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CAMX Conference Proceedings. Dallas, TX, October 26-29, 2015. CAMX – The Composites and Advanced Materials Expo.
PROCESSING AND PERFORMANCE OF OUT-OF-
AUTOCLAVE BISMALEIMIDE COMPOSITE SANDWICH
STRUCTURES
S. Anandan, G. S. Dhaliwal, and K. Chandrashekhara
Department of Mechanical and Aerospace Engineering
Missouri University of Science and Technology, Rolla, MO 65409
T. R. Berkel
Boeing Research & Technology, St. Louis, MO 63166
D. Pfitzinger
GKN Aerospace, St. Louis, MO 63042
ABSTRACT
Composite sandwich structures offer several advantages over conventional structural materials
such as lightweight, high bending and torsional stiffness, superior thermal insulation and
excellent acoustic damping. In the aerospace industry, sandwich composites are commonly
manufactured using the autoclave process which is associated with high operating cost. Out-of-
autoclave (OOA) manufacturing has been shown to be capable of producing low cost and high
performance composites. Unlike the autoclave process, OOA processing avoids the issue of core-
crushing due to high pressure. However, Bismaleimide (BMI) prepregs require high cure and
post-cure temperatures which can lead to high internal core pressures, core-to-facesheet
disbonding and voids. In the current work, OOA sandwich composite panels are manufactured
using aluminum honeycomb core, BMI adhesive film and carbon/BMI prepregs. Two vacuum
levels were used during OOA processing, full vacuum (100 kPa) and partial vacuum (80 kPa).
Adhesive bond quality was evaluated using flatwise tensile and fracture toughness tests.
Mechanical performance was evaluated using edgewise compression. It was observed that
vacuum level variation during processing had no significant effect on mechanical properties of
manufactured laminates. Tests are performed at room temperature and 177 °C (350 °F). All
manufactured laminates exhibited room temperature flatwise tensile strengths comparable to
those of aerospace grade epoxy adhesives. Sandwich mechanical properties reduced when test
temperature was increased.
1. INTRODUCTION
High performance composite materials are conventionally processed in autoclaves which are
associated with high capital requirements and operating costs. Part size is also limited by the
autoclave chamber volume. Out-of autoclave (OOA) processing has emerged as an alternative to
autoclave processing [1, 2]. OOA involves curing at atmospheric pressures which obviates the
need for an autoclave and provides greater design flexibility for production of large and complex
shaped composite components. It also reduces capital and operating costs. In an OOA prepreg
process, removal of entrapped air from the prepreg layup is critical which can otherwise lead to
voids which degrade mechanical properties of the composite. Sandwich components can be co-
cured, a process where adhesive and the prepreg are cured simultaneously, resulting in time and
cost reduction. Manufacturing co-cured sandwich panels out-of-autoclave, eliminates core
crushing which can occur at autoclave pressures [3] and enables the use of lighter cores.
In a composite sandwich structure, the facesheet absorbs in-plane loads while the core is resistant
to shear loading. The composite facesheet is bonded to the core using a film adhesive. The
adhesive bond between core and facesheet is of great importance in the design of sandwich
structures. The relation between the adhesive fillet and sandwich properties was described by
Grimes [4]. The mechanical performance of a sandwich composite depends on the quality of the
core-to-facesheet adhesive bond. In the case of honeycomb cores, the cell walls provide a
relatively small area for bonding. Structural strength depends on the presence of a well-formed
fillet at the interface of the core cell wall and facesheet. Larger adhesive fillets absorb a higher
amount of energy upon fracture, due to which they have high fracture toughness [5]. Grove et al.
[6] showed that the higher debonding energy was obtained with larger, regular-shaped adhesive
fillets between the honeycomb cell walls and the skin. Rion et al. [7] showed the failure occurs in
the adhesive meniscus when low weight adhesive is used. Butukuri et al. [8] evaluated the effect
of the reticulation technique on properties of sandwich constructions. Core reticulation resulted
in improved fillet geometry and flatwise tensile strength. Hou et al. [9] reported that thermal pre-
treatments can reduce adhesive foaming in OOA cured composite sandwich structures.
Nagarajan et al. [10] studied the effect of processing variables on OOA sandwich panels with
carbon/epoxy facesheets. Presence of dissolved moisture in the adhesive layer was found to
increase adhesive foaming.
