Samik Das

Locating point of impact in anisotropic fiber reinforced composite plates

The conventional triangulation technique cannot predict the point of impact in an anisotropic composite plate because the triangulation technique assumes that the wave speed is independent of the direction of propagation which is not the case for anisotropic plates. An alternative method based on the optimization scheme was proposed by Kundu et al. [T. Kundu, S. Das, K.V. Jata, Point of impact prediction in isotropic and anistropic plates from the acoustic emission data, J. Acoust. Soc. Am. 122, 2007, 2057-2066] to locate the point of impact in plates by analyzing the time of arrival of the ultrasonic signals received by the passive sensors attached to the plate. In this paper, that objective function is modified further to overcome the inherent difficulties associated with multiple singularities and to maximize the efficiency of the acoustic emission data for multiple receiving sensors. With this modified objective function the impact point on an anisotropic composite plate is predicted from the acoustic emission data. Experiments are carried out by dropping steel and ping pong balls on a graphite-epoxy composite plate and recording acoustic signals by passive transducers adhesively bonded to the plate at three different locations. The impact point is predicted by the proposed method and compared with the actual location of impact.

Point of impact prediction in isotropic and anisotropic plates from the acoustic emission data

It is shown in this paper that the conventional triangulation technique is not very reliable for locating the impact point even in isotropic plates when the sensors are placed close to the point of strike for two reasons: First, it is difficult to pinpoint the exact time of arrival of the signal and, second, the Lamb modes in a plate are dispersive. Dispersive signals attenuate differently at various frequencies and propagate with different speeds causing distortions in the received signals, and thus introduce error in the time of flight measurement. The triangulation technique assumes that wave speeds in all directions are the same, which is not true for anisotropic plates. Here an alternative approach based on an optimization scheme is proposed to locate the point of impact in isotropic and anisotropic plates. A formulation is presented for the general anisotropic case. Experiments are carried out with an aluminum plate by dropping balls on the plate and picking up acoustic signals at different locations. The impact points predicted by the conventional triangulation technique and the proposed modified method are compared for this isotropic plate. Then it is investigated how the prediction would change if the plate is assumed to have some anisotropy.

Detection of the point of impact on a stiffened plate by the acoustic emission technique

The applicability of guided waves to predict the point of impact in a stiffened plate is investigated. The conventional triangulation technique cannot predict the point of impact in a stiffened plate because the triangulation technique assumes that the wave speed is independent of the direction of propagation, which is not true for stiffened plates. An alternative method based on the optimization scheme was proposed by Kundu et al (2007?J. Acoust. Soc. Am.?122?2057?66) to locate the point of impact in plates by analyzing the time of arrival of the ultrasonic signals received by passive sensors attached to the plate. After successful extension of this technique to predict the point of impact in anisotropic but homogeneous composite plates (Kundu et al 2008?Ultrasonics?48?193?201) it is investigated in this paper whether this technique works well for predicting the point of impact in an inhomogeneous plate where the stiffeners make the structure inhomogeneous. Experiments are carried out by dropping ping pong and metal balls on the plate to simulate the impact phenomenon and recording acoustic signals by passive transducers adhesively bonded to the plate at three different locations. The impact points are predicted and compared with the actual locations of impact.

DPSM Modeling for Studying Interaction Between Bounded Ultrasonic Beams and Corrugated Plates with Experimental Verification

Periodically corrugated structures play an important role in the field of vibration control and for designing structures with desired acoustic band gaps. Analytical solutions for corrugated plates are available for well-defined, smooth corrugations, such as sinusoidal corrugations that are not very common in the real world. Often corrugated plates are fabricated by cutting grooves at regular intervals in a flat plate. No analytical solution is available to predict the wave propagation behavior in such a periodically corrugated plate in which the equation of the plate surface changes periodically between a planar flat surface and a nonplanar parabolic groove. This problem is solved here for steady-state case by a newly developed semianalytical technique called distributed point source method (DPSM), and the theoretical predictions are compared with the experimental results generated by reflecting a bounded 2.25 MHz ultrasonic beam by a fabricated corrugated plate. The main difference that is observed in the reflected beam profile from a flat plate and a corrugated plate is that the back- scattering effect is much stronger for the corrugated plate, and the forward reflection is stronger for the flat plate. The energy distribution inside the corrugated plate also shows backward propagation of the ultrasonic energy.

Health Monitoring of a Thermal Protection System using Lamb Waves

The applicability of guided waves as a structural health monitoring (SHM) tool to predict the point of impact and detect delamination in a thermal protection system (TPS) is studied. A model TPS was designed by bonding ceramic porous tiles to a 2.2 mm thick 2124-T351 aluminum alloy plate. The delamination defect may be caused by the impact phenomenon or due to other reasons such as manufacturing defect, thermal, or mechanical fatigue, etc. Impact phenomenon is simulated by dropping a ping pong ball on the tiled structure. The delamination at the interface between the ceramic tile and the aluminum plate is simulated by removing the adhesive bond at the selected interface regions during the specimen fabrication process. The conventional triangulation technique cannot predict the point of impact in a tiled plate structure because the triangulation technique assumes that the wave speed is independent of the direction of propagation which is not the case for ceramic tile mounted plates. An alternative method based on the optimization scheme was proposed by Kundu et al. [1] to locate the point of impact in plates by analyzing the time of arrival of the ultrasonic signals received by passive sensors attached to the plate. This objective function based method is used to locate the impact point on the tiled plate. Experiments are carried out by dropping a ping pong ball on the tiled structure and recording acoustic signals by passive transducers adhesively bonded to the plate at three different locations. The impact point is predicted and compared with the actual location of impact. Delamination detection studies were conducted in the pitch-catch mode, by changing the angle of strike and the frequency of the transducer excitation to generate the appropriate guided wave mode. The delamination defect could be detected and the impact point could be identified underlining the importance of the use of guided waves as an SHM tool for TPS.

