2006-01-2285
Micrometeoroid and Orbital Debris Enhancements of Shuttle
Extravehicular Mobility Unit Thermal Micrometeoroid Garment
Robert Jones, David Graziosi, Jinny Ferl, Keith Splawn,
David Zetune and David Cadogan
ILC Dover LP
Eric L. Christiansen
NASA Johnson Space Center
Copyright © 2006 SAE International
ABSTRACT
As NASA is preparing to extend man’s reach into space,
it is expected that astronauts will be required to spend
more and more time exposed to the hazards of
performing Extra-Vehicular Activity (EVA). One of these
hazards includes the risk of the space suit bladder being
penetrated by hypervelocity micrometeoroid and orbital
debris (MMOD) particles. Therefore, it has become
increasingly important to investigate new ways to
improve the protectiveness of the current Extravehicular
Mobility Unit (EMU) against MMOD penetration.
ILC Dover conducted a NASA funded study into
identifying methods of improving the current EMU
protection. The first part of this evaluation focused on
identifying how to increase the EMU shielding, selecting
materials to accomplish this, and testing these materials
to determine the best lay-up combinations to integrate
into the current thermal micrometeoroid garment (TMG)
design. Part of this study included using extensive
hypervelocity testing to identify potential candidate
materials. The last part of this study expanded on the
previous results by conducting a more thorough
investigation into the performance of the top three
candidate lay-ups for micrometeor protection. The ability
to manufacture the candidates into the current TMG and
their effects on the torque of a mobility joint were the
main focus points. This paper summarizes the findings
of this study.
INTRODUCTION
In an attempt to better understand and mitigate the risk
of micrometeoroid and orbital debris penetrating the
bladder of the EMU (see Figure 1), NASA implements a
probability model to estimate the protection provided by
the outer layers of the suit to the bladder. The estimate
Fig 1: Extravehicular Mobility Unit (EMU)
of protection is reflected in a Probability of No
Penetration (PNP) value. The current level of acceptable
PNP through the EMU bladder is .91 which was defined
by the prediction model, “ORDEM 96”. Recently, a new
MMOD Environmental Model, “ORDEM 2000”, was
developed to refine the PNP value and replace the
previous ORDEM 96 model. However before the new
model was finished, NASA expressed concern that new
data and a refinement to the prediction model may
indicate that the current level of protection to the EMU
bladder maybe insufficient. To prepare for the results of
the new model, NASA initiated a study through ILC
Dover into how to increase the current level of protection
provided by the EMU outer layers.
Thus far, this study has been divided into three phases.
The first phase began by reviewing literature and new
technology on how to improve the current EMU to
withstand impact. Two methods were studied: 1.) the
use of a self-sealing bladder and 2.) adding more
shielding to the EMU TMG.
The self-sealing bladder approach was reviewed in 2003
by ILC. In that study, methods of containing self-sealing
material were reviewed to evaluate implementation into
the current restraint/bladder fabrication. A self-sealing
bladder with gores was fabricated and integrated into a
lower arm assembly. The result of that evaluation
proved that a sealing bladder is possible but it is difficult
to manufacture and adds weight. (1)
The first part of the study into improving the shielding
effectiveness of the TMG began by examining current
MMOD shielding found on satellites, the ISS, and the
current TMG lay-up to observe how they were fabricated.
It was noted that the majority of the MMOD protection
system designs are based on the Whipple Bumper
effect. In a Whipple Bumper, hypervelocity particles are
broken apart by an outer “smasher” layer, the particles
are dispersed through a “spacer” layer, and are finally
absorbed by an “absorber” layer. This type of shielding
is already built into the current TMG.
into a number of different combinations to be compared
using hyper velocity impact (HVI) testing at the NASA
facility in White Sands New Mexico. The purpose of this
testing was to find the ballistic limit of each material to
identify which material provided the best protection.
From this testing, it was found that Silicone Coated
Nextel, Silicone Coated Kevlar, and Neoprene Coated
Nylon all have similar ballistic limits. Therefore, the
Neoprene Coated Nylon was selected as the absorber
material because it is already used on the current TMG.
The ballistic testing concluded that the Prima Loft
performed as good as the Open Cell Foam did but
because of undesirable thermal limitations with Prima
Loft, the OCF was selected as the best spacer
candidate.
