AN ULTRASONIC GUIDED WAVE TOMOGRAPHIC IMAGING APPROACH FOR CHARACTERIZING BALLISTIC DAMAGE IN COMPOSITE STRUCTURES

Through Small Business Innovative Research (SBIR) support from the U.S. Army, an industry partner has explored the possibility of using an ultrasonic guided wave computed tomography (CT) imaging approach to detect and characterize ballistic damage to composite armor panels that are commonly used in ground vehicles. Laboratory tests have been conducted and shows that the guided wave CT approach can indeed be applied to these complex structures to provide accurate damage mapping potential. Analytical analysis and finite element method (FEM) modeling has been used to aide in understanding guided wave propagation behavior in these anisotropic structures. The work presented herein clearly shows great potential for using a guided wave sensing approach to locate and image ballistic damage in composite armor panels as well as the ability to predict wave propagation and scattering in these complex structures that could be used in the future to predict optimal sensor geometry, configuration, defect type, and detection sensitivity. INTRODUCTION There currently exists a need to develop new technologies for real time health monitoring of composite armor panels on ground vehicles in order to inform decision makers on board or at remote locations as to the severity of damage that may occur. Ultrasonic guided waves offer an attractive solution for embedded structural health monitoring (SHM) of such structures because of their ability to travel long distances and to inspect inaccessible and hidden areas from locations where access is achievable. The guided wave sensors can be used during field deployment to detect an impact event and then determine the location and size of the resulting damage. The guided wave sensors can be embedded in the composite panels during manufacture. Key to the development and optimization of the monitoring system is the development of sensors based on a theoretical understanding of the guided wave mode selection possibilities in these complex structures. Analytical and FEM modeling work has been used to study the complex wave propagation phenomenon. Comparisons with experiment have shown promising correlation. Guided wave tomography experiments have been carried out on an actual armor panel with surface mounted sensors and accurate mapping the location of simulated damage was observed. Further, a representative composite panel with embedded piezoelectric ceramic sensors has been fabricated. Experiments to date show great promise for mapping ballistic damage with the embedded sensing approach. EXPERIMENTS ON AN ACTUAL COMPOSITE PANEL Tests were carried out on a composite specimen. Specific details of the make-up of the panel are not known but in general the panel is ~1 ft. x 1 ft. , ~0.59” thick, and it is Proceedings of the 2009 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) An Ultrasonic Guided Wave Tomographic Imaging Approach for Characterizing Ballistic Damage in Composite Armor Structures, Royer et al. UNCLASSIFIED: Dist A. Approved for public release Page 2 of 7 assumed that is comprised of glass fiber reinforced polymer (FRP) layers, ceramic cylinders, and epoxy resins as reported in the literature for composite armor panels [1]. Initial testing on this panel were conducted to show that guided wave energy could propagate well in all directions and through the entire thickness of the panel. Both normal beam incidence and angle beam incidence tests were conducted in a through transmission approach. The transducer setup for normal incidence tests is shown in Figure 1a. Two 500 kHz center frequency transducers were used in through-transmission (pitch-catch) mode. The transducers were 8” apart. In this case, the wave propagation direction was defined as the 0 o direction. The transmission signal obtained in the experiment using a 10 cycle 500 kHz tone-burst as the input is shown in Figure 1b. As can be observed, two clear wave packages were received by the receiving transducer. To determine whether the received wave packages were guided waves, the receiver was moved to the opposite surface of the plate, as illustrated in Figure 2a. The corresponding received signal is shown in Figure 2b. It was demonstrated that ultrasonic wave energy can still be well-received with the transmitter and receiver transducers on different surfaces of the composite panel. The received wave packages look similar to what was received when the two transducers were on the same surface, which proved that the received ultrasonic waves were ultrasonic guided waves whose energy floods the entire thickness of the composite panel. Ideally, since the panel is symmetric with respect to the center plane of the panel, guided waves in the panel should be either symmetric or antisymmetric modes, which should yield either the same waveform or out-of-phase but with the same amplitude waveforms for the two cases shown in Figures 1 and 2. The discrepancy in waveforms shown here was mainly due to the misalignment of the transducers and variances in transducer coupling conditions.