waves along the wood grain of timber specimens. Axial damage was simu- 

 lated by cutting slots in wood blocks of various depths. Laboratory 

 tests were conducted on wood blocks with slots equal to 0, 25, 50, 75, 

 and 100% of the block thickness. The blocks were submerged in seawater. 



Measurements of the RMS (root-mean-square) of the received ultra- 

 sonic signal were taken between two fixed times. An increase in the 

 attenuation of the acoustic waves was expected with an increase in slot 

 depth because of the impedance mismatch between water and wood in the 

 axial direction. It was also expected that a portion of the wave energy 

 would be delayed in time. Test results, summarized in Table 2, show a 

 decrease in RMS amplitude with an increase in slot depth. 



However, the decrease in RMS is not linearly related to the increase 

 in damage. SWRI concluded "that the RMS of the transmitted signal between 

 two fixed points in the time window decreases with an increase in slot 

 depth, but there is no consistent relationship between slot depth and 

 RMS amplitude." 



Table 2. RMS Amplitude of Ultrasonic Signals for Various 

 Slot Depths 



[The transducers are placed on the 

 opposite side of the slots.] 



Slot Depth (%) 



Signal Amplitude 



(3.5 in.) 



Millivolt RMS 







113.8 



25 



20.02 



50 



17.5 



75 



5.0 



100 



2.8 



Impact Analysis 



The impact/resonance technique evaluated by SWRI during the contract 

 differed from the technique evaluated at NCEL. SWRI ' s impact resonance 

 tests were to determine the effect that axial holes in a wood pole had 

 on vibration damping. In the SWRI tests, two accelerometers were placed 

 on one side of a wood pole with axial holes simulating borer damage 

 drilled into one end. The damping of vibrations generated by an impact 

 located diametrically opposite each of the accelerometers was evaluated 

 from the frequency response of the accelerometer output. No difference 

 in modal frequencies between "solid" and "damaged" wood was noticed. 



The impact testing technique evaluated at NCEL is an extension of 

 the hammer sounding technique currently used by inspection divers in the 

 field. By striking a piling with a hammer and listening to the quality 

 or tone of the acoustic response, a diver can tell if there is extensive 

 internal damage. After discussions with several experts in the wood and 



