This paper presents a feasibility study for the visualization of hidden damage in aluminum plates using the integration of a digital camera, projected speckles, and a baseline-free wavelet transform mode shape curvature (WT-MSC) damage index. To capture out-of-plane motion in the plates, off-axis 2D digital image correlation (DIC) is applied. With the camera at an angle with respect to the plate surface normal, a component of the higher-amplitude transverse displacements can be captured. Compared to 3D DIC, the system is less complex, there is no need for camera calibration, and the 2D DIC algorithm is more computationally efficient. A major limitation of DIC for practical applications is the need to apply a speckle pattern to the surface to introduce trackable features in the image for tracking displacement. Projected speckles replace the need for surface-applied speckle patterns. Thus, minimal surface preparation is required. Two geometrically identical 305-mm x 305-mm aluminum plates with thinning defects of different sizes and depths were used to demonstrate the system. Through the excitation of a 20 Hz to 1 kHz chirp signal in a single-edge-clamped plate, the first 12 transverse vibration modes of the plate were sensed. These mode shapes were recreated with the off-axis 2D DIC system, and a wavelet transform mode shape curvature (WT-MSC) damage index was applied for damage imaging. This index is sensitive to irregularities in the higher mode shapes caused by differences in geometry in the damaged regions. The system provided clear damage images with a clear correlation with actual damage geometry regardless of plate orientation. This system serves as a preliminary study for the eventual application of imaging barely visible impact damage in composite plates using projected speckles.
This paper presents an integrated system capable of damage imaging of barely visible impact damage (BVID) in composite structures. This system applies guided-wave-based structural health monitoring using 3D digital image correlation, or GWSHM-3D DIC to produce a map of subsurface damage using a short video from a stereo pair of synchronized digital cameras. The proposed system overcomes many limitations of previous efforts of GWSHM that used 2D digital image correlation (DIC). First, 3D DIC can capture the higher-amplitude out-of-plane displacements associated with the anti-symmetric wave mode, lowering the spatial resolution requirements of the cameras. Second, a total wave energy (TWE) imaging condition is employed that uses the monogenic signal via a Reisz transform to obtain the local instantaneous amplitude as a contribution to wave energy. This condition can highlight local resonance in the damage region without the need for high frame rates to fully reconstruct the wavefield. With significantly lowered spatial and temporal resolution requirements of the cameras, high-stiffness materials like composites can be inspected or monitored with a larger field-of-view (FOV). Additionally, signal enhancement techniques intended to increase the effective resolution of the camera are no longer necessary, which reduces the data acquisition time from many hours to a few seconds. To demonstrate this integrated dual-camera concept with the TWE imaging condition, the system was used to image damage in a CFRP composite sandwich panel that had been subjected to a low-velocity impact. Initial damage maps produced for a 100-mm ´ 100-mm FOV using a three-second video pair show precise damage imaging ability that is comparable to benchmark ultrasonic and x-ray scans. This efficient and reliable integrated system demonstrated high potential for in-time damage inspection on composite aircraft and other critical structures.
This paper presents a feasibility study for the visualization of hidden damage using an integrated highspeed stereo-camera system. The reconstructed wavefields produced by the 3D-enabled stereo-camera system were compared to those from a single-camera system, which is limited to 2D sampling. To demonstrate this stereo-camera concept, guided waves were generated in a thin aluminum plate with surrogate damage using a surface-mounted piezoelectric actuator under continuous sinusoidal excitation. The resulting wavefield was captured with two calibrated high-speed cameras and reconstructed through stereo-digital image correlation. After signal or image filtering and processing, the reconstructed wavefield was produced. To overcome the limited data rate of state-of-the-art high-speed cameras, sample interleaving was implemented to effectively increase the sampling rate. For the single-camera 2D-DIC case, image stitching was needed to increase the field of view, which left artifacts in the damage image due to slight misalignments between samples. The implementation of two high-speed cameras for stereo-digital image correlation enables the full-field sampling of both in-plane and out-of-plane components of the wavefield displacement. Because the out-of-plane displacement is typically much larger than other wave modes due to the thin plate dimensions, this advantage of capturing out-of-plane vibrations unique to the stereo-camera setup lowered the required resolution and enabled the capture of a larger field-of-view without the need for image stitching.
In this paper, the feasibility and effectiveness of a guided-wave approach are investigated as a first step toward the development of an automated non-contact ultrasonic system for inspecting railroad spikes to locate cracks and breaks. An automated system is desired that can perform the inspection more effectively and reliably than current manual tapping or pulling methods. The goal of this research is to detect damage in railroad spikes in a well-controlled laboratory environment. A recent study using contact ultrasonic transducers for spike inspection revealed many limitations, including high signal attenuation, high sensitivity to transducer positioning, and the need to use an ultrasonic couplant. To circumvent these limitations and provide a more robust inspection method, an “airborne ultrasound” system comprised of an air-coupled transducer (ACT) for actuation and a laser Doppler vibrometer (LDV) for sensing is proposed and evaluated. The inspection system employs the concept of guided waves, which propagate with less attenuation at lower ultrasonic frequencies and, hence, a larger amplitude than that from the bulk waves typically used for ultrasonic nondestructive inspection (NDI). The detection of broken spikes is based on the premise that the wave energy will be attenuated in the transmitted wave inhibited by the damage and amplified in the reflected wave from additional waves scattered from the damage. Initial tests on cut spikes demonstrated that the proposed system performs with confidence of damage detection based on baseline comparisons with pristine spikes.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.