Mason C. Conrad

Case Western Reserve University, Cleveland, Ohio

Christopher C. Daniels and Marta B. Bastrzyk

The University of Akron, Akron, Ohio

An Experimental Investigation of Silicone-toMetal Bond Strength in Composite Space

Docking System Seals

NASA/CR—2010-216886

October 2010

AIAA–2009–5318
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Mason C. Conrad

Case Western Reserve University, Cleveland, Ohio

Christopher C. Daniels and Marta B. Bastrzyk

The University of Akron, Akron, Ohio

An Experimental Investigation of Silicone-toMetal Bond Strength in Composite Space

Docking System Seals

NASA/CR—2010-216886

October 2010

AIAA–2009–5318

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared under Contracts NNX08AU01H and NNC08CA35C

Prepared for the

45th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

Denver, Colorado, August 2–5, 2009
Acknowledgments

The material is based upon work supported by the National Aeronautics and Space Administration under Contract Numbers 

NNX08AU01H and NNC08CA35C. The authors greatly appreciate the contributions of Richard Tashijan for his technical 

support. The authors also wish to thank the Parker-Hannifi n Corporation for providing the seals.

Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by NASA technical management.
The National Aeronautics and Space Administration (NASA) is currently developing

a new universal docking mechanism for future space exploration missions called the Low

Impact Docking System (LIDS). A candidate LIDS main interface seal design is a composite assembly of silicone elastomer seals vacuum molded into grooves in an electroless

nickel plated aluminum retainer. The strength of the silicone-to-metal bond is a critical

consideration for the new system, especially due to the presence of small areas of disbond

created during the molding process. In the work presented herein, seal-to-retainer bonds

of subscale seal specimens with different sizes of intentional disbond were destructively

tensile tested. Nominal specimens without intentional disbonds were also tested. Tension

was applied either uniformly on the entire seal circumference or locally in one short circumferential length. Bond failure due to uniform tension produced a wide scatter of observable

failure modes and measured load-displacement behaviors. Although the preferable failure

mode for the seal-to-retainer bond is cohesive failure of the elastomer material, the dominant observed failure mode under the uniform loading condition was found to be the less

desirable adhesive failure of the bond in question. The uniform tension case results did not

show a correlation between disbond size and bond strength. Localized tension was found

to produce failure either as immediate tearing of the elastomer material outside the bond

region or as complete peel-out of the seal in one piece. The obtained results represent a

valuable benchmark for comparison in the future between adhesion loads under various

separation conditions and composite seal bond strength.

Nomenclature

CEV Crew Exploration Vehicle

GRC Glenn Research Center

LIDS Low Impact Docking System

LSAM Lunar Surface Access Module

NASA National Aeronautics and Space Administration

AS Adhesive failure at Seal-to-retainer bond

CS Cohesive failure in Seal at seal-to-retainer bond

M Material failure

CRR Cohesive failure at RTV-to-seal bond in RTV

CRS Cohesive failure at RTV-to-seal bond in Seal

An Experimental Investigation of Silicone-to-Metal Bond 

Strength in Composite Space Docking System Seals 

Mason C. Conrad 

Case Western Reserve University 

Cleveland, Ohio 44106 

Christopher C. Daniels and Marta B. Bastrzyk 

The University of Akron 

Akron, Ohio 44325 

NAS
I. Introduction

The National Aeronautics and Space Administration (NASA) is currently developing a universal nextgeneration docking system to meet the demands of the Constellation program. The Low Impact Docking

System (LIDS)1 design, shown in Figure 1, uses an extendable load-sensing electromagnetic capture ring

and closed-loop feedback control to join two round docking interfaces at the periphery, allowing cargo and

crew passage through the tunnel that is formed. During the mating of two vehicles, each equipped with a

LIDS interface, one side assumes an active role and the other a passive role. The active LIDS extends its

capture ring and attaches to the opposing passive unit. After a “soft” electromagnetic capture is achieved, the

interface is aligned with a system of actuator arms, and then is retracted. At this point, the pressure seals are

compressed, structural attachments are made, and the docking process is completed. This sequence decreases

the necessary contact velocity as compared to other docking systems currently in use2 and thus decreases

the risk of collision, undesired vibrations to sensitive onboard equipment, and vehicle course alterations.

