Quantitative Damage Identification for Prestressed Tendon Anchorage using PZT Interface Technique and Model Updating

Thanh-Canh Huynh1), Ngoc-Loi Dang2), Joo-Young Ryu2)

and *Jeong-Tae Kim3)

1), 2, 3) Department of Ocean Engineering, Pukyong National University, Busan, Korea *) idis@pknu.ac.kr

ABSTRACT

As the impedance monitoring technique has been successfully implemented on various types of structures, the quantitative damage detection by the impedance-based technique has become an important issue for complex structural connections. Since the impedance-based technique is not based on any particular physical models, it is very difficult to quantitatively correlate the change in impedance signatures to the change in structural properties. In this study, the severity of damage in tendon anchorage caused by the loss of tendon forces is quantitatively identified by using the PZT interface-based impedance monitoring technique. Firstly, a prototype of PZT interface is designed for the impedance monitoring of tendon anchorage. Next, impedance signatures are experimentally measured from a lab-scaled tendon-anchorage system under different prestress-force levels. Finally, damage severities of the tendon anchorage caused by the loss of prestress-force are quantitatively identified from a phase-by-phase model updating process, in which changes in the measured impedance signatures are correlated to changes in structural properties. 1. INTRODUCTION

Many researchers have studied the impedance-based damage detection in

structural systems during the past decade. The primary advantages of the impedance-based technique include (1) the technique is not model-based, so it can be easily applied to complex structures; (2) the technique uses small, cheap, lightweight, fast response, and non-instructive sensors; (3) the technique itself is very sensitive to small damage-size; and (4) the technique can be integrated with a wireless impedance sensing network to perform auto-monitoring. By taking the above advantages, the impedance-based technique has been applied for structural health monitoring of tendon anchorage systems (Liang et al. 1994, Kim et al. 2010, Nguyen and Kim 2012, Huynh

1) PhD 2) Graduate Student 3) Professor

and Kim 2014, Huynh et al. 2014, 2016). As the impedance monitoring technique has been successfully implemented on the tendon anchorage, the quantitative damage detection by the impedance-based technique has become an important issue. Most of experimental studies used the impedance technique for qualitative health monitoring (Chaudhry et al. 1996, Park G. et al. 2001, Kim et al. 2010, Park et al. 2015, Huynh and Kim 2016, Nguyen et al. 2017). In these applications, the impedance signatures have been qualitatively classified for the alert of damage occurrence due to the target structure’s complexity and the difficulty in high-frequency modelling.

Up-to-date, few researchers have reported on the quantitative health monitoring in structural joints using the impedance-based method. Ritdumrongkul et al. (2004) employed the spectral element method to model the PZT-bolted joint and applied a mathematical technique to quantitatively estimate structural parameters of the joint. In their study, the considered frequency band was relatively low, less than 2.5 kHz, as compared to the ones commonly used in reality. Lopes et al. (2000) and Min et al. (2012) combined the impedance-based technique with the ANN (artificial neural network) to estimate damage types and severities in joints. However, a large amount of the impedance records should be conducted on the target structure under both intact and damage states that could limit the application of their methods to realistic structures.

In this study, the severity of damage in tendon anchorage caused by the loss of tendon forces is quantitatively identified by using the PZT interface-based impedance monitoring and model updating technique. Firstly, a prototype of PZT interface is designed for the impedance monitoring of tendon anchorage. Next, impedance signatures are experimentally measured from a lab-scaled tendon-anchorage system under different prestress-force levels. Finally, damage severities of the tendon anchorage caused by the loss of prestress-force are quantitatively identified from a phase-by-phase model updating process, in which changes in the measured impedance signatures are correlated to changes in structural properties. 2. PZT INTERFACE-BASED IMPEDANCE MONITORING METHOD

2.1 Prototype of Mountable PZT Interface To predetermine effective frequency bands in impedance-based damage

detection, a mountable PZT interface device was developed by Huynh and Kim (2014). The interface prototype is a beam-like structure with a PZT patch at the center, see Fig. 1(a). The interface device has a flexible beam section in the middle and the fixed-fixed boundary by two outside contact bodies. The flexible section, where the PZT sensor is installed, is intentionally designed to allow flexural vibrations of the interface according to piezoelectric deformations of the PZT. The two contact bodies of the PZT interface allows it to be easily attached and detached from the target structure.

As shown in Fig. 1(b), the PZT interface is surface-mounted on the host structure to monitor the changes in its structural properties which can be induced by any structural damage. Under the excitation of the PZT sensor, the PZT interface interacts with the mother structure via the coupling vibrations. If the mother structure is damaged, the coupling dynamic responses could be altered, making the variation of the EM

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Table 1 Peak frequencies of PZT interface under various tendon-force levels

Case Tendon Force

Level (kN) Peak 1 Peak 2

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4. QUANTITATIVE DAMAGE IDENTIFICATION OF TENDON ANCHORAGE Damage severities induced by the prsestress-loss in the tendon anchorage

should be quantified to estimate the variation of structural performance with regards to its designed state. This goal is achieved by the following steps: (1) a FE model of the PZT interface-tendon anchorage system is simulated; (2) a phase-by-phase model updating process is designed to fine-tune the FE model; and (3) the model updating is performed to correlate the change in impedance signatures to the change in structural properties, both of which are induced by the variation of prestress forces.

