SHM can be described as the process of implementing a damage identification strategy. Significant research efforts, which are highly motivated by safety and cost of ownership concerns, have been employed for the SHM of aerospace, civil, and mechanical engineering infrastructures in the last decade. SHM systems have found interesting applications in innovative new structures and ageing structures.
Ageing structures with known problems can benefit from SHM systems to increase system reliability and extend the useful lifespan. The basic premise of an SHM system is that damages alter stiffness, mass, or damping of a structure and in turn cause changes in the system performance as well as dynamic response. Multiple techniques, such as statistical pattern recognition and vibration analysis, have been applied to develop customised SHM systems for different applications by detecting, quantifying, and localising the damage. Different sensing techniques, including piezoelectric (PZT) transducers, optic fibre Bragg gratings (FBG), and accelerometers, have been integrated in advanced SHM systems. Therefore, the research on SHM is a multidisciplinary research area that requires contributions from mechanical engineering, electrical engineering, civil engineering, computer science, etc.
The complete health state of a structure can be determined based on presence, location, type, and severity of damages, and the estimation of remaining useful life. The first key issue of SHM is to identify the existence of damage in structures so that required maintenance can be conducted and catastrophic failures can be prevented. This procedure can be referred to as damage detection. Localisation is an extension of damage detection in the process of SHM by estimating the possible damage position. For a large structure, it is necessary to localize the damage position when it is necessary to predict its growth rate and path.
The location of existing damage can also determine the propagation rate of the damage under certain environmental and loading conditions. The damage type can be identified by clustering the sensing features as well as modelling the structural response. This procedure is named damage classification. Damage quantification, which is the last step to assess the damage condition, is to quantitatively estimate the severity of damage. The process of damage detection, localisation, classification, and quantification is defined as diagnosis of a structure. Once the state of a structure is identified, the residual useful life can be estimated. Successful damage prognosis will eventually lead to the decision making for maintenance and service requests.
Currently, the maintenance paradigm is a mix of preventative (schedule-based) and corrective repair and replacement. The schedule-based system requires that maintenance action be taken at fixed time intervals regardless of the state of the system. Once a part has failed, corrective maintenance is performed. However, this type of maintenance can often lead to excessive system downtime, particularly if replacement parts require lead time to be procured. The schedule-based maintenance system has been the preferred mode because it safeguards against the failure of a critical system or subsystem that can lead to significant economic and human loss. The frequency of this type of inspection involves a trade-off between the cost of inspection and the risk of induced damage, which takes into account the expected loads and environmental conditions of the structure. Unavoidably, schedule- based maintenance can result in unnecessary components replacement even though they still have considerably safe operating life remaining. This is because a thorough inspection is expensive and time consuming, and replacement is often a cheaper alter- native. Condition-based maintenance (CBM) is the process of using the actual state of the system to conduct service and maintain performance. If implemented, such a system has the potential to dramatically reduce life cycle cost and improve safety. In addition, CBM increases productivity since the sys- tem would have less down time due to unnecessary maintenance. The logistics enterprise required to maintain a large structure can also be streamlined so that necessary parts can be ordered just before they are needed, reducing inventory cost and lead time.
The performance of SHM systems depends on the damage features extracted from sensing signals. The capabilities of sensors significantly influence the application of SHM in complex engineering structures. The most popular sensors used in SHM include PZT transducers, optic FBG sensors, accelerometers, strain gages, acoustic emission sensors, etc. PZT transducers are widely used since they can be used to generate guided wave (GW) signals in structures for detection. FBG sensors are considered to be useful because multiple sensors can be integrated into one optical fibre cable and embedded in reinforced composite structures. All the data generated needs to be processed with advanced algorithms and integrated on a platform that enable the share of information to all stakeholders.
Stratosphere S.A. is a global provider of integrated health management solutions. The robustness and integration of its solutions is the pinacle of SHM systems.
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