Figure 1. Illustration of the active LIDS

The LIDS must be able to withstand multiple

docking and undocking cycles on orbit in order to

support the exploration mission scenarios planned

for the Constellation program. For example, the

lunar mission scenario requires docking the Orion

Crew Exploration Vehicle (CEV) with the Lunar

Surface Access Module (LSAM) in Earth orbit, undocking the crafts in lunar orbit to allow descent

and landing with the LSAM, and finally, re-docking

in lunar orbit to allow crew return to Earth in the

CEV.3 The operational requirements of the LIDS

introduce the need for sophisticated components at

all levels of the system.

The successful operation of the LIDS requires an

advanced sealing system at the mating interface to

prevent the loss of breathable cabin air to vacuum.

The LIDS main interface seal is being developed at

NASA Glenn Research Center (GRC) in Cleveland,

OH. A key design challenge for the main interface

seal is its operating environment, which includes prolonged exposure to atomic oxygen, ultraviolet and

ionizing radiation, vacuum pressure, and potential micrometeoroid and orbital debris impact damage, as

well as an expected exposure temperature range of −100∘C to 125∘C. In addition, NASA requires that

all spacecraft materials exposed to hard vacuum meet low-outgas requirements,4 as liberated material can

condense on other external objects and affect their functionality. The operating environment and the lowoutgas requirements have led to the selection of the silicone elastomer S0383-70 manufactured by Parker

Haniffin Corporation as a candidate material for use in the LIDS main interface seal. This particular

material has been found to meet the low-outgas requirements, as well as exhibit desired behaviors when

tested in subscale seal configurations exposed to simulated space environment conditions.5–8

Upon undocking, the rigid latches securing the interface are released and spring-loaded mechanisms

provide a separation force. It is critical that the seal material is not dislodged during this maneuver, as this

could liberate orbital debris and damage the seal, potentially impairing sealing ability and redundancy. Seal

fragments could also be left behind on the mating flange, disabling the docking port. Quantifying the amount

of separation force required to undock the seal from its flat metal mating surface is a part of ongoing research.6

The S0383-70 compound has been found to develop considerable adhesion when compressed against a flat

metal surface,7, 8 thus introducing the possibility that the seal-to-retainer bond of the composite seal might

fail before the seal detaches from the flat metal mating surface at the interface.

The objective of the work presented herein was to quantify the strength of the silicone-to-metal bond

in the candidate composite seal assembly design. In particular, the objective of this work was to determine

the amount of force required to extract molded-in-place silicone elastomer seals from their grooves in the

retainer. Moreover, the observed bond failure modes were examined to provide a thorough understanding

of the extraction behavior. The presented data was acquired from tests on individual seals of subscale (12

inch diameter) seal assemblies at ambient conditions. Results of tests on seal assembly specimens fabricated

using nominal procedures, as well as specimens fabricated with three levels of intentional flaw (i.e. disbond)

are reported.

NASA/CR—2010-216886 2
II. Description and Methodology of the Experiments

A. Test Specimens

Figure 2. Illustration of the upper portion

of a test specimen cross section showing two

elastomer seals in grooves in an electroless

nickel plated aluminum retainer.

The test specimens were custom designed seal assemblies manufactured by the Composite Sealing Systems Division at Parker

Hannifin Corporation. The test specimens considered herein

were manufactured according to a candidate Gask-O-SealTM

design with multiple elastomer seals molded into an aluminum

retainer, as shown in Figure 2. The upper portion of the the

seal assembly consists of a primary (inner) seal and a redundant

(outer) seal, providing single fault tolerance.

