Axiom I: All materials have inherent flaws or defects.Several fundamental axioms, or general principles, have emerged: 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. Two techniques in the field of SHM are wave propagation based techniques and vibration based techniques. In rotating machinery, vibration monitoring has been used for decades as a performance evaluation technique. 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. Qualitative and non-continuous methods have long been used to evaluate structures for their capacity to serve their intended purpose. This process may involve induced-damage testing, fatigue testing, corrosion growth, or temperature cycling to accumulate certain types of damage in an accelerated fashion.
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. 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. 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. 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. 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. SHM is intended to provide reliable information regarding the integrity of the structure in near real time. After extreme events, such as earthquakes or blast loading, SHM is used for rapid condition screening. Long term SHM outputs periodically updated information regarding the ability of the structure to continue performing its intended function. In an operational environment, structures degrade with age and use. 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. For the journal, see Structural Health Monitoring.
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