates, and angle-ply laminates are modeled using three-dimensional spectral finite elements. Wave propagation characteristics in intact composite laminates are investigated, and the effectiveness of the method is validated by comparison of the simulation results with analytical solutions based on transfer matrix method. Different Lamb wave mode interactions with delamination are evaluated, and it is demonstrated that symmetric Lamb wave mode may be insensitive to delamination at certain interfaces of laminates while the antisymmetric mode is more suited for identification of delamination in composite structures.

Transient Elastic Thermal Stress Development During Laser Scribing of Ceramics

2000 ◽  
Vol 123 (1) ◽  
pp. 171-177 ◽  
Author(s):  
Michael F. Modest ◽  
Thomas M. Mallison
Keyword(s):  
Thermal Stresses ◽  
Continuous Wave ◽  
Elastic Stress ◽  
Three Dimensional ◽  
Thick Layer ◽  
Temperature Fields ◽  
Subsurface Cracks ◽  
Laser Scribing ◽  
Micro Cracks ◽  
Cutting Speeds

Lsaers are emerging as a valuable tool for shaping and cutting hard and brittle ceramics. Unfortunately, the large, concentrated heat flux rates that allow the laser to efficiently cut and shape the ceramic also result in large localized thermal stresses in a small heat-affected zone. These notable thermal stresses can lead to micro-cracks, a decrease in strength and fatigue life, and possibly catastrophic failure. In order to assess where, when, and what stresses occur during laser scribing, an elastic stress model has been incorporated into a three-dimensional scribing and cutting code. First, the code predicts the temporal temperature fields and the receding surface of the ceramic. Then, using the scribed geometry and temperature field, the elastic stress fields are calculated as they develop and decay during the laser scribing process. The analysis allows the prediction of stresses during continuous wave and pulsed laser operation, a variety of cutting speeds and directions, and various shapes and types of ceramic material. The results of the analysis show substantial tensile stresses develop over a thick layer below and parallel to the surface, which may be the cause of experimentally observed subsurface cracks.

scholarly journals Working Mechanism of Pile Group with Different Pile Spacing in Dense Sand

2019 ◽  
Vol 2019 ◽  
pp. 1-16 ◽  
Author(s):  
Yadong Chen ◽  
Fan Lu ◽  
Abdoullah Namdar ◽  
Jiangdong Cai
Keyword(s):  
Three Dimensional ◽  
Interaction Mechanism ◽  
Pile Group ◽  
Resistance Pattern ◽  
Pile Groups ◽  
Pile Spacing ◽  
Dense Sand ◽  
Energy Mechanism ◽  
Tip Resistance ◽  
Displacement Zone

Complex interaction mechanism exists between the pile group and soil. To realize the pile-soil load transmission mechanism in detail, the failure pattern of pile groups installed in dense sand considering different pile spacing was investigated by means of laboratory experimental model test and three-dimensional discrete element method. The results suggested that the narrow pile spacing was beneficial to the development of the pile tip resistance, and it enhanced the bearing performance of the pile group at the initial stage of settlement. The pile spacing changed the shaft resistance pattern with modification of the strain energy mechanism released within the subsoil. The pile group with 6b pile spacing had higher composite group efficiency. A joint fan-shaped displacement zone was formed beneath the pile tip for the pile group with 3b pile spacing; this pile foundation presented the block failure mechanism. The sand displacement beneath the cap for the pile group with 6b pile spacing mainly located on the upper part of the piles, the sand displacement around both sides of the piles presented asymmetric, and a relatively independent fan-shaped displacement zone was formed beneath the pile tip.

scholarly journals Study of Internal Flows in a Mixed-Flow Pump Impeller at Various Tip Clearances Using 3D Viscous Flow Computations

1990 ◽  
Author(s):  
Akira Goto
Keyword(s):  
Reverse Flow ◽  
Three Dimensional ◽  
Interaction Mechanism ◽  
Secondary Flows ◽  
Wake Flow ◽  
Navier Stokes ◽  
Tip Leakage Flow ◽  
Mixed Flow ◽  
Pump Impeller ◽  
Flow Pump

The complex three-dimensional flow fields in a mixed-flow pump impeller are investigated by applying the incompressible version of the Dawes’ 3D Navier-Stokes code. The applicability of the code is confirmed by comparison of computations with a variety of experimentally measured jet-wake flow patterns and overall performances at four different tip clearances including the shrouded case. Based on the computations, the interaction mechanism of secondary flows and the formation of jet-wake flow are discussed. In the case of large tip clearances, the reverse flow caused by tip leakage flow is considered to be the reason for the thickening of the casing boundary layer followed by the deterioration of the whole flow field.

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