Christopher T. Dunn

Optimization of Lamb wave actuating and sensing materials for health monitoring of composite structures

In a continuing effort to examine the effectiveness of Lamb wave methods for the health monitoring of composite structures, this paper presents the conclusions of an analytical and experimental study optimizing piezoelectric patches to detect damage within composite laminates. Previous research has demonstrated the ability of Lamb waves to provided useful information about the presence of damage in simple narrow coupons, and they have yielded the possibility of estimating severity and location of damage as well. During the course of this NRO funded research program, several types of piezoelectric materials in various configurations were analyzed in order to produce the highest force actuator and best resolution sensor at the lowest power level. Consideration was also placed towards directionality of wave propagation, and durability, reliability and reproducibility of the sensing patch itself. Experiments were then carried out on narrow coupon laminates to qualify and tune these actuating/sensing patches. New algorithms were used to filter and decompose the resulting signals to more efficiently detect the presence of damage for automated use, and gather information relating to the damage type, severity and location. SHM technologies will enable condition-based maintenance for efficient structural design, will reduced overall life-cycle costs, and eliminate scheduled inspections.

Packaging of structural health monitoring components

Structural Health Monitoring (SHM) technologies have the potential to realize economic benefits in a broad range of commercial and defense markets. Previous research conducted by Metis Design and MIT has demonstrated the ability of Lamb waves methods to provide reliable information regarding the presence, location and type of damage in composite specimens. The present NSF funded program was aimed to study manufacturing, packaging and interface concepts for critical SHM components. The intention is to be able to cheaply manufacture robust actuating/sensing devices, and isolate them from harsh operating environments including natural, mechanical, or electrical extremes. Currently the issues related to SHM system durability have remained undressed. During the course of this research several sets of test devices were fabricated and packaged to protect the piezoelectric component assemblies for robust operation. These assemblies were then tested in hot and wet conditions, as well as in electrically noisy environments. Future work will aim to package the other supporting components such as the battery and wireless chip, as well as integrating all of these components together for operation. SHM technology will enable the reduction or complete elimination of scheduled inspections, and will allow condition-based maintenance for increased reliability and reduced overall life-cycle costs.

Carbon Nanotube Appliques for Fatigue Crack Diagnostics

The present research targets hot-spot monitoring for ageing aircraft. The sensor itself is a sheet of CNT sandwiched between two electrically isolating adhesive layers. Any growth of flaw would disrupt the CNT electrical network, therefore increasing the network resistance. This effect would be accentuated if the flaw actually tears through the CNT patch, however since the CNT is piezoresistive, even damage growing under the patch would be measured due to the effective residual strain imparted. Using orthogonal parallel pairs of electrodes, one would be able to not only measure extent of a crack, but also deduce the orientation based on relative changes seen by each pair; a flaw growing towards an electrode would have a small effect, while growing parallel to an electrode pair would offer a significant measured change. Analytical results are presented along with experimental data for calibrated simulated crack growth in addition to actual fatigue crack growth in aluminum specimen. doi: 10.12783/SHM2015/207