The seals were made of S0383-70 silicone elastomer compound. The 6061-T651 aluminum retainer of the seal assembly

had an outside diameter of 12.0 inches. The expected outside

diameter of the full-scale LIDS interface seal is currently 58

inches.

In the nominal fabrication process for the seal assembly, the

metal retainer is machined and electroless nickel plated, after

which a priming agent is applied to the groove surfaces prior

to silicone vacuum-molding. Once the silicone cures in place

in the grooves, it is permanently bonded to the retainer. Unintentional localized disbond occurs where the

silicone separates from the metal, as seen in Figures 3 and 4.

Figure 3. Photograph of unintentional localized

disbond at an arbitrary location.

Figure 4. Photograph of unintentional disbond

occurring at bolt hole locations due to molding

process.

Two different kinds of test specimens were fabricated for this study: nominal and intentionally flawed.

The nominal test specimens were manufactured using standard processes expected to take place during the

full-scale seal assembly fabrication. The intentionally flawed test specimens were fabricated with a controlled

flaw (i.e. disbond) in the seal-to-retainer bond of both seals by masking a specified length of the retainer

grooves when applying the priming agent prior to the molding process. Three levels of intentional flaw, along

with the nominal configuration, were considered. The lengths of the intentional disbonds in the received

test specimens are summarized in Table 1. It should be noted that the molding process typically produces

some level of disbond visible at the edge of the groove on the side wall, as was shown in Figure 4. Although

unintentional disbond could occur at the bottom of the groove, it is very unlikely that a seal produced with

the standard process would have both side walls and the entire groove bottom completely disbonded at

the same circumferential location. For that reason, the test specimens manufactured with the intentional

disbonds represent a worst-case scenario of what might occur during seal assembly fabrication. In addition,

NASA/CR—2010-216886
Table 1. Seal Specimen Types and Corresponding Circumferential Flaw Lengths

Specimen Fabrication Flaw

Type Process Length, inch

A Nominal 0

B1 Flawed 0.25

B2 Flawed 0.50

B3 Flawed 1.0

the disbond lengths in Table 1 have been shown to be detectable by nondestructive inspection techniques,

with the maximum disbond length (1.0 inch) representing an extreme case that would most likely lead to

part rejection.

B. Test Fixture Assembly

Custom steel rings were designed to allow “gripping” of the seals without actually pinching or squeezing the

S0383-70 material. Different ring sizes were made so that only one seal, inner or outer, would be tested at a

time. These rings were bonded to the top surface of the seal using a room temperature vulcanizing (RTV)

silicone adhesive. The rings were designed to match the seal surface profile in order to maximize the area

available for bonding with the RTV, as shown in Figure 5. The RTV was applied to the elastomer surface

and the rings were placed onto the seals. The rings and the seal assembly were then fastened to identical

1.0 inch thick stainless steel platens and the RTV was cured for a period of approximately five days. A

schematic of the resulting test fixture assembly is shown in Figure 6.

Figure 5. Section view of the test fixture used

to maximize adhesive bonding area by matching

the seal profile. The seal-to-retainer bond being

tested is shown in dashed yellow lines.

Figure 6. Illustration of the test fixture showing

the seal specimen, rings (blue), and platens (yellow) with an inset view of the RTV interface.

The specimens were tested under two different loading scenarios: uniform tension and localized tension.

In the uniform tension case, the RTV adhesive was applied to the entire seal circumference, therefore stressing

the entire seal-to-retainer bond simultaneously. The localized tension case was considered to represent the

worst-case scenario for the seal-to-retainer bond, where the RTV adhesive was applied only over a 1.0 inch

circumferential length of the seal. For intentionally flawed specimens, the adhesive area coincided with the

intentional disbond area.

