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Jerry D. Stachiw.

Development of a spherical acrylic plastic pressure hull for hydrospace application online

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a distance of 12.000 inches from the center of the mold as would be expected
from a sphericity-measuring technique that used the surface of the mold at
that location as reference datum. Second, the deviations from the nominal
sphericity of the formed spherical sector followed closely the trend of the
deviations of the surface mold. But since the sphericity of the sector was
measured on its concave surface, and the thickness of the sector varied
somewhat from point to point (Figure 30), a quantitative comparison
between the sphericity of the convex surface on the sector and the spher-
icity of the concave surface on the mold could not be made accurately.
Third, the radius of curvature on the concave surface of the sector appeared
to be greater than the 30.500 inches specified.



54




Figure 26. Sawing acrylic plastic plate stock into circular discs.




Figure 27. Turning edge of circular disc prior to placement in mold.



55




Figure 28. Circular disc after thermoforming at 300 F into spherical sector with
a 33-inch radius of curvature; note thermocouples embedded in acrylic
plastic for measurement of temperature.



33.000-inch-long arm
with dial indicator




5 5

Lateral Radius (inches)



Figure 29. Actual sphericity of the mold for thermoforming sectors with
33-inch radius of curvature.



56



30.500-in.-long arm
with dial indicator




Measurements were taken along the same
meridian as on the mold.
The deviations shown are with respect to an
arc described by a 30.500-in. radius indexed
with respect to the reference datum on con-
cave surface.
I I I I I



10



10



5 5

Lateral Radius (inches)
Figure 30a. Typical deviations from nominal 33-inch radius of curvature on
thermoformed sector 18 for the 66-inch-diameter capsule.




hickness Before


Thickness After


Forming (in.)


Forming (in.)


2.390


2.362


2.467


2.556


2.502


2.592


2.477


2.593


2.404


2.477


2.430


2.410


2.432


2.425


2.425


2.404


2.430


2.420



Note: The increase in thickness is caused by lateral
shrinking of previously "unshrunk" grade G
Plexiglas plate after it was subjected to 300 F
forming temperature. The thinning out of the
formed sector at the center is due to stretching
of acrylic as the previously flat plate is forced
to conform to the spherical surface of the mold.



Figure 30b. Effect of thermoforming on thickness of sector 18 for the
66-inch-diameter capsule.



57



In view of the fact that the deviations from sphericity of the formed
sector were less than 1.0% of the specified radius, no real incentive existed to
modify at that time the existing curvature of the mold by remachining it to an
exact 33-inch radius. Besides, it was predicted (and subsequently verified by
actual measurements) that the deviations from specified sphericity during the
thermoforming process are minor compared to the deviations resulting from
machining of the pentagons. However, the generated data on deviations in
sphericity proved conclusively that specifying the nominal radius of curvature
of the mold 33.187 instead of 33.000 inches produced thermoformed sectors
with radii of curvature larger rather than smaller than the specified 33.000
inches. Thus, it is recommended that before additional 66-inch acrylic plastic
spheres are fabricated, the curvature of the female mold be remachined
accurately to the specified radius of curvature for the sphere.

Machining. The transformation of the spherical sectors into spherical
pentagon modules was performed in two steps. The first step consisted of band-
sawing the spherical sector into an oversized spherical pentagon (Figure 31).
The second step, reducing the oversized spherical pentagon to the dimensions
calculated for the spherical pentagon module, was performed on a vertical mill.
For machining of model spherical pentagons, a manually operated mill was
utilized (Figure 32), while for the machining of full-scale pentagons, a magnetic-
tape-controlled mill was chosen (Figure 33). To facilitate the holding of the
33-inch-radius spherical sector in the bandsaw and in the vertical mill, a vacuum
chuck (Figure 34) was designed and built by the shop personnel at the Pacific
Missile Range. This chuck not only held the spherical sector in place but also
served as a pneumatically operated indexing head for accurately locating the
five straight beveled edges of the pentagon.

