Abstract
In the prefabricated concrete structures with wet connection,the interface of the pre-cast layer and the cast-in-place layer is a relative weak part for environmental and load resistance[1].The fatigue damage evolution of the new-old concrete interface dominates the structure damage under cyclic load.It's significant to use an appropriate damage indicator to quantify the degree of the fatigue damage considering the interfacial degradation.There have been many experimental studies carried out to characterize fatigue damage by using traditional indicators such as stiffness,deflection,energy and strain[2-4].However,there are still many deficiencies in using the traditional fatigue indicators to characterize the fatigue damage of the composite members.For example,traditional non-destructive testing has difficulty to detect composite member damage in its early stage due to unobvious changes in microscopic and macroscopic physical parameters[5].The accuracy to quantify damage,especially for interfacial damage and the information available from the curve of macro-level traditional indicators such as stiffness is limited.In this study,a magnetic signal detection method based on piezomagnetic effect is proposed as a new damage indicator to characterize the fatigue damage of prefabricated composite beams.The mechanism of piezomagnetism is that the microplasticization process can cause slip dislocation inside the ferromagnetics under cyclic loading,which changes the texture,voids and flaws of the material.The magnetic domain structure of the material can be transformed,which appears as magnetization state changing[6].Relevant experimental studies have proved that magnetic signal can indicate the fatigue damage process of ferromagnetic materials[7,8].However,there is lack of relevant experiments using magnetic signal indicators to measure the development of interfacial damage of composite members.
Static tests and fatigue tests were carried out on 15 T-shaped prefabricated concrete composite beams with 3 different sections designed according to the code of JGJ 1—2014[9].As shown in Figure 1,PCB indicates the prefabricated composite beam;the following letter is K or S designating the shear key or the pre-cast slab.The cyclic load level is divided into 50%,60%,70%,and 80% of the static ultimate strength.
Figure 1 Cross section of composite beams(unit:mm)
For failure mode,two kinds of debonding types:full debonding and partial debonding were observed according to the level of the interfacial debonding,which occurred at new-old concrete interface or beam-slab interface.The damage development of the composite members is characterized by the mid-span deflection,stiffness,energy consumption,reinforcement strain and magnetic signal.The traditional damage indicators were compared with magnetic signal as a new indicator.
From time-varying curves it can be found that the shape of the deflection and the strain curve exhibits a sinusoidal waveform,which has little change during the loading process,while as shown in Figure 2,magnetic signals change obviously during cyclic loading and can reveal more information about the fatigue properties of the composite members.
Figure 2 Time-varying curves of magnetic signal in different fatigue stages
The damage degree is quantified by using four indicators of stif fness,deflection,magnetic signal and energy consumption.Based on the Lemaitre damage formula,there has:
where X0 is the initial value of each indicator in the first cycle,XN is the processing value and 
is the value at the designated end of damage evolution.Fatigue damage curves were given using damage indicators of stiffness,deflection,magnetic signal and energy consumption.Figure 3 contains damage curves of PCBSK-50 using different indicators shown as an example.The dif ferences among several indicators,as well as the sensitivity of parameters such as interfacial damage degree,load level,and setting of shear key were compared.
Figure 3 Damage curves of PCBS-K-50 using different indicators
The results show that the magnetic signal can reflect the fatigue damage process of composite members.Setting the shear key at the interface has a great effect on the inhibition of interfacial debonding,of which the performance improvement can be shown more obviously using the magnetic signal.For prefabricated composite members,the interface debonding damage has a great contribution to the overall damage of the composite members,which can influence the whole composite member performance and should be paid more attention.Compared with the traditional indicators,the fatigue damage detection based on the piezomagnetic ef fect is more sensitive to interface fatigue damage since the piezomagnetic effect and the fatigue damage are at the same mesoscopic level[8,10].However,the magnetic signal is greatly affected by the external magnetic field environment and is too sensitive to the magnetic field change[11,12].The signal stability problem needs to be solved by subsequent research.