Structural health monitoring (SHM) involves the observation and analysis of a system over time using periodically sampled response measurements to monitor changes to the material and geometric properties of engineering structures such as bridges and buildings. In one of the foundational papers of the field, it is defined as ``[t]he process of implementing a damage identification and characterization strategy with the eventual goal to predict the remaining life of the structure."
In an operational environment, structures degrade with age and use. Long term SHM outputs periodically updated information regarding the ability of the structure to continue performing its intended function. After extreme events, such as earthquakes or blast loading, SHM is used for rapid condition screening. SHM is intended to provide reliable information regarding the integrity of the structure in near real time.
The SHM process involves selecting the excitation methods, the sensor types, number and locations, and the data acquisition/storage/transmittal hardware commonly called health and usage monitoring systems. Measurements may be taken to either directly detect any degradation or damage that may occur to a system or indirectly by measuring the size and frequency of loads experienced to allow the state of the system to be predicted.
To directly monitor the state of a system it is necessary to identify features in the acquired data that allows one to distinguish between the undamaged and damaged structure. One of the most common feature extraction methods is based on correlating measured system response quantities, such a vibration amplitude or frequency, with observations of the degraded system. Damage accumulation testing, during which significant structural components of the system under study are degraded by subjecting them to realistic loading conditions, can also be used to identify appropriate features. This process may involve induced-damage testing, fatigue testing, corrosion growth, or temperature cycling to accumulate certain types of damage in an accelerated fashion.
Introduction Qualitative and non-continuous methods have long been used to evaluate structures for their capacity to serve their intended purpose. Since the beginning of the 19th century, railroad wheel-tappers have used the sound of a hammer striking the train wheel to evaluate if damage was present. In rotating machinery, vibration monitoring has been used for decades as a performance evaluation technique. Two techniques in the field of SHM are wave propagation based techniquesRaghavan, A. and Cesnik, C. E., Review of guided-wave structural health monitoring," Shock and Vibration Digest, vol. 39, no. 2, pp. 91-114, 2007. and vibration based techniques.Montalvao, D., Maia, N. M. M., and Ribeiro, A. M. R., A review of vibration- based structural health monitoring with special emphasis on composite materials," Shock and Vibration Digest, vol. 38, no. 4, pp. 295-326, 2006.Fan, W. and Qiao, P. Z., Vibration-based damage identification methods: A review and comparative study," Structural Health Monitoring, vol. 10, no. 1, pp. 83-111, 2010. Broadly the literature for vibration based SHM can be divided into two aspects, the first wherein models are proposed for the damage to determine the dynamic characteristics, also known as the direct problem, and the second, wherein the dynamic characteristics are used to determine damage characteristics, also known as the inverse problem.
Several fundamental axioms, or general principles, have emerged: Axiom I: All materials have inherent flaws or defects; Axiom II: The assessment of damage requires a comparison between two system states; Axiom III: Identifying the existence and location of damage can be done in an unsupervised learning mode, but identifying the type of damage present and the damage severity can generally only be done in a supervised learning mode; Axiom IVa: Sensors cannot measure damage. Feature extraction through signal processing and statistical classification is necessary to convert sensor data into damage information; Axiom IVb: Without intelligent feature extraction, the more sensitive a measurement is to damage, the more sensitive it is to changing operational and environmental conditions; Axiom V: The length- and time-scales associated with damage initiation and evolution dictate the required properties of the SHM sensing system; Axiom VI: There is a trade-off between the sensitivity to damage of an algorithm and its noise rejection capability; Axiom VII: The size of damage that can be detected from changes in system dynamics is inversely proportional to the frequency range of excitation.
These Axioms were challenged in another paper to give a concise set of three Axioms:
Axiom AD1: A perfect material is a theoretical construct; real materials require significantly less energy to initiate damage than their ideal counterpart. Axiom AD2: The assessment of damage is dependent on the definition of system performance parameters. The clarity of this assessment depends on the precision of parameter definitions. Axiom AD3: The more sensitive a measurement is to damage, the more sensitive it is to changing operational and environmental conditions affecting the sensing parameter, and the more vulnerable it is to corruption by noise.
