dP dh
√π
√A P
24.5h2p. (2-8)
A Hysitron Triboindenter was used to conduct the nanoindentation tests. All nano-indentation tests and their evaluation were carried out at the Research School of Physics and Engineering from the Australian National University (ANU) in Canberra in collaboration with Mr. Sherman Wang.
2.2.5 Scanning electron microscopy
SEM was used for visual inspection of the adhesion of differently sized carbon fibers to the selected polyamide grades. Before inspection via SEM, the fractured surface of four-point bend and DCB test specimens were sputtered using the sputter coater BAL-TEC SCD 005 at 40 mA for 40 sec in an evacuated chamber that was flushed with argon prior to the sputtering step. Thereby, a thin gold film of about 100 nm is created to enhance the electrical conductivity for SEM measurements. The used SEM was a JSM 6060/6060LV from Jeol. The sputter coating and the image ac-quisition were conducted in collaboration with Mrs. Susanne Schnell-Witteczek from the IMETUM Institute of Medical Engineering of the Technical University of Munich.
2.3 Sample preparation
2.3.1 Intermediate production
The powder-coating technique allows the investigation of different material combi-nations due to its high flexibility and is characterized by full comparability when similar particle size distributions of the powder are used. In addition, the DOIi
of 0 % allows the observation of the impregnation progress throughout processing.
Therefore, the powder-coating method was selected as production technique for intermediates within this work.
A laboratory prepreg line (LPL), located at SGL Carbon GmbH, was used to produce the various material combinations. The schematic of the LPL used for
production of the powder-coated tows is presented in Figure 2-6. For each of the material combination that are summarized in Table 2-3, nine carbon fiber tows were spread simultaneously to a width of nominally 200 mm and fed into the LPL.
The powder was dried at 80 °C for at least 4 h before coating to avoid bubble
for-Spread carbon fiber tows
Infrared heat source
Heat
Powder-coated tows Powder reservoir
Needle roller
Figure 2-6 Schematic of the prepreg line used to produce powder-coated tows on a laboratory scale.
mation arising from heating up non-dried, hydrophilic powder. To begin with, the spread fibers were coated with 50 g/m2 of polymer powder on one side by means of a needle roller. Before powder application, the exact width of the spread fibers was measured and the amount of powder was adjusted if necessary to keep the areal weight constantly at 50 g/m2. The powder was fused by passing an infrared source incorporated into the LPL. The coated tows were then solidified by running through a pair of calender rolls and wound up. In a second run, the opposite side of the spread fibers was coated with 50 g/m2. Therefore, double-sided powder-coated tows with an areal weight of 235 g/m2 were produced. The velocity of the labora-tory prepreg line was 5 m/min.
Epoxy-sized carbon fibers could be spread to a width of 205 mm and CF-TP fibers to a width of 180 mm. The reduction in width resulted in an increase in thickness of the powder-coated tows. Therefore, powder-coated tows with increased thickness require a lay-up of less plies than tows that could be spread to 205 mm. Different lay-ups arise from the difference in spreading behavior to generate a comparable thickness of 2±0.2 mm for four-point bend testing and 3±0.2 mm for DCB testing within the given tolerances, as presented in Table 2-3.
2.3.2 Test panel production
After manufacturing, the powder-coated tows were cut to the desired length and stacked in accordance to the lay-up stated in Table 2-3. Aluminum foils were used to produce test panels with varying size as shown in Figure 2-7. During the lay-up
Table 2-3 Investigated material combinations produced by the powder-coating technique along with required lay-up to obtain desired test panel thickness for four-point bend and DCB testing.
Material Combination Four-point bend DCB
Fiber-Sizing/Matrix Lay-up Thickness [mm] Lay-up Thickness [mm]
CF-TP/B3S [012] 1.98±0.02 [018] 2.94±0.02 CF-EPY/B3S [013] 2.14±0.17 [020] 3.16±0.05 CF-TP/B3L [012] 2.05±0.01 [020] 2.86±0.03 CF-TP/B40 [013] 2.08±0.01 [020] 3.15±0.05 CF-TP/C2000 [011] 1.88±0.02 [020] 3.08±0.03 CF-EPY/C2000 [012] 2.17±0.03 [022] 3.27±0.01 of the DCB test panels, a polyimide film (Upilex) with a thickness of 25 μm and coated with high temperature resistant release agent (Frekote 55 NC) from both sides was placed at the midplane of the laminate to generate the initial delamination length a0.
Aluminum foil
b) Processing in a static press
a) Stacking c) Produced test panel
Polyimide foil Powder-coated tows
Figure 2-7 a) Stacking, b) processing in a static press and c) produced test panel.
Subsequently, the stacks of powder-coated tows were dried at 80 °C in a vacuum oven for at least 4 h. After drying, each stack was transferred to a press WKP 1000s from Wickert heated to 260 °C (B3S, B3L, B40) and 300 °C (C2000). After the stacks have reached the processing temperature, a pressure of 10 bar was applied.
Pressure and temperature were kept constant for 60 min (B3S, B3L, B40) and 30 min (C2000) and cooled to 80 °C with a cooling rate of 20 °C/min under pressure.
2.3.3 Test specimen preparation
Test samples were cut to the required dimensions by using a water-cooled circular diamond saw. The DCB specimens were coated with white water-based fluid and
marked according to ASTM D 5528 [63] to enable observation of the crack propa-gation during testing. After cutting, the test specimens were dried under vacuum at 80 °C (PA6) and 110 °C (PA10T/X), respectively for at least 60 h to ensure complete water removal. Storage in hermetically sealed aluminum foils prevented moisture absorption prior to testing.
2.3.4 Micrographs
Micrographs were prepared by extracting rectangular-shaped specimens from test panels and potting them in transparent epoxy resin to enable visual inspection of the laminate quality. After curing the potting resin, the potted specimens were ground with silicon carbide sand papers with a grain size from 180 to 2400 by using the polishing machine Struers TegraPol-21. Final surface finish was achieved by polishing the ground specimens with several suspensions and was completed with a chemical suspension with particles smaller than 1 μm. Micrographs were taken by using an Olympus BX41-M. Besides visual inspection, potted and polished specimens extracted from DCB test panels were used to carry out nano-indentation tests.