SAMPE Journal - January/February 2023 - 48

FEATURE / INTER-PLY AND TOOL-PLY FRICTION
button load cell provided by Transducer Techniques
(Temecula, CA) (Figure 6E part number LBC-750)
is located directly behind the dimple plate and
mounted to the angle adjuster wheel. Force is
directly transmitted through the keyed shaft to the
platen assembly as the load cell button is in direct
contact with the shaft, which is allowed to protrude
from the angle adjuster wheel. The root-meansquare
(RMS) error of the load cell is 0.2565% of the
rated output.
Two pressure regulators are used to control the
output force of the tie rod air cylinder. To account
for unexpected fluctuations in the supply pressure
a coarse control pressure regulator (Figure 6F part
number SMC AR20K-N01B-Z-B) with ± 0.01 MPa is
used to regulate the incoming supply flow which
proceeds to a holding reservoir. The purpose of
which is to maintain an adequate head of air as the
air cylinder draws or releases air during actuation.
Past the reservoir a second pressure regulator (fig.
6G part number OMEGA PRG200-120H) with ±
0.0069 MPa and supply pressure effect < 3.5E-5
MPa is used to finely adjust pressure supplied to
the air cylinder.
As mentioned earlier in the discussion, it is
important to ensure accurate temperature control
and uniform temperature distribution throughout
the test specimen for a pull-through test setup. The
prepreg is conductively heated by contact with the
platens which are heated using 250 W electrically
resistive cartridge heaters. The sample drawn
through the fixture when in contact with the platens
is heated conductively by the same means, whereas
the free hanging length is convectively heated
by the " tail-heater " . This consists of two sheet
metal clamshell halves that form a small chamber
directly below the platens, equipped with two
7.62 cm (3.0 in) 22.5 W square silicon pad heaters
(part number SH-3x3-2.5-115A). An exposed
thermocouple collects the temperature in the
middle of the enclosed chamber. During a test the
sample therefore maintains uniform temperature
equal to the portion in contact with the platens.
This ensures that as the exposed sample is drawn
through the platen, resin viscosity of the sample
remains uniform along the length of the platen and
consistent characterization is achieved. A Watlow
(St. Louis, MO) proportional-integral-derivative
(PID, part number PM6C1CA-EAAAPWP) controller
equipped with thermocouple probes (part number
40EJFGB036A) for feedback was used to control the
temperature of both the platens and " tail heater " .
The PID can alternate power to the 250 W cartridge
heaters every 0.05 s. Altogether this allows for an
accuracy of ± 1°C.
Figure 6. Pneumatic assembly (A) tie rod air cylinder, (B) threaded piston
conical adapter, (C) 5/2 air directional control valve, (D) load cell dimple
plate, (E) button load cell, (F) coarse pressure regulator, (G) fine pressure
regulator.
A complete view of the fixture and
accompanying PID controllers is shown in Figure 7.
EXPERIMENTATION
Preliminary tests are performed with uncured
commercial Hexcel IM7 and HexPly® 8552 resin
continuous prepregs as proof of concept and to
demonstrate the use of the newly developed unit.
A sample of uncured prepreg ply is wrapped
around a metal insert and is then heat pressed.
Prior to clamping the specimen, the wrapped end
is secured between cardstock tabs with fast curing
adhesive, while the other end remains free. For plyply
tests, two prepreg plies are wrapped around the
platens and secured by the clamping bars. Pre-test
with pressure sensitive film is performed before
each set of tests to ensure proper block alignment
and thus uniform pressure distribution at the
contact interface. Once the middle composite ply
is placed correctly into the tensile test machine,
platens are heated at the desired temperature
the tail heater clamshells are closed around the
lower end of the sample and the platen assembly
48 | SAMPE JOURNAL | JANUARY FEBRUARY 2023
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Figure 5. Angle adjust assembly (A) clamping assembly, (B) keyed shaft,
(C) lower clamping block, (D) guide rail, (E) angle adjuster wheel, (F)
retractable spring plunger, (G) upper clamping block.
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SAMPE Journal - January/February 2023

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