Distributed point source method for modeling scattered ultrasonic fields in the presence of an elliptical cavity

Scattering of ultrasonic waves by an elliptical cavity is modeled. The ratio of the semimajor to semiminor axes is varied from 1 to model a cylindrical cavity with a circular cross section to a large value to approximately model a Griffith crack. The distributed point source method (DPSM), which is a Green’s function-based semi-analytical technique, is adapted in the present modeling. DPSM generated results for slit opening and elliptical cavity are compared. In the elliptical cavity model, the entire cavity surface is considered traction free, whereas in the slit model, two parallel crack surfaces are assumed traction free and no special consideration is given to the crack tips. Because a Griffith crack under tensile loads opens like an ellipse, the elliptical cavity with a large semimajor to semiminor axes ratio can be considered as an open Griffith crack. Elliptical cavity model clearly shows large stress values near the crack tips. Earlier DPSM models did not consider these high stress concentrations. The assumption was that it would have negligible effect on the scattering pattern at the far field. The numerical results are presented for a cavity of length 4 mm when bounded ultrasonic beams of 1 and 2.25 MHz frequency strike the solid half-space.

An improved technique for locating the point of impact from the acoustic emission data

Triangulation technique for impact point location works very well when the acoustic emission sensors are placed at a relatively large distance from the point of impact. In this situation the time of arrival measurement is not affected significantly by the small error that might arise from not being able to pinpoint the exact time of arrival of the acoustic signal. The conventional technique also requires that the wave speed in the medium is well-known and non-dispersive in the frequency range of interest. If the receiving wave is a P-wave or S-wave or a non-dispersive Rayleigh wave then the conventional triangulation technique is reliable. In this paper it is shown that the conventional triangulation technique is not very reliable for locating the impact point in a plate when the sensors are placed close to the striking point for two reasons - first, it is difficult to pin point the exact time of arrival of the signal and secondly the Lamb modes in a plate are dispersive. Dispersive signals attenuate differently at various frequencies and propagate with different speeds causing distortions in the received signals and thus introduce more error in the time of flight measurement. In this paper an alternative approach is proposed to locate the impact point more accurately. Experiments are carried out with an aluminum plate. The impact points predicted by the conventional triangulation technique and the proposed modified method are compared.

An integrated health management system for real-time impact monitoring and prediction of impact-induced damage on composite structures

Next generation technology of integrated health management systems for air-transportation structures will utilize SHM methods in combination with simulation techniques for the prediction of structural degradation induced by adverse events such as impacts. The contribution focuses on the development of an advanced real-time monitoring system for impact loads using passive sensing networks. Starting from the fundamental approach of real-time monitoring based on system identification models, problems of model order, signal conditioning and efficient model training will be addressed. Finally, the load monitoring system is interactively linked to a damage prediction module based on numerical failure analysis employing composite failure criteria. The utilization of appropriate database techniques allows a real-time prediction of impact induced damage after detection of any adverse impact event making information available on developing degradation at the earliest possible state.

Mesh-free Modeling of Ultrasonic Wave Fields in Damaged Layered Half-spaces

Modeling of an ultrasonic wave field inside a layered half-space is carried out by using the mesh-free semi-analytical Distributed Point Source Method (DPSM). The complete field is computed in a layered half-space in presence and absence of defects. The layered structure is excited by a bounded ultrasonic beam generated by a finite-sized transducer. It is important to have theoretical models to predict the ultrasonic fields in damaged and damage-free structures for its nondestructive evaluation. Numerical exercises can be carried out aided by these theoretical models to determine the area of the most distorted ultrasonic field in presence of an internal anomaly. Several numerical examples are provided for an aluminum half-space attached to layers made of two different materials. The structure is excited by a bounded ultrasonic beam. Influence of the material properties and internal anomaly on the ultrasonic field pattern is demonstrated here.

Real-time prediction of impact-induced damage for composite structures based on failure analysis and efficient database methods

This contribution presents a novel strategy to achieve real-time prediction of non-penetrating impact-induced damage, especially delamination between plies of composite laminated structures. The proposed strategy aims to create an optimum-sized database of simulated damage information on a given laminated structure using numerical failure models and pattern recognition technique. A multi-stage data clustering algorithm is implemented to identify regions in the structural Finite Element (FE) model that have similar damage signatures. The generated database is linked to the real-time estimate of impact location and load-time history obtained from piezoelectric transducers based passive impact monitoring system. A composite stiffener panel is selected as a model problem and it is shown that the proposed strategy based on pattern recognition will result in large savings in computational cost for the database generation besides providing real-time damage diagnostic capabilities for in-service adverse impact events within certain confidence bounds.