Internal core pressure in the cells of a honeycomb core has been shown to affect adhesive
foaming and fracture toughness in sandwich composites with carbon/epoxy facesheets [11]. Core
pressure fluctuations were observed when internal core pressures during cure equaled
atmospheric pressure. Increased adhesive permeability due to perforation resulted in enhanced
fracture toughness of the adhesive layer. The optimal range of honeycomb core pressure during
processing was estimated to be 40-70 kPa, to reduce adhesive outgassing and ensure good fillet
formation [12].
OOA epoxy prepreg systems have previously been used in the Boeing wing spar, Advanced
Composite Cargo Aircraft (ACCA) and the Ares V interstage and payload shroud [13, 14]. OOA
Bismaliemide (BMI) systems were evaluated as a part of NASA CoEx (Composites for
Exploration) project [15]. Due to tooling limitations, the cure temperature was fixed at 177 °C
(350 °F) followed by a freestanding post-cure at elevated temperatures. BMI prepreg systems
from Renegade, Stratton Composites and Tencate were evaluated. Two adhesive systems namely
Cytec FM 2550 and Renegade RM 3011 were evaluated. Sandwich structures exhibited no
warping on post-cure, and flatwise tensile strength obtained was comparable to aerospace grade
epoxy adhesives.
In the current work, sandwich composites were manufactured using aluminum honeycomb,
carbon/BMI facesheets and a toughened BMI adhesive system. The sandwich composite was
cured via the OOA prepreg process. One set of panels was fabricated under full vacuum pressure
(100 kPa). Remaining panels were manufactured using a reduced vacuum pressure of 80 kPa.
This level was chosen to ensure that the internal core pressure does not exceed atmospheric
pressure during the high temperature cure. The effect of varying vacuum pressure on mechanical
properties of the sandwich composite was evaluated. Adhesive bond quality was evaluated using
flatwise tensile tests and adhesive bond fracture toughness tests. Load bearing capability
structure was evaluated using edgewise compression tests. Tests were performed at room
temperature as well as an elevated temperature of 350 °F (177 °C).
2. MATERIALS
The sandwich facesheets were fabricated using unidirectional IM7/AR4550 prepreg, a toughened
carbon/BMI system suitable for OOA curing. It has a resin content of 35% and a prepreg areal
weight of 304.8 gsm. Cytec Metlbond 2550 film adhesive was used to bond the facesheet to the
honeycomb core. Metlbond 2550 is a modified BMI film adhesive with good high temperature
properties. The core material was aluminum honeycomb. Materials are listed in Table 1.
Table 1. Materials used
Name
Supplier
Specifications
Carbon/BMI
prepreg
(IM7/AR4550)
Aldila Composite Materials
Recommended cure cycle:
143.33 °C (290 °F) for 1 hour
followed by 190.5 °C (375 °F)
for two hours. Post cure at 232
°C (450 °F).
BMI film adhesive
(Metlbond 2550)
Cytec Engineered Materials
Recommended cure cycle:
177 °C (350 °F) for 4 hours. Post
cure at 226.6 °C (440 °F).
Aluminum
honeycomb
Hexcel
Density: 8.1 pcf (129.7 kg/m3)
Thickness: 1 in. (25.4 mm)
Cell size: 1/8 in. (3.175 mm)
3. MANUFACTURING
Honeycomb sandwich composite panels were manufactured using an Out-of-Autoclave (OOA)
manufacturing process. One set of panels of size 304.8 mm. x 304.8 mm (12 in. x 12 in.) and
facesheet configuration [0 °/90 °]s were manufactured, which yielded samples for flatwise tensile
and edgewise compression tests. A second set of sandwich panels 304.8 mm. x 152.4 mm (12 in.
x 6 in.) and facesheet configuration [0 °/90 °]2s was manufactured for adhesive bond fracture
toughness evaluation.
An aluminum mold was cleaned of surface defects using sand paper, and a non-porous Ethelyne
tetrafluoro ethelyne (ETFE) release film was placed. The prepreg layers were cut into required
dimensions and laid up according to the required facesheet configuration. Rollers along with
hand pressure were used to press the prepregs, starting from the center of the layup and moving
progressively towards the edges. This process was repeated for all the prepreg layers to remove
entrapped air as well as folds or wrinkles. The tool-side and bag-side facesheet layups were
debulked at room temperature under full vacuum for 60 minutes to ensure good compaction.
The schematic of the bagging procedure employed for the OOA process is shown in Figure 1.