The next part of this phase involved defining how much
and where these materials needed to be integrated into
the TMG. Three combinations of lay-ups were proposed
as samples to be tested, which are identified as thin,
medium, and thick.
Layers
Outer Layer
Material
Orthofabric
MLI (5 - 7 layers)
Neoprene Coated Nylon
Radiation
Liner Layer
Baseline MMOD Lay-up
Fig 2: Functions of current TMG layers – MMOD
specific
Layers
Outer Layer
Material
Orthofabric
MLI (7 layers)
Radiation
The Whipple Bumper design was broken apart to identify
areas where the TMG could be modified to enhance the
effect. The amount of lofting, the number of particle
destroying layers, the tenacity of the fabric layers, the
stopping effectiveness of the outer layer, the thickness,
and the mass of the layers were all identified as potential
items for improvement. These properties were then
rated according to how much they would improve the
current TMG against penetration. A trades study was
performed on different types and lay-ups of materials to
identify which could yield the largest gains in protection
while also maintaining the mobility of the TMG for flexing
parts.
This study aided in the down selection of materials to be
evaluated as possible candidates to enhance the TMG.
It was decided early on that the Orthofabric currently
used on the TMG was a good choice as an outer
“smasher” layer due to its ideal combination of
properties. These properties include protection against
atomic oxygen and abrasions, strength, resistance
against wear and tear, electrostatic charge dispersion,
and protection against chemical influences such as
spacecraft fuel remainders. Prima Loft, a spun-laced
polyester, and Open Cell Foam (OCF), a urethane, were
suggested as possible “spacer” layers. Finally, silicone
coated Kevlar, silicone coated Nextel, Spectra 1000,
silicone coated Vectran, and Neoprene Coated Nylon
were reviewed as possible “absorber” layers.
Phase II involved the testing of these selected materials.
The absorber and spacer candidates were constructed
Absorber Layer
Neoprene Coated Nylon
Liner Layer
Neoprene Coated Nylon
Thin MMOD Lay-up
Layers
Outer Layer
Material
Orthofabric
MLI (7 layers)
Open Cell Foam (OCF)
Neoprene Coated Nylon
Neoprene Coated Nylon
Radiation
Spacer Layer
Absorber Layer
Liner Layer
Medium MMOD Lay-up
Layers
Outer Layer
Material
Orthofabric
MLI (7 layers)
Open Cell Foam (OCF)
Neoprene Coated Nylon
Open Cell Foam (OCF)
Neoprene Coated Nylon
Radiation
Spacer Layer
Absorber Layer
Spacer Layer
Liner Layer
Thick MMOD Lay-up
Fig 3: Proposed Lay-ups of Selected Materials
(Note: At the time these lay-ups were proposed, it was
undecided where the best location was for the OCF.
Testing found that the location of the OCF was best
between the two-absorber layers. This also provided
protection from the OCF abrading the MLI).
Using this data, the new MMOD Environmental Model,
and these proposed lay-ups, a sensitivity study was
performed on various regions of the EMU. This study
enabled a determination of which areas of the EMU are
most vulnerable to penetration and therefore which areas
are in greatest need of improvement. A finite element
model of the EMU was used to perform this analysis.
The waist, thigh, leg, boot, shoulder, and PLSS were all
identified as areas with the greatest MMOD risk.
The prediction model was updated to reflect the
prototype lay-ups covering critical parts of the EMU. The
thick lay-up was not considered in this part of the study
due to the determination that it was too thick and bulky
for mobility and manufacturability. See the following
figures for the results of this model.
As can be seen from the above figures, the new MMOD
model predicted a doubling of the amount of protection to
the EMU with most of the vulnerable parts being covered
with the thin combination. Also, it predicted a tripling of
the protection with most of the vulnerable parts being
covered with the medium lay-up. These findings led to
further testing for verification of the model’s results which
Phase III of this evaluation addressed.
CURRENT MMOD EVALUATION
The goals of Phase III of this study were to validate the
predictions made by the MMOD Environmental Model,
manufacture a mock-up leg TMG to evaluate
manufacturability and impact to torque and to provide
recommendations on what changes to make to the
current EMU. All of these goals were accomplished.
