Pallets, as a base or load carriers for transport and storage, play an important role in ensuring the safety of goods and improving the efficiency of logistics activities (Naves et al., 2019; Gzara et al., 2020). With the rapid development of the logistics industry, traditional manual operations are gradually being replaced by automatic and mechanized ones. Emergency logistics further promote the pallet market (Sun et al., 2022). During the height of the COVID crisis, the turnover of pallets greatly increased, partly reducing the risk of infection to the personnel and improving the value of pallet operation. Pallets are also widely used in the tobacco industry and commerce (Zhang et al., 2017), medicine and health (Deck et al., 2022), cold chain transportation, and other industries because of their ability to safely circulate goods (Loisel et al., 2022). Various materials have been used to make pallets (Zacchei et al., 2022). The wooden pallet is one of the most widely used pallets. They are lightweight and recyclable and have a large load capacity, simple manufacturing process, low cost, and convenient maintenance (Alanya-Rosenbaum et al., 2021). Solid wood pallets account for approximately 90% of the pallets in the United States (Handoko et al., 2021). About 20% of the total wood consumption goes into making and packaging the pallets, and approximately 400 million wooden pallets are produced every year (García-Durañona et al., 2016). The market share of both wooden and plastic pallets is > 80% in China (Sun et al., 2022). As wooden forest products, wooden pallets will occupy a dominant position in the pallet market for a long time (Buehlmann et al., 2009).
However, continuous use of wooden pallets often results in damages, such as splitting, falling of deck boards, and loosening and bending nails (Patricio et al., 2007) owing to accumulated stress, uneven force, violent vibration (Zacchei et al., 2022) and other factors. Physical reasons for the damage to wooden pallets include the drying treatment of wood that often causes shrinkage and deformation in different directions during pallet production, leading to buckling of the pallet structure and shortening the service life. Wooden pallets are prone to expansion and deformation after absorbing moisture because of their porosity, and deformed wooden pallets easily produce stress concentrations when carrying goods. When the load exceeds the stress limit of the wood, cracks or even splits occur in the pallets (Tang et al., 2012). During their use, the main damage types of wooden pallets include cracked components, missing deck boards, and loose rivets. Among them, splitting, fracture, and missing components were about 61%, and deck board damage accounted for approximately 59% of all damages. According to the damaged parts, the damage related to the block, stringer, or stringer board accounts for approximately 21%, whereas the damage caused by fastener nails accounts for 19% of all damages (Tang et al., 2012). Of these, the deck board is most likely to be damaged. Failure to predict or detect the damage in time, it could cause serious economic losses to the carried goods. Thus, it is necessary to perform extensive work on the defect or damage detection in wooden pallets.
The histogram of connected elements (HCE) concept (Patricio et al., 2007) was presented and applied in some manufacturers' vision-based inspection systems to complete the automated inspection of used wooden pallets, thereby reducing the error rate caused by manual detection. The damage in aircraft pallets can be successfully located with an error of less than 2 cm using the damage subarea identification method and probabilistic diagnostic imaging (Liu et al., 2019). Li et al. (2020) developed a device for detecting damaged wooden pallets by combining image processing and detection technology, further improving the detection accuracy of broken wooden pallets. Schmoldt et al. (1996) designed an automated pallet inspection system and distinguished different wooden pallet defect types by measuring the ultrasonic time of flight (TOF) in a pitch-catch arrangement.
In recent years, with the application of smart material technology, research on piezoelectric-based structural health monitoring (SHM) technology has gradually developed (Junior et al., 2019). The processed piezoelectric signal can not only determine the damage location and quality grade of materials but also realize real-time health monitoring of some structural components, including strengthened concrete frames (Narayanan et al., 2020). Different approaches based on the piezoelectric effect have been proposed for defect detection. Chien-Kuo et al. (2019) studied the application of piezoceramic transducers to detect crack damage in shear-critical high-strength reinforced concrete column members and obtained the relationship between the maximum residual crack width and damage index, monitoring the health of the structure. To promptly monitor the internal damage in concrete, Kocherla et al. (2020) used embedded PZT patches to record the changes in electrical impedance (EI) under applied stress and internal damage in the concrete. The results suggested that the appearance of internal damage occurred significantly earlier than the presence of surface cracking. Piezoelectric technology can also be used to identify damage to wood or its connecting parts (Dziendzikowski et al., 2016). Han et al. (2019) used two PZTs as actuators and sensors to detect damage in four wood connection structures and explored the relationship between the degree of damage and energy change. They successfully demonstrated that PZT can be used to monitor the damage to wood connection structures. The combination of piezo-ceramic sensing technology with the vision classification algorithm could effectively evaluate timber damage conditions, and the disposal of spectrogram visualization could transform sensing signals into images of time and frequency characteristics of different timber crack conditions (Xiong et al., 2021).
In the logistics automation production line, pallet stacks made by the manipulator could be unbalanced due to damage to the deck board. Damage to the stringer can cause conveying failure or incorrect stacking and storage (Li et al., 2020). As wood is the raw material, its strength and durability can be tested in a nondestructive manner (Gao et al., 2015; Fang et al., 2017). Careful storage and material handling process, timely detection, and repair in the early damage stage can effectively promote the recycling of wooden pallets and reduce economic losses. Therefore, this study aims to determine the variation in piezoelectric eigenvalues under different degrees of damage and build the relationship between piezoelectric characteristic values and the maximum number of falls before the wooden pallet breaks to build a reference method for nondestructive testing of wooden pallet damage based on piezoelectric technology.