NASA/CR—2010-216886 4
C. Bond Strength Tensile Test System

The bond strength tensile tests were performed using an Instron 5584 electromechanical load frame. The

frame was equipped with an alignment fixture for both angular and concentricity adjustments of the upper

actuator rod with respect to the lower actuator rod. Load during separation was measured using a 33700 lbf

capacity Instron 2525 − 171 load cell with an accuracy of 0.30% of the full range. A position encoder within

the load frame was used to measure displacement of the crosshead to an accuracy of 0.001 inch. Sources

for measurement uncertainty include linearity, repeatability, and hysteresis, as well as the off-axis nature of

loading the circumference of the seal. The speed at which the platens were separated was a function of time

simulating the expected LIDS undocking rate. The test fixture assembly was mounted in the load frame as

shown in Figure 7.

Figure 7. Photograph of the test specimen assembly mounted in the load frame.

D. Failure Mode Identification

Adhesion is defined as the attraction between two different substances because of the intermolecular forces

between the materials.9 Cohesion, on the other hand, describes the attractive forces between molecules

within a single substance.9 At a bonded interface of two different materials (in this case silicone elastomer

and aluminum retainer), bond failure can occur in two distinct ways differentiated by location. Adhesive

failure occurs in the interfacial layer between the bonded materials, indicating that the bond is the weaker

intermolecular force. Cohesive failure occurs within one of the two materials but not at the interface itself.

Cohesive failure is the desirable failure mode for an engineered bonded joint because it indicates that the

bond achieved with a material is stronger than the material itself. Both cohesion and adhesion are dependant

on local conditions, meaning some areas can fail cohesively and others adhesively on the same test specimen.

Five failure modes were defined in this study:

1. Adhesive failure at the molded seal-to-retainer bond (AS),

2. Cohesive failure in the S0383-70 at the molded seal-to-retainer bond (CS),

3. Material failure in the S0383-70 away from the surfaces (M),

4. Cohesive failure in the RTV adhesive at the top surface of the seal (CRR),

5. Cohesive failure in the S0383-70 at the top surface of the seal (CRS).

The adopted nomenclature is also shown in Figure 8 along with a schematic of a seal molded into a retainer

at the bottom and attached to two rings at the top using RTV. All of the defined failure modes represented

either adhesive or cohesive failure in a general sense. However, the area of failure had different implications

for the LIDS main interface seal and thus they were differentiated for clarity. Careful selection of the RTV

adhesive successfully eliminated two other possible failure modes: adhesive failure at the top surface of the

seal and adhesive failure at the RTV-to-retainer bond. The relative strengths of the materials precluded any

cohesive failure within the metal components.

NASA/CR—2010-216886 5
Figure 8. Schematic showing the location of the five failure modes observed.

III. Results and Discussion

Results of the seal-to-retainer bond strength tests are reported from quantitative and qualitative perspectives. The results are divided into the two loading cases: uniform tension and localized tension. Analysis of

each of the cases includes both acquired data quantifying the bond strength and observations of the failure

surfaces to determine the failure modes.

A. Uniform Tension Case

The uniform tension case was a simulation of an extreme adhesion load during separation due to prolonged

contact between the S0383-70 material and a metal surface in the compressed (i.e. docked) state. In this

case, as previously mentioned, the RTV adhesive was applied on the entire seal circumference, therefore

stressing the entire seal-to-retainer bond simultaneously.

1. Failure Mode Characterization

Failure in the specimens tested in the uniform tension case occurred primarily at the seal-to-retainer bond,

with the exception of one test where a majority of the RTV failed. In the event of RTV failure the information

produced was the strength of the test adhesive, not the seal-to-retainer bond performance, and thus was

treated as a minimum strength value of the seal-to-retainer bond.

Bonds are commonly given bond ratings which represent the percentage of bond area exhibiting each

failure mode. A summary of bond ratings using the failure modes defined above is shown in Figure 9 for

the uniform tension case specimens. The most dominant failure mode observed was adhesive failure of the

seal-to-retainer bond (AS), notably on specimens which exhibited greater than 99% adhesive failure (Type

B1 specimens). A representative area of the groove surface exhibiting adhesive failure (AS) is shown in

Figure 10, where the retainer surface was clear of any seal residue. In addition to adhesive failure, all of

the other failure modes were observed. During the Type A Inner 2 seal test, a majority of the RTV failed

(CRR) and the specimen exhibited all five of the failure modes to some extent. This result highlighted the

variability that is possible of the seal-to-retainer bond, even within a single seal specimen.