HYPERVELOCITY TESTING
Fig 4: Extravehicular Mobility Unit
Baseline Lay-up
PNP: 0.938 Risk: 6.2% Odds: 1 in 16
Gray = Baseline, Aqua = Thin, Blue = Medium
Fig 5: Extravehicular Mobility Unit
Option 1 Thin Lay-up
PNP: 0.971 Risk: 2.9% Odds: 1 in 34
Gray = Baseline, Aqua = Thin, Blue = Medium
Fig 6: Extravehicular Mobility Unit
Option 2 Medium Lay-up
PNP: 0.978 Risk: 2.2% Odds: 1 in 45
Gray = Baseline, Aqua = Thin, Blue = Medium
The first goal of this phase was met by manufacturing
and testing another round of samples using the
Hypervelocity Impact (HVI) testing to determine more
precise ballistic limit equations to input into the bumper
model.
This testing was coordinated by the JSC
Hypervelocity Impact Technology Facility (HITF) and was
conducted during May and June of 2005. ILC Dover
manufactured ninety (90) test articles used for
investigating their ballistic limits. The HITF supported the
testing by conducting the tests, analyzing the results, and
writing a report that summarized the results.
HITF’s test objectives were to perform hypervelocity
impact tests with different sizes and types of projectiles,
assess the damage done, determine the critical projectile
diameter, and determine the ballistic limit(s) of the layups by simulating MMOD damage to the EMU TMG.
Also, they were asked to determine if the addition of 2
more layers of aluminized Mylar insulation (MLI) would
significantly alter the ballistic limit of the TMG and to
determine the optimum location for the OCF in the layup. During testing, they rated the results of each sample
and gave it a “pass/fail” by inspecting the damage
inflicted to the Urethane Coated Nylon Bladder. They
defined failure as “an impact that penetrates the
Urethane Coated Bladder layer of the sample lay-up and
causes the Leak-Tec to bubble out when the bladder is
pressurized to 2-psi or it is clear that the bladder has
been penetrated and there is impact residue on the
Plexiglas witness plate in such case no pressure test is
warranted.”
90 test articles were fabricated which were comprised of
fifteen EMU TMG Thin Lay-ups, thirty-nine EMU TMG
Thin-2 Lay-ups, and thirty-six EMU TMG Medium Layups.
The Thin-2 Lay-ups were a result of some
questions that arose during testing concerning the
amount of standoff between materials that the current
EMU contains.
They were fabricated in order to
represent the current TMG configuration more
accurately. Each combination was constructed into a 6”
x 7” panel with a Plexiglas back to view bladder damage.
Figure 9: Front of EMU TMG
Medium Lay-up Test #48
Urethane Coated Nylon (Bladder) – Front
Fig 7: Test Article Assembly
Below are some photographs taken after a shot was
fired. The pictures included below represent the medium
lay-up. As can be seen, this lay-up was able to
adequately protect the Urethane bladder and thus was
given a “pass” rating.
This testing resulted in a large matrix that outlines the
ballistic limits for all of the lay-ups and is included in the
Appendix “A.1 – Phase II Hypervelocity Impact Testing
Results” at the end of this report. From this testing, it
was also determined that the addition of 2 extra layers of
aluminized Mylar insulation did not improve the
protection significantly. In the case of both the five layer
samples and the seven layer samples, the bladder was
perforated each time. It was also determined that the
two neoprene coated nylon layers separated by the open
cell foam allowed the debris cloud to better disperse.
Therefore, the optimal medium lay-up was proven to be
Orthofabric, MLI, Neoprene Coated Nylon Absorber,
Open Cell Foam, Neoprene Coated Nylon, Dacron
Polyester Restraint, and Urethane Coated Nylon Bladder.
After testing was completed, the results were input into
the bumper model to refine the risk estimates. This
analysis was run in parallel with the next step of this
phase.
MANUFACTURING
Fig 8: Front of Post-Test EMU TMG
Medium Lay-up Test #48
The second goal of this phase was to manufacture a
mock-up leg TMG to assess the best method for
integrating these new materials into the current TMG. A
leg TMG was chosen because it is a flexing part and an
increase in TMG thickness would increase the torque, a
critical design consideration. At this point in the study,
the bumper model and results from the ballistic testing
indicated that the level of protection of the EMU could
increase three times if the majority of the most
vulnerable parts were covered with the medium lay-up.
