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Engineering Evaluation of International Low Impact Docking System Latch 
Hooks 

J. Martinez 1 , R. Patin 1 , J. Figert 1 

1 Structural Engineering Division, NASA Lyndon B. Johnson Space Center Houston, TX 77058 

The international Low Impact Docking System (iLIDS) provides a structural arrangement that 
allows for visiting vehicles to dock with the International Space Station (ISS) (Fig 1). The iLIDS 
docking units are mechanically joined together by a series of active and passive latch hooks. In 
order to preserve docking capability at the existing Russian docking interfaces, the iLIDS latch 
hooks are required to conform to the existing Russian design. The latch hooks are classified as 
being fail-safe. Since the latch hooks are fail-safe, the hooks are not fracture critical and a fatigue 
based service life assessment will satisfy the structural integrity requirements. 

Constant amplitude fatigue testing to failure on four sets of active/passive iLIDS latch hooks was 
performed at load magnitudes of 10, 11, and 12 kips. Failure analysis of the hook fatigue failures 
identified multi-site fatigue initiation that was effectively centered about the hook mid-plane 
(consistent with the 3D model results). The fatigue crack initiation distribution implies that the 
fatigue damage accumulation effectively results in a very low aspect ratio surface crack (which can 
be simulated as thru-thickness crack). Fatigue damage progression resulted in numerous close 
proximity fatigue crack initiation sites. It was not possible to determine if fatigue crack coalescence 
occurs during cyclic loading or as result of the fast fracture response. The presence of multiple 
fatigue crack initiation sites on different planes will result in the formation of ratchet marks as the 
cracks coalesce. Once the stable fatigue crack becomes unstable and the fast fracture advances 
across the remaining ligament and the plane stress condition at a free-surface will result in failure 
along a 45 deg. shear plane (slant fracture) and the resulting inclined edge is called a shear lip. The 
hook thickness on the plane of fatigue crack initiation is 0.787”. The distance between the shear lips 
on this plane was on the order of 0.48” and it was effectively centered about the mid-plane of the 
section. The numerous ratchet marks between the shear lips on the fracture initiation plane are 
indicative of multiple fatigue initiation sites within this region. The distribution of the fatigue 
damage about the centerline of the hook is consistent with the analytical results that demonstrate 
peak stress/strain response at the mid-plane that decreases in the direction of the hook outer surfaces. 
Scanning electron microscope images of the failed sections detected fatigue crack striations in close 
proximity to the free surface of the hook radius. These findings were documented at three locations 
on the fracture surface : 1) adjacent to the left shear lip, 2) adjacent to the right shear lip, and 3) near 
the centerline of the section. The features of the titanium fracture surface did not allow for a 
determination of a critical crack size via identification of the region where the fatigue crack 
propagation became unstable. 

The fracture based service life projections where benchmarked with strain-life analyses. The strain- 
range response in the hook radius was defined via the correlated finite element models and the 
modified method of universal slopes was incorporated to define the strain-life equation for the 
titanium alloy. The strain-life assessment confirmed that the fracture based projections were 
reasonable for the loading range of interest. Based upon the analysis and component level fatigue 



test data a preliminary service life capability for the iLIDS active and passive hooks of 2 lifetimes is 
projected (includes a scatter factor of 4). 


References: 

[1] Dowling, Norman E., Mechanical Behavior of Materials : Engineering Methods for 
Deformation, Fracture, and Fatigue, Prentice Hall, 1993, pp 154-155. 

[2] Richards, R. Jr., Principles of Solid Mechanics, CRC Press, 2000, pp 243-245. 

[3] MIL-HDBK-5H, Metallic Materials and Elements for Aerospace Vehicle Structures, 1998, pg. 
5-54. 

[4] Gallagher, Giessler, and Berens, USAF Damage Tolerant Design Handbook: Guidelines for 
the Analysis and Design of Damage Tolerant Aircraft Structures, AFWAL-TR-82-3073, 
1984, Section 3.4. 



Fig 1. iLIDS solid model image that depicts the active 
and passive latch hook assemblies. 



Fig 3. Post test photographs of the active hook test articles. 



Fig 5. Active hook (10 kip fatigue loading) fracture photograph. 


Fig 2. Russian drawings of a hook set with one pair of hooks 
latched and the other unlatched. 




Fig 6. SEM image of fatigue striations adjacent to the 
shear lip-fatigue transition region. 