The machining of the large pentagons was performed with a 2-inch-
diameter, helical-type milling cutter rotating at 3,800 rpm and fed at 10 in./min
(Figure 33). A detergent and water cutting fluid was used to keep the cutter
and the work piece cool so that serious stress concentrations would not be
introduced into the spherical pentagon. The selected cutter, rotational speed,
feed rate, and coolant resulted in a 63-rms finish for the beveled edges of the
spherical pentagon.

Regardless of whether the pentagons were machined manually, as was
the case with the model scale ones, or by preprogramed tape, as with the large
pentagons, each pentagon had to be dimensionally checked before acceptance.
The dimensional check consisted of measuring the linear distance between
opposite tips of the pentagon with a specially built caliper having a 46-inch
throat (Figure 35). Only if all the measurements between the opposite tips
of the pentagon were within the specified dimensional range was the pentagon
considered completed. To reduce the influence of temperature on the mea-
surements, all readings were taken in the 65 to 75°F range. Although in the



58



beginning of the machining operation grave doubts existed whether pentagons
could be machined to the dimensional tolerances specified (Appendix A) for
the prototype 66-inch sphere, they soon were dispelled by these dimensional
checks: the dimensional tolerances specified could be maintained. After the
dimensions were checked, the large spherical pentagons were placed into con-
toured, padded plywood boxes (Figure 36); the model pentagons were stacked
in shop baskets.




Figure 31 . Sawing spherical sectors into rough spherical pentagon shape
for the 66-inch-diameter capsule.



Annealing. No annealing of the model pentagons was performed after
machining, as there were no contoured boxes or portable molds to support
them during annealing. The annealing of the model pentagons was performed
only after complete assembly and bonding of the model spheres, when the
individual spherical pentagons could not readily change their curvature.

Large-scale spherical pentagons with a 33-inch spherical radius were
annealed after completion of all the machining operations. This was per-
formed to decrease the residual stresses introduced into the acrylic plastic
by thermoforming, bandsawing, and milling. It was hoped that the decrease,
if not complete elimination, of tensile stresses on the acrylic plastic surfaces
would eliminate the tendency of acrylic plastic to craze on the machined
surfaces.



59




Figure 32. Milling the edges of spherical pentagon for the 15-inch-diameter
capsule.




Figure 33. Milling the edges of spherical pentagon for the 66-inch-diameter
capsule.



60




Figure 34. Indexing head with a vacuum operated chuck for the large
spherical pentagons.




Figure 35. Dimensional check of large spherical pentagon prior to removing
it from chuck.



61




Figure 36. Storage of completed large pentagons between individual steps of
the fabrication process.



Annealing of the large pentagons was performed in a walk-in oven.
Each pentagon was placed there individually for 24 hours either in a wooden
storage box, or in the mold, while the temperature was maintained at 160°F.
After the annealing period, the temperature in the oven was slowly reduced
to 100°F at which time the pentagon was removed from the oven.

Quality Control. Since it was predicted prior to the machining and
annealing operations that some realignment of the curvature would take
place on the full-scale pentagons when residual stresses were relieved, a
dimensional check was made on all pentagons after all major steps in their
fabrication were completed (Appendix A). Thus, a measurement of the
sphericity and thickness was made on all spherical pentagons after annealing,
but prior to assembly and bonding into a sphere. The effects of bandsawing,
milling, and annealing on the sphericity of the pentagon can be seen by com-
paring the sphericity of the first thermoformed spherical sector (Figure 30a)
to the sphericity of the annealed pentagon made from that sector (Figure 37).



62



e K = 144°




angle
O = 0°0'
</>! = 10°30'
2 = 21°O'



4 = 31°3O'

5 = 37°1O'



1 . Points A and B were marked to give orientation.

2. Angles 1 through 0g are angles through the center
of the sphere.

3. Angles By through B^q are measured from the
center of the pentagon.

4. Pentagons are for the prototype 66-inch NEMO
capsule.

5. AR denotes change in internal surface curvature from
specified 30.500 inches; + indicates shorter radius,
while - indicates longer radius.



