A Study Of The Acoustic Emission Monitoring And The Fracture Mechanism As Demonstrated In The Damaging Process Of Reinforced Concrete

Acoustic emission is a newly-emerged technology used in dynamic non-destructive testing. With simple operation, high sensitivity and real-time monitoring, it is widely applied in the property evaluation of materials and the structural monitoring of civil engineering. To solve the problem lying in the coupling between the metal-packaged sensor probe and the civil engineering structure, 1-3 type cement/polymer based piezoelectric composite was made with double cutting-pouring method and PZT-5 piezoelectric ceramics, cement base and cement/polymer base as raw materials, in which process, the property of smart materials was given full consideration. The 1-3 type cement/polymer based piezoelectric composite was then used as piezoelectric element to make embedded acoustic emission sensor. Detailed research and achievements made thereby are as follows:The tests and analysis on the property of 1-3 type cement/polymer based piezoelectric composite suggests that 1-3 type cement/polymer based piezoelectric composite possesses stable piezoelectric voltage constant, low mechanical quality factor and high electromechanical coupling factor, thus a remarkable fall in acoustic impedance and high compatibility with concrete.Tests and analysis on the property of acoustic emission sensor suggests that the resonant frequency and center frequency of the acoustic sensor were within the range of concrete health monitoring. Moreover, it has a relatively large frequency bandwidth and an acoustic impedance compatible with concrete, which completely meet the demand for the health monitoring of concrete. Analysis of the comparative experiment on metal-packaged sensor suggests that the acoustic emission sensor made above is better compatible with concrete. Therefore, it is suitable for health monitoring embedded within the concrete.Home-position lead-break experiment was carried out on test pieces, during which process pulse signals of acoustic emissions were collected. Risetime and duration of waveforms were read by the data cursor installed with acoustic emission collecting device so as to set the parameter time for acoustic emission. Finally, the PDT was set at 230μs, the HDT 460μs and the HLT roughly at 700μs. Through model loading experiment on test pieces, the threshold valve was set at 40 dB.The ready-made acoustic emission sensor was then embedded into the reinforced concrete beam. Three point bending, fatigue cycle, 3D positioning and verification test of Kaiser Effect were carried out on the test piece with MTS fatigue testing machine, and acoustic emission signals were collected with PCI-2 acoustic emission system. The damage and fracture mechanism of the reinforced concrete beam were studied under different stress conditions. The results suggest:Three point bending test on the reinforced concrete beam suggests as follows:(a) Three stages of acoustic emission signals during the loading process can be identified, namely the initial stage, the development stage and the disappearing stage;(b) Judged from the load-displacement curve, the deformation of reinforced concrete beam is a process of continuous macro deformation. Massive data from the acoustic emission test prove the intermittency, coupled with discontinuity and heterogeneity, of the micro deformation of the reinforced concrete beam under three point bending.(c) The existence and depth of cracks exerted significant impact on the limit load of the reinforced concrete beam. As the depths of cracks grow, the limit load of the reinforced concrete beam remarkably decreases. When it reached the limit load, the displacement reduces, suggesting the increase in fragility.3D positioning was conducted on the crack source of reinforced concrete beam underthree point bending.An analysis of the progress chart containing hit-tine-amplitude shows that the positioning well matched the features seen from the test, which suggests that acoustic emission technology can be used in the positioning of acoustic emission source.The constant-amplitude low fatigue cycle test suggests that there are three stages of acoustic emission in the whole process of fatigue cycle, including active stage, stabilizing stage and disappearing stage. The acoustic emission signals from the whole process remain uninterrupted which suggests the continuity of the damaging within the test piece.Kaiser effect exists in reinforced concrete beam, but only within a certain load range. At the lower level of the load, the accuracy of Kaiser Effect is also low due to the fact that acoustic emission signals are affected by frictional acoustic emission. When the load reaches a higher level where frictional acoustic emission remarkably reduces and acoustic emission significantly increases, the stability and accuracy of Kaiser Effect evidently rises. The fitting of regressive curve suggests that Kaiser Effect does exist during the loading process, but only on a low stress level. During the loading process of the reinforced concrete beam, Felicity Effect is closely connected to the size of the loading. When the load was low, Kaiser Effect was at work. As the load increased, Felicity ratio started to drop. The more it drew near the limit load, the faster the Felicity ratio dropped, which suggests that the growth of damaging in reinforced concrete beam increased the irreversibility of AE.