Table 2 summarizes the calculation for the total UF to be applied to the nominal preload loss at room  temperature for the heat strap flight joints at specified intervals after re-torque. It is noted that re-torque  decreases UF− from 0.69 to 0.52 for the expected minimum preload after 5 years. Calculations are based  on a torque/preload UF of 0.36, which was determined from a separate nut factor test for this bolt/insert  flight configuration.  Table 2. Total Preload UF for Flight Joints After Re-torque  Coupon Retorque Max Preload  (lbf) % Preload Loss from CF  after 24 hrs after 5 years after 13 years  Flight-1 Y 354.7 4.9% 22.9% 25.1%  Flight-2 Y 344.5 3.1% 20.1% 22.3%  Flight-3 Y 341.4 1.6% 19.1% 21.3%  Average 3.2% 20.7% 22.9%  Sigma 1.7% 2.0% 2.0%  B-basis Prealod UF (Torque/Preload) 0.36 0.36 0.36  B-basis Preload UF (Loss) 0.14 0.14 0.16   Preload UF (Relax) 0.39 0.40 0.40  UF(+) 0.36 0.36 0.36  UF(-) 0.41 0.52 0.54  Conclusion  A joint relaxation test was completed, specific to the JWST heat strap attachment joints. The flight joints  include threaded inserts embedded in Al-1100, washers bearing directly on high purity aluminum foils, and  indium gaskets. Al-1100 is known to creep under load at room temperature and indium gaskets can  gradually flow out of a bolted interface. The results of the test and post-test processing of the data show  that the flight joints can lose ~ 25% of its initial preload (average+2-sigma) in 5 years after initial torque  application. A relative high torque/preload UF of 0.36 was also determined based on a separate test for  bolts with threaded inserts in Al-1100. Combining the results from the joint relaxation test and a separate  torque/preload test, a total uncertainty factor of 0.52 was calculated for the minimum expected preload in a  flight joint five years after initial torque-up. In other words, the minimum expected preload five years after  initial torque application is 52% of the nominal installation preload. The test and data analysis results  presented in this report has been applied to the JWST heat strap attachment joint analysis for predicting  preloads and checking thermal performance requirements.  Several observations are noted from the test results and curve fits. The largest relaxation drivers in the heat  strap joints are (1) threaded inserts embedded in Al-1100 and (2) washers bearing directly on high purity  aluminum foils. Also, pull-out strengths for inserts in Al-1100 are low due to the very low shear strength of  the parent material which further reduces the design space for the attachment joints. It is recommended  that threaded inserts in Al-1100 are avoided if possible in future joint designs. Alternate options for joints  requiring Al-1100 members for thermal performance could include nuts or nut plates. The test results also  demonstrate the benefit of re-torqueing bolts in joints that include indium gaskets. For the indium and flight  joint coupons, re-torque after initial torque-up reduced the total preload loss (5 years out) by ~50%.  Caution must be exercised when applying the results from this test to joint configurations that deviate from  the JWST heat strap joint designs (material, initial torque, indium gasket, bolt size, etc.). It is recommended  that the results in this report are not directly applied to other joints configurations, but may be used for  guidance and insight when designing joints with similar configuration.  References  1. NASA, "Requirements for Threaded Fastening Systems in Spaceflight Hardware”, NASA-STD-5020 NASA/CP—2018-219887 210