Point


Pentagon 18


AR


Thickness


e 1 -<t>o


+0.007


2.385


e 1 -0,


-0.018


2.405


By -02


-0.045


2.435


01 -04


-0.089


2.485


e 1 ~<t>5


-0.127


2.510


&2 -0i
&2 -02



-0.015


2.405
2.435


#2 "04


-0.065


2.465


e 3 -0 1


+0.015


2.400


e 3 -0 2


+0.013


2.405


e 3 -0 4


-0.038


2.435


e 3"*5


-0.083


2.470


8 4 -01


+0.023


2.390


4 -0 2


+0.017


2.400


e 4 "04


-0.024


2.420


05-0!


+0.018


2.395


6 5 -0 2


+0.017


2.405


5 -0 4


-0.039


2.450


e 5"05


-0.100


2.475


e 6"*1


+0.004


2.395


6 -0 2


+0.020


2.400


00 -0 4


-0.073


2.460


^7-01


-0.010


2.400


e 7 -0 2


-0.014


2.430


»7-0 4


-0.134


2.570


e 7 _ 05


-0.187


2.550


e 8 -0i


-0.023


2.415


e 8 -0 2


-0.070


2.485


08~04


-0.168


2.555


e 9~01


-0.028


2.428


0g-0 2


-0.074


2.485


0g-0 4


-0.140


2.545


e 9"05


-0.180


2.560


010-01


-0.029


2.415


1o -0 2
010-04


-0.065
-0.120


2.410
2.515



Figure 37. Typical deviations from nominal 33-inch radius of curvature on a
completed spherical pentagon for a 66-inch diameter capsule.



63



Several conclusions can be made from that comparison. First, the
deviations from specified sphericity are larger after machining operations
than prior to them by a factor of 2 to 4. Second, even with the considerable
increases in spherical deviations, they are less than 1% of specified radius.

Similar measurements (Appendix C) were taken on model pentagons
(Figure 38). The only difference here was that the measurements did not
reflect any deviations from sphericity due to annealing process, as the model
pentagons were not annealed until they were assembled and bonded into
spheres.

Bonding. Initially the assembly of both the models and the 66-inch
spheres was planned to be by the solvent-cement technique. The arguments
for this bonding technique were that (1) the tensile strength of solvent-bonded
joints can be made to equal that of the parent material by proper curing
schedule, (2) all the operational parameters of this bonding technique are
well known, and (3) the joint, if properly made, is completely colorless and
transparent.

Although the arguments cited in support of this technique are valid,
the results from bonding the first model capsule were quite disappointing.
Either the joints were full of air cavities (Figure 39) and surfaces unwetted
by the solvent because of insufficient clearance between the individual pen-
tagon for penetration of solvent, or the joints were completely wetted by
the solvent, but because of too large a separation between pentagons, the
solvent upon drying left a very weak joint.

The reasons for this unsatisfactory performance of solvent-cement
technique were many. The major ones were ( 1 ) all the pentagons had to be
bonded simultaneously so that mismatches between individual pentagons
could be distributed over the whole sphere, (2) edges of pentagons were not
presoaked in solvent prior to assembly because of physical impossibility of
presoaking the five edges of all pentagons simultaneously prior to assembly,
(3) joint clearances and clamping forces varied from point to point depending
on the dimensional deviation from nominal pentagon dimensions, and (4) it
was nearly impossible to apply a calibrated clamping restraint on the whole
sphere during bonding.

In order to improve the quality of the joints, an attempt was made to
control the clearances in the joints so that complete penetration of the joints
by solvent would take place. After an exploratory investigation into this pro-
blem, it was found that a 0.005-inch clearance was required to create the
capillary force that would draw the solvent into all the joint spaces.
Unfortunately, the joint with a 0.005-inch clearance maintained by spacers
had nearly zero strength as the evaporation of the solvent from the wide
joint resulted in very inferior joint bond strength. The decrease of the joint
clearance to 0.001 inch by placement of 0.001-inch-thick spacers resulted in
higher joint bond strength, but did not permit the solvent to wet all the joint
surfaces.



64




i=o u



9 7 = 216



e 10 = 324°



Pentagon Number 6



angle
O = 0°0'
0., = 10°30'

2 = 21°O'

3 = 31°3O'

4 = 37° 10'



Note:

1 . Mark on each pentagon the pentagon number at center
and angle numbers at edge. These numbers should be
maintained throughout fabrication of complete sphere.