A representative image of significant cohesive failure (CS) in the bottom of the groove of a Type A

specimen is shown in Figure 11. For this particular section of the seal, it was not immediately clear whether

cohesive or adhesive failure was the dominant failure mode. The groove bottom was covered in S0383-70

residue in patterns along the circumference, possibly indicating that priming agent was brushed in this

direction, leading to distributed spots of superior bond formation and resulting in cohesive failure. This

observation suggests the sensitivity of bond quality to fabrication processes. To explore this failure in more

detail, the surface was examined with an optical microscope. Under 63X magnification, areas which appeared

to the naked eye as sparse spots of residue, were actually interspersed with fields of much smaller residue

NASA/CR—2010-216886 
Figure 9. Bar chart summarizing the bond ratings of uniform tension case test specimens.

spots with size on the order of 100μm (Figure 12). This led to the observation that cohesive failure may

be more prevalent than is visible with the naked eye. Specimens that failed in the adhesive (AS) were also

examined under magnification to confirm the visual assessment as shown in Figure 10.

Figure 10. Photograph of a representative area

showing exclusively adhesive failure i.e. the seal

material is completely removed.

Figure 11. Photograph of a fracture surface showing significant cohesive failure, along with adhesive failure.

2. Failure Load Measurements

Representative load-displacement results of an inner and outer seal of a Type A specimen are shown in

Figure 13. These particular test results produced the highest measured failure loads of the uniform tension

test series, with the peak load of 4576 lbf for the outer seal and 3842 lbf for the inner seal. As seen in Figure 13,

the load-displacement curves of the inner and outer seal tests agree closely until the point of failure in the

inner seal test. In the first stage of the separation (approximately 0.030 inch) a linear relationship between

load and deflection was observed. At the initiation of failure, a flattening of the curve is noted as small areas

began to fail and could no longer carry the load. The first steep drop in load represents the initial bond

failure in part of the seal and this peak load was the parameter of interest. A chart summarizing the peak

measured load divided by the seal circumference for all specimens in the uniform tension case is shown in

NASA/CR—2010-216886 
Figure 12. Optical micrograph of fracture surface under 63X magnification showing small spots of cohesive

failure invisible to the naked eye.

Figure 14. The second steep drop indicates that most of the remaining bonded area had failed. The slightly

downward slope in the final region indicates that the last remaining bonded section of the elastomer was

stretching and/or tearing away from the retainer. The failure surface image shown in Figure 11 corresponds

to the outer seal data shown in Figure 13.

Figure 13. Graph of measured results for uniform

tension case exhibiting significant cohesive failure.

Figure 14. Bar chart representing failure load per

circumferential inch of a single seal. The error bars

indicate load measurement uncertainty.

Based on the peak measured tensile load values shown in Figure 14, there was no clear correlation between

intentional flaw length and failure load. The two significantly lower values of Figure 14 can be explained by

failure mode observations. The second Type A inner seal exhibited mostly RTV failure (CRR), and thus

the seal-to-retainer bond had at least that level of strength. In the Type B1 Inner seal test a portion of

the circumference initially failed at the RTV-to-seal interface (CRR and CRS), as shown in Figure 15, while

most of the seal failed adhesively at the seal-to-retainer bond (AS). The subsequent separation removed

the rest of the seal circumference in adhesive failure (AS). This partial RTV failure is characterized by

a lower peak load measured, followed by a long and slow decline in the measured load as the remaining

length of the seal peeled out. Aside from these lower values, there was no clear relationship between the

failure load and the failure modes exhibited. The Type A seal specimens exhibited slightly higher load

measurements and more cohesive failure (CS and M). However, the load to failure was not significantly

different for seals with intentional flaws, though they exhibited more adhesive (AS) failure. It was originally

anticipated that the intentional disbond location would be the initiation point of rapid peel-out of the seal.

