For a pipeline structure, the local failure due to connection damages might lead to the local force unbalance of the connection surface or body, resulting in the severe failure of the whole pipeline structure. However, there is the lack of effective method for monitoring the connection damage because of the complicated connecting forms and damage types such as defects, fatigue, crack, and buckling etc. In the paper, based on the experiment research, an equivalent relationship between the signal energy transmission and the connection damage such as bolt loosening is proposed by dealing the pipeline connection with a black box, to make a quantitative monitoring and evaluation of the connection damages. Meanwhile, piezoelectric ceramic patches used as transducers will be applied to generate detection signals, to set up the connection monitoring model based on the signal energy method and conduct relevant experiment for validation. The relationship between the test signal energy and the bolt torque which represents the degree of bolt loosening can be used for damage identification of pipeline structure connection. The proposed method in the paper will realize the efficient and real time intelligent monitoring and damage evaluation for pipeline structure connection.
In pipeline structures, flange connections with bolts are widely used because they have advantages of good sealing performance, convenient installation and construction. However, after a long-term use, pipeline structures might generate corrosion, crack, fatigue and other damages. A defective pipeline without a long-term detection or timely maintenances will eventually lead to the pipeline failure, resulting in leakage accidents. Among them, leakage accidents due to the pipeline connection problem occupy a high proportion. For a flange joint of a pipeline, the contact issue between the planes is closely related to the bolt working performance. Actually, in the structure, bolt loosening due to the above-mentioned impacts is the key influencing factor on the connection failure. Since the influence of the surface roughness of the steel plate and the rubber packing, the contact issue between the planes has the strong nonlinear characteristics named touch nonlinearity and makes the damage identification complicated for using PZT-based ultrasonic wave method because of altering the signal propagating path. In the study of the contact mechanism between planes, the contact area in a realistic structure is usually less than the theoretical one. The rough characteristics between the planes' interface also make the contact of interfaces very complex. Therefore, so far, there is not a very effective way to monitor the flange connection of pipeline structures. Most of the bolted flange connection failures are caused by failure of bolt load (Zhu, 2012), and a survey in 2008 showed that this factor could reach 83%. It is of importance and meaningful to monitor the bolted flange connection of pipeline structures. Wu, Liu and He (2010) realized the health monitoring of pipe flange structure based on electromechanical impedance technology, through the study on the influence of bolt torque loads on the impedance of pipe flange structure, and the relationship between the number of loose bolts and the impedance variation of pipe flange structure. Zhu et al. (2014) found that the bolt loose will cause the stiffness of the pipeline structure to change, and the change of the stiffness can be reflected by the change of the natural frequency of the pipeline. The method can be used to monitor the bolt loosening of the pipe structure connected by flanges.