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This paper aims at experimental study of delamination process in multidirectional composite laminate exhibiting elastic couplings by using the acoustic emission (AE) technique. The mode I double cantilever beam (DCB) test was conducted on carbon/epoxy laminate with the bending-twisting elastic couplings according to the ASTM D5528 Standard. Experimental research was supported with visual high resolution registration of propagated crack tip, as well as with measurements of elastic waves generated by developing internal damages by using the acoustic emission set Vallen AMSY-5 equipped with two piezoelectric sensors. Prior to test, values of fundamental Lamb wave modes velocities were estimated and the dispersion curves were calculated. Delamination initiation point was determined by using two different AE criteria. The mode I critical strain energy release rates were calculated by using three different data reduction schemes. Different damage mechanisms during crack propagation process were quantitatively evaluated by fundamental waveform features divided into three characteristic frequency bands groups. In addition, signal energy concentration regions were assessed by using the wavelet transforms. The outcomes revealed various damage mechanisms related with delamination process, even though the matrix cracking found to be the dominating mode of failure. • Application of the acoustic emission technique to examine of delamination process in elastically coupled laminates. • Determination of total fracture toughness by using two different AE criterion. • Classification of damage mechanisms by using waveform characteristic features. • Analysis of failure mechanisms by using wavelet transform. [ABSTRACT FROM AUTHOR]

The characteristics of the multi-mode resonance behavior of AlN Lamb wave resonators (LWRs) are theoretically and experimentally investigated for the first time in this paper. Adler’s approach and finite element method (FEM) are used to calculate the dispersive characteristics of the phase velocity ($v_{p}$), group velocity ($v_{g}$), effective coupling coefficient ($k^{2}_{\text {eff}}$), and temperature coefficient of frequency for the first eight Lamb wave modes with different transducer configurations. The FEM is performed to take an insight into the mode shapes of the S0mode and S1mode specifically: the S0mode is more contour-like and exhibits the largest $k^{2}_{\text {eff}}$ when $h_{\text {AlN}}/\lambda $ is close to 0.5; the S1mode is strong in vertical direction and can enable high resonance frequency ($f_{s}$) and large $k^{2}$ simultaneously when AlN thickness is very thin. Experimentally, AlN LWRs with different AlN thicknesses are designed and fabricated. The measured results are fitted into the multi-resonance BVD model so that the device performance parameters, as well as the equivalent, lumped element values are extracted and compared. By choosing different normalized AlN thicknesses, the performance of different Lamb wave modes varies largely due to the dispersive characteristics and agrees well with theoretically predicted acoustic characteristics. This paper lays the foundation for characterizing the multi-resonance behaviors of AlN LWRs and gives guidance on choosing the optimal design parameters and Lamb wave modes for different applications. [2018-0040] [ABSTRACT FROM AUTHOR]

The development of fatigue cracks at fastener holes represents a common maintenance problem for aircraft. High frequency guided ultrasonic waves allow for the monitoring of critical areas without direct access to the defect location. During cyclic loading of tensile, aluminum specimens fatigue crack growth at the side of a fastener hole was monitored. The changes in the energy ratio of the baseline subtracted reflected guided wave signal due to the fatigue damage were monitored from a stand-off distance using standard ultrasonic pulse–echo measurement equipment. Good sensitivity for the detection and monitoring of fatigue crack growth was found. [ABSTRACT FROM AUTHOR]

Abstract: Several Lamb wave modes can be coupled to a particular structure, depending on its geometry and transducer used to generate the guided waves. Each Lamb mode interacts in a particular form with different types of defects, like notches, delamination, surface defects, resulting in different information which can be used to improve damage detection and characterization. An image compounding technique that uses the information obtained from different propagation modes of Lamb waves for non-destructive testing of plate-like structures is proposed. A linear array consisting of 16 piezoelectric elements is attached to a 1mm thickness aluminum plate, coupling the fundamental A0 and S0 modes at the frequencies of 100kHz and 360kHz, respectively. For each mode two images are obtained from amplitude and phase information: one image using the Total Focusing Method (TFM) and one phase image obtained from the Sign Coherence Factor (SCF). Each TFM image is multiplied by the SCF image of the respective mode to improve contrast and reduce side and grating lobes effects. The high dispersive characteristic of the A0 mode is compensated for adequate defect detection. The information in the SCF images is used to select one of the TFM mode images, at each pixel, to obtain the compounded image. As a result, dead zone is reduced, resolution and contrast are improved, enhancing damage detection when compared to the use of only one mode. [Copyright &y& Elsevier]

Using the finite element method, we have calculated the transmission spectra of Lamb wave modes which propagate in one-dimensional three-component Fibonacci quasiperiodic composite plates made of three different materials, and analyzed the influence of filling fraction, the ratio of the thickness of the plates to the lattice period and especially the number of generations on the band gaps of Lamb wave modes. The band gap splitting depends on the number of generations which is different from those of one-dimensional two-component Fibonacci composite plates. Engineering band gaps can be obtained by turning different parameters and the number of generations. [ABSTRACT FROM AUTHOR]