“…Compared to current methods of detecting bolt looseness in industries, such as the vision method (Huynh et al, 2019; Lei et al, 2018; Wang et al, 2019a) and percussion approach (Kong et al, 2018; Wang et al, 2019b), the emerging structural health monitoring (SHM) approaches (Annamdas et al, 2017; Kuok and Yuen, 2019; Li et al, 2019a; Pan et al, 2019) that employ integrated sensors (Chen et al, 2018; Liao and Chiu, 2019; Xu et al, 2019) have attracted much attention since they have capacity to implement online or real time monitoring (Feng et al, 2018; Mańka et al, 2016; Wang et al, 2018a). Among the sensors used in SHM, piezoelectric transducers have received much attention because of its wide bandwidth (Kong et al, 2017a; Lu et al, 2018; Xu et al, 2018), energy harvesting (Brenes et al, 2019; Park et al, 2008; Wang, 2013; Wang and Shen, 2019), and dual sensing and actuation functions, which enable piezoelectric transducers’ ability to generate and detecting stress waves (Dziendzikowski et al, 2018; Huo et al, 2017a; Kong et al, 2017b; Li et al, 2019b). Stress wave–enabled methods have been widely used in SHM (Fan et al, 2016; Jung and Park, 2018; Wang et al, 2009; Wang and Hao, 2014; Zhang et al, 2018).…”