SAMPE Journal - May/June 2016 - 16

Feature Article
between hours 1.5-2.5, vacuum level rose by a factor
of 2.5, reducing the available consolidation pressure
to 90% of ambient (1025 mbar).
All sensors showed an increase in measured
pressure during the temperature ramp, and a period
of constant pressure existed as the part temperature
equilibrated. During the dwell, Sensors 1, 2, and 4
showed a pressure decrease, but Sensor 3 measured
constant pressure. At Sensor 3, the part geometry
required excess release film to prevent consumable
bridging. The perforated release ply likely became
folded in that area and, as the resin viscosity decreased
during the temperature ramp, the resin filled the
perforations. As a result, the high-pressure gas within
the core could not escape, and the pressure remained
constant at that location during the dwell. The
resulting increase in core pressure further reduced
the consolidation pressure by 25-50%.
Figure 10. Cured demonstrator: (A) OML with sensor
locations circled, (B) IML with approximate sensor locations
outlined, and (C & D) ply bridging defects in female corners.

The honeycomb core pressure during the pre-cure
vacuum hold is shown in Figure 8. The decrease in
core pressure proceeded quickly through the spiked
prepreg layers, and falls below 100 mbar within 3 h.
Pressure fluctuations were observed on Sensors 1
and 3 during the vacuum hold. A possible explanation
is that a local pressure gradient exists within nearby
cells, and as the permeability increased in that cell,
gas moved from a high-pressure to a low-pressure
region. The overall trend is rather encouraging, all
sensors are converging towards the bag vacuum level
and are within 50 mbar of each other.
A previous thermal survey revealed that an
overshoot was required for the OML skin to reach the
target 120°C cure temperature near resin gelation,
at roughly 2.5 h into the cure cycle. Additional
thermocouples were used to measure the IML and
OML skin temperatures during cure, and are shown
in Figure 9, alongside the pressure sensor readings.
As the part temperature increased, the honeycomb
core pressure increased from below 100 mbar to
peak between 360 and 480 mbar. The bag vacuum
level rose as the honeycomb core pressure decreased
throughout the dwell. This indicates that gas flowed
from the core into the skins during processing.
A vacuum pump/system capable of drawing and
maintaining high vacuum through the cure cycle is
an important consideration. In the resin flow window
16

Cured Demonstrator
Photographs of the OML and IML sides of the cured
demonstrator are shown in Figure 10. Some of the
surfacing film from the OML skin remained on the
mould after cure. Additional visible defects in the
part were caused by ply bridging, as shown in Figure
10C/D. Other than the tight radii in the flat-bottom
depression and the joggle between the flat and raised
regions of the mould, the OML surface was smooth
and without surface porosity. The IML side looked
very acceptable excluding the ply bridging shown in
Figure 10D. This defect was caused by a poor chamfer
angle on the core below the bridging.

Infrared Thermography Inspection
Infrared thermography images of the IML and
OML skins are shown in Figure 11 and Figure 12,
respectively. The more interesting features of the
demonstrator were observed in the double-curvature
region, and near surface features were more evident.
For clarity, Figure 11 and Figure 12 show the quarterdepth thermography images (in-between plies 1 and
2 from the mould line of interest).
The IML thermal image in Figure 11 indicates the
presence of discontinuities within the demonstrator.
This includes the transitions between monolithic
and core sections, ply-darts, and interlaminar voids.
Interlaminar voids appear as black spots, primarily
located within the top-half of the double-curvature
honeycomb insert in Figure 11. The ply darts are
observed on the bottom right-hand corner of the
image, and correspond to those of ply 3 and 4 from
Figure 4. The ply-bridging observed in Figure 10D
SAMPE Journal, Volume 52, No. 3, May/June 2016

Table of Contents for the Digital Edition of SAMPE Journal - May/June 2016