Rapid diagnostics and virtual imaging of damages in complex structures like folded plate can help reduce the inspection time for guided wave based NDE and integrated SHM. Folded plate or box structure is one of the major structural components for increasing the structural strength. Damage in the folded plate, mostly in the form of surface breaking cracks in the inaccessible zone is a usual problem in aerospace structures. One side of the folded plate is attached (either riveted or bonded) to adjacent structure which is not accessible for immediate inspection. The sensor-actuator network in the form of a circular array is placed on the accessible side of the folded plate. In the present work, a circular array is employed for scanning the entire folded plate type structure for damage diagnosis and wave field visualization of entire structural panel. The method employs guided wave with relatively low frequency bandwidth of 100-300 kHz. Change in the response signal with respect to a baseline signal is used to construct a quantitative relationship with damage size parameters. Detecting damage in the folded plate by using this technique has significant potential for off-line and on-line SHM technologies. By employing this technique, surface breaking cracks on inaccessible face of the folded plate are detected without disassembly of structure in a realistic environment.
Composite T-joints are commonly used in modern composite airframe, pressure vessels and piping structures, mainly to
increase the bending strength of the joint and prevents buckling of plates and shells, and in multi-cell thin-walled
structures. Here we report a detailed study on the propagation of guided ultrasonic wave modes in a composite T-joint
and their interactions with delamination in the co-cured co-bonded flange. A well designed guiding path is employed
wherein the waves undergo a two step mode conversion process, one is due to the web and joint filler on the back face of
the flange and the other is due to the delamination edges close to underneath the accessible surface of the flange. A 3D
Laser Doppler Vibrometer is used to obtain the three components of surface displacements/velocities of the accessible
face of the flange of the T-joint. The waves are launched by a piezo ceramic wafer bonded on to the back surface of the
flange. What is novel in the proposed method is that the location of any change in material/geometric properties can be
traced by computing a frequency domain power flow along a scan line. The scan line can be chosen over a grid either
during scan or during post-processing of the scan data off-line. The proposed technique eliminates the necessity of
baseline data and disassembly of structure for structural interrogation.
Stiffener is one of the major components of aircraft structures to increase the load carrying capacity. Damage in the
stiffener, mostly in the form of crack is an unavoidable problem in aerospace structures. Stiffener is bonded to the inner
side of the aircraft panel which is not accessible for immediate inspection. A sensor-actuator network can be placed on
the outer side of the panel that is accessible. Ultrasonic lamb waves are transmitted through stiffener using the sensoractuator
network for detecting the presence of damages. The sensor-actuator network is placed on both halves of the
stiffened section on the accessible surface of the plate. Detecting damage in stiffener by using this technique has
significant potential for SHM technology. One of the major objectives of the present work is to determine the smallest
detectable crack on the stiffener using the proposed technique. Wavelet based damage parameter correlation studies are
carried out. In the proposed scheme, with increase in the damage size along the stiffener, it is found that the amplitude of
the received signal decreases monotonically. The advantage of this technique is that the stiffened panels need not be
disassembled in a realistic deployment of SHM system.