Microscopy & Micr 

Symposium P06 - Failure An 
s 06 AugiiS 


nter national 


Impact Doc kin 


J.E. Martinez, R.M. Patin, J.D. Figert 
Structural Engineering Division 
Lyndon B. Johnson Space Center 
Houston, TX 77058 



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iLIDS Latch Hook Service Life Presentation Overview: 

• Introduction 

• Static Strength Testing 

• Component Level Fatigue Testing of Latch Hooks 

• Fractography of Latch Hooks 

• Updated Analysis Methodology Based Upon Fractographic Results 

• Definition of Latch Hook Mission Capability 

• Summary 


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Introduction: 


The international Low Impact Docking System (iLIDS) provides a structural arrangement that allows for 
visiting vehicles to dock with the International Space Station (ISS). 


The iLIDS docking units are mechanically joined together by a series of 12 sets of active and passive latch 


12 sets of active/passive 
latch hook assemblies. 


Generic Vi&iting Vehicle (VV) 


Androgynous 

iLIDS 


CDA Tunnel 



PaSSlv# CBM 


Active CBM 


Internet ion al Spate Station (ISS) 



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Introduction: 



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In order to preserve docking capability at the existing Russian docking interfaces the iLIDS latch hooks are 
required to conform to the existing Russian design. 



Latched 
hook set 


Unlatched 
hook set 


Active Hook 


Housin 


Passive Hook 


Interlace 


Active Hook 


Passive Hook 


Eccentric 


The passive hook is stationary with a series of Bellville springs located on the mounting stem. The Bellville 
spring compliance allows for the resulting hook loads to be more uniformly distributed throughout the 12 sets 
of latch hook assemblies. The active hook is driven by a motor that rotates the hook through a small angular 
displacement followed by an inward translation which allows for engagement and preloading with the 
passive hook. 


The latch hooks are classified as ‘fail-safe’ due to 
the structural redundancy that exists. 


Since the hooks are not fracture critical (failure 
does not directly results in a catastrophic event) 
only a fatigue based service life assessment will 
satisfy the structural integrity requirements. 



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I iLIDS Latch Hook Static Strength Testing: 

The static strength testing was performed at 3 different temperatures in each of the 2 material candidates. 

The STA static strength failure loads exceed that of the annealed alloy - this result trend is consistent with the 


Passive latch hook static load failure location at the hook ledge transition radius. 


Active latch hook static load failure location at the hook ledge transition radius. 


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Latch Hook Fatigue Testing: 


Constant amplitude fatigue testing of 4 sets of active and passive hooks to failure was conducted 
at hook loads of 10, 11, and 12 kips (2 hook sets were fatigue tested at 11 kips). Fatigue failure 
occurred in active hook at the contact shelf transition radius (consistent with the static strength 
failure location) for each test conducted. 


1 


Passive 

Hook 


Active 


Hook 


Active 
Hook 
Failed in 
Fatigue 


Latch hook latigue test configuration 


Latch hook fatigue failure - active hook 


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Latch Hook Fatigue Testing - (cont): 


Frame by frame photographs of the 10 kip fatigue failure event are provided below. 


lA/lPtest 
hooks prior 
to active 
hook fatigue 
failure. 


Unstable 
fracture 
propagating 
across active 
hook. 


Active hook 
unstable 
fracture 
traversing the 

remaining 

ligament. 


Active hook 

failed 

section 

being 

ejected. 




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Active 


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Latch Hook Fatigue Testing - (cont): 


Passive 

Hook 

3A 

1 1 kip 
cyclic 
load 


Passive hook set 
contact region & 
witness marks. 


Active hook set 
contact region & 
witness marks. 


Post fatigue test photographs of the active and passive hook test article set are provided above. 


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Fractography of Latch Hooks: 


A schematic representation of a mode-I (tensile opening) fatigue failure is provided below [2] . The 
presence of multiple fatigue crack initiation sites on different planes will result in the formation 
of ratchet marks as the cracks coalesce. Once the stable fatigue crack becomes unstable and the 
fast fracture advances across the remaining ligament the plane stress condition at a free-surface 
will result in failure along a 45° shear plane (slant fracture) and the resulting inclined failure 
region is called a shear lip. 


