Symmetrical Signal Method And Its Application On Structural Health Monitoring

Structural Health Monitoring has emerged as an important research area incivil engineering. Damage detection technique based on changes of structuraldynamic properties is the core of structural health monitoring. However, most ofthe existent methods of damage detection and location depend on the originalmessage with the damaged message to identify the damage position. Such baselinemay result in identification error due to inconsistent noise between pre-damageand after-damage.In order to overcome this disadvantage, the method of symmetrical signal ispresented and applied to identify crack damage. This new method is based onvibration test and mode analysis, so it possesses simple convenient and highlysensitive merits. It is very important that the new method decreases errors due tothe incomplete original parameters and only uses the on-site structural responsesdamage within the structure to be identified and located.Structural Health Baseline (SHB) based on the symmetrical principle isestablished and proved. In fact, symmetrical principle is widely used in the most ofstructure design, especially those formed slim bar. Assume "the same damageoccurs simultaneously on the symmetrical bars(suppose they are identical ingeometry, material, boundary conditions, etc.) of structure" is an event of smallprobability, regard these bars as beams supported by nodes, then the modalfrequency of the vibration response of the beams is equal. Otherwise, the one withlower modal frequency is probably damaged. The steel truss model is designedand fabricated, the four symmetrical diagonal bars which is subjected to a tensileforce in the truss are studied. The crack damage is located in the middle of the bar.The vibration test shows that (1) the local frequency of the symmetry bar withoutcrack is invariable, but the one with crack decreases as the crack deeper, (2) fromthe local and global frequencies under different damage extent, the globalfrequency reduces very slowly, but the local frequency reduces very fast with thedamage extent increasing. In other words, the local frequency is more sensitive todamage.