Monitoring the structure of various systems, including civil and aerospace infrastructures, over a period of time provides users with valuable information on the structural integrity and performance of these systems. This is not only useful in understanding and assessing the life expectancy of the structure, but can also provide real-time feedback of any structural abnormalities.
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Structural Health Monitoring (SHM) Technologies
Technologies capable of improving the safety of the structures and personnel working within them is critical in any industry. As a result, structural health monitoring (SHM) has emerged as a crucial aspect of numerous applications during the past few years.
SHM technologies are comprised of a network of sensors that collectively work together to monitor the health of connected systems. Several new SHM technologies have been further improved through the incorporation of various types of sensors, such as those capable of performing a nondestructive evaluation of both existing and new infrastructures.
Advantages of SHM Methods
Unfortunately, it is not uncommon that both aerospace and civil infrastructures are used past their design lifetime. While functional use of existing structures is cost-effective, ensuring the safe and efficient use of these infrastructures is paramount, especially once they out-live their design life.
SHM methods can easily be incorporated into existing infrastructures to monitor their performance and provide feedback on potential structural flaws or deformations that occur over time. Compared to existing structures that have been upgraded with SHM, new structural designs that have been integrated with SHM sensors and sensory systems provide several advantages, including a significant reduction in the cost, weight and size of the infrastructure.
Nondestructive Techniques for Damage detection
Ultrasonic techniques, such as nondestructive evaluation (NDE), nondestructive inspection (NDI) and nondestructive testing (NDT), are widely used for the detection of flaws in metallic structures, as well as for the identification of local damages in structures. Each of these damage detection techniques primarily work by identifying disturbances in the wave field.
The sensors used in such testing utilize ultrasonic techniques such as pulse echo, pitch-catch and pulse-resonance methods to record various measurements. These measurements can include time of flight (TOF), path length, frequency, angle of wave reflection, angle of refraction, impedance and phase angle. Variance in these parameters caused by the structural flaws or local damages can be recorded by the sensors, thereby facilitating nondestructive evaluation.
Ultrasonic Nondestructive Techniques in SHM
Researchers at the University of Southern California (USC) demonstrated the use of piezoelectric wafer active sensors (PWAS) for nondestructive damage detection. PWAS are small, inexpensive and non-intrusive sensors that can detect cracks, corrosion, delamination and disbands in the structure by sending and receiving ultrasonic Lamb waves. These incredible sensors can be seamlessly integrated into various structural components.
Adrian Cuc and Victor Giurgiutiu’s team at USC has used this ultrasonic sensor to develop an embedded SHM. Their embedded SHM utilizes an array of sensors interconnected with data concentrators and wireless communicators to monitor and detect any structural damages that may occur during the lifetime of the infrastructure.
The team has demonstrated the efficiency of their developed SHM by integrating the sensors and testing the system’s efficiency in various specimens, including an aluminum lap-joint, a helicopter blade and a spacecraft panel. By sending and receiving guided Lamb waves through the use of various wave propagation methods, the USC researchers validated the successful integration of this technology into adhesively bonded materials.
Conclusion
As exemplified in various other areas, nondestructive testing methods are excellent ways to non-intrusively detect damages in systems. Monitoring the integrity of structures over a period of time also allows for identification and correction of minor structural issues before such damages present a major challenge. Integrated SHM techniques have several advantages and could have the potential to transform the design of future infrastructures.
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