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Material Tensile Strength
While product manufactures provide their own material property values, each product can have variations. Hence, we tested the tensile strength of the HDPE sheets
Results
The experimentally determined elastic modulus of the 1/4 inch HDPE sheet was 408.41 MPa and the tensile strength was 25 MPa.
Buoy Housing Pod Ballasting
The buoy housing needs to float with at least 3 inches above water in order to allow for communication with an internal GPS tracker to allow for retrieval of the pod. Different ballasting weights were added to the bottom of a buoy housing pod and the corresponding height above water was recorded, and balance was observed.
Buoy Housing Ballasting #1
buoy no ballast.mp4Buoy with no ballasting weight
5lb concentrated in the center_Trim.mp4Buoy with concentrated ballasting weight
5lb self-correcting_Trim.mp4Buoy with distributed ballast weight
Conclusion of Buoy Housing Ballasting
In the first testing, it was discovered that 1/2 lb of weight was enough to ballast a buoy housing pod balanced and upright. It was also discovered that when the weight was concentrated in the center, the green buoy floated 5 inches out of the water and while it floated 4 inches when the weight was distributed on the corners of the buoy housing. Hence, in the final design, ballasting weights were distributed to the four corner of the bottom platform of the buoy pod as this placement minimized the amount of weight required to properly balance the pod. Additionally, as a results of this test, the height of the buoy housing pod was reduced to move the center of mass closer to the frame, thus reducing the amount of weight required to ballast the buoy.
Buoy Housing Ballasting #2
IMG_8030.MOVBuoy with no ballasting weight
IMG_8034.MOVBuoy with ballast weight
IMG_8035.MOVBuoy self correcting when initially upside down
Conclusion of Buoy Housing Ballasting
In this second testing, the buoy housing pod was shorter and stainless steel nuts were used as ballast weights. When the buoy housing had one nut on each corner, the buoy was 4 inches above the surface of the water. When the buoy was flipped over it quickly corrected itself and also resurfaced 4 inches above the surface of the water. When two nuts were added on each corner of the buoy, it also resurfaced the same 4 inch height, but was more robust to turbulence than the previous test. Total ballasting weight added to each pod was approximately 52.8g
GTR Presoaking + Testing
Galvanic Time Releases (GTR) corrode through an electrochemical process in salt water. GTR can be made so they corrode from hours to weeks at a time. The shortest time available were 2 hours (AA0.5 GTR) and 2.5 hours (AA1 GTR). However presoaking them can reduce the length of time they are intended to corrode at.
Presoaking of AA0.5
AA0.5 GTRs are rated for 2 hours. The AA0.5 GTRs were zip tied to HDPE sheets with approximate 5lbs of tension. Five GTRs were put in in separate containers. In 20 minute intervals, each were taken out and blow-dried for one minute.
Testing of AA0.5
Presoak Time Rating : 20 min
Actual Release: N/A GTR broke before submerging
Presoak Time Rating : 40 min
Actual Release: 26 min
Presoak Time Rating : 60 min
Actual Release: 59 min
Presoak Time Rating : 80 min
Actual Release: 70 min
Presoak Time Rating : 100 min
Actual Release: 95 min
Conclusions
Overall, presoaking method demonstrated great reduction in time to release, however as the presoaking time was increased this resulted in more error.
GTR Presoaking + Testing AA1
AA1 GTR are rated for 2.5 hours. The AA1 GTRs were zip tied to HDPE sheets. Five GTRs were put in in separate containers. In 20 minute intervals, each were taken out and blow-dried for one minute. Left in tension "dry" for over 72 hours except for 40 min presoaked GTR.
Testing of AA1
Salt water was stored in ice chest for 24 hours and the properties measured included the temperature of 72 degrees, salinity of 10 ppt, TDS 10ppt and pH 7.72.
AA1_Release.mp4
Presoak Time Rating : 50 min
Actual Release: 42 min
Presoak Time Rating : 70 min
Actual Release: 81 min
Presoak Time Rating : 90 min
Actual Release: 78 min
Presoak Time Rating : 110 min
Actual Release: 118 min
Presoak Time Rating : 130 min
Actual Release: 133 min
Conclusions
Overall, presoaking method demonstrated great reduction in time, however since these GTR were preset to corrode in 2.5 hrs unlike the AA0.5, these would most likely be soaked for longer.
Frame Testing
Frame Testing in Scripps Laboratory
Testing Goals
Test sinking operation of the frame
Test release of one buoy pod from the frame
Test release of frame from ballast weights
Results
Central column weights released from the frame and frame successfully resurfaced.
Buoy pod released after 39 mins after being presoaked for 1 hours and 10 mins when it was predicted to release after 40 minutes.
Weights from frame released after 1 hour 56 minutes with a GTR that was predicted to release in 2 hours
Frame Testing in Deerwood Pool
Testing Goals
Test that the buoy in the central column attached to frame floats above water enough to be GPS tracked.
Results
With the addition of two 6lb buoyant diver floats on the central column, the central buoy was able to rise above the water 4 inches, enough for the GPS tracker to work.
Full Unit Testing in NIWC
Testing Goals
Test full unit with five buoy pods in delayed releases
Test central weight release
Test frame recovery with chain and mat attached
Results
Test 1
30 minutes pod resurfaced after 42 minutes
45 minute pod resurfaced after 45 minutes
60 minute pod resurfaced after 60 minutes
75 minutes pod resurfaced after 68 minutes
Full unit initially floated
mat and chain weight in buoyancy calculations was before cutting into shape
Test 2
Polypropylene straps securing the weights to the central column failed, causing the GTRs connecting the pods to the frame to come in contact with seawater, thus skewing release times
Full unit failed to sink with all five buoy pods attached
Full unit successfully sunk with four pods attached
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