2. Angles 0-| through 04 are angles through center of sphere.

3. Angles 0-j through 0-jq are measured from the center of
pentagon.

4. Ar denotes change in internal surface curvature from
specified 7.000 inches; + indicates shorter radius, while
- indicates longer radius.



Point


Ar


Thickness


e 1-«0


+0.003


0.485


• l-#!


+0.0015


0.475


e 1 -02


0.000


0.475


e 1 ~<t>3


-0.006


0.475


01 -0 4


-0.018


0.485


$2 - 0-|


+0.0015


0.475


&2 -02


0.000


0.475


2 -0 3


-0.01 1


0.475


e 3 -0 1


+0.0015


0.470


e 3 -0 2


0.000


0.472


e 3 -0 3


-0.004


0.475


e 3 -0 4


-0.018


0.485


e 4 -0i


+0.002


0.472


4 -0 2


0.000


0.472


64-03


-0.009


0.470


5 -01


+0.0015


0.475


5 -0 2


0.000


0.475


5 -0 3


-0.004


0.485


65-04


-0.010


0.485


05-0,


+0.003


0.475


6 -0 2


0.000


0.485


0g-0 3


-0.009


0.485


e 7 -0i


+0.001


0.472


By -0 2


0.000


0.475


0y-0 3


-0.005


0.485


7 -0 4


-0.009


0.485


0g-0 1


+0.003


0.475


0g-0 2


0.000


0.482


0g-0 3


-0.007


0.485


©g-0!


+0.0015


0.475


0g-0 2


0.000


0.480


0g-0 3


-0.004


0.487


0g-0 4


-0.009


0.492


9 10"*1


+0.0025


0.475


1o -0 2


0.000


0.475


01Q-0 3


-0.009


0.480



Figure 38. Typical deviations from nominal 7.5-inch radius of curvature on a
completed spherical pentagon for 15-inch-diameter capsule model.



65



A procedure was finally developed for solvent bonding model capsules
that produced high-strength bonds over 100% of the joint area (Figure 40):

(1) assemble the model capsule with 0.005-inch spacers between pentagons,

(2) place a clamping restraint on it in the form of rubber bands cut from auto
inner tubes, (3) introduce solvent into the joint by means of a syringe (results
in complete wetting of joint surfaces by capillary action), and (4) rapidly
remove the spacers before the joint surfaces are bonded, thus permitting the
clamping forces to bring the joint surfaces into intimate contact. This proce-
dure was used to assemble and bond model NEMO capsules 1 through 1 1 .

The annealing of the solvent-bonded model capsules consisted of
placing them in an oven heated to 175°F and leaving them there for 24 hours
at that temperature. The annealing process, however, did not eliminate com-
pletely the residual stresses in the joints introduced into the capsule by the
bonding process (Figure 41). Since it was felt that the remaining residual
stresses in the model capsule (approximately 1,300-psi shear stress in the
joint area) would not significantly decrease the implosion pressure of the
capsule, no further effort was made to eliminate the residual stresses after
the single annealing operation described.




Figure 39. Typical air bubbles in an improperly solvent-bonded joint on the
15-inch-diameter capsule model.



66




Figure 40. Typical sample of a properly solvent-bonded joint on a 15-inch-diameter
capsule model.

Although the procedure developed for solvent bonding model capsules
produced satisfactory joints, grave doubts existed whether this technique would
produce satisfactory results for the large-scale capsule. It appeared that in order
to solvent bond the large pentagons, the dimensional control of individual pen-
tagons would have to be so strict as to make the fabrication cost unacceptable.
Also, the cost of a clamping system for placing the solvent-bonded joints in a
large sphere under uniform compression would, in all probability, be higher
than the cost of fabrication up to the bonding stage. In view of these problems,
it was decided to find a different joint bonding technique that, although being
as costly as the solvent bonding technique for the models, would be distinctly
less for the large-scale capsule.