Progressive flat 
fatigue fracture 
with curved 
beach marks 


Ratchet 
mark 
Origin 2 


Shear lip 
(slant fracture) 


'Fast 

overload 

fracture 


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Fractography of Latch Hooks - (cont.): 


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Low magnification photos of the 2A hook are provided below. The numerous ratchet marks 
between the shear lips on the fracture initiation plane is indicative of multiple fatigue initiation 
sites within this region. The distribution of the fatigue damage about the centerline of the hook is 
consistent with the analytical results that demonstrate peak stress/strain response at the mid-plane 
that decreases in the direction of the hook outer surfaces. 


atchet marks ■ region of multiple fatigue initiation sites 


2A: 1 1 kip fatigue fracture surface 


2 A: ratchet mark zone ; crack initiation plane 


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i n 







1.00mm 


S-4800 15.0kV 10.5mm x30 SE(M) 


'1 4/201 2 


1.00mm 


S-4800 15.0kV 11.1mm xl.OOk SE(U,LA10) 2/14/2012 


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Fractography of Latch Hooks - (cont.): 


Low and high magnification images of 
the 1 A shear lips on the crack initiation 
plane. The dimple rupture features within 
the slant fracture region confirm the 
overload induced shear lip formation. By 
fracture topography and similarity with 
the 2A, 3A, and 4A specimens shear lips 
exist at these locations in all the active 
hook fatigue test failures. 


S-4800 15.0kV 11.1mm x30 SE(M) 2M 4/2012 






S-4800 15.0kV 10.9mm xl.OOk SE(U, LAI 0)' 2/14/201 2 ' ' ' ' 50.0um 


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surface dir 


500um 


500um 


surface dir 


The 1 A fracture surface and scanning 
electron microscope images of regions 
adjacent to the shear lip regions. 

Active hook 1 A (10 kip fatigue loading). 
Scanning electron microscope images of 
fatigue cracks adjacent to the shear lip / 
fatigue transition region. 


S-4800 15.0kV 11.6mm xlOO SE(W) 1/26/2012 


S-4800 15.0kV 11.0mm xlOO SE(M) 1/26/2012 




S-4800 15.0kv 12.7mm x5.00k SE(M) 2/1/2012 


S-4800 15.0kV 11.3mm x50.0k SE(U,LA10) 1/26/2012 I .OOum 


Local striation spacing » 2.9e-5 in/cycle 
(surface direction) 


Local striation spacing « 9.3 e-6 in/cycle 
(depth direction) 


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Local striation spacing » 9.0e-5 in/cycle 
(depth direction) 





S-48U 

IU lo.ukv ll .umm Xl.UUk bt(M,LA1U) 1/26/2U12 

50.0um 


50.0um S-4800 15.0kV 11.0mm xlOO SE(M) 1/26/2012 


Active hook 1A (10 kip fatigue loading). 
Scanning electron microscope images of 
fatigue in the centerline region of the 
hook initiating surface. 


ri - r .- / > 


S-4800 15.0kV 10.9mm xl.OOk SE(M,LA10) 1/26/2012 

50.0um 


Local striation spacing « 9.8e-5 in/cycle 
(depth direction) 


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Wmi 


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Fractography of Latch Hooks - (cont.): 


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Scanning electron microscope (SEM) images of the 2 A fracture surface adjacent to the shear lips 
and near the centerline of the section are provided below; striations (microscopic tear ridges 
resulting from cyclic loading induced crack front advancement) were detected all along the crack 
initiation front. iSiM ======= ^ = i^^^^B I r 






1» 


C 

r ' w 


} > 


| S-4600 15 OkV 1 1 5mm x2 50k S£(M) 2 


Local st nation spa eng 
(depth direction) 


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Crack 
Depth (a) 


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Fracture Mechanics Analysis Update: 


The failure analysis investigation demonstrates that the fatigue damage progression results in numerous close proximity 
fatigue crack initiation sites distributed over a length of approximately 0.48” centered over the 0.787” section width. Close 
proximity coplanar fatigue cracks will coalesce into a larger effective crack with increased surface length with respect to the 
crack depth (crack aspect ratio = crack depth (a) over crack half length (c) ; (a/c) « 1). The crack tip stress intensity factor 
for a very low aspect ratio is dominate in the depth direction and approaches that of a thru-thickness crack of the same 
depths. 


Outer surface 




Inner bore 


Crack Half-Length (c) ; total length = 2c 


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Latch Hook Mission Capability - Room Temp / Lab Air 

The latch loading spectrum and resulting mission capability are provided below. 