The majority of the vulnerable parts are flexing parts and
therefore are susceptible to a change in torque. As a
result, it was decided that the medium lay-up would be
used for the mock-up TMG. Due to its robustness over
the thin lay-up, it was surmised that the medium lay-up
would yield the largest gains for the scope of the project.
The manufacturing study began by fabricating sample
gores to assess how the layers would interact with each
other and what technique would minimize the amount of
torque the additional layers would add. Since the TMG
lay-up would remain relatively the same with the
exception of an additional layer of Neoprene Coated
Nylon, the issue to resolve was how to integrate the
Open Cell Foam.
Thirteen concepts were fabricated, each one detailing a
different method of attaching the OCF. Methods were
derived from other soft good products that ILC has
manufactured and used at other locations on the space
suit.
After these concepts were fabricated, a trades study was
conducted to down select to the best manufacturing
method. The chosen concept would be made into a fullsized mock-up. A trades matrix was generated to rate all
thirteen concepts according to ten categories that were
created to compare the concepts’ attributes in all areas.
For each category, a specific concept was given a grade
on a scale of 1 to 10 on how well it satisfied the category.
The higher the grade a concept got for a category, the
better it was compared to the other concepts. Grades
were assigned keeping in mind the objectives of this
evaluation and the concept’s impact on the current EMU
design and fabrication. This grade was then multiplied
by a weighting factor in order to enable certain
categories to have more weight in the final rating of the
concept. For instance, a concept’s effect on torque was
weighted more heavily than its required manufacturing
time. This weighting aided in making each category
impact the final score according to its importance. The
weighted grade for each respective category was added
up for each concept to form the total rating, which
determined each concept’s ranking with respect to one
another.
The higher the total rating, the better the
concept accomplished the goal while decreasing the cost
of adding extra layers.
ILC Dover compiled a trades matrix for this study using
engineering judgement and experience to rank each
concept. This trades matrix is included in Appendix “A.2
– MMOD Trades Matrix for TMG Thickening of Knee
Joint.”
With torque being one of the most heavily weighted
categories in the study, it was decided that all of the
concepts that rated high in the torque category could be
eliminated.
chosen concept and also allow comparison testing of
each concept to help to better understand the unknowns
involved.
For fabricating layers in the current TMG such as the
Orthofabric layer and the MLI layers, work instructions
were followed as closely as possible maintaining all of
the required tolerances and precision. For each new
OCF concept, detailed notes were kept on the fabrication
of the new layers to document the process and then
work instructions for those layers were generated. This
was done for each concept so that after manufacturing
was completed each process could be reviewed to
assess its difficulty compared to the others. Also,
pictures were taken detailing the techniques used in the
process to capture what each concept looked like.
The completed concepts were weighed to compare them
to a traditional TMG as a first step in analyzing how
much the additional layers would impact the weight of the
EMU.
Fig 10: Completed TMG Mock-ups
TMG
Weight (lb)
Baseline*
1.18
3
1.71
8
1.72
13
1.73
*Average of 5 Class I TMG's
Table 1: Weight Comparison
The evaluation showed that multiple concepts appeared
to be good and ranked high and almost equal in the
matrix. One reason why it was difficult to narrow it down
to one concept was that, as with any trades study
performed on a prototype, some categories must be
ranked on an educated guess rather than knowledge
because this type of manufacturing had never been done
before. Therefore, it was decided that it was within the
scope and timetable of this evaluation to fabricate and
test three full-sized leg TMG mock-ups to aid in the
selection of one best method. Three mock-ups would
allow the complete fabrication of a TMG using each
The average percent weight increase of the prototypes
over the baseline was 45.8%. By completing these three
concepts, it was found that each concept had its own
unique challenges to building it into a TMG and the
manufacturing categories listed in the trades matrix
could further be defined.
It was decided from a
manufacturing standpoint that Concept 13 seemed the
best concept to recommend for this evaluation.
The third goal of this phase was to provide a torque
evaluation of the selected construction method verses a
baseline Class I leg TMG. Because three TMG mockups were constructed each representing a different
manufacturing concept, the torque evaluation was also
used to determine if one manufacturing method was
better for torque over the others. The thought was that
this would also aid in selecting one best concept.
The torque evaluation began immediately after the
completion of the three TMG mock-ups. The testing was
performed in the ILC Test Lab. To serve as a baseline
measurement, a Class I leg TMG was used. It was
decided to use a Class I item because of its relatively low
amount of wear. It was a concern that a “broken in”
TMG would yield lower torque measurements than a new
one.