4.1 FE Analysis of Tendon Anchorage with PZT Interface Figure 7 shows a FE model of the tested tendon anchorage with the PZT

interface which was established by using COMSOL Multiphysics. The dimension of the FE model was consistent with the actual size of the target structure. The presence of the tendon force can be equivalently treated as the contact stiffness of the bearing plate (Nguyen and Kim 2012, Huynh et al. 2015). The loss of the prestress force can be simulated by the reduction in the contact stiffness. Hence, an equivalent contact spring system kx, ky and kz was simulated for the test structure (see Fig. 7(a)). The loss factor of the contact damping is assumed as 0.02. The FE model was meshed by 1463 solid elements.

The bonding layers between the PZT patch and the interface were not considered for the simplification. The material properties of the aluminum interface were input to the FE model, as described in the previous section. The steel tendon-anchorage has Young's modulus E = 200 GPa, Poisson's ratio = 0.3, mass density = 7850 kg/m3, and damping loss factor = 0.02. The material properties of the PZT patch can be found in Huynh et al. (2015). The initial contact stiffness parameters were selected as: kx = ky = 0 and kz = 106 N/m/m2. A harmonic voltage of 1V-amplitude was simulated to the top of PZT, and the bottom one was set as the ground electrode.

Figure 8 shows real impedance signatures which were generated from the initial FE model. According to the figure, two resonant impedance peaks were identified at f1 = 18.16 kHz (Peak 1) and f2 = 36.96 kHz (Peak 2). As compared to the experimental results shown in Fig. 6, the two calculated impedance peaks were respectively consistent with the two experimental ones, though the peak frequencies and peak

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4.3 Identified Contact Stiffness of Tendon-Anchorage The model updating process was carried out for the tendon anchorage using the

impedance signatures measured from the PZT interface. At first, the model updating was performed for the tendon force T1 = 48.1 kN. For the case T1, three phases were performed to match the numerical impedance peaks (Peaks 1 and 2) to the experimental results. Figure 10 shows convergences of two peak frequencies of impedance signatures during model updating for the case T1. In Phase 1, the contact stiffness kz was first updated. Parameter kz was went up from kz = 106 N/m/m2 to kz = 4x1012 N/m/m2 while kx and ky were remained as zero. After Phase 1, the frequency error of Peak 1 was converged from 5.7 % to 0.58 % while that of Peak 2 was converged from 0.58 % to 0.33 %. In Phase 2, the contact stiffness kx and ky were updated. The contact stiffness was gradually increased from kx = ky = 0 to kx = ky = 2.6x1012 N/m/m2 while kz remained unchanged. After Phase 2, the frequency error of Peak 1 was decreased from 0.58 % to 0.12 % while that of Peak 2 remained unchanged as 0.33 %. In Phase 3, kz was again updated, and the contact stiffness was changed from kz = 4x1012 N/m/m2 to kz = 5.15x1012 N/m/m2 while kx and ky were remained unchanged. After Phase 3, the frequency error of Peak 1 was converged from 0.58 % to 0 % while that of Peak 2 was reduced from 0.33 % to 0.16 %.

From the model updating for T1 = 48.1 kN, it is observed that the two peak frequencies of the selected impedance signatures were changed in their frequencies during the model updating (see Fig. 10). The frequency differences of Peak 1 and Peak 2 were decreased as the iteration C24 was reached (see Fig. 10). The stiffness parameters were identified as the converged values when the peak frequencies of the updated FE model became identical to the experimental peak frequencies.

The same updating process was repeated for all tendon force cases T2-T4, from

Measure Experimental Local Dynamics Measure experimental impedance: Zexp

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impedance signatures and analyzing equivalent structural parameters corresponding to the damaged state. It is also noted that the relationship between the identified contact stiffness and the prestress force may also be applicable for similar cable anchorage systems. 5. CONCLUSIONS

In this study, the severity of damage in a lab-scaled tendon anchorage caused by the loss of tendon forces was quantitatively identified by using the PZT interface-based impedance monitoring technique. The change in impedance signatures was correlated to the change in tendon forces by the phase-by-phase model updating process. Then the relationship between the prestress force and the contact stiffness was identified for the tendon-anchorage system. By using the relationship, the prestress force could be estimated from the measured impedance signatures. From the findings, it is concluded that the proposed approach has the potential to quantitatively estimate the damage severity in the tendon anchorage which is caused by the tendon force-loss. ACKNOWLEDGMENT

This work was supported by Basic Science Research Program of National Research Foundation of Korea (NRF 2016R1A2B4015087). The post-doctoral researcher and graduate students were also supported by the Brain Korea 21 Plus program of Korean Government. REFERENCES Chaudhry, Z., Lalande, F., Ganino, A., and Rogers, C. (1996), "Monitoring the integrity

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