Summary of Impact Test Results 



Like the ultrasonic tests, the impact tests were evaluated on accu- 

 racy, range of damage detection, repeatability of readings taken, and 

 operational simplicity. 



The accuracy associated with the impact tests is ±50%. Frequency 

 analysis of the impact response can be used only to detect internal 

 damage between 50 and 100% of the total cross-sectional area. The range 

 of detection of external damage, however, is not limited to severe 

 damage; a decrease in amplitude and frequency was observed for external 

 damage constituting only a 15% cross-sectional area loss. 



The impact tests were not reproducible. The variance in frequency 

 shifts between repeated tests was extremely high (300 to 700 Hertz) . 



From the standpoint of assessing the operational simplicity of the 

 impact inspection technique, the true test was actual use in Port Hueneme 

 harbor. Although the impact procedure was relatively simple to perform, 

 diver technique greatly influenced the signal generated and received. 

 This adversely affected the reliability and consistency of the results. 



CONCLUSIONS AND RECOMMENDATIONS 



Three timber inspection techniques - computerized axial tomography, 

 ultrasonic, and impact - have been presented. The following conclusions 

 and recommendations are made: 



1. At this time, only the conceptual design of an underwater computerized 

 axial tomography inspection system can meet the accuracy requirements 

 (Ref 1) for detection of internal defects in wood. Because of the devel- 

 opmental nature of underwater tomography and cost, further work on devel- 

 opment of an underwater CAT system has been postponed. 



2. A decrease in standard deviation of the received ultrasonic signal 

 with an increase in percent cross-sectional area loss has been observed 

 for a few of the laboratory and harbor test piles, but a consistent cor- 

 relation has not been found. A shift in standard deviation reading from 

 the average reading taken on a solid section for each pile could be an 

 indication of internal or external damage, a knot, or a change in grain 

 orientation. Therefore, a shift in ultrasonic reading indicates a poten- 

 tially damaged region but must be confirmed with a visual inspection or 

 other NDT technique. 



3. The ultrasonic test results indicate the low frequency ultrasonic 

 system cannot meet the accuracy requirements in Reference 1. However, 

 it would be advantageous to develop an ultrasonic system similar to the 

 NCEL system to increase the confidence and reliability of inspection 

 data compared to a visual inspection. In addition, the probability of 

 detecting internal damage is greater during an ultrasonic inspection 

 than during a visual inspection. 



4. Frequency analysis of the acoustic response after impact can only be 

 used to detect excessive internal damage (50% or more) . Many factors 

 involved in the impact testing procedure affect the signal received: 



50 



