Accurately predicting fastener preload relaxation in the James Webb Space Telescope (JWST) heat strap assemblies is essential to insure adequate thermal performance during its mission lifecycle. The mechanisms for preload relaxation in the strap joints include Al-1100 material creep, indium gasket flow-out, and embedment of the joint faying surfaces. This report documents the results from a bolted joint relaxation test, including analysis and curve fitting of the test data for predicting preloads five years after initial torque application. The report also includes the derivation of a preload uncertainty factor enveloping both torque/preload application scatter and expected preload relaxation at the end of mission life

Bartoszyk, Andrew

Davis, Jody

Hemminger, Edgar

Hylan, Jason

Krom, Jason

Walsh, Thomas

Williams Walsh, Patrick

NASA Technical Reports Server

Investigation of Bolt Preload Relaxation
for JWST Thermal Heat Strap Assembly Joints
with Aluminum-1100 and Indium Gaskets
Andrew Bartoszyk (NASA Goddard Space Flight Center)
Thomas Walsh (Stinger Ghaffarian Technologies)
Jody Davis (NASA Goddard Space Flight Center)
Jason Krom (Sigma Space Corporation)
Patrick Williams Walsh (Stinger Ghaffarian Technologies)
Jason Hylan (NASA Goddard Space Flight Center)
Edgar Hemminger (Ares Corporation)
44th Aerospace Mechanism Symposium
https://ntrs.nasa.gov/search.jsp?R=20180003015 2019-08-31T16:27:23+00:00Z
JWST and Heat Strap Bolted Joint Overview
244th Aerospace Mechanism Symposium JWST/ISIM (Heat Strap Recovery Review v5.pptx)
Al-1100
Al-1100
High Purity Aluminum Foil
Indium
Threaded Insert 
JWST
(James Webb Space Telescope)
Observatory
ISIM
(Integrated Science Instrument Module)
Instrument to Radiator 
Heat Strap Assembly
Heat Strap Bolted Joint
Heat Strap Bolted Joint Design Challenge
• Heat strap joints must meet minimum preload requirements 
on-orbit 5 years after initial torque application.
• Al-1100 joint members prohibit large preloads, drastically 
reducing the design space for the initial torque spec.
• Joint preload decreases under cool down and must be 
compensated for by temperature compensation washers.
• Several mechanisms for preload relaxation exists:
– Al-1100 Creep at RT
– Indium Flow-out
– Joint Interface Embedment
• Difficult to mathematically model joint preload relaxation.  
A preload relaxation test and curve fitting of test data is 
required for predicting preload during mission life.
344th Aerospace Mechanism Symposium JWST/ISIM (Heat Strap Recovery Review v5.pptx)
Preload Relaxation Test Overview
• Six coupon configurations were tested in order to isolate 
different drivers for preload relaxation.
• Each configuration consisted of 6 coupons (3 re-torqued 
after 24 hours and 3 without re-torque).
• Bolted coupons were torqued to design specs and included 
in-line load cells for preload monitoring.
• Preloads were sampled every 10 minutes for ~8 months.
444th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
Preload Relaxation Test Configurations
• Coupons included six different configurations:
1. Control: 6061 plate bolted into 6061 plate  
2. Indium in Compression: 0.005” Indium between 
6061 plates
3. 1100 Series Aluminum in Compression: 1100 plate 
bolted into 6061 bottom plate
4. 1100 Series Aluminum in Tension: 6061 plate 
bolted into 1100 plate (threads in tension)
5. Aluminum Foil in Compression: High purity 
Aluminum (0.040” total stack-up) in direct compression 
against 6061 bottom plate.
6. Flight: Combines Aluminum foil in compression, 1100 
plate in compression, Indium in compression, 1100 
plate in tension.
544th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
6061
6061
6061
6061
Indium
6061
1100
1100 w/ inserts
6061
6061
Foil
1100 w/ inserts
1100
Indium
Foil
Test Data Curve Fit 
• The natural logarithm function proved to be the most successful 
function for fitting the data across the various test configurations:
– Test data was processed in Excel and the software’s Solver Tool was run 