Test plugs were machined for the test stand and a Test
Plan was generated to outline the scope of the test. In
the test, each TMG including the baseline was laced onto
a leg restraint and bladder that had been down-graded
and given to Engineering for the testing. The leg was
inflated to 4.3 psig to simulate pressures used in the
EMU during EVA.
It was found in the
Specification/Assembly Drawing (S/AD) requirements for
the lower torso assembly (LTA) of the current EMU, that
the knee joint should not exceed 155 in-lbs. of torque
when flexed to 75 degrees.
The outputs from a
rotational encoder and a force transducer were fed into a
DAQ system and the real-time results such as angular
velocity and rotation could be viewed during testing. This
also aided in maintaining consistency from trial to trial for
the test to be fair.
could flex the leg while monitoring the data on the output
screen.
Second, each TMG was flexed from an initial starting
position to the maximum deflection that the restraint and
bladder would allow, and then back to the initial position.
The maximum angle was determined to be 75 degrees,
which is the point where the cylinder began to break
down and the gore pattern was no longer effective. This
range was completed five times for each TMG. Data
from both ranges of motion was collected so that the
torque values of each trial could be averaged and
compared.
Shown here are the results of the torque testing from
averaging the data for each TMG:
Compare Results of 55 Degrees Torque Testing
200
150
100
Torque (in-lb)
TORQUE EVALUATION
Concept 3 Avg.
Concept 8 Avg.
Concept 13 Avg.
Class I Avg.
50
0
0
10
20
30
40
50
60
70
80
90
-50
-100
Rotation (deg)
Fig 12: Graph Comparing Results of 55 Degree
Torque Testing
*Note: Red line indicates 75 in-lbs of torque but is
shown in this graph merely to aid in visual
comparison of the data
Compare Results of Max Degrees Torque Testing
200
150
Fig 11: Test Stand / Installed TMG
Each TMG was subjected to two ranges of motion for the
purpose of comparing them at two different points. First,
each TMG was flexed from an initial starting position,
marked by 0 degrees and which was determined by
pressurizing the restraint and bladder and allowing them
to inflate without guidance to their natural resting
position, to a maximum rotation of 55 degrees and then
back to the initial position. This range was completed
seven times for each TMG so data could be averaged to
eliminate any operator-induced results such as a sudden
increase in angular velocity, which would cause an
unnatural spike in the recorded torque. Also as each trial
was run, an attempt was made to hold the angular
velocity constant at 2°/sec to maintain consistency. This
value was established after performing several test trials
to determine the optimal rate at which the test operator
Torque (in-lb)
100
Concept 3 Avg.
Concept 8 Avg.
Concept 13 Avg.
Class I Avg.
50
0
0
10
20
30
40
50
60
70
80
90
-50
-100
Rotation (deg)
Fig 13: Graph Comparing Results of Maximum
Degree Torque Testing
*Note: Red line indicates 75 in-lbs of torque but is
shown in this graph merely to aid in visual
comparison of the data
Several results came from performing this testing. First,
it is clearly shown in the graphs that the torque required
to flex the studied TMG’s resulted in a hysteresis pattern.
This could be felt in the TMG as it was flexed to its
farthest rotation and back as it had a “spring-back” feel
to it. The hysteresis was a result of the “memory” of the
material and its tendency to return to its natural, unflexed
state similar to how a spring behaves. However, due to
the construction of the TMG which has multiple layers
rubbing against each other and creating friction, the leg
assembly tended to come to rest before returning to its
starting position therefore requiring a negative torque to
push it back into place. This is why the hysteresis curves
begin and end at different values on the graph. This
effect was expected as it has been observed during
torque studies performed on other EMU parts.
Second, during testing it was noted that the way the test
operator applied force to the handle of the test stand
varied the torque output in a way not related to the
construction of the TMG. While this does not apply
directly to the outcomes of interest for this study, it is
important to note here to show that during testing a more
consistent method of testing was applied to the trials and
therefore created the need for several trials to be rerun.
This insured that all testing was completed accurately
and that the data reflects a fair comparison. Initially, the
operator was holding the handle of test stand in a grip,
which applied some downward force thus effectively
reducing the amount of torque in the direction of the bent
knee. After realizing this, a new method of applying
force with one finger only in the direction of rotation was
applied. This allowed for much more uniform results.
evaluation because the method of fabrication does not
change the torque significantly enough for it to determine
which method to use.
