Research On Nondestructive Testing Technology Of Concrete Beam Cracks Based On Piezoelectric Wave Method

Since the reform and opening up,all kinds of large infrastructure continue to emerge,and engineering accidents inevitably occur,threatening the safety of people’s lives and property.Therefore,the public pay more attention to the structural safety assessment.In recent years,in view of the complexity of the conventional detection method of concrete structure,high investment cost,unable to real-time monitoring,experts and scholars have began to study the health monitoring technology based on intelligent sensor.Intelligent structural monitoring can monitor the physical properties of structures in real time.In this paper,based on the theory of concrete monitoring based on piezoelectric wave method,numerical simulation and experimental methods were used to study the performance of piezoelectric intelligent aggregate,concrete strength and crack defects.The specific research works were as follows.Firstly,theoretical research on concrete monitoring based on piezoelectric wave method.The main performance parameters of piezoelectric ceramics(PZT)were studied to determine the types of piezoelectric ceramics used in subsequent simulation and experiment,and PZT47 was selected to make piezoelectric intelligent aggregate.Theory of stress wave monitoring concrete strength and crack was studied to determine the factors that affected the receiving effect of PZT driving,such as concrete strength,different states of concrete structure(health,crack damage).Secondly,a multi-physical field coupled finite element model of piezoelectric ceramic concrete beam was established.Matlab was used to generate the external excitation signal,and ABAQUS was used to establish the finite element model to study the influence of PZT shape and size,concrete crack depth and location on the receiving effect of PZT drive.The results showed that the relationship between the frequency and the peak value of the output signal was not simply proportional or inversely proportional,and there was an optimal excitation frequency.The output amplitude of the circular PZT with the same area was slightly better,and the thicker the thickness was,the larger the output amplitude would be.The transmission performance of the F1B2 C chip was the best.The amplitude of the received signal decreased gradually with the increase of the crack depth.The closer the distance between the through crack and the transmitting sensor was,the stronger the signal received by the receiving sensor.On the contrary,the attenuation degree increased.Thirdly,the properties of the embedded piezoelectric intelligent aggregate and the properties of piezoelectric ceramics before and after packaging were made.In this paper,the embedded piezoelectric intelligent aggregate was selected for the experiment.The change of capacitance and resistance of piezoelectric intelligent aggregate before and after encapsulation was tested.The reason for the change was that after PZT embedded in the cement mortar,due to the hydration reaction of the cement,the internal piezoelectric ceramic was squeezed and the thickness decreased,so capacitance value of the encapsulated piezoelectric intelligent aggregate increased.Fourthly,based on the piezoelectric wave method,the static monitoring test of precast concrete beam cracks was carried out to study the influence of different curing age,crack location and crack depth on the receiving effect of PZT driving.The results showed that with the increase of concrete curing age,the compressive strength increased gradually,and the harmonic response amplitude decreased continuously.The harmonic response amplitude of piezoelectric sensor decreased rapidly in the first week,and the piezoelectric amplitude changed greatly.After a week,with the cement hydration process slowing down,the signal amplitude also decreased slowly,and the piezoelectric amplitude changed slightly.The amplitude of the output signal of the concrete beam was the highest in the healthy state.With the increase of the crack depth,the attenuation degree of the signal was also increasing,and the damage index was also increasing.Meanwhile,the energy of the small wave packet was gradually decreasing with the increase of the crack depth.Fifthly,dynamic crack propagation test of concrete beam under external load was made.With the help of 50 T self-balancing structure loading reaction system,four point bending was applied to the reinforced concrete beam,and pressure was applied at two different loading points(from the middle to both sides).Three-point bending was applied to the reinforced concrete beam,and load was applied in the middle of the beam,so as to study the piezoelectric signal changes of cracks under different propagation states.The results showed that the amplitude of the dominant frequency decreased with the increase of loading level before the occurrence of micro cracks.When micro cracks appeared,the amplitude corresponding to the dominant frequency decreased to the minimum.When the micro cracks continued to expand,the corresponding amplitude of the dominant frequency increased suddenly.With the increase of the loading level,the amplitude of main frequency has no obvious change.