Interaction of Rayleigh Waves Induced by Interdigital Transducer with Fatigue Crack
Compared to bulk waves, Rayleigh waves can propagate much longer distances with lower signal attenuation levels, which are nominally pro portional to r / 1 for a given frequency (r=distance). In addition to the small attenuation, Ra yleigh waves are also non-dispersive and sensitive to surface-breaking defects. The surface penetra tion depth of Rayleigh waves is also approximately one wavelength, which can be beneficial for many NDT applications. However, one major drawback of using Rayleigh waves for nondestructive testing has been the difficulty of generating Rayleigh waves, where the most commonly used method involves the combination of a plastic wedge and a conventional longitudinal mode trans ducer. Unfortunately, a number of wedges with different angles are needed to generate Rayl eigh waves in different materials or at different operating frequencies to satisfy the critical Rayleigh launch angle requirement. In this study, instead of using wedge transducers, we have designed, fab ricated, and tested narrow-band Interdigital Transducers (IDTs), where precision elect rode patterns were etched on piezoelectric ceramic substrate materials using micro laser machining t echniques. These IDTs can be directly applied to the surface of test specimens to generate Rayle igh waves in MHz range. A laser interferometry system was used to characterize the I DT sensors, where a well-defined and directional Gaussian beam profile was observed. Inves tigations were also made for Rayleigh waves interacting with fatigue cracks in aircraft qualifi ed metallic alloys. It was observed that some of the Rayleigh wave energy can propagate through a tight crack at both normal and oblique incidence.
Structural Health Monitoring of M1114 High Mobility Multipurpose Wheeled Vehicle Armor System
Abstract : The M1114 High Mobility Multipurpose Wheeled Vehicle (HMMWV) has been the workhorse vehicle of the U.S. Armed Forces in Afghanistan and Iraq. Donald Rumsfeld, Secretary of Defense, was faced with massive public criticism in 2004 for not equipping our military personnel in Afghanistan and Iraq with M1114s that had the proper ballistic armor. In May 2004, a $618M Senate Bill was passed to increase the production level of HMMWVs and improve their ballistic protection capabilities while minimizing additional weight. While the military is taking advantage of using composite armor on the HMMWV, it does not have a rigorous method to detect, locate, and quantify damage on a two-layer composite armor system. Structural Health Monitoring (SHM) is the process of implementing a damage detection and characterization strategy for engineering structures. Damage is defined as changes to the material and geometric properties of a structural system, including changes to boundary conditions and system connectivity, which adversely affect the system's performance. An active SHM system was developed to detect, locate, and quantify damage on a two-layer composite armor (HJ1 composite with ceramic frontal plates) that encounters impact from a 0.30 caliber armor piercing projectile. An adaptive version of a one-at-a-time experiment was used during this research. Base line testing was performed to provide information on the structural properties and wave propagation characteristics of the material. Ballistic testing was completed to replicate David Fecko's experiment of maximum V(sub 50) velocity of 947 meters per second and a ceramic-to-composite ratio of 60/40%.
CONVOLUTION AND TIME INVERSION USING PARAMETRIC INTERACTIONS OF ACOUSTIC SURFACE WAVES
We report on the observation of acoustic convolution, time inversion, and electronically variable delay in real time using 105‐ and 220‐MHz surface acoustic waves in YZ‐cut LiNbO3 delay rods. The time inversion is a special case of correlation, where a short ``pip'' pulse will create a reflection in time for the injected input signal pulse. The conversion losses in the processing were 64–74 dB.