The compacted tool side facesheet layup was laid on the aluminum mold which was covered
with the ETFE release film. A layer of film adhesive was placed on the prepreg layup. The
honeycomb core was placed on the adhesive layer followed by the bag-side adhesive layer and
facesheet stack. Edge breathing using EB1590 edge bleeder was used to evacuate entrapped air
from the sandwich layup. A layer of ETFE release film was placed on the sandwich layup
followed by an aluminum caul plate. Airweave N10 breather was used to distribute the vacuum
pressure evenly. The entire layup was sealed using a double vacuum bagging scheme.
Figure 1. Schematic of bagging layup
The layup was debulked at 49 °C (120 °F) for two hours. The sample was cured according to the
cure cycle shown in Figure 2, and allowed to cool down to room temperature. This was followed
by a free-standing post-cure at 232 °C (450 °F) for two hours. The prepreg cure cycle was used
to manufacture the sandwich panel. The samples did not exhibit warping following post cure.
Figure 2. Cure Cycle
4. EXPERIMENTS
4.1 Thermogravimetric Analysis
Outgassing of prepreg and adhesive during cure can lead to voids in facesheets and adhesive
fillets. These volatiles need to be removed during cure. The evolution of volatiles from the
prepreg and adhesive was studied using Thermogravimetric analysis (TGA). The adhesive was
removed from storage and allowed to warm up to room temperature. A small sample was
removed and the prepreg cure cycle was simulated in a TGA. Air at a flow rate of 40 mL/min.
was used as the sample purge gas and the weight loss was recorded.
4.2 Flatwise Tensile Test
Flatwise tensile (FWT) strength primarily serves as a quality control parameter for bonded
sandwich panels. This test produces information on the quality and strength of the core-to-facing
adhesive bond. The flatwise tensile test for sandwich samples was done according to ASTM
C297. Sandwich samples had a facesheet configuration of [0°/90°]s. This test method consists of
subjecting a honeycomb sandwich samples to a uniaxial tensile force normal to the plane of the
sample. Honeycomb sandwich samples of 50.4 mm x 50.4 mm (2 in. x 2 in.) were cut from the
manufactured panel. The sample edges were polished progressively using 80-220 grit sandpaper.
The facings of the samples were lightly roughened using 80 grit sand paper. For RT testing,
aluminum blocks of 50.4 mm x 50.4 mm x 50.4 mm (2 in. x 2 in. x 2 in.) were bonded to the
sample facings using a high strength epoxy, 3M Scotch-Weld DP-460NS. The adhesive was
allowed to cure at room temperature for 24 hrs and was post cured at 121 °C (250 °F) for two
hours. Samples for elevated temperature testing at 177 °C (350 °F) were bonded to aluminum
blocks using Metlbond 2550 film adhesive. Test samples were conditioned at 177°C (350°F) in
the thermal testing chamber for 2 hours prior to testing. Average relative humidity was
maintained at 50%. Sample loading rate was 0.5 mm/minute. Each test was replicated four times.
4.3 Edgewise Compression Test
Edgewise compression test is used to measure the compressive properties of a sandwich
specimen in a direction parallel to the facing plane. This test is a measure of the load bearing
capacity of the sandwich structure. The edgewise compression test for sandwich samples was
performed according to the ASTM C364. Sandwich samples had a facesheet configuration of [0°
90°]s Sandwich specimens were cut into 50.4 mm x 127 mm (2 in. x 5 in.) sizes. Tests were
conducted at room temperature as well as 177 °C (350 °F). Test samples were conditioned at 177
°C (350 °F) in the environmental testing chamber for 2 hours prior to testing. Average relative
humidity was maintained at 50%. A loading rate of 3 mm/min. was used. Mechanical load was
applied along the core ribbon direction. Each test was replicated three times.
4.4 Adhesive Bond Fracture Toughness Test
The interfacial fracture toughness was measured using a modified cracked sandwich beam (CSB)
specimen [16]. Sandwich composites were manufactured with [0° 90°]2s facesheet orientation,
using the manufacturing procedure mentioned in section 3. Specimens of size 25.4 mm x 254
mm (1 in. x 10 in.) were extracted from the manufactured panel. An initial crack of 25.4 mm (1
in.) was cut as shown in Figure 3. Aluminum blocks, with hinges, were bonded onto the sample
as shown in Figure 4. A roller support was added at the free end to prevent large rotations.