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16 


15 


14 


2D thm-erk si F {with taper); eifs^c.ooqi’ 1 

SC1.1 3D stress; test correlated ; S-N EIFS 

+ Latch Hook Fatigue Test Data ; R=0 


_ IS 


12 


Hook Limit Load = lL2401b 


11 ■ L 


10 - 


HookPreload= 9.96S lb 





2DSIF 

2DSIF 

Test Correl Test Correl 

Min 

Max 

No. 

Taper End Taper End SC113D 

SC11 3D 

Load 

Load 

of 

Min Stress Max Stress Min Stress 

Max Stress 

(lbs) 

(lbs) 

Cycles 

(ksi) 

(ksi) 

(ksi) 

(ksi) 

0 

9968 

20 

0.00 

304.02 

0.00 

117.75 

9968 

10032 

222141 

304.02 

305.98 

117.75 

118.50 

9968 

10095 

109858 

304.02 

307.90 

117.75 

119.25 

9968 

10159 

38537 

304.02 

309.85 

117.75 

120.00 

9968 

10222 

11291 

304.02 

311.77 

117.75 

120.75 

9968 

10350 

84 

304.02 

315.68 

117.75 

122.26 

9968 

10477 

21 

304.02 

319.55 

117.75 

123.76 

9968 

10604 

11 

304.02 

323.42 

117.75 

125.26 


10731 

10858 

10985 

11113 

11240 

11240 


304.02 

304.02 

304.02 

304.02 

304.02 

0.00 


327.30 

331.17 

335.04 

338.95 

342.82 

342.82 


Service Life Prediction 
Room Temperature 

16 Lifetimes : SF= 1 

4 Lifetimes ; SF= 4 


Service Life Prediction 
Room Temperature 

1? Lifetimes : SF= 1 

4.75 Lifetimes ; SF= 4 


100 


1000 


10000 


Constant Amplitude Cycles to Failure ; (scatter factor = 1) 


Ground cycling 
usage 20 cycles 
(lx) 


> 


126.76 
128.26 

129.76 
131.27 

132.77 
132.77 J 


On-oxfait 
spectrum : 
Defines 1 
mission w' a 
full range 
limit load 
cycle on la st 
step ; (25x) 




Loading 
blocks define 
a latch ho ok 
lifetime and 
are repeated 
until 

analytical 
failure is 
achieved. 


J 


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j Latch Hook Mission Capability - Final Result: 

The latch hook operational temperature regime of -100°F/+163°F in a vacuum environment does 
not result in a detrimental fatigue damage accumulation trend with respect to the lab air room 
temperature result. 

The analytical model that is slightly conservative with respect to the component level fatigue test 
data is utilized for the final service life projection. This is due to the complexity associated with 
achieving accurate similitude between the test configuration and the actual flight units with 
respect to boundary conditions, alignments, tolerances, reactions, loadings, time, environments, 
and potential operational usage variations. 

Based upon the analysis and component level engineering testing performed a 4x lifetime 
capability is assigned to the iLIDS active/passive latch hooks (includes a scatter factor of 4; 1 
lifetime = ground test cycles + 25 missions). 



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Summary - iLIDS Latch Hook Service Life Analysis: 

The prescribed usage for iLIDS active/passive latch hook components results in appreciable 
localized plastic straining at the contact shelf transition radius in both hooks. The latch hooks are 
fail-safe and thus definition of the fatigue capability is all that is required (non- fracture critical). 

Static strength testing was performed that defined the stiffness and static strength capability of 
mated hooks. 

Constant amplitude fatigue testing of 4 latch hook sets to failure was performed (1 @ 10 kip, 2 @ 
11 kip, and 1 @ 12 kip). The component level fatigue test data fell between the analysis 
established upper and lower bounds. 

Failure analysis of the hook fatigue failures identified multi-site fatigue initiation that was 
effectively centered about the hook mid-plane. This implies that the fatigue damage 
accumulation effectively results in a very low aspect ratio surface crack. 


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Summary - iLIDS Latch Hook Service Life Analysis (cont): 

The room temperature latch hook mission capabilities based upon a correlation through the test 
data points and for the slightly conservative 2D thru-crack cases were generated (170 and 140 
missions respectively; with a scatter factor of 4). 

