REXIS/ TiNi Telecon

REXIS/TiNiTelecon Harrison Bralower Rebecca Masterson 5/30/13

Agenda • Discussion of previous Frangibolt failures • Root causes of failure • Surrogate fasteners, reset procedures • Joint design • Check-in on existing procurement • TiNi washers • Review of proposed testing procedures • Additional Questions • Shock data for #4 Frangibolt • Scaling of acceptance test time with temperature • Lubrication of fasteners

Background • The REXIS team wanted to test out its prototype radiation cover deployment mechanism using modified 316 stainless steel 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 broke two Frangibolt actuators in this process and believe we’ve determined the causes that led to failure

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 This surface was not flat

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

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 • 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

Believed Causes of Actuator Failure • After discussion with TiNi and material scientists we have identified several potential root causes of the Frangibolt failure • Uncontrolled reset of the actuator may have damaged the memory state of the SMA • 316 stainless steel is more ductile and work hardens compared to titanium, so these surrogates would require more stroke than a titanium bolt for the same breaking force • Overloading the actuator while hot (austenitic crystalline structure) also likely caused heterogeneities in the SMA when it cools down to a different crystalline structure (martensitic) • Future uses of the Frangibolt will follow the reset ICD and will only use TiNi-provided fasteners to mitigate these risks

Believed Causes of Actuator Failure • There was concern from TiNi that the joint design may prevent successful operation or damage the Frangibolt • We did successfully actuate this prototype with the older model Frangibolt • Are there tolerances on dimensions or surface finishes that we should know about?

Existing PO & Washers • What is the estimated delivery date on the existing PO? • CAD sent to TiNi to assist with design and procurement used models of non-TiNititanium washers • Previous tests with Frangibolts, which included a successful deployment with a TiNi fastener, also used McMaster washers • Is there a specific need for custom TiNi washers? Are we running a risk of damaging the Frangibolt without these? • If yes, is there still time to add washers to the current procurement?

Proposed Testing Setup • Test bench shown at right will be used to test proposed mechanism • Intend to test mechanism at room temperature, 0C, and -20C with three actuations each • Test goals • Determine dynamic parameters (inertia, friction, spring constant) of this mechanism • Characterize shock levels from Frangibolt actuation and cover rebound • Demonstrate mechanism functionality over a range of expected on-orbit temperatures (-20C-0C; testing at room temp is more to verify general function) • Current instrumentation can measure: • Cover position (shaft encoder) • Cover acceleration (accelerometer) • Temperatures at various locations • Would like to characterize Frangibolt shock as well, but we currently don’t have proper sensors for that Shaft encoder Backstop Cover prototype Frangibolt Vacuum chamber (transparent) Thermal isolation Mounting plate Copper cold sink (attached to LN2 sink, not shown)

Proposed Testing Procedures • Measure Frangibolt length with calipers and record • Measure Frangibolt primary and secondary heater resistances • Install Frangibolt and fastener (see next slide) • Cool mechanism to test temperature • Apply 9V to Frangibolt, cut power after successful actuation or allotted time is reached (depends on test temperature) • Remove Frangibolt from test article • Place lower jaw of TiNi-provided reset fixture into a vice, making sure not to clamp down on the fixture’s hinge • Place Frangibolt into reset fixture and torque fixture to 20±1lbf • Measure Frangibolt length with calipers and record

Proposed Fastener Installation Procedure • Current plan is to provide a jig to hold down the cover, freeing both hands for reset • Installation steps • Lubricate fastener (if advisable) • Install fastener, washers, Frangibolt, and nut into joint by hand • Take up running torque • Thoughts on how to do this? • Hold fastener with tool while torqueing nut with a break-over torque wrench to 50in-ozf (or other torque specified in fastener ICD) Fixture will attach hereto hold down the cover during reset One hand used to install nut to proper torque with break-over torque wrench One hand used to hold fastener on flats with a custom tool http://www.newark.com/productimages/nio/standard/4417772.jpg

Proposed Testing Procedures • The above is a sample table of data we intend to collect as part of our tests to ensure proper handling of the Frangibolt • Is this enough data to ensure that the Frangibolt isn’t damaged?

Shock Data • Concerned about shock damage to the instrument • Frangibolt is next to sensitive imaging detectors • Is shock data available for the #4 actuator? • Time-series or shock response spectra Bonded jointFastened jointShock path

Scaling of Acceptance Time w/ Temp • Acceptance test pass time as a function of temperature fits extremely well to a linear curve (t(T) = -.55T + 52.67) • REXIS will operate the Frangibolt on-orbit between -20C and 0C, which using this fit places the pass time for tests at these temperatures at 64s and 53s, respectively • Is this an appropriate way to scale the time with temperature?

Lubrication of Fasteners • We’ve received advice that it’s a good idea to lubricate TiNi-provided fasteners in order to both control running torque and provide a known surface finish in order to minimize uncertainty in the torque/preload relationship • Does TiNi recommend a particular lubricant? • We’ve been advised to use Braycote 600EF or 601EF • Would using such a lubricant change the installation torque specified on the ICD for our fastener? • Relatedly, how does TiNi measure running torque in test setups?

Backup Slides

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

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

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

Inspection of Broken Frangibolts

REXIS/ TiNi Telecon