Predetermined crack lengths were marked on the specimen and mechanical load was applied
using an Instron 5985 universal testing machine. This procedure was repeated for multiple
loading/unloading cycles. Crack growth was along the core ribbon direction.
Figure 3. Schematic of modified CSB test
(a) Test setup (b) Crack propagation on mechanical loading
Figure 4. Crack propagation in a modified CSB test
5. RESULTS AND DISCUSSION
5.1 Thermogravimetric Analysis
Results of TGA are shown in Figure 5. The temperature was increased to 143.3 °C (290 °F) and
held for 60 minutes. This isothermal hold is required to maximize the mobility of reacting
groups. An initial mass loss of 1.8 % was seen during the first isothermal hold. The temperature
was then increased to the cure temperature of 190.5 °C (375 °F) at 10 °C/min. An adhesive mass
loss of 1.7 % was observed during this period. Mass loss during cure is due to adhesive
outgassing which can lead to foaming defects in the adhesive layer.
Figure 5. Mass loss of adhesive during TGA
5.2 Flatwise Tensile Test
Results of the FWT tests are shown in Table 2. All samples failed in a combination of cohesive
and adhesive failure mode. Values depicted in Table 2 are comparable to previously reported
FWT properties of BMI sandwich structures [15]. Samples cured at full vacuum pressure and at
partial vacuum exhibited slightly higher FWT strengths. The adhesive bond strength reduced by
23% (full vacuum) and 21% (partial vacuum) when samples were tested at 177 °C (350 °F). The
room temperature FWT strength values compare well with the FWT strength of 6167.7 kPa
reported by Butukuri et al. [8] for OOA cured epoxy sandwich systems using Cycom 5320
facesheets (Cytec), Cycom FM 300-2U epoxy adhesive (Cytec) and aluminum honeycomb core
(1/8 in. cell size, 12 pcf). Adhesive fillets are distributed on either side of the cell wall (Figure
6).
Table 2. FWT test results
Vacuum pressure
Temperature
Flatwise tensile strength
(kPa)
°C/°F
Average (kPa)
S.D. (%)
Full vacuum (100)
24/75
6848.77
11.8
Full vacuum (100)
177/350
5270.99
15.3
Partial vacuum (80)
24/75
6311.39
7.6
Partial vacuum (80)
177/350
4968.23
7.3
Figure 6. Adhesive fillet formation in sandwich composites manufactured under full vacuum
pressure
Optical photographs of fracture surfaces are shown in Figures 7 and 8. A few voids are visible on
the facesheet side fracture surface of all samples (Figures 7 (a), 7 (b),8 (a) and 8 (b)). This can be
a result of adhesive outgassing during cure. Voids and air packets are seen along the fillet
regions (Figures 7(c), 7 (d),8 (c) and 8 (d)). These defects may be a result of low internal core
pressures which have been linked with adhesive foaming in previous studies [11]. The presence
of these defects did not have a significant effect on the FWT strength. While cohesive mode of
failure dominates in all specimens, regions of adhesive failure are also seen (Figure 8 (b)).
(a) Facesheet side (b) Facesheet side
(c) Core side (d) Core side
Figure 7. Representative images of fracture surfaces of samples cured under full vacuum at
different locations
(a) Facesheet side (b) Facesheet side
(c) Core side (d) Core side
Figure 8. Representative images of fracture surfaces of samples cured under partial vacuum at
different locations
5.3 Edgewise Compression Test
In the present study, edgewise compression test was conducted under room temperature and 177
°C (350 °F). Results of edgewise compressive test are shown in Table 3. Similar compressive
strengths were observed in samples cured under full vacuum and partial vacuum. When test
temperature was raised to 177 °C, the edgewise compression strengths dropped by 20.8 % and
32.2 % respectively. This can be an effect of reduced core stiffness under elevated temperatures.
Table 3. Edgewise compression test results
Vacuum pressure
Temperature
Edgewise compressive strength
(kPa)
°C/°F
Average (MPa)
S.D. (%)
Full vacuum (100)
24/75
12343.78
5.6
Full vacuum (100)
177/350
9781.33
4.5
Partial vacuum (80)
24/75
12825.73
2.2
Partial vacuum (80)
177/350
8689.49
5.9
A representative load-displacement plot of edgewise compression test is shown in Figure 9. The
load increases gradually until it reaches a peak value. Progressive failure initiates at this point.
The exact location of facesheet failure was unknown. Continued loading resulted in stable end
crushing of the sandwich specimen (Figure 10).