The component level fatigue test data and analysis predictions are at room temperature in a lab 
air environment. The operational environment of the latch hooks is the vacuum of space (hooks 
are located outboard of the pressure containment seals) over a temperature min/max range of - 
100°F/+163°F. Literature data demonstrates that the material fatigue properties improve at 
temperatures less than room temperature and that fatigue crack growth rates in argon (inert gas) 
demonstrate an insensitivity to temperatures above room temperature beyond the currently 
defined upper limit. The same reference also demonstrates a markedly improved fatigue crack 
growth response for material alloy in the vacuum environment over the inert argon result. 

Based upon the analysis and component level engineering testing performed a 4x lifetime 
capability is assigned to the iLIDS active/passive latch hooks (includes a scatter factor of 4 ; 1 
lifetime = ground test cycles + 25 missions). 



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References: 


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1. Lawrence, Victor, iLIDS Hook Pull Test Report, ESCG-4520-11-TL-MEMO-0002, 1-24- 
2011. 

2. Lund, R.A., Fatigue Fracture Appearances, ASM Handbook, Volume 11: Failure Analysis 
Prevention, W.T. Becker, R.J. Shipley, editors, 2002, p627-640. 

3. K.S. Ravichandran, Effect of Crack Shape on Crack Growth, ASM Handbook, vol. 19, 
Fatigue and Fracture, 1996, p 161. 

4. International Docking System Standard (IDSS) Interface Definition Document (IDD), Rev. A, 

2011, pg. 


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] ! B ack-Up Reference Slide: j 

• Dowling, Norman E., Mechanical Behavior of Materials : Engineering Methods for Deformation, Fracture, and Fatigue, 

Prentice Hall, 1993, pp 154-155. 

• Richards, R. Jr., Principles of Solid Mechanics, CRC Press, 2000, pp 243-245. 

• Stowell, R.E., Stress and Strain Concentration at a Circular Hole , NACA TN 2073, 1 950. 

• MIL-HDBK-5H, Metallic Materials and Elements for Aerospace Vehicle Structures, 1998, pg. 5-54. 

• Gallagher, Giessler, and Berens, USAF Damage Tolerant Design Handbook: Guidelines for the Analysis and Design of 
Damage Tolerant Aircraft Structures, AFWAL-TR-82-3073, 1984, Section 3.4. 

• Newman, J.C., Jr., Phillips, E.P, Swain, M.H., and Everrett, R.A., Jr., “Fatigue Mechanics : An Assessment of a Unified 
Approach to Life Prediction,” Advances in Fatigue Lifetime Predictive Techniques, ASTM 1122, M.R. Mitchell, and R.W. 
Landgraf, Eds., American Society for Testing and Materials, Philadelphia, 1992, pp. 5-27. 

• Bannantine, Comer, and Handrock, Fundamentals of Metal Fatigue Analysis, Prentice-Hall, 1990, pp 130. 

• Manson, S.S., Halford, G.R., Fatigue and Durability of Structural Materials, ASM International, 2006, pp 54-55, 81. 

• Dowling, N. E., “Local Strain Approach to Fatigue,” Chapter 4.03, Volume 4, Cyclic Loading and Fatigue, R. O. Ritchie and Y. 
Murakami, volume editors, part of the 10-volume set, Comprehensive Structural Integrity, B. Karihaloo, R. O. Ritchie, and I. 
Milne, overall editors, Elsevier Science Ltd. Oxford, England, 2003. 

• Bell, R.P, Shah, S., and Alford, R.E., Equivalent Initial Flaw Size Using Small Crack Data, Structural Integrity Department, 
Lockheed Martin Aeronautical Systems, Marietta, Georgia, USA and C-141 System Program Office, WR-ALC, Robins AFB, 
Georgia, Small Fatigue Cracks: Mechanics, Mechanisms and Applications, R.O. Ritchie and Y. Murakami (Editors), 1999 
Elsevier Science Ltd. 

• Wang, D.Y., A Study of Small Crack Growth Under Transport Spectrum Loading, AGARD-CP-328, Behaviour of Short 
Cracks in Airframe Components, 1983, pp. 14-1 thru 14-15. 

• IDSS IDD, International Docking System Standar (IDSS) Interface Definition Document (IDD), Rev. A, May 13, 2011, pg. 21. 

• AFML-TDR-64-280, Cryogenic Materials Data Handbook, August 1968, pp 721 . 

• S. Lampman, Fatigue Data Book: Light Structural Alloys, ASM International, 1995, pp. 292-293. 



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