for minimizing the chi square values.
– Coefficient of determination (R2) was calculated for each curve fit.
– Curve fit drift was also monitored throughout test progression.
644th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
Curve Fit Example – Flight 1 Coupon 
744th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
Curve Fit Drift Example – Flight 1 Coupon
• Curve fit drift slowed down considerably near the end of testing.
844th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
CF w/ 1 month of data
CF w/ 2 months of data
CF w/ 3 months of data
CF w/ 8 months of data
Test Data Curve Fit Results Summary
• Error bars shown are 2-sigma errors.
• Al-1100 in tension (insert pull-out in 1100) is the biggest driver for relaxation.
• Indium flow-out benefits the most from a re-torque.
• Flight joints can lose up to ~25% of preload (+2-sigma) 5 years after re-torque.
944th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
Test Results Application
• Expansion of preload uncertainty factor to include both 
torque/preload scatter and preload relaxation
1029-April-2015 Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
𝑃𝑃𝑃𝑃𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
𝑃𝑃𝑃𝑃𝑃𝑃𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑟𝑟𝑟𝑟
𝑃𝑃𝑃𝑃𝑃𝑃𝑟𝑟𝑙𝑙𝑙𝑙𝑙𝑙
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑖𝑖𝑙𝑙𝑛𝑛 ∗ %𝐿𝐿𝑃𝑃𝐿𝐿𝐿𝐿𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑖𝑖𝑙𝑙𝑛𝑛𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟_𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑛𝑛𝑖𝑖𝑖𝑖
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑛𝑛𝑖𝑖𝑖𝑖 = 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑖𝑖𝑙𝑙𝑛𝑛 1−%𝐿𝐿𝑃𝑃𝐿𝐿𝐿𝐿𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟 1 − 𝑈𝑈𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑛𝑛𝑖𝑖𝑖𝑖 = 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑖𝑖𝑙𝑙𝑛𝑛 1−%𝐿𝐿𝑃𝑃𝐿𝐿𝐿𝐿𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟 − 𝑈𝑈𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 +%𝐿𝐿𝑃𝑃𝐿𝐿𝐿𝐿𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟𝑈𝑈𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑛𝑛𝑖𝑖𝑖𝑖 = 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑖𝑖𝑙𝑙𝑛𝑛 1− 𝑈𝑈𝑃𝑃−
𝑈𝑈𝑃𝑃− = %𝐿𝐿𝑃𝑃𝐿𝐿𝐿𝐿𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟 + 𝑈𝑈𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 −%𝐿𝐿𝑃𝑃𝐿𝐿𝐿𝐿𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟𝑈𝑈𝑃𝑃𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
𝑈𝑈𝑃𝑃−
Test Results Application
1144th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints
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 0.16 0.17
Preload UF (Relax) 0.39 0.40 0.40
UF(+) 0.36 0.36 0.36
UF(-) 0.41 0.52 0.54
• Calculated 0.52 uncertainty factor that is applied to the nominal preload 
accounting for both initial preload scatter and preload relaxation 5 years 
after initial torque and re-torque application.
Summary and Lessons Learned
• Largest driver for preload relaxation in the JWST/ISIM heat 
straps is Al-1100 in tension (insert pull-out in 1100).
– Nuts or nut-plates is preferred over inserts for reducing 
relaxation.
• Indium flow-out benefits the most from re-torqueing.
• The B-basis preload uncertainty factor for the JWST/ISIM 
heat strap joints is 52% (5 years after initial torque and re-
torque application).
• The results from this test apply directly to the JWST/ISIM 
heat strap joint design (materials, size, bolt, torque spec, 
etc.).  The results from this test should not be applied 
directly to other joint configurations, but may be 
used for guidance and insight when designing other 
joints with similar configuration.
1244th Aerospace Mechanism Symposium Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints


Investigation of Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints with Aluminum-1100 and Indium Gaskets

Williams, Patrick

1 
 
Investigation of Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly 
Joints with Aluminum-1100 and Indium Gaskets 
 
