Bacterial Infected Lumber Testing

Weakening of rough cut hardwoods such, as red oak, by bacteria can lead to cracking and splitting as the result of kiln drying.  This significantly decreases the value of the end product and reduces the productivity of  valuable kiln space.  Early identification of infected and damaged lumber is desirable so that it can either be removed prior to drying, or an alternate kiln drying process can be applied to the degraded wood. 

As part of a Department of Agriculture sponsored program, IMS Inc., has been exploring the use of ultrasound to measure the bacterial degradation of green lumber prior to kiln drying.  IMS has successfully completed the initial phase of the program that demonstrated a correlation between high ultrasound attenuation and wood quality. 

Figure 1: Schematic Illustrating the  Basic Concept of the Ultrasonic Testing System

Figure 2: Test fixture for holding samples and transducers

The measurement concept, which is analogous to a sonar ranging system is illustrated in Figure 1 and 2. A high frequency pulse of sound is emitted from one sound source and received by another sound source after it has traversed the board. By measuring either the speed or the intensity, one can estimate the wood quality.

Figure 3: Average Attenuation (Signal Loss) on "Good" and "Failed"
Groups for Various Ultrasonic Modes

Figure 3 shows the average results obtained on laboratory test samples for a variety of ultrasonic wave modes evaluated in our Phase I study. These results were obtained by measuring the attenuation (loss of intensity) on each sample prior to drying. Then the samples were dried. Those that cracked or showed ring separation during the drying process and had microscopic evidence of bacterial damage were segregated into one group (“failed”). A second group, “normal” were comprised of those samples which maintained their integrity during the drying process. These results show that bacterial damage is evidenced by increased attenuation.

Figure 4: Attenuation vs. Velocity for One of the p-wave Modes. 
Dark Squares Show the Failed Components

Figure 4 looks at a subset of this same data in a different way, where the attenuation vs. velocity for all of the 30 measured points is plotted.  Those in the failed group of wood showed consistently higher attenuation. The fact that these points show considerable “horizontal” spread simply indicates that the velocity varies considerably in the infected group.

Figure 5: Concept Sketch showing Rolling Contact Fixture

In order to demonstrate the potential to measure these attenuation (signal loss) variations in a test situation which approaches that found in a saw mill, a bench scale prototype test fixture was developed.  This is shown schematically in figure 5.  The system consists of ultrasonic sources which are fixed to the hub of an aluminum wheel which is free to rotate.  Ultrasound is propagated through the wheel, into the board and then received by an identical wheel located on the opposite side of the board.