SHM System's elements typically include: Structure Sensors Data acquisition systems Data transfer and storage mechanism Data management Data interpretation and diagnosis: System Identification Structural model update Structural condition assessment Prediction of remaining service life
An example of this technology is embedding sensors in structures like bridges and aircraft. These sensors provide real time monitoring of various structural changes like stress and strain. In the case of civil engineering structures, the data provided by the sensors is usually transmitted to a remote data acquisition centres. With the aid of modern technology, real time control of structures (Active Structural Control) based on the information of sensors is possible
Health assessment of engineered structures of bridges, buildings and other related infrastructures Commonly known as Structural Health Assessment (SHA) or SHM, this concept is widely applied to various forms of infrastructures, especially as countries all over the world enter into an even greater period of construction of various infrastructures ranging from bridges to skyscrapers. When damages to structures are concerned, there are stages of increasing difficulty that require the knowledge of previous stages, namely:
Detecting the existence of the damage on the structure Locating the damage Identifying the types of damage Quantifying the severity of the damage
It is necessary to employ signal processing and statistical classification to convert sensor data on the infrastructural health status into damage info for assessment.
Operational evaluation
Operational evaluation attempts to answer four questions regarding the implementation of a damage identification capability:
i) What are the life-safety and/or economic justification for performing the SHM?
ii) How is damage defined for the system being investigated and, for multiple damage possibilities, which cases are of the most concern?
iii) What are the conditions, both operational and environmental, under which the system to be monitored functions?
iv) What are the limitations on acquiring data in the operational environment?
Operational evaluation begins to set the limitations on what will be monitored and how the monitoring will be accomplished. This evaluation starts to tailor the damage identification process to features that are unique to the system being monitored and tries to take advantage of unique features of the damage that is to be detected.
Data acquisition, normalization and cleansing
The data acquisition portion of the SHM process involves selecting the excitation methods, the sensor types, number and locations, and the data acquisition/storage/transmittal hardware. Again, this process will be application specific. Economic considerations will play a major role in making these decisions. The intervals at which data should be collected is another consideration that must be addressed.
Because data can be measured under varying conditions, the ability to normalize the data becomes very important to the damage identification process. As it applies to SHM, data normalization is the process of separating changes in sensor reading caused by damage from those caused by varying operational and environmental conditions. One of the most common procedures is to normalize the measured responses by the measured inputs. When environmental or operational variability is an issue, the need can arise to normalize the data in some temporal fashion to facilitate the comparison of data measured at similar times of an environmental or operational cycle. Sources of variability in the data acquisition process and with the system being monitored need to be identified and minimized to the extent possible. In general, not all sources of variability can be eliminated. Therefore, it is necessary to make the appropriate measurements such that these sources can be statistically quantified. Variability can arise from changing environmental and test conditions, changes in the data reduction process, and unit-to-unit inconsistencies.
Data cleansing is the process of selectively choosing data to pass on to or reject from the feature selection process. The data cleansing process is usually based on knowledge gained by individuals directly involved with the data acquisition. As an example, an inspection of the test setup may reveal that a sensor was loosely mounted and, hence, based on the judgment of the individuals performing the measurement, this set of data or the data from that particular sensor may be selectively deleted from the feature selection process. Signal processing techniques such as filtering and re-sampling can also be thought of as data cleansing procedures.
Finally, the data acquisition, normalization, and cleansing portion of SHM process should not be static. Insight gained from the feature selection process and the statistical model development process will provide information regarding changes that can improve the data acquisition process.
Feature extraction and data compression
The area of the SHM process that receives the most attention in the technical literature is the identification of data features that allows one to distinguish between the undamaged and damaged structure. Inherent in this feature selection process is the condensation of the data. The best features for damage identification are, again, application specific.