Abstract
L-shaped bolt lap joints are commonly used in aerospace and civil structures. However, bolt joints are frequently subjected to loosening, and this has a significant effect on the safety and reliability of these structures. Therefore, bolt preload monitoring is very important, especially at the early stage of loosening. In this paper, a virtual time reversal guided wave method is presented to monitor preload of bolted L-shaped lap joints accurately and simply. In this method, a referenced reemitting signal (RRS) is extracted from the bolted structure in fully tightened condition. Then the RRS is utilized as the excitation signal for the bolted structure in loosening states, and the normalized peak amplitude of refocused wave packet is used as the tightness index (TIA). The proposed method is experimentally validated by L-shaped bolt joints with single and multiple bolts. Moreover, the selections of guided wave frequency and tightness index are also discussed. The results demonstrate that the relationship between TIA and bolt preload is linear. The detection sensitivity is improved significantly compared with time reversal (TR) method, particularly when bolt loosening is at its embryo stage. The results also show that TR method is an effective method for detection of the number of loosening bolts.
Highlights
Bolted joints are widely used in engineering structures such as aerospace and civil structures because of their ease of assembly and high load carrying capacity
The referenced reemitting signal (RRS) is utilized as the excitation signal for the bolted structure in loosening states, and the normalized peak amplitude of the received focus wave packet is used as the tightness index (TIA )
The relationship between bolt torque and the peak amplitude of refocused wave packet measured by the virtual time reversal (VTR) method is almost linear
Summary
Bolted joints are widely used in engineering structures such as aerospace and civil structures because of their ease of assembly and high load carrying capacity. Bolts are frequently subjected to loosening due to inappropriate preloads during installation, time varying external loads during service, or other environment factors [1]. Bolts loosening may lead to the failure of entire structure. It is critical to monitor bolt preload at an early stage to ensure the safety and reliability of structures [2]. Structural heath monitoring (SHM) techniques enable effective monitoring of bolt preload [3]. Impedance and guided wave-based damage detection techniques have been widely used for SHM [4]. Electromechanical impedance (EMI) is sensitive to minor changes in the bolt preload
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