Figure 9. Load vs Extension in edgewise compression test at 177 °C (350 °F ) of sandwich
samples cured under full vacuum
Damage initiation in the sandwich composites was in the form of facesheet failure, because the
failure strain of the core is much higher than that of the facesheets. The composite facesheets can
exhibit various failure modes such as elastic microbuckling, plastic microbuckling, fiber
crushing, splitting, buckle delamination and shear band formation. On continued loading after
failure initiation, sandwich structures can exhibit various failure modes such as (1) Unstable
buckling of the sandwich column; (2) Facesheet delamination followed by unstable buckling in
opposite directions; and (3) Stable end crushing of sandwich panels [17, 18].The unstable failure
modes are seen when the bending strength of the facesheets is high, compared to the strength of
the fracture interface. The sandwich falls apart rapidly as the crack in the interface propagates
along the length of the sandwich specimen. When the interface strength is high compared to the
bending strength of the facesheets, crack propagation is more stable. The crack arrests after a
short propagation distance and other failure modes such as facesheet bending, delamination and
core crushing begins to dominate.
(a) Front view (b) Isometric view
Figure 10 (a) and (b). Stable end crushing failure
5.4 Adhesive Bond Fracture Toughness Test
A sample load/displacement curve is shown in Figure 11. All tests were conducted at room
temperature. The load increases until it reaches a critical value and crack begins to propagate. As
the crack propagates, load reduces. Critical strain energy release rate was calculated from three
loading/unloading cycles for each specimen.
Figure 11. Loading–unloading cycle in modified CSB test of a sample cured under partial
vacuum
The critical strain energy release rate, , can be expressed as,
=
(1)
where, P is the external load; is the displacement; B is the width of specimen and isthe
incremental crack length during the process of test. The energy required for crack growth, ,
can be calculated by calculating the area enclosed by the loading-unloading path. The area under
the loading/unloading path is obtained by integration using the trapezoid rule. The average
interface fracture energy release rate for samples fabricated under full vacuum and partial
vacuum is shown in Table 4. About 10% variation was noticed in samples fabricated under full
vacuum and partial vacuum.
Table 4. Adhesive bond fracture toughness test results
Vacuum pressure
Strain energy release rate
(kPa)
Average (J/m2)
S.D. (%)
Full vacuum (100)
335.19
17.3
Partial vacuum (80)
301.80
11.6
6. SUMMARY AND CONCLUSIONS
High temperature sandwich composite panels were manufactured using the OOA prepreg
process. Two levels of vacuum pressure were studied, full vacuum (100 kPa) and partial vacuum
(80 kPa). The prepreg cure temperature were used to manufacture the panels. No warping was
visible after post cure at 232 °C (450 °F), which implies that the panels had sufficient green
strength to enable a free-standing post-cure. Outgassing of adhesive during the cure cycle was
studied using TGA. A mass drop of 1.7 % was observed during the ramp and cure isotherm.
These volatiles need to be removed during cure to avoid foaming issues. Adhesive bond strength
was evaluated using FWT tests and adhesive bond fracture toughness tests. The samples
manufactured using full vacuum pressure during cure exhibited in a 6-8.5 % increase in FWT
strength compared to those manufactured at a partial vacuum pressure. FWT strength reduced
when samples were tested at 177 °C (350 °F). Cohesive failure dominated in all samples while
some regions of adhesive failure were also present. Sandwich samples with [0°/90°]2s facesheet
orientation were fabricated and a modified CSB test was conducted at room temperature.
Samples exhibited a strain energy release rate of 335.19 J/m2 (full vacuum) and 301.80 J/m2
(partial vacuum). Edgewise compression tests were conducted on samples with facesheet
orientation [0°/90°]s. Vacuum pressure levels used in this study, did not have a significant effect
on edgewise compressive strength. Increase in test temperature to 177 °C (350 °F) resulted in a
reduction in edgewise compressive strength. This can be a result of reduced core stiffness at
increased temperatures. Measured FWT strengths at room temperature were comparable to those
mentioned in previous studies involving aerospace grade epoxy adhesives.
7. ACKNOWLEDGEMENTS
This research is sponsored by the Industrial Consortium of the Center for Aerospace
Manufacturing Technologies (CAMT) at Missouri University of Science and Technology. The
authors would like to thank Stratton Composite Solutions and Cytec Engineered Materials for the
materials supplied.
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