Andrew Bartoszyk*, Thomas Walsh**, Jody Davis*, Jason Krom+, Patrick Williams**, Jason Hylan*, Edgar Hemminger++ 
ABSTRACT 
Accurately predicting fastener preload relaxation in the James Webb Space Telescope (JWST) heat strap 
assemblies is essential to insure adequate thermal performance during its mission lifecycle. The 
mechanisms for preload relaxation in the strap joints include Al-1100 material creep, indium gasket flow-
out, and embedment of the joint faying surfaces. This report documents the results from a bolted joint 
relaxation test, including analysis and curve fitting of the test data for predicting preloads five years after 
initial torque application. The report also includes the derivation of a preload uncertainty factor enveloping 
both torque/preload application scatter and expected preload relaxation at the end of mission life. 
INTRODUCTION 
The JWST thermal control system includes Al-1100 heat straps for controlling heat flow to and from 
temperature critical instruments in order to meet instrument performance requirements and optimize 
observatory science capability. Critical to heat strap performance is preload maintenance at the bolted 
joint interfaces over mission lifetime. To improve thermal conduction through the bolted interfaces, Al-
1100 and indium gaskets are integral in many of the joints. Although these materials are common for 
improving thermal conduction in aerospace bolted joints, Al-1100 creeps under load at room temperature 
and indium gaskets can gradually flow out. Creep and flow-out will relax joint preload and therefore 
degrade joint thermal conductance over time. Many of the heat strap joints also include threaded inserts 
in Al-1100, resulting in high stresses in the parent material around the insert and exacerbating Al-1100 
creep. Figure 1 illustrates a representative flight joint for the JWST heat strap assemblies. 
  
Figure 1. Representative Heat Strap Bolted Joint Interface 
Because Al-1100 is relatively low in strength, compression of Al-1100 under washers and threaded insert 
pull-out limit the design space for the initial bolt preload. This limited design space necessitates a rigorous 
approach for predicting joint preloads in the heat straps throughout the joint life cycle.  
In order to quantify preload loss over time, a comprehensive joint relaxation test was completed. The test 
includes a set of representative flight like joints, a control set, and various joint configurations in order to 
compare preload loss between different drivers for joint relaxation. The different joint coupons were 
torqued to the same nominal preload and the preload relaxation was measured over a period of eight 
                                                     
* NASA Goddard Space Flight Center, Greenbelt, MD, USA 
** Stinger Ghaffarian Technologies, Greenbelt, MD, USA 
+ Sigma Space Corporation, Lanham, MD, USA 
++ Ares Corporation, Greenbelt, MD, USA 
 
Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018 
Al-1100
Al-1100
High Purity Aluminum Foil
Indium Gasket
Threaded Insert 
https://ntrs.nasa.gov/search.jsp?R=20180003004 2019-08-31T16:25:32+00:00Z
2 
 
months. The test data was then processed in an Excel spreadsheet for future preload loss predictions.  
Uncertainty factors were also derived for calculating max and min expected preloads, accounting for both 
scatter in initial preload application and joint relaxation over time. 
TEST DESCRIPTION 
Table 1 lists the joint configurations tested. For each group, with the exception of the control group, a set 
of 3 coupons were tested with the bolts re-torqued 24 hours after the initial torque-up and a set of 3 
coupons were not re-torqued. For the control group, only 3 coupons were tested without any re-torqueing 
of the bolts. The coupons included #10-32 A-286 fasteners, phosphor-bronze threaded inserts (2D 
length), and lubrication of external/internal threads with Braycote. All coupons were torqued to a nominal 
preload of 1550 N (350 lbf). 
Table 1. Coupon Test Configurations 
 
All test coupons included a FUTEK piezo-electric load cell for preload monitoring. The data acquisition 
system included the SENSIT Test and Measurement Software. The program was set to sample load cell 
outputs every 0.1 seconds during initial torque-up for each joint. After initial torque-up, the load cells were 
sampled every 10 minutes. Test data was downloaded on a daily basis for monitoring and analysis. A 
separate load cell was put under a known weight and monitored for several months to confirm that the 
load cell read-out did not drift over time. 
JOINT RELAXATION TEST DATA ANALYSIS 
The physical mechanisms driving preload loss in a thermally conductive bolted joint with Al-1100 and an 
indium gasket are varied and complicated, making it difficult to derive a simple mathematical model for 
predicting joint relaxation. An alternative approach is to curve fit data from a joint relaxation test and 
extrapolate the empirical model for predicting future preload loss. After curve fitting the test data with 
several candidate functions, the natural logarithm function proved to be the most accurate for fitting the 
data across the various coupon test configurations.       
Using the method of least squares, the data was processed in Excel and the software’s Solver Tool was 
applied for minimizing the sum of the chi square values (the square of the difference between the real 
data and the predicted data). Since the data acquisition system sampled preload every 10 minutes for 32 
joint coupons and the test duration was more than 8 months, the spreadsheet was set up to sample the 
Configuration Description 
Control Al-6061 top plate compressed against an Al-6061 bottom plate 
 
