Piezoelectric Smart Aggregate Based Concrete Complex Dynamic Stress Monitoring And Fatigue Damage Analysis

In civil engineering,many large-scale and complex concrete structures are not only subjected to static loads,but also long-term dynamic complex loads from multiple directions.Such stress state will cause the performance of concrete members or structures to degrade,even lead to sudden failure,resulting in serious losses.Although there have been a lot of tests and theoretical studies on concrete performance under fatigue load at home and abroad,there is still a lack of direct and effective method for concrete structures to monitor such dynamic and complex stress state as well as fatigue damage evolution process.Therefore,the following research work has been carried out in this paper:(1)A piezoceramic-based,three-dimensional normal stress sensor,also called piezoelectric smart aggregate,is proposed.It can be embedded in concrete and adapted to multi-axial dynamic complex stress environment.The cross-sensitivity matrix of the sensor is obtained by finite element analysis and calibration test respectively.By observing the matrix elements,it is found that the cross-interference between different monitoring directions of the sensor is very small and can be ignored.Each monitoring direction is equivalent to working independently,and they have a consistent sensitivity.In the experiment,the sensor has a fast response and a good linear relationship with the external load.Moreover,there is only 8.8% difference between the sensitivity obtained by finite element analysis and the test results,indicating the reliability of the results.(2)Through the mesoscopic finite element model,the initiation and extension process of micro-cracks within concrete under uniaxial compression were observed.The vertical and transverse stresses of the aggregates in the whole process were extracted.The variation degree of the stress distribution is quantitatively obtained by using the correlation coefficients between the stress distribution data at different stress state and the initial stress state.The results show that the vertical and transverse correlation coefficients will decrease with the increase of concrete internal damage.The faster the damage develops,the faster the correlation coefficient will decrease.Taking into account the similarity of the mechanism of concrete failure in the process of static compression and fatigue compression(both of which are caused by the accumulated micro-cracks,and the final failure pattern is similar),the damage analysis method is also applied to the fatigue damage monitoring test.(3)The designed piezoelectric smart aggregate is embedded in the concrete specimen for uniaxial constant amplitude fatigue test.The experimental results show that it is possible to establish the relationship between the output voltage of the sensor and the external load.This way,the complex loading process experienced by the actual structure can be recorded,which would be of great benefit to the assessment of the healthy state of the structure.The change of the vertical peak voltages of sensors in the fatigue process reflects the stress redistribution phenomenon in concrete.It is found that the correlation coefficient of the vertical peak voltage presents a three-stage decline with the increasing of cycle times,which provides a new characteristic parameter for evaluating fatigue damage in the monitoring of concrete structures.Under uniaxial fatigue,although the correlation coefficient of the transverse peak voltage of the sensor is decreasing continuously,there is no three-stage pattern.Multi-axial fatigue test is needed to study the effectiveness of this three-dimensional normal stress sensor in monitoring concrete multi-axial fatigue damage.