NASA/CR—2010-216886 8
Figure 15. Video frame of the initial partial

RTV failure in a uniform tension case test leading to an altered load-displacement curve. The

remaining bonded section was peeled out adhesively.

It was assumed that, considering the mechanics of separation,

the short section of disbonded seal would arc upward and

begins to stretch as the interface separated until enough of

the tension in the seal concentrated at the seal-to-retainer

bond line to commence peel-out.

However, this behavior was not observed in the tests because failure initiated at a displacement less than that required for lifting and stretching the unbonded section of the

seal. As a result, the initial bond failure occurred at an arbitrary location and not necessarily at the disbond. Therefore,

there was not a stark difference in failure load values or bond

ratings between Type A and Type B seals.

The uniform tension case led to the important observation that a bond which eventually fails adhesively can still

be strong in terms of the load it can withstand before failure. This suggested that although the bond strength may be

acceptable for the loads generated by the LIDS system, the dominant observed mode of failure may not be

desirable.

B. Localized Tension Case

The localized tension case can be viewed as the worst-case scenario for the seal-to-retainer bond as it

represents increased localized seal adhesion coinciding precisely with an area of complete disbond. The

potential for increased localized adhesion stems from surface contamination of the interface and material

variation in the elastomer, which likely would be non-uniform on the surface. This introduces the possibility

that some areas would be preferentially adhesive with the S0383-70 and therefore develop isolated higher

adhesion loads.

Figure 16. Bar chart summarizing the bond rating of localized tension case test specimens.

1. Failure Mode Characterization

A summary of the bond ratings for the localized tension case is presented in Figure 16. Five of the eight

seals tested in this case produced predominantly adhesive failure (AS). The seals in this category detached

NASA/CR—2010-216886 9
from the retainer by way of slow, steady peel-out (Figure 17) along the entire circumference, resulting in

the complete removal of the seal in one piece as shown in Figure 18. The bottom of the grooves in these

tests exhibited the same conditions as the adhesive failure mode exhibited by tests of the uniform tension

case. Inspection of the groove side walls on several tests revealed small spots of apparent cohesive failure

(CS) as well. The remaining three tests failed significantly within the S0383-70 material (M) rather than at

Figure 17. Video frame of an inner seal pullout and peel from the retainer groove.Figure 18. Photograph of the complete removal of the seal in one piece due to localized

tension.

the seal-to-retainer bond. This was a more desirable failure mode then adhesive failure in the LIDS main

interface seal as it prevents the seal from being loosely attached at only one point. The distinction between

the material (M) and adhesive (AS) failure modes has important implications for the LIDS application. In a

localized material failure, some fragments could be left behind on the mating surface after the failure. Upon

a subsequent docking operation, it would be possible to achieve some level of seal, although the redundancy

provided by the two seals would be lost. In a completely adhesive failure, where the entire seal peeled out

of the groove (as was shown in Figure 18), it would most likely lie across the interface, preventing any kind

of seal from being attainable.

Material failure in the localized tension case progressed along the top of the seal for a distance before

release. A sequence of photographs detailing the material failure process is outlined in Figure 19. The failure

shown in Figure 19 is analogous to the propagation of peel around the circumference, except that the failure

occurred within the S0383-70 elastomer.

Figure 19. Video frame sequence showing the progressive material failure along the top of an inner seal. The

reference mark on the lower platen shows the edge of the 1.0 inch disbond.

NASA/CR—2010-216886 10
2. Failure Load Measurements

Representative load-displacement curves for Type B1 localized tension case specimens which failed adhesively

(AS) are shown in Figure 20. Peel out of the seal began at a displacement of about 0.06 inches and continued

at negligible peel loads until the entire seal circumference was extracted from the retainer groove. Complete

extraction required nearly 15 inches of total displacement, but only the first inch is shown in Figure 20 for

visual clarity. These results suggested that once peel had initiated, the seal was likely to be completely

extracted from the groove.

