13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fracture damage identification of pile using element strain energy method based on sensitive modals Wenjuan Yao1,*, Xiaozhong Zhang1,2 1 Department of Civil Engineering, Shanghai University, Shanghai 200072, China 2 School of Architectural Engineering ,Quzhou University,ZheJiang,Quzhou,324000,China * Corresponding author: wenjuan@mail.shu.edu.cn Abstract A sensitive identification method for fracture damage identification of pile was established using the rate of change of natural frequency, element modal strain energy method and fracture mechanics theory. According to the structural vibration equation of damaged pile, a structural damage identification equation was deduced which containing modal damage sensitivity factor, fracture injury factor of pile and element modal strain energy change ratio. Sensitive damage identification modals were selected applying modal damage sensitivity factor, then damage warning indicators of pile were established according to the strain energy changes ratio, and the fracture damage location identification of pile was implemented using wavelet analysis techniques. On the basis of the damage location identification, the pile fracture damage size was identified using structural damage identification equation. Pile damages of Wenzhou City Stadium were identified applying the proposed method. The comparation of the results using proposed method and low strain test results verified the correctness and practicability of the proposed method. This study provided a new and efficient method for structural damage identification for pile. Keywords sensitive modals; element modal strain energy method; Fracture damage identification of pile; low strain test 1. Introduction In service process, pile foundation’s quality is influenced by factors related to the conditions of geotechnical engineering, structural design, construction quality, and the external environment, it is difficulty to found quality problems and hard to deal with the accident. For pile foundation, the characteristics of hidden and complexity, the limitations of the existing identification methods, and some other uncertainty factors make its damage detected in both theory and practice to face many problems[1-4]. How to early detect the crack damage of the pile foundation and how to develop the relevant prevention measures become a challenging topics for the structural health evaluators. A variety of pile detection technology at home and abroad have been developed to solve the pile quality problems in the construction. Chen studied the application of core drilling method for testing the damage of piles[5], but the core drilling method can not accurately judge the fracture and the development of piles foundation in service. Xiao studied the application of ultrasonic transmission method for the pile foundation defect detection[6]. Liu et al. [7]applied the low strain method to detect the defects of piles. Wu et al.[8] proposed a approach of foundation piles test which combines application of core drilling method , ultrasonic transmission method and low strain integrity testing. Actually the results demonstrated the reliability of the technical advantages and the measures. The above studies are directed in the pile foundation detection of bridges or buildings under construction. Pile foundation construction of bridges or buildings, when the need to determine its integrity, due to the limitations of the superstructure, the usual pile testing methods are not fully applicable. Johnson et al. [9] and Luo et al.[10]gradually proposed the low strain dual velocity method, super shock, equality seismic detection methods to detect damage of the pile foundation in service. In 1995, Japanese scientists explore the application hole camera technology roughly detect the foundation piles cracking and damage after southern Hyogo earthquake, but they did not achieve good results for various reasons[11]. Yuan et al. [12] determined the integrity of the bridge piles in service, applying combined horizontal impact load displacement signal collected under static
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