Accelerometer Description . The equipment used for the accelerometer 

 impact tests was exactly the same as described for the hydrophone impact 

 tests except an accelerometer was used in place of the hydrophone (Fig- 

 ure 22). A quartz accelerometer was supplied with the instrumented PCB 

 hammer. This accelerometer offers exceptionally good low frequency re- 

 sponse. The accelerometer parameters are: 



• Range ±500g 



• Resolution of 0.02g 



• Sensitivity of 10 mV/g 



• Resonant frequency 45 kHz 



• Frequency range of 1 to 5000 Hz 

 Laboratory Procedure 



Laboratory impact testing was conducted on the same test piles 

 described earlier in the NCEL laboratory ultrasonic tests. The initial 

 preparations made were also the same as those made before ultrasonic 

 testing began. 



In the general procedure for the impact tests, the timber pile was 

 hit in the desired, previously marked location, and the acoustic response 

 of the timber pile was picked up with the hydrophone or accelerometer at 

 a set distance away from the impact position. Both the hydrophone and 

 accelerometer were hand-held. The hydrophone was held approximately 

 2 inches away from the timber pile. The accelerometer was held in inti- 

 mate contact with the timber pile to pick up the impact-generated signal. 



One technician and one engineer performed the impact tests. The 

 technician struck the timber pile with the PCB hammer, held the hydro- 

 phone approximately 2 inches from the pile or held the accelerometer 

 directly against the pile in the desired location, and kept the test 

 pile from moving during the test. The test pile lay horizontally in the 

 saltwater test tank. 



Both the impact signal of the hammer and the received outgoing sig- 

 nal were sent directly to the programmable oscilloscope through connecting 

 cables. The digitized waveforms for both the impact and received signals 

 were displayed on the oscilloscope CRT view screen. A schematic of the 

 impact test equipment configuration is shown in Figure 23. Digitized 

 waveforms of both the input and output signals were stored on a magnetic 

 tape for later analysis. 



The distance between the impact position and the placement of the 

 hydrophone or accelerometer depended on the test pile being evaluated. 

 The procedure was to hit the test pile at every marked line and to pick 

 up the received signal at the succeeding line. The length of the stan- 

 dard pile was marked with 20 lines spaced 3 inches apart; the other test 

 piles (5x, 4x1, 8335D, 8323E) were marked lengthwise into 6-inch incre- 

 ments. Thus, the localized axial response between the marked lines was 

 received. Each impact signal and received signal was repeated to verify 

 the reproducibility of the test results. 



39 



