T. H. Ju

Bio: T. H. Ju is an academic researcher from University of Colorado Boulder. The author has contributed to research in topics: Composite plate & Plane stress. The author has an hindex of 2, co-authored 4 publications receiving 53 citations.

Scattering of an Impact Wave by a Crack in a Composite Plate

Abstract: Ultrasonic waves provide an efficient means of characterizing defects in structures. For this purpose it is necessary to analyze scattering by such defects. However, scattering by crack-like defects in a plate-like structure is a complicated phenomenon and the problem is made more difficult if it is a composite plate. In recent years considerable progress has been made toward understanding wave propagation in anisotropic composite plates [1–5], but not much work has been done on the scattering by cracks in a composite plate. Recently Karim and Kundu [6] and Karim et al. [7] studied scattering of elastic waves in a layered half-space and in layered fiber-reiforced composite plates by interface cracks using a boundary integral formulation. They considered antiplane motions. Although this method can be extended to plane strain motion the computional effort is considerably amplified if one considers a plate geometry. Besides, the method used by these authors is limited to planar defects. For arbitrarily shaped scatterers Sanchez-Sesma [8] reviewed various applicable methods. Most of these numerical methods require considerable computational effort to evaluate the response. Their applicability to layered and anisotropic medium is also limited.

Scattering of impact wave by a crack in composite plate

Abstract: The surface responses due to impact load on an infinite uniaxial graphite/epoxy plate with and without delamination cracks are investigated both in time and frequency domain by using a hybrid method combining the finite element discretization of the near-field with boundary integral representation of the field outside a contour enclosing completely the crack. This combined method leads to a set of linear unsymmetric complex matrix equations which are solved to obtain the response in the frequency domain by biconjugate gradient method. The time domain response is then obtained by using an FFT. In order to capture the time-domain characteristics accurately, high order finite elements have been used. Also, both the six node singular elements and eight node transition elements are used around the crack tips to model the crack-tip singularity. It is shown that from the numerical results for surface responses both depth and length of this crack can be identified.

Transient response of a laminated composite plate

Abstract: Propagation of guided waves in a laminated plate is of interest for ultrasonic nondestructive evaluation of defects and for material characterization. There is a need for a thorough understanding of the wave propagation characteristics in such a plate in order to use ultrasonic means to determine the material properties, assess damage, and characterize defects. The problem is also of interest for study of acoustic emission.

Strip Element Method to Analyze Wave Scattering by Cracks in Anisotropic Laminated Plates

Abstract: The strip element method (SEM) has been extended to investigate wave scattering by cracks in anisotropic laminated plates. The cracked plates are divided into domains in which the extended SEM is applied. For each domain a set of SEM equations is obtained which gives a relationship between the traction and displacement vectors on the vertical boundaries. These equations are solved by using the conditions at the junctures of the domains. To obtain the time domain response a Fourier transform technique is used, and an exponential window method is introduced to avoid singularities in the Fourier integration. For composite plates with horizontal and vertical cracks, scattered wave fields in both the time and frequency domains are computed, and discussed in comparison with results for uncracked plates. A technique for determining the length of a crack in a plate is also presented. It is shown that the SEM is an efficient technique for the calculation of elastodynamic fields in cracked anisotropic laminated plates.

Crack characterization using guided circumferential waves.

Abstract: This paper examines the propagation of guided circumferential waves in a hollow isotropic cylinder that contains a crack, with the goal of using these guided waves to both locate and size the crack. The crack is sized using a modified Auld's formula, which relates the crack's length to a reflected energy coefficient. The crack is then located by operating on the backscattered signal with a time-frequency digital signal processing (DSP) technique, and then comparing these results to those obtained if the cylinder is perfect. The guided circumferential waves are generated with a commercial finite element method (FEM) code. One objective of this work is to demonstrate the effectiveness of using sophisticated DSP techniques to describe the effect of scattering on dispersive waves, showing it is possible to characterize cracks systematically and accurately by quantifying this scattering effect. The results show that the need for high frequency signals to detect small cracks is significantly decreased by using these techniques.

Lamb wave generation and reception with time-delay periodic linear arrays: a BEM simulation and experimental study

Abstract: A time-delay periodic linear array model has been proposed for Lamb wave generation and reception in plates. The unilateral guided wave emitting and receiving have been achieved by applying the interference principle in the array designs. A hybrid BEM technique has been developed and applied to simulate the wave generation procedure with such arrays and to analyze the performance. Experimental results also are presented for two typical time-delay periodic arrays to qualitatively validate the theoretical designs. The effects of the array parameters on the array performance, such as the selectivity of Lamb modes and effectiveness of Lamb wave generation, are investigated through the 2-D phase velocity-frequency spectrum analyses as well as Lamb mode wave structure calculations.

Acoustic Emission Signals in Thin Plates Produced by Impact Damage

Abstract: Acoustic emission (AE) signals created by impact sources in thin aluminum and graphite/epoxy composite plates were analyzed. Two different impact velocity regimes were studied. Low-velocity (less than 0.21 km/s) impacts were created with an airgun firing spherical steel projectiles (4.5 mm diameter). High-velocity (1.8 to 7 km/s) impacts were generated with a two-stage light-gas gun firing small cylindrical nylon projectiles (1.5 mm diameter). Both the impact velocity and impact angle were varied. The impacts did not penetrate the aluminum plates at either low or high velocities. For high-velocity impacts in composites, there were both impacts that fully penetrated the plate as well as impacts that did not. All impacts generated very large amplitude AE signals (1-5 V at the sensor), which propagated as plate (extensional and/or flexural) modes. In the low-velocity impact studies, the signal was dominated by a large flexural mode with only a small extensional mode component detected. As the impact velocity was increased within the low velocity regime, the overall amplitudes of both the extensional and flexural modes increased. In addition, a relative increase in the amplitude of high-frequency components of the flexural mode was also observed. Signals caused by high-velocity impacts that did not penetrate the plate contained both a large extensional and flexural mode component of comparable amplitudes. The signals also contained components of much higher frequency and were easily differentiated from those caused by low-velocity impacts. An interesting phenomenon was observed in that the large flexural mode component, seen in every other case, was absent from the signal when the impact particle fully penetrated through the composite plates.

Hybrid Signal Processing Technique to Improve the Defect Estimation in Ultrasonic Non-Destructive Testing of Composite Structures.

Abstract: This work proposes a novel hybrid signal processing technique to extract information on disbond-type defects from a single B-scan in the process of non-destructive testing (NDT) of glass fiber reinforced plastic (GFRP) material using ultrasonic guided waves (GW). The selected GFRP sample has been a segment of wind turbine blade, which possessed an aerodynamic shape. Two disbond type defects having diameters of 15 mm and 25 mm were artificially constructed on its trailing edge. The experiment has been performed using the low-frequency ultrasonic system developed at the Ultrasound Institute of Kaunas University of Technology and only one side of the sample was accessed. A special configuration of the transmitting and receiving transducers fixed on a movable panel with a separation distance of 50 mm was proposed for recording the ultrasonic guided wave signals at each one-millimeter step along the scanning distance up to 500 mm. Finally, the hybrid signal processing technique comprising the valuable features of the three most promising signal processing techniques: cross-correlation, wavelet transform, and Hilbert-Huang transform has been applied to the received signals for the extraction of defects information from a single B-scan image. The wavelet transform and cross-correlation techniques have been combined in order to extract the approximated size and location of the defects and measurements of time delays. Thereafter, Hilbert-Huang transform has been applied to the wavelet transformed signal to compare the variation of instantaneous frequencies and instantaneous amplitudes of the defect-free and defective signals.