Background

1. Background • The REXIS team wanted to test out its prototype radiation cover deployment mechanism using modified COTS fasteners and Frangibolts on loan in order to gain schedule margin and refine the design before TiNi delivers actuators and custom bolts to us in mid-June • We have broken two Frangibolt actuators in this process and are attempting to determine how they failed and how to modify our test plans

2. Test Conditions • We decided to make surrogates out of 316 stainless steel • Lower CTE than aluminum and closer to that of titanium • Lower thermal conductivity than aluminum and closer to that of titanium • Sized notch depth such that bolt would break at 500lbf—this was a mistake on our part confusing the specs of the different models • We used COTS titanium washers from McMaster since SDL did not provide enough TiNi washers for our purposes • Wider than TiNi washers, but same thickness • Reference design and solid model provided to TiNi by MIT uses these washers • We also used an MS21043 lock nut as recommended by TiNi • For all tests bolts were torqued to ~4in-lbf, consistent with TiNi specs for SDL, with a standard torque wrench fitted with a hex head • Took up the running torque by hand, then tightened to final torque with the torque wrench for all tests • Although not best practices, we had difficulty finding a nut-driving bit with enough length to torque the nut instead of the bolt

3. Comparison of Material Properties • Notes: • All values are representative for the material and taken from Matweb • Aluminum COTS bolts from McMaster are not rated for any properties • 316 COTS bolts from McMaster are rated for UTS >483MPa and Rockwell B hardness >70

4. First Set of Tests • Used a loaned #4 Frangibolt FD04 from Utah State’s Space Dynamics Laboratory • Older model Frangibolt • Only had a primary heater, no secondary • Rated bolt breaking strength is 400lbf, not 500lbf • Operated at 9W (9V @ 1A) • Ran one test before realizing mounting surface for Frangibolt was not flat, then milled the surface down and tried again several more times • No tests were successful with the surrogate bolts, which in hindsight is to be expected, although we did hear some snapping/popping noises during one test • All tests were carried out for 60-90s • One test with an SDL-provided TiNi bolt was successful, which verifies the mechanism concept and design • This actuation took 60s • Is it possible that the actuator may have broken in these configurations? This surface was not flat

5. Reset Issue • The SDL actuator was always reset in the configuration shown at the top, except for once, with no issues • We used the ¼” wrench shown, not a torque wrench, in every scenario • In the bottom configuration the actuator failed in the reset jig and completely fractured • Was this due to overtorquing or is it possible the previous tests may have damaged the actuator?

6. Second Set of Tests • We replaced the broken actuator with the newest FD04 model loaned from another project in our lab • We first reset the actuator in the manner shown at the top of the previous slide • We ran a test using a stainless bolt designed to fail at 400lbf; applied 15W at 9V to the primary heater for just over 30s and the bolt did not fail, then found that the actuator had only gone through half its rated stroke (up to 0.520” instead of 0.540”) • Snapping/popping was also heard during this test

7. Second Set of Tests • After this test we reset as normal and the actuator fractured in the reset jig at a much lower torque than we had come to expect from previous resets • Is there anything from this description that could have permanently damaged the actuator?

8. Inspection of Broken Frangibolts • We had a materials expert at MIT Lincoln Labs inspect the newer model FD04 and discussed possible failure modes with him • Newer-model Frangibolt bore was elliptical, not circular at the time of inspection—this could have been due to differential heating after the insulating jacket ripped • Popping/snapping sounds during actuation may have been a phase or shape change in the SMA • Initial crack that propagated under compression could have come from a scratch already present in the bore or from a burr on the bolt during bolt installation scratching the bore • Over-compression of Frangibolt in reset fixture could have prevented actuator from reaching full stroke on actuation

9. Inspection of Broken Frangibolts

10. Storage of Newer-Model Frangibolts • Frangibolts received at MIT late Dec. 2012 • Transported in original packaging and pelican case to and from Albuquerque 1/7/13 and 1/12/13, respectively • Frangibolts were taken out of the Pelican case but not the packaging • Frangibolts not opened or used until 4/18/13

11. Changes in Test Procedures • We’ve already recognized the need for several changes in our procedures • Use of a torque wrench with the reset fixture as recommended by the reset ICD • Shortening of surrogate bolts for use with proper torque wrench and proper installation procedure • Lubrication of parts to control running torque • Logging of data found in conformance tests • Actuator length before test/after reset • Actuator length after test/before reset • Actuation time • Keeping notes for all tests, whether they are experimental ad-hoc tests or formal demonstrations • Other suggestions would be very welcome