Surface and Subsurface Quality Evaluation of Engineered Wood Products by Ultrasonic Means

Introduction

Nondestructive evaluation techniques are somewhat limited for wood and wood products. The application of ultrasonic technologies to these materials has been substantially aided by developments in assessing the integrity of Fiber-Reinforced Plastics (FRP), which have many characteristics similar to those of wood. FRP, like wood, requires special techniques to detect internal flaws, in contrast to metals where X-ray techniques are particularly useful. Two of the key ultrasonic technologies used for wood-based materials are Acoustic Emission (AE) and Acousto-Ultrasonics (AU). Each has a reasonable history of use in laboratory research and in the past several decades there has been progress toward industrial use. There is a substantial trend from solid wood to engineered wood as structural material for both residential and non-residential structures. Figure 1 provides an excellent overview of the maturity of engineered wood products. For example, in 1980, North American Oriented Strandboard (OSB) panel production was 0.7 Mm3. By 1990, this figure was 7.0 Mm3 and by the end of 2005, it had grown to 22.1 Mm3 (TECO, 2011). I-joists (most with OSB Webs) have a 40% market share of wood floors in the United States and Canada, with a 2005 production of 0.38 Gm. As the second generation of structural composite lumber products, Oriented Strand Lumber (OSL) has been recognized by the International Code Council and will likely become a beam and header product that will take share away from many of the products already in the market, including solid sawn lumber, glulam, and Laminated Veneer Lumber (LVL). Twenty-five years ago, engineered wood (mostly glulam, I-joists, and LVL) was less than 1% of the combined total of structural lumber and engineered wood. By 1990, engineered wood had grown to over 1.6 Mm3, representing 1.7% of structural wood products. In 2005, engineered wood was estimated to be nearly 6%, over 7 Mm3. It has been projected that by 2100, solid wood will be virtually absent from structural wood materials. The transition to structural engineered composites poses a challenge for quality assessment and for determining mechanical properties. These materials obtain their strength and stiffness by randomly distributing defects (largely the ends of the shorter raw material constituents), by the uniform distribution of the appropriate level of adhesive, and consistency of in curing and bonding. Because many of these materials are or will be made in continuous processes, the critical need is to assure consistency of desired material properties in the products using nondestructive evaluation techniques that can identify critical defects (e.g., voids, non-uniform distributions of adhesive, etc.) and monitor consistency of material properties such as density. For example, the typical quality control procedure when producing OSB is to sample two panels in a 12-h shift, which is about 0.03% of the total number of boards produced. The visual grading process that has been historically used for solid wood is no longer useful for these reconstituted materials, and the reliance is on destructive tests of selected samples. Because of the severe environments where large volumes of these materials are manufactured, there are currently no reliable nondestructive testing systems capable of providing on-line sensing for purpose of quality assurance.

Figure 1.

Transition of solid wood to structural composites. SCL is structural composite lumber.

The key challenges of ultrasonic nondestructive evaluation of the developing composite structural wood materials is the need to cope with the inherent density gradients and elastic properties, and to be able to interrogate using non-contacting techniques. The major objective of the chapter is to provide an overview of past developments and a review of more advanced ultrasonic techniques that can be applied to advanced wood products.