Study On Debonding Detection Approaches For Concrete-filled Steel Tube Using Stress Wave Measurement And Its Mechanism With Multi-scale Simulation

Entering the 21 st century,the construction scale of super high-rise buildings and large-scale engineering structures has been rapidly increased with the development of the national economy.The steel-concrete composite structure represented by the concrete-filled steel tubes(CFST)has been widely applied in the complex civil infrastructures.Therefore,it is urgent to develop the nondestructive testing(NDT)method for the structural health monitoring of CFST components to guarantee the mechanical performance and structural safety.In this study,a novel Piezoelectric Lead Zirconate Titanate(PZT)based NDT method is proposed,for the purpose of achieving the interfacial debonding detection in CFSTs.The detection mechanism of this approach is numerical analyzed,considering the piez oelectric effect and the converse piezoelectric effect and the PZT-CFST coupling effect.As a typical multiphase composite material,the concrete is composed of coarse and fine aggregates,mortar,interfacial transition zone(ITZ),initial defects and voids at meso-level.The stress wave excited with high-frequency signals will be easily influenced by the mesostructure variation of concrete.For establishing the multiscale modeling for the multi-physics coupling analysis,the random aggregate method(RAM)is further introduced to the geometric modeling of the concrete core to validate the effectiveness of the proposed NDT method.The feasibility of multiscale modeling techniques is comprehensively discussed from material level(static and dynamic loading)to structural member(column and bridge pile)level.The interfacial debonding can be effectively identified using the body wave-based NDT technology with PZT ceramics.However,the PZT smart aggregates(SA)have to be pre-embedded in the concrete core,which makes the practical application difficult to be extended to the existed projects.Therefore,it is necessary to further develop the NDT detection method for debonding detection with the aid of surface wave measurement and the multi-channel analysis of surface wave(MASW).The main research contents of this paper are as follows:(1)By using the PZT smart material as actuator and sensor,the detectability of interfacial debonding defects in the CFSTs is experimentally and numerically studied.Three sets of CFST specimens with different debonding defects are designed and tested.The influence of debonding defects on the PZT measurement is comparatively discussed.For discovering the detection mechanism,the PZT-CFST multi-physical coupling models are constructed.Based on the homogeneous material assumption,the propagation process of the stress wave in the cross-section of CFSTs with various geometries is compared.The numerical findings agree well with the experimental observation,which validated the feasibility of the proposed NDT method using body wave.(Chapter 2)(2)For studying the influence of mesostructure variation of the concrete core on the stress wave propagation the CFST at meso-level,the numerical concrete is modeled with the random aggregate method and is adopted to investigate the static and dynamic behavior of concrete to validate the practicability of the mesoscale modeling approach and the multiscale modeling technique from material level.The influence of aggregates' shape and position,the ITZ and other factors on the macroscopic stress-strain curves of the concrete block subjected to axial tensile and compressive loads are systematically discussed.The effects of different mesoscopic parameters on the fracture initiation and evolution process in concrete are comparatively analyzed.The similarities and differences between the numerical prediction of the rate-dependent effect of mesoscale numerical concrete samples by using the explicit and implicit dynamic analysis solver in Abaqus are investigated.Furthermore,the effects of the variation of aggregates,the dynamic increase factor(DIF)parameters in the concrete damaged plasticity model(CDP)model,the loading method and the viscosity coefficient in implicit dynamic analysis on the cra ck morphology,macro-mechanical response and DIF-strain rate relationship are discussed.(Chapter 3)(3)The structural behavior of typical steel reinforced concrete(RC)components is studied using multiscale simulation.The uniaxial constitutive subroutin es(Umat and Vumat),are compiled for 3D beam elements in Abaqus,to establish the multiscale numerical models of RC components based on the CDP model and the extended finite element method(XFEM).The generation and development of cracks until the whole process of structural failure of the RC column and the mechanical behavior of pier column-drilled shafts interface with inconsistent geometrical shapes are investigated.The differences led by varying the confined concrete constitutive models and the variation of mesoscopic parameters in predicting the macroscopic behavior of RC columns are further compared and analyzed.The cracking behavior of pier column-drilled shafts interface with inconsistent geometrical shapes is experimentally investigated and the tested specimens are modeled with multiscale modeling approaches.The effects of the transverse stirrup ratio and the overlap length of longitudinal bars and dowel bars on the fracture morphology and the macroscopic mechanical response are discussed.(Chapter 4)(4)The multiscale models for PZT-CFST multi-physics coupling analysis are established.The propagation process of the stress wave in the cross-section of CFST is analyzed,and the influence of mesostructure parameter variation on the wave field distribution and output voltage signal is systematically studied.The feasibility of PZT-based stress wave measurement on the segregation identification of concrete core is further investigated.(Chapter 5)(5)The NDT method based on the surface wave measurement for the interfacial debonding detection is proposed.CFST specimens with interfacial debonding defects at different levels are designed for experimental study.Under the excitation of continuous sinusoidal and pulse voltage,the signal amplitudes of the output voltage obtained from the PZT sensors corresponding to the CFST specimens with and without debonding defects are compared.The 3D mesoscopic models of CFST components are established to investigate the influence of aggregate distribution on wave field distribution.Moreover,the difference led by the interfacial debonding defects on the time-history propagation process of surface wave is studied.The rationality of the experimental observations is verified by the multi-physics coupling numerical analysis,and the physical mechanism of the debonding detection based on surface wave measurement is further discussed.(Chapter 6)(6)The multichannel analysis of surface wave(MASW)method is introduced to investigate the dispersion characteristics of steel-concrete composite structures with and without debonding defects.The feasibility and effectiveness of the MASW method on debonding identification for CFST members are validated with multiscale numerical simulation.The influence of debonding defects on the time-history curve of the output signal from MASW measurement is studied.For recognizing the wave attributes variation of surface wave,the fundamental mode(M0)in dispersive image calculated with the forward analysis is compared with the theoretical dispersion curve of the Rayleigh wave and Lamb wave.The homogenization approach is further introduced to the multiscale simulation on the MASW-based debonding detection for CFSTs,for accelerating the time-history analysis on propagation process of stress wave(Chapter 7)...