However, the tests also showed that altering the current
TMG lay-up to reflect the proposed medium MMOD layup would increase the torque at least 13 in-lbs. at a
rotation of 55 degrees.
CONCLUSION
This study was initiated due to concerns that the current
EMU may not provide an acceptable level of MMOD
protection. Therefore, the overall goal of this study was
to evaluate how to increase the protection provided to
the bladder in the EMU.
In the first two phases of this evaluation, materials were
selected and tested to determine the order of the various
layers that would provide the optimal MMOD protection.
Orthofabric, Open Cell Foam (OCF) and Neoprene
Coated Nylon were the materials selected from this
testing. After these materials were identified, they were
combined into lay-ups of varying thickness.
The purpose of the third phase was to evaluate the
manufacturability and performance of the medium
thickness lay-up selected in the previous phases.
Thirteen methods of fabricating a full-scale mock-up
were evaluated, and three were selected. After further
evaluation, the final method chosen was the one where
assembling the mock-up closely resembled the
assembly of production line TMGs.
A weight comparison was performed. The mock-up TMG
weighed 46% more than a baseline TMG.
Figure 14: Proper Technique Used for
Consistent Torque Testing
Finally, the most interesting conclusion from this testing
pertains to the goal of this study: to find out how the
additional protection to the TMG affected the torque and
which fabrication method reduced the torque increase
the most. This testing indicated that there were no
significant differences in torque among these three
manufacturing methods.
For the 55-degree range
comparison, all three prototypes reached an almost
identical maximum torque of 87 ft-lbs compared to the 74
ft-lbs reached by the baseline. This was a 17.6%
increase. For the 75-degree comparison, they again
tested very similarly with only Concept 8 reaching a
slightly less maximum torque value.
The average
percent increase of the prototypes from the baseline for
this degree range was a 35.4% increase. Therefore, it
can be concluded that the method of fabrication for this
new lay-up should be based solely on the manufacturing
When torque evaluations were performed on the mockups based on the three methods mentioned above, data
indicated that there were no significant differences in
torque among the three manufacturing methods.
However, the mock-ups required an average of 35%
more torque than the baseline TMG. Since the new layup exceeds the current maximum allowable torque for
the knee joint, a reassessment of the current
requirement would be needed before this lay-up could be
incorporated into the suit.
During this evaluation, there was a concurrent effort to
improve the risk predictions made by the MMOD
probability model. The outcome of that effort was that
th
after re-sizing the EMU model for a 95 percentile male,
the PNP (probability of no penetration) increased to
0.938, which is higher than the previous value and
therefore meets the current requirement. Thus, no
changes to the current EMU are required by NASA.
Furthermore, the model shows that by modifying some of
the non-mobility areas of the TMG the PNP can be
increased two- to three-fold. While this degree of
protection may not be required currently, future EVA
activity may call for a suit that offers such protection.
REFERENCES
1. D. Cadogan, C. Shear, A. Dixit, J. Ware, J. Ferl, E.
Cooper, P. Kopf, “Intelligent Flexible Materials For
Deployable Space Structures (INFLEX).” 36th ICES
Conference, SAE, 2006
CONTACT
Bobby Jones
ILC Dover
302-335-3911 x512
[email protected]
DEFINITIONS, ACRONYMS, ABBREVIATIONS
EMU - Extravehicular Mobility Unit
EVA – Extra Vehicular Activity
HITF - Hypervelocity Impact Technology Facility
HVI - Hyper Velocity Impact
MLI – Multi-Layer Insulation
MMOD - Micrometeoroid and Orbital Debris
OCF - Open Cell Foam
PNP - Probability of No Penetration
TMG - Thermal Micrometeoroid Garment
APPENDIX
A.1
PHASE II HYPERVELOCITY IMPACT TESTING RESULTS
A.2
MMOD TRADES MATRIX FOR TMG THICKENING OF KNEE JOINT
(*Note: This trades matrix was generated during the study in an effort to down select to a few candidates for
integrating the foam into the TMG. Weighted categories were chosen to reflect differences due to manufacturing
techniques and do not include all issues evaluated in this study.)