Investigations On The Structural Health Monitoring System Identified Of Cantilever Beam Based On The Intrinsic Vibrational Frequency

Nowadays,since the scale of engineering structures is becoming larger and larger,correspondingly the structures are becoming more and more complex and diverse,for instance,the aircrafts,buildings,bridges,military equipment and other objects with huge sizes.Safety has always been the most concerned issue.In facing how to avoid catastrophic accidents caused by the material structural aging,environmental impact,fatigue effect and other physical and chemical effects in complicated working environment,structural health monitoring（SHM）technology has gradually become an important research topic in the field of damage monitoring.The service life of these large sized structures such as houses,bridges,important equipment and instruments are often used as long as several decades and even hundreds of years.If the sudden accidents caused by the structural damage can not be foreseen,it is likely to lead to a deadly impact and result in a remarkable threat to people’s lifes,property’s safety and even national security.With the progresses in the applications of AI,sensor technology and Internet of things,"wearable" SHM systems,namely contact sensing systems,have also been further developed.For currently used SHM systems,there usually exist lots of drawbacks,such as low monitoring accuracies,low signal-to-noise ratios and large errors of analyzed results.The structure of cantilever beam is simple,and it is convenient to pursue the investigations after being equivalent to various structures in the experiment.For SHM systems based on the intrinsic frequency identification,the contact sensors often exert a certain impact on the testing structures.Therefore,aiming at the selection and preparation of sensing materials and the optimization of data acquisition system,the theory,experiments and simulation of cantilever beams with different cantilever materials and various damaged structures will be investigated in this thesis.The main research contents and outputs are listed as follows:The research work will design a set of SHM system based on the intrinsic frequency identification from the characteristics extracted from several kinds of damages on the cantilever.1.The principle of detecting object vibration signal using a piezoelectric sensor is introduced.The vibration equations of system with a single degree of freedom and continuous system are established,and the theoretical model of damage identification by intrinsic frequency is deduced.2.An smart sensing element with a light weight,good ductility and wide frequency range was fabricated.Poly（vinylidene fluoride）（PVDF）nanofiber membranes were prepared using the electrospinning approach,the properties of the membrane were characterized.The relationship between the piezoelectric effect and the PVDF concentration was explored.Experimental results indicated that β phase ratio in PVDF can be enhanced using the electrospinning process,which is in favor of the piezoelectric effect.For instance,a sensitivity of 200 m V/N in the force/electric test can be obtained for 15 wt% of PVDF solution.And in the cyclic test,the output is relatively more stable and the macroscopic piezoelectric characteristics are better than those with other PVDF concentrations.3.The weak charge signal generated by the cantilever vibration was amplified and input into the microprocessor for charge/voltage conversion.In the designing of hardware circuits,the A/D conversion module,filter module and boost module were reasonably designed.The generated continuous time signals were converted into discrete digital sequences,and input into PC for further processing.4.The dynamic test and analysis system is composed of PC platform,necessary intelligent sensing elements,charge conversion circuit and other hardware circuits.The Lab VIEW software was developed on the computer,which can correctly record,store,export and process the signals in discrete time sequences.5.The equivalent model of cantilever beam with single degree of freedom was established.Two dynamic test systems were built based on the piezoelectric sensor and grating sensor.The vibration was analyzed according to the vibration theory,and the characteristic parameters of damages on the cantilever are extracted.6.It was observed in our experiments that the frequency changes were 14 and 16 Hz for carbon steel and aluminum plate,respectively,when a damaged length was 120 mm.In addition,the frequency change had a monotonic relationship with the damaged length,which could be used as reference to estimate the frequency change values in between the measured points.