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LANL researchers investigated integrated damage identification processes in structures based on self-sensing impedance methods, acoustic emission, and Lamb wave propagation methods. In particular, the impedance method has been used for detecting joint connection damage and the Lamb wave propagation method has been used for identifying surface cracks. The acoustic emission techniques are used to identify the location and severity of impact loadings on the structure. The integration procedure of these techniques is relatively straightforward because the same PZT patches can be used for these methods. The combination of the local impedance method with the wave propagation based approach allows a better characterization of the system’s structural integrity. The coupling of these approaches also makes the damage identification process more redundant such that the loss of a particular patch does not render the health monitoring system nonfunctional.
Once structural damage has initiated and is detected, numerical and predictive models are used to capture the influence of damage on the structural system and predict future performance. This procedure necessitates the measurement of system level response. A unique multi-scale monitoring can be achieved by using the same active sensors/actuators used for the local damage identification as passive sensors, similar to commercial piezoelectric accelerometer, to monitor the global response of the structure in an effort to quantify the effects of local damage on the global system response (e.g. how local debonding in a composite wing affects the flutter characteristic of the aircraft). Because active sensing systems can be easily embedded into the structure and provide data directly correlated to the dynamic stress-strain field in global scale, this versatile sensing system is effective for global on-line response monitoring. Successful completion of multi-scale monitoring will result in a system capable of assessing the behavior of damaged components and the implications of that damage on system performance.
Structural health monitoring sensors/actuators must be rugged to withstand operational environments. Traditional PZT wafers used for active SHM methods are, however, brittle and require careful treatment; they are especially vulnerable under impact loadings. Therefore, LANL has extensively investigated the use of newly developed flexible piezoelectric-fiber composites, in particular Macro-fiber composite actuators, as SHM sensor/actuators. Our research showed that the MFC is more rugged, and hence more reliable, compared to conventional brittle piezoceramic counterparts under harsh environment.
LANL has also developed a unique approach to diagnose the functionality of piezoelectric sensors. The premise of this technique is to track the changes in the capacitive value of piezoelectric materials resulting from the degradation of the mechanical/electrical properties of a PZT and its attachment to a host structure.
In many areas of active-sensing technology, hundreds, even thousands of sensors/actuators are needed to truly make damage identification feasible in a real-world environment. Because interrogating such a large number of sensors is both time and cost prohibitive, it becomes necessary to utilize unique hardware that can quickly and efficiently interrogate large numbers of active sensor. LANL has developed a relay-based approach that can serve as both a multiplexer and a general-purpose signal router with special consideration given to piezoelectric active-sensing approaches. We have also implemented this device as an expandable design that allows for easy scalability depending upon the size of the structure. Therefore, by using this hardware in conjunction with a centralized monitoring station, large numbers of active-sensors can be monitored effectively.
In structural health monitoring and damage prognosis applications, it is desirable to have autonomous, self-contained sensor systems. Systems that depend on batteries have maintenance requirements and can fail at inconvenient times. If power can be supplied by ambient sources then the system has a potentially unlimited life span. As power requirements for electrical devices decrease, the possibility exists that localized power generation using ambient sources will be adequate to meet these power requirements. The process of acquiring ambient energy is called power or energy harvesting. EI researchers are investigating to electrically harness energy from ambient vibration and thermal energy. This capture energy can be used to prolong the life of the power supply or, in the ideal case, provide unlimited energy for the lifespan of the SHM/DP sensing systems. EI researchers are also investigating the feasibility of using Radio Frequency (RF) signals to wirelessly deliver electrical energy to power SHM sensing systems. In this approach, the required energy will be periodically and wirelessly delivered as needed. Our study shows that it is quite possible to operate wireless active-sensing nodes completely from the wirelessly delivered RF energy. Current studies include the hybrid scheme that integrates the energy harvesting and delivery schemes to result in more autonomous SHM/DP sensing systems.
