Structural Health Monitoring

There is a critical need for integrated system health management (ISHM) approaches to asset maintenance. Ideally, ISHM methodologies would track the system usage and the associated loads, monitor the system degradation and materials state, monitor relevant environmental parameters and their effects on system degradation, detect insipient system damage, diagnose failure mode, predict future system performance, and recommend maintenance actions. Even though there has been considerable progress in many subareas of ISHM over the past years, there is still ample room for future improvements in all technological aspects affecting ISHM. In fact, progress in ISHM has not been uniform. Some subsystems have experienced a far greater degree of development than others. For example, engine and machinery health monitoring and diagnostics, due to its criticality, has evolved at a faster pace than structural health monitoring. This article will review some of the aspects that need to be addressed in order to make structural health monitoring (SHM) of military systems a reality in the near future.

The structural health monitoring (SHM) is an important requirement to handle military aircraft safety. In contrast with civil aircraft, the missions, configurations and environments are changing frequently and therefore also the load spectra, which lead to a various life consumption. To ensure the structural integrity of the airframe and structural systems by SHM systems the following four functions are essential: Event and fatigue monitoring, including remaining life assessment and damage detection monitoring. This is the basis for an economic and safety in-service operation of a flying weapon system.

This article shows the way from a simple monitoring system to a state-of-the-art support system based on examples from the German Air Force (GAF). From a military aircraft manufacturer’s point of view, the pro and contra of these systems based on in-service experience of various aircraft types are listed. Referring to this experience the benefit of the modernization of SHM-System will be presented and the requirements on a modern SHM-System especially for the ground support system and the logistic and in addition for new monitoring areas will be worked out. The approach for hot spot monitoring for damage detection is explained. A special view will be given on the possible integration of modern damage detection methods in existing and new SHM-Systems. Furthermore the combination of damage detection system and existing usage monitoring systems opens the way for enhanced monitoring systems.

Structural health monitoring (SHM) technology has been successfully applied to understand the loads, environment actions, and behaviors of a structure subjected to various actions through solving a reverse problem. The sensing technology is a critical part of SHM. In this article, the development of advanced sensing technology and sensors in mainland China in the past decade, such as optic fiber sensing technology, wave propagation-based piezoelectric ceramic (PZT) sensing technology, smart cement-based sensing technology, and corrosion detection technology, have been critically reviewed. In addition, the article also summarizes the application of SHM technologies in earthquake engineering, wind engineering and life-cycle performance evaluation and corresponding progress achieved in mainland China. The challenges and future trends in the development of sensing technology and SHM are put forward in this paper.

Obtaining the strain data all along the optical fiber, with adequate spatial resolution and strain accuracy, opens new possibilities for structural tests and for structural health monitoring. Formerly, only point sensors, as strain gages or fiber Bragg grating, were available, and information about the response to loads was restricted only to those points on which the sensors were bonded. Unless a sensor was located near the damage initiation point, details about the failure initiation and growth were lost. With a distributed system, the information is given as an array of data with the position in the optical fiber and the strain or temperature data at this point. In this article, the physical principles underlying the different techniques for distributed sensing are discussed, a classification is done based on the backscattered wavelength; this is important to understand its possibilities and performances. The definition of performance for distributed sensors is more difficult than for traditional point sensors because the performance depends on a combination of related measurement parameters. For example, accuracy depends on the spatial resolution, acquisition time, distance range, or cumulated loss prior to measurement location. The field of applications of this new technology is very wide; results of the structural tests of a 40 m long wind turbine blade, detecting the location and load of onset of buckling, and the results of the delamination detection in a composite plate, are presented as examples.

The role of the adhesive layer on PZT-induced Lamb wave propagation in structures exposed to elevated temperatures is presented in this article. Both experiments and numerical simulations were performed to study the effects of the adhesive layer on sensor signal at elevated temperatures. Experimentally, signals from PZT transducers with different adhesive thicknesses (40 and 120 μm) were investigated up to 500 kHz. In model simulations, the spectral element package (PESEA), which was developed previously, was adopted to simulate the test results. The simulations agreed with the experimental data quite well. Parametric studies were was then performed using PESEA to evaluate the effect of adhesive layer on PZT-induced Lamb wave propagation at elevated temperatures as compared to other mechanical properties of the host structure and PZT materials; these studies revealed that the stiffness change of adhesive layer due to temperature is the most influential parameter for the change in sensor signals as compared to other mechanical properties, and that the thickness of the adhesive layer can affect a sensor signal in a different manner at elevated temperatures. This study shows PESEA can reasonably simulate the adhesive layer effect at elevated temperatures and hence can be a useful tool for understanding the behavior of Lamb wave propagation generated by adhesively bonded PZTs on structures.

One practical issue that must be addressed prior to the implementation of a vibration-based structural health monitoring system is the influence that variations in the structure’s environmental and boundary conditions can have on the vibration response of the structure. This issue is especially prominent in the structural health monitoring of aircraft, which operate in a wide variety of different environmental conditions and possess complex structural components connected through various boundary conditions. However, many types of damage introduce nonlinear stiffness and damping restoring forces, which may be used to detect damage even in the midst of these varying conditions. Vibro-acoustic modulation is a nondestructive evaluation technique that is highly sensitivity to the presence of nonlinearities. One factor that complicates the use of vibro-acoustic modulation as a structural health monitoring technique is that the amount of measured modulation has been shown to be dependent on the frequency of the probing signal. The frequency dependence of the modulation was investigated and the magnitude of modulation was found to be correlated with the underlying vibration characteristics of the structure, which are influenced by environmental and boundary condition variations. To facilitate the use of nonlinear vibro-acoustics for the health monitoring of complex aerospace components in varying environments, a vibro-acoustic modulation technique utilizing a swept probing signal has been developed. The developed method was demonstrated on a steel beam in varying operational conditions. The presence of a crack in the beam was detected both through an increase in the amount of normalized modulation and without the use of historical data by utilizing generalized extreme value statistics.

In this article, we present experimental investigations using energy harvesting and wireless energy transmission to power wireless structural health monitoring sensor nodes. The goal of this study is to develop sensing systems that can be permanently embedded within a host structure without the need for an on-board power source. With this approach the required energy will be harvested from the ambient environment, or periodically delivered by a radio-frequency energy source to supplement conventional harvesting approaches. This approach combines several transducer types to harvest energy from multiple sources, providing a more robust solution that does not rely on a single energy source. Both piezoelectric and thermoelectric transducers are considered as energy harvesters to extract the ambient energy commonly available on civil structures such as bridges. Methods of increasing the efficiency, energy storage medium, target applications and the integrated use of energy harvesting sources with wireless energy transmission will be presented.