Indium 0.005” thick Indium foil between Al-6061 plates 
 
1100C Al-1100 plate compressed against an Al-6061 bottom plate 
 
1100T Al-6061 plate bolted into an Al-1100 plate, putting threads in tension 
 
Foil 
Multiple layers of 0.002” thick high purity aluminum (99.999% Al) in 
direct compression against an Al-6061 bottom plate 
 
Flight 
Combines multi-layer aluminum foil in compression, 1100 plate in 
compression, Indium in compression, 1100 in tension 
 
6061
6061
6061
6061
Indium
6061
1100
1100 w/ inserts
6061
6061
Foil
Foil
1100 w/ inserts
1100
Indium
3 
 
test data every 24 hours. This minimized the processing time for each Solver Tool run and kept the 
spreadsheet at a manageable size. A number of curve fits were also processed with higher analysis 
sampling rates in order to check the nominal analytical sampling rate. Additionally, the coefficient of 
determination (R2) was calculated for each curve fit to quantify how well the test data fit the curve. Figure 
1 is a plot of the test data and curve fit for the Flight-1 coupon.  
 
Figure 1. Test Data and Curve Fit for Flight-1 Coupon 
JOINT RELAXATION TEST RESULTS 
Figure  plots the average preload predictions after 5 years, with 2-sigma error bars calculated from the 
test data. The results from this test are specific to the joint configurations tested (materials, thicknesses, 
configurations, initial nominal preload, etc.) and apply only to these specific configurations. Caution 
should therefore be exercised when extrapolating the results to design deviations from the specific 
 
Figure 3. Joint Relaxation Preload Loss Predictions (5 years out) 
354.7
280
290
300
310
320
330
340
350
0 1000 2000 3000 4000 5000 6000
Pr
el
oa
d 
(lb
f)
Time (hrs)
Test Data and Curve Fit (Flight - 1)
Test Data CF
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
Control 1100C Indium Foil 1100T Flight
%
 P
re
lo
ad
 L
os
s
Coupon Type
% Preload Loss (5 years out)
No Retorque 1 Retorque after 24 Hrs
4 
 
configurations tested. As can readily be seen in Figure , the flight configuration (with 1 re-torque), is 
predicted to lose ~25% of its initial preload (average + 2-sigma). Without the re-torque, the preload loss is 
>40%. For comparison, the control configuration, a typical joint for structural applications in flight and 
ground support hardware, is predicted to lose ~5% of its initial preload after 5 years. The test results also 
show that the biggest drivers for joint relaxation are the Al-1100 in tension (inserts embedded in Al-1100) 
and the high purity aluminum foil in compression. Joint relaxation from indium flow-out appears to benefit 
the most from a single re-torqueing of the bolts (from ~25% to <10%).   
The plot in Figure  compares the curve fits for the different configurations. The plots only include the 
coupons that were re-torqued, with the exception of the control coupons. Figure  shows the curve fits for 
preload loss after approximately 6000 hours of testing (~8.3 months), the flight coupons were losing 
~0.25 lbf of preload per week and the control group was losing <0.1 lbf of preload per week. 
 
