lumber NDT industry, it appeared that, potentially, a technique could be 

 developed for correlating changes in the frequency spectrum of impact- 

 generated acoustic signals with the amount of cross-sectional wood loss 

 in marine pilings. The NCEL impact testing technique is more thoroughly 

 discussed in the IMPACT TESTING section of this report. 



X-ray Tomography 



Computerized axial tomography (CAT) systems currently used in the 

 medical fields for imaging and diagnostics use a photon source and a 

 bank of receivers. With this system a two-dimensional map of the cross- 

 section of a specimen is reconstructed, using mathematical algorithms 

 and data obtained from measuring the adsorption of the gamma rays trans- 

 mitted through the material. The use of X-ray tomography in CAT scanners 

 for inspection has been documented widely in the literature. Scientific 

 Measurement Systems (SMS) of Austin, Tex., developed a CAT scanner to 

 investigate preservative penetration in wood poles. Limited laboratory 

 tests were conducted by SMS using their CAT system on sections of timber 

 pilings with simulated marine borer holes drilled in them. The piling 

 sections were submerged in water, but the holes were filled with air and 

 the CAT system was completely dry. A CAT system has never been used 

 underwater to date. 



In a simulation study under contract to NCEL, AMETEK Offshore 

 Research and Engineering Division, Santa Barbara, Calif., showed that 

 X-ray tomography is feasible for detecting water-filled cavities in wood 

 when at least a 0.5% density difference between wood and water is assumed 

 (Ref 9). AMETEK also developed a conceptual design for an underwater 

 computerized tomography inspection system (see Ref 10) . 



The work performed by SMS, AMETEK and SWRI indicated that an under- 

 water computerized axial tomography system is technically feasible. 

 However, definition of the packaging and operational requirements for a 

 CAT system for effective use in a waterfront environment is needed. 



Real Time X-ray Imaging 



The real time X-ray imaging technique, which is based on X-ray radi- 

 ography, has been used to determine the internal condition of living 

 trees (Ref 11). However, no work has been done on inspecting borer- 

 damaged wood piles, particularly under water; therefore, because of the 

 great effort necessary to develop this technique, it was determined to 

 be beyond the scope of the contract. 



Acoustic Emission 



SWRI evaluated an acoustic emission technique in the laboratory as 

 part of the timber NDT assessment contract; it is based on the belief 

 that a bending load applied to a wooden pile would produce sufficient 

 stress to break wood fibers where the piling was weakened by the tun- 

 neling activity of marine borers. Also anticipated was that a higher 

 acoustic emission (AE) count would result for an equivalent load from a 

 pile damaged by marine borers than from an undamaged pile. 



10 