The alternate bonding technique investigated was the cast-in-place
adhesive. The prime characteristics of this bonding technique are: (1 ) the
pieces to be bonded are maintained in a fixed relationship to each other by
spacers that do not permit the pieces to contact each other, (2) the space
between the acrylic plastic members is made fluid tight by placement of
adhesive tapes over both sides of the space, (3) the adhesive is poured into
the taped-over space by means of a funnel or squeeze bottle, and (4) filling
the joint space is done in more than one pour, with the mechanical spacer
being removed from the joint after the first increment of adhesive-filled joint
has set, but before the succeeding increment of adhesive covers it (Figure 42).



67




Figure 41. Residual stresses in a solvent-bonded 15-inch-diameter capsule not

removed by annealing operation; maximum shear stress approximately
1,300 psi.

The advantages of this bonding technique are: ( 1 ) the dimensional
tolerances of spherical pentagons being bonded need not be tight because
reasonable dimensional deviations are taken care of by the 0.100-to-0.150-
inch-wide joint space, (2) there is no requirement for clamping forces on the
members being bonded, (3) irregularly shaped surfaces can be joined, providing
the joint space can be sealed on both sides with adhesive tape, (4) there is no
requirement for fine finish on the acrylic plastic surfaces forming the two
sides of the joint, and (5) the bonding of a large structure can proceed by
small increments with each joint increment becoming set before the next
pouring is performed.



68




Adhesive Pour No. 1




Adhesive Pour No. 2




Adhesive Pour No. 3

!■•■•■ •■■•■•■■■ ■•! Fresh, liquid adhesive
y//////////\ old, solid adhesive

Figure 42. Procedure for casting of
self-polymerizing joints
in acrylic plastic capsules.



In view of the many
advantages that such a bonding
technique possesses for fabrica-
tion of large-scale acrylic plastic
capsules, it was decided to
evaluate this technique first
on small-scale capsules and on
joint-test specimens simulating
the joint dimensions of the 66-
inch capsule. Four cast-in-place
adhesives were investigated:
epoxy, polyester, PS-18 acrylic
cement, and PS-30 acrylic cement.
The epoxy and polyester adhe-
sives were almost immediately
discarded as the tensile strength
of a joint between acrylic plastic
members filled with these adhe-
sives was less than 1 ,500 psi for
epoxy and 1,000 psi for polyester.
Although the compressive strength
of these two adhesives was indis-
tinguishable from that of the
acrylic plastic parent material, the
low tensile strength just made
these adhesives noncompetitive
with the PS-18 and PS-30 acrylic
cements. When 2.5-inch-wide
acrylic plastic blocks were bonded
with acrylic cement in 0.125-inch-
wide joints, it was found that the
tensile strength for PS-18 was
consistently higher than 5,000
psi, while for PS-30 it was higher
than 4,000 psi (Table 3). Com-
pressive strength of these joints
was found to be of approximately
the same magnitude as that of the
parent acrylic plastic material, but
their deformation under load was
somewhat higher.



69





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70



Since it was known that both PS-18 and PS-30 adhesive joints
deteriorate in strength with age when exposed to atmospheric environment,
it was questioned whether immersion in seawater accelerates this aging pro-
cess further. To answer this question, a bonded acrylic plastic block was
placed in the Pacific off Point Mugu at a depth of 120 feet. After 13 months
of continuous submersion, the acrylic plastic block was retrieved, and material
test specimens were machined from it. Because the block was rather small,
and only a limited number of test specimens could be machined from it, only
tensile and compressive test specimens were cut from it. These test specimens
were tested at 70°F to establish the ultimate tensile and compressive strength
of the PS-18 and PS-30 cast joint after 1 year's exposure to seawater.

When the test specimens were tested to destruction, it was found
(Table 3) that the average tensile strength of the 0.125-inch-wide, adhesive-
filled joints was 8,890 psi for PS-18 and 5,710 psi for PS-30 joints. Since no
control block was bonded at the time when the ocean test specimen was
fabricated and placed in the ocean, no accurate comparison can be made
with test specimens exposed for 1 year to atmospheric environment. A
general comparison, however, can be made with tensile and compressive
strengths of PS-18 and PS-30 adhesive-bonded joints tested several days
after bonding. The comparison between strengths of the fresh joints and


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Online LibraryJerry D. StachiwDevelopment of a spherical acrylic plastic pressure hull for hydrospace application → online text (page 5 of 14)