Nondestructive Evaluation of Defects in Wood Pallet Parts by Ultrasonic Scanning

Ultrasonic scanning experiments were conducted for detecting defects in wood pallet parts using rolling transducers. The characterization of defects is important for sorting and grading pallet parts, as well as for manufacturing quality and durable pallets. This paper reports the scanning results for stringers and deckboards – the two main components of pallet for red oak and yellow-poplar species. Sound and unsound knots, bark pockets, decay, splits, holes, and wane were characterized using several ultrasonic parameters. Each ultrasonic waveform collected was characterized using eight ultrasonic variables – three involving time of flight, two involving ultrasound pulse energy, one using ultrasound pulse duration, and peak frequency. The amplitude of ultrasound signals was significantly reduced by defects. The degree of dispersion of the power spectrum mostly depends on the severity and type of defects. The energy loss parameters are more sensitive to defects compared to time of flight measurements. Unsound knots, bark pockets, decay, holes, and splits are easily detectable and distinguishable using power spectrum, energy loss, and peak frequency parameters. Two-dimensional images were constructed from multi-line scanning data for each sample. The constructed images are able to show the exact location and area of the defects. Scanning properties of this prototype system suggest that an on-line system to inspect, sort, and grade wooden pallet parts is possible using rolling transducer ultrasonic inspection. Introduction Detection of defects in wood pallet parts is required to sort and grade components. High-quality pallet parts produce high-grade pallets, which ultimately extends a pallet’s life and promotes multiple use. Typically, a wood pallet consists of two parts – stringers, the structural center members that support the load and deckboards and the top and bottom members that provide dimensional stability and a platform for product placement. The most common defects in pallet parts are knots, cross grain, bark pockets, insect holes, splits, decay, shake, wane, etc. Usually, pallet parts are produced from solid wood (lumber) or from the center cant material of logs. These cants have a high percentage of defects and have less market value for other solid wood products. An economic analysis by Schmoldt et al. (1993) showed that improved pallet durability and performance imparts much greater value to carefully manufactured pallets. Description of allowable defects for minimum pallet component quality are shown in Table 1 (Anon 1994). Manual grading and sorting of pallet parts is a slow and inaccurate process, which depends on the individual skill of the grader. Moreover, the presence, location, and extent of defects in pallet parts are often difficult to ascertain accurately, making grading complicated. Detection of defects in wood by nondestructive ultrasonic methods have been investigated by many researchers with a variety of ultrasonic parameters (McDonald 1980; Patton-Mallory and DeGoot 1990; Ross et al. 1992; Fuller et al. 1995; Niemz et al. 1999; Raczkowski et al. 1999; Karsulovic et al. 2000). Most of this work was carried out using ultrasonic waveform parameters such as time of flight (TOF) or propagation velocity measurements for the detection of defects. The basis of these studies is that the defects in wood changes ultrasonic signal propagation. Simple ultrasonic propagation velocity, however, may not be suffiKabir and Araman • 203 Table 1. ~ Minimum pallet component quality.