Figure 20. Graph of measured results for a localized tension case specimen exhibiting adhesive

failure.

Figure 21. Graph of measured results for a localized tension case specimen exhibiting material

failure.

Figure 22. Bar chart representing failure loads measured during localized tension tests using 1.0 inch of

RTV adhesive. The error bars indicate load measurement uncertainty.

Figure 23. Photograph of material failure of

a test specimen with a 1.0 inch intentional

flaw.

NASA/CR—2010-216886 11
Results from two localized tension cases which demonstrated failure in the seal material (M) are plotted

in Figure 21. The “sawtooth” pattern evident in both the inner and outer seal curves indicates start-stop

failure. In other words, the failure progressed for a distance, then stalled until enough stress built up again

to recommence the failure propagation. Figure 22 shows a summary of the peak loads measured in the

localized tension case. An image of the surface of material failure is shown in Figure 23.

The Type A, B1, and B2 specimens resulted in peak measured load between approximately 75 and 225

lbf, but there is not a clear correlation between disbond length and peak measured load. However, the Type

B3 specimens are both less than 30 lbf due to the fact that only the 1.0 inch disbond was pulled with RTV

adhesive. Therefore, the seal-to-retainer bond was stressed in peel almost immediately, leading to material

failure or peel progression.

IV. Summary

The experiments presented in this paper were part of the Low Impact Docking System (LIDS) main

interface seal development. The key concern addressed in this work was whether the seal-to-retainer bond

in a composite seal assembly can be relied on for elastomer seal retention during interface separation. The

bond strength between the elastomer and retainer was quantified on both nominal and intentionally flawed

specimens by simulating high adhesion loads during LIDS separation. The measured failure load and observed

manner of failure were used to characterize the bond strength.

The seal assemblies were tested under both uniform and localized tensile loads. The uniform case represented the normal LIDS separation scenario where the S0383-70 develops nearly uniform adhesion to the

mating surface. The localized case represented a worst-case scenario, where an area of increased localized

adhesion coincides precisely with a disbond. In the uniform tension case, the bond was found to fail mainly

in the less desirable adhesive mode (clean pull-out of the elastomer from the retainer). The results of the

localized tension tests showed adhesive peel-out of the entire seal similar to the uniform case, as well as

S0383-70 material failure.

Neither tensile case indicated a strong correlation between the size of intentional disbond in the seal

assembly and the failure load measured, or the failure modes observed. Failure loads on the order of 4000

lbf were measured for uniform tension case specimens. In the localized tension case, the Type B3 specimens

showed lower failure loads because 1.0 inch disbonds were pulled with 1.0 inch lengths of RTV adhesive which

represented a worst-case scenario and led to immediate peel initiation. However, the lack of a relationship

in the other localized case tests between failure load measured and flaw level suggested that material and

bond variability are more influential than the length of the disbond. Failure initiation loads on the order of

200 lbf were observed for specimens that failed in a relatively desirable material failure mode in the localized

tension scenario. Analysis of the localized tension case results also suggested that once peel is initiated by

high localized adhesion, there is a significant possibility of the entire seal being extracted.

References

1Lewis, J. L., Carroll, M. B., Morales, R. H., and Thang, D. L., “Androgynous, Reconfigurable Closed Loop Feedback

Controlled Low Impact Docking System with Load Sensing Electromagnet Capture System,” 2002.

2Zimpfer, D., Kachmar, P., and Tuohy, S., “Autonomous Rendezvous, Capture and In-space Assembly: Past, Present and

Future,” 1st Space Exploration Conference, 2005.

3“NASA’s Exploration Systems Architecture Study Final Report,” 2005.

4National Aeronautics and Space Administration, “NASA-STD-(I)-6016:Standard Materials and Processes Requirements

for Spacecraft,” 2007.

