Study On Mechanism And Method Of Bolt Connection Status Monitoring Based On Piezoelectric Active Sensing

The bolted joints is one of the most common elements in mechanical, chemical, traffic, electric power, aerospace and civil engineering. Its health is the crucial factor to the reliability and safety of the whole equipment and structure. Hence, in order to avoid the serious accident and ensure the equipment safety, monitoring the bolt connection status is significant and valuable. On this account, this dissertation presents a novel theoretical basis to the bolted joints real time monitoring and the bolt pretension measurement by integrating theory derivation and experiment research. These works include the research on the surface microscopic features of the bolted connection boundary, the study on the fractal contact model of the bolted joints interface, the investigation on the ultrasonic propagation properties between the bolted connection surfaces, the energy technique and time reverse by applying the piezoelectric active sensing method, and experiment research using the frequency tracking method. Accordingly, the main researches of this dissertation are as followed:(1) The study on contact model of the bolted joints interfaceThe effects of fractal dimension to the tradition roughness parameter is investigated. Then, this study verifies that the fractal dimension can partly represent the traditional roughness of the engineering roughness surface. After researching the elastic deformation, the elastic-plastic deformation, and fully plastic deformation of the contact point(i.e. micro-bulge), the real contact area fractal model of the bolted connection surface is set up. That is, the real contact area will go up with the increase of the total contact load. Under the same load, the smoother the surface, the bigger the real contact area. This study hence provides the theory basis to disclose the transmit mechanism of the ultrasonic energy between the bolted joints interface.(2) The investigation on the ultrasonic propagation properties between the bolted connection surfacesThis work establishes the ultrasonic transmissive wave energy model using the law of reflection and refraction. This transmit model demonstrates the relationship among energy, real contact area and pretension. In fact, when a fractal surface contacts an ideal flat surface, with the raise of the pretension and the real contact area, the received signal energy will be increased. This conclusion reveals the ultrasonic energy transmit mechanism between the bolted connections surface.(3) The bolt connection status monitoring based on energy methodThis work indicates that the response signal energy has a good relativity with the pretension torque. Hence, energy method has the capability to monitor the bolted joints condition without considering the structural parameters and the specification of the piezoelectric materials. This study discloses the influence of the bolted connection surface roughness on the signal energy. As a result, energy method is useful for the equipment foundation and steel structure contact surface which has a high roughness.(4) The bolt connection status monitoring based on the time reversal approachThis research shows that the focus signal peak(signal energy) has a good dependence with the preload. Bolt connection status can be determined through the analysis of the peek. By comparison with the response signal, the focus signal has high Signal to Noise Ratio and anti-interference capability. Thus, time reversal approach is an effective method for detection of bolt load loss.(5) The bolt connection status monitoring based on the frequency tracking methodThis research establishes the normal contact stiffness model of the bolted joints interface based on fractal theory. The relationship among the bolt pretension, connection interface normal contact stiffness and mode frequencies is then discussed. When the bolt pretension is going up, the stiffness is increased as well as mode frequencies. The experiment verifies that the frequency tracking approach is easy to record the change trend of preload corresponding to the bolted joints without considering the real bolted joints contact area, contact stiffness, mode frequencies and the specification of the piezoelectric materials. Accordingly, this technique is very convenient to apply in the bolted joints condition monitoring system.(6) The bolt pre-tightening force monitoring based on the macro controlled contact surfaces using piezoelectirc active sesningAccording to above researched several monitoring methods, the surface roughness of bolt connection interface affected the monitoring range: if the contact interface is smooth, with the increasing of the bolt axial force, the received signal will saturate, and the preload monitoring range will be limited. Thus, this study puts forward a approach of the bolt preload measuring using the macro controlled contact surface based on piezoelectric active sensing, which has a wide measurement range and a good anti-noise performance. Consequently, this will has a good potential application in the bolted joints condition monitoring.