In the SWRI laboratory tests, three timber piles with varying amounts 

 of wood removed were submerged in water and loaded in a three-point bending 

 configuration. Good correlation between the AE counts and loss of cross 

 section was observed. A sensor submerged in water was found more sensi- 

 tive in AE detection than a sensor mounted directly on the piling. How- 

 ever, because of the high ambient noise level in most harbor environments, 

 it was questionable whether or not the acoustic emission from wood piling 

 could be detected. SWRI proposed subtracting the background noise from 

 the AE rate to obtain the AE rate alone. Yet, based upon work with the 

 passive sonic testing technique, distinguishing AE counts in a noisy 

 harbor environment is still considered unlikely. 



Dielectric Measurement 



In this technique the change in the dielectric constant of a timber 

 pile is measured and compared to the amount of wood and water-filled 

 marine borer tunnels. Damage is indicated by the change in capacitance 

 of a wood pile between two conducting plates. For the SWRI tests a flat 

 plate capacitor was built by placing two copper plates on opposing sides 

 of a wood pole. According to theory, the dielectric constant of the 

 piling should increase with an increase in volume fraction of water 

 caused by borer tunnels. Results showed a consistent increase in the 

 dielectric constant with an increase in loss of cross section. 



In these tests, a pair of copper foils, used as electrodes, must 

 create a watertight fit around the pile: excluding all water between 

 the copper foils and a saturated wood piling in the field does not ap- 

 pear feasible at this time. Therefore, NCEL decided not to pursue this 

 technique further. 



EVALUATION OF TIMBER INSPECTION TECHNIQUES 



Based upon the contract results from SWRI, three of the six orig- 

 inally recommended techniques were selected for further evaluation at 

 NCEL: X-ray tomography, indirect ultrasonic testing, and impact testing. 



TOMOGRAPHY 



The technique that holds the greatest potential for satisfying the 

 accuracy requirements (Ref 1) for the detection of hidden defects in 

 timber to this date is the CAT. Tomography, a radiographic technique, 

 differs from a conventional radiograph in that it appears to be a cross- 

 section photograph of the inspected object, taken as if the object had 

 been sliced in a plane. A conventional radiograph appears as a shadow 

 of the interior structure of the specimen. A tomograph is reconstructed 

 from a set of projections of the specimen taken through that slice plane 

 (Figure 3) . Each projection is representative of the relative density, 

 as a function of the angle through the slice plane, at each viewing posi- 

 tion. The tomograph allows detailed visualization of the interior of 

 the specimen (Figure 4) . 



11 