5Daniels, C. C., deGroh, III, H. C., Dunlap, Jr, P. H., Finkbeiner, J. R., Steinetz, B. M., Bastrzyk, M. B., Oswald,

J. J., Banks, B. A., Dever, J. A., Miller, S. K., and Waters, D. L., “Characteristics of Elastomer Seals Exposed to Space

Environments,” 43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, No. AIAA 2007-5741, 2007.

6Smith, I., Daniels, C. C., Dunlap, Jr, P. H., and Steinetz, B. M., “Performance of Sub-scale Docking Seals Under Simulated

Temperature Conditions,” 44th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, No. AIAA 2008-4713,

2008.

7Daniels, C. C., Oswald, J. J., Bastrzyk, M. B., Smith, I., Dunlap, Jr, P. H., and Steinetz, B. M., “Experimental Investigation

of Elastomer Docking Seal Compression Set, Adhesion, and Leakage,” AIAA SPACE 2007 Conference and Exposition, No.

AIAA 2007-6197, 2007.

8deGroh, III, H. C., Miller, S. K. R., Smith, I., Daniels, C. C., and Steinetz, B. M., “Adhesion of Silicone Elastomer Seals

for NASA’s Crew Exploration Vehicle,” Tech. Rep. NASA TM-2008-215433, National Aeronautics and Space Administration,

2008.

9Petrie, E. M., Handbook of Adhesives and Sealants, McGraw-Hill Professional, 2nd ed., 2006, pp. 7–8.

NASA/CR—2010-216886 
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1. REPORT DATE (DD-MM-YYYY)

01-10-2010 

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Final Contractor Report 

3. DATES COVERED (From - To)

4. TITLE AND SUBTITLE

An Experimental Investigation of Silicone-to-Metal Bond Strength in Composite Space 

Docking System Seals 

5a. CONTRACT NUMBER

NNX08AU01H; NNC08CA35C 

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Conrad, Mason, C.; Daniels, Christopher, C.; Bastrzyk, Marta, B. 

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

WBS 644423.06.31.04.01.03.22 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Case Western Reserve University 

8. PERFORMING ORGANIZATION

 REPORT NUMBER

E-17465 

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National Aeronautics and Space Administration 

Washington, DC 20546-0001 

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NASA 

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Subject Category: 18 

Available electronically at http://gltrs.grc.nasa.gov 

This publication is available from the NASA Center for AeroSpace Information, 443-757-5802

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The National Aeronautics and Space Administration (NASA) is currently developing a new universal docking mechanism for future space 

exploration missions called the Low Impact Docking System (LIDS). A candidate LIDS main interface seal design is a composite assembly 

of silicone elastomer seals vacuum molded into grooves in an electroless nickel plated aluminum retainer. The strength of the silicone-tometal bond is a critical consideration for the new system, especially due to the presence of small areas of disbond created during the molding 

process. In the work presented herein, seal-to-retainer bonds of subscale seal specimens with different sizes of intentional disbond were 

destructively tensile tested. Nominal specimens without intentional disbonds were also tested. Tension was applied either uniformly on the 

entire seal circumference or locally in one short circumferential length. Bond failure due to uniform tension produced a wide scatter of 

observable failure modes and measured load-displacement behaviors. Although the preferable failure mode for the seal-to-retainer bond is 

cohesive failure of the elastomer material, the dominant observed failure mode under the uniform loading condition was found to be the less 

desirable adhesive failure of the bond in question. The uniform tension case results did not show a correlation between disbond size and 

bond strength. Localized tension was found to produce failure either as immediate tearing of the elastomer material outside the bond region 

or as complete peel-out of the seal in one piece. The obtained results represent a valuable benchmark for comparison in the future between 

adhesion loads under various separation conditions and composite seal bond strength.

15. SUBJECT TERMS

Docking; Adhesion; Seals; Elastomers; Spacecraft docking 

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