Figure 4. Curve Fits for Preload Loss Over Time 
PRELOAD UNCERTAINTY FACTOR 
A torque/preload uncertainty factor (UF) is typically included in bolted joint analysis for calculating the 
expected maximum and minimum preloads after torque application to a design nominal preload. The UF 
for a given joint configuration can be recovered from torque/preload test data with the assumption that the 
preload probability distribution function (PDF) is Gaussian. NASA-STD-5020 [1] recommends the 
recovery and use of a B-basis (90% of population with 95% confidence) UF for bolted joint analysis.  
In addition to the torque/preload UF, the calculated max/min preload must also account for preload loss 
over time, especially when preload is critical for joint performance. A total UF, accounting for the scatter in 
preload from both the initial torque-up of a bolt and relaxation over time, is derived in order to simplify the 
final bolted joint analysis and retain the use of existing in-house analysis programs. Preload changes due 
to temperature effects are not included in the UF since it is calculated separately. 
280
290
300
310
320
330
340
350
360
0 1000 2000 3000 4000 5000 6000
P
re
lo
ad
 (
lb
f)
Time (hrs)
Preload vs Time (Curve Fit Data)
Control Flight Indium 1100C 1100T Foil
5 
 
In order to easily combine the preload scatter for the nominal installation preload and joint relaxation, the 
PDF for preload loss from joint relaxation is assumed Gaussian and independent from the initial torque-up 
preload PDF. This is a reasonable assumption if the torque/preload UF is low. The average relaxed 
preload after torque-up and preload loss is simply the nominal initial preload minus the average predicted 
preload loss across all coupons for a given configuration. 
𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑟𝑒𝑙𝑎𝑥_𝑎𝑣𝑒𝑟𝑎𝑔𝑒 = 𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑛𝑜𝑚(1 − %𝐿𝑜𝑠𝑠𝑎𝑣𝑒𝑟𝑎𝑔𝑒)                                            (1) 
Since the initial preload and preload loss are uncorrelated random variables and their PDFs are 
Gaussian, the B-basis UF for the relaxed preload PDF is simply the RSS (root sum square) of the UF 
values for the initial preload PDF and preload loss PDF. 
𝑈𝐹𝑟𝑒𝑙𝑎𝑥 = √𝑈𝐹𝑖𝑛𝑖𝑡𝑖𝑎𝑙
2 + 𝑈𝐹𝑙𝑜𝑠𝑠
2                                                                      (2) 
A total UF− can then be derived for the minimum expected preload. 
𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑚𝑖𝑛 = 𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑛𝑜𝑚(1 − %𝐿𝑜𝑠𝑠𝑎𝑣𝑒𝑟𝑎𝑔𝑒)(1 − 𝑈𝐹𝑟𝑒𝑙𝑎𝑥)                                      (3) 
𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑚𝑖𝑛 = 𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑛𝑜𝑚(1 − %𝐿𝑜𝑠𝑠𝑎𝑣𝑒𝑟𝑎𝑔𝑒 − 𝑈𝐹𝑟𝑒𝑙𝑎𝑥 + %𝐿𝑜𝑠𝑠𝑎𝑣𝑒𝑟𝑎𝑔𝑒𝑈𝐹𝑟𝑒𝑙𝑎𝑥)                    (4) 
𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑚𝑖𝑛 = 𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑛𝑜𝑚(1 − 𝑈𝐹
−)                                                         (5) 
𝑈𝐹− = %𝐿𝑜𝑠𝑠𝑎𝑣𝑒𝑟𝑎𝑔𝑒 + 𝑈𝐹𝑟𝑒𝑙𝑎𝑥 − %𝐿𝑜𝑠𝑠𝑎𝑣𝑒𝑟𝑎𝑔𝑒𝑈𝐹𝑟𝑒𝑙𝑎𝑥                                                         (6) 
Figure  illustrates the relationships between UF− and the nominal preload. 
 
Figure 5. UF− and Nominal Preload 
UF+ is simply equal to the torque/preload UF, since the max preload for any given time is before joint 
relaxation progresses. 
𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑚𝑎𝑥 = 𝑃𝑟𝑒𝑙𝑜𝑎𝑑𝑛𝑜𝑚(1 + 𝑈𝐹
+)                                                                   (7) 
𝑈𝐹+ = 𝑈𝐹𝑖𝑛𝑖𝑡𝑖𝑎𝑙                                                                                                                     (8) 
Table 1 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. 
6 
 
Table 1. 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 
 


Investigation of Bolt Preload Relaxation for JWST Thermal Heat Strap Assembly Joints with Aluminum-1100 and Indium Gaskets

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server

NASA Technical Reports Server