2.4 Results
2.4.1 Influence of the sizing
Aliphatic polyamide
Focusing on the aliphatic polyamide grade B3S, the transverse flexural stress σf2 is plotted against the strain of the outer fiber f2 for epoxy-sized carbon fibers CF-EPY/B3S and polyamide-based sized fibers CF-TP/B3S in Figure 2-8. The polyamide-based sizing seems to promote brittle failure at high stresses. In con-trast, the epoxy-based sizing leads to an early failure at considerably lower stresses.
Plotting the opening mode I interlaminar fracture toughnessGIc and the propaga-tion valuesGI against the crack length yields the R curve that is compared for B3S specimens with EPY and TP sizing in Figure 2-9. The GIc of B3S combined with
0 1 2 3 4 5
Figure 2-8 a) Stress-strain curves of CF-EPY/B3S and b) CF-TP/B3S.
epoxy- and polyamide-based sizing are very similar. However, a significant increase in the propagation valuesGI is observed for CF-TP/B3S that is attributed to fiber bridging and stick-slip behavior noticed during testing.
50 60 70 80 90 100 110
Figure 2-9 R curves for a) CF-EPY/B3S and b) CF-TP/B3S.
Figure 2-10 compares the mean transverse flexural strengthσf2along with the mean opening mode I interlaminar fracture toughness GIc of specimens with CF-TP and CF-EPY fibers impregnated with B3S. Comparing the results from mechanical test-ing, the polyamide sizing strongly influences transverse flexural properties. σf2 is increased by 152 % when polyamide-sized fibers are used and lies above the matrix yield stress indicating a strong adhesion between fibers and B3S. The effect of the TP sizing has a less pronounced effect on GIc revealing an improvement by 11 %.
Inspection of the fracture surfaces of tested DCB specimens by SEM (Figure 2-11) reveals that B3S peels off of the epoxy-coated carbon fibers implying a weak adhesion between fibers and matrix. In contrast, B3S almost covers the complete carbon fiber with polyamide-based sizing and shows cohesive failure. In addition,
CF-EPY/B3S CF-TP/B3S 0
20 40 60 80 100 120
Meanσ f2[MPa]
a
CF-EPY/B3S CF-TP/B3S 0
400 800 1200 1600 2000
MeanG Ic[J/m2 ]
b
Figure 2-10 a) Mean transverse flexural strengthσf2and b) mean mode I interlaminar fracture toughnessGIc of CF-EPY/B3S in comparison to CF-TP/B3S.
the matrix area around polyamide-sized carbon fibers exhibits a higher degree of deformation compared to epoxy-sized fibers.
a) CF-EPY/B3S b) CF-TP/B3S
Figure 2-11 Fracture surface analysis of tested DCB specimens made of a) CF-EPY/B3S in comparison to b) CF-TP/B3S.
Co-polyamide
Although the thermoplastic-based sizing originally has been designed to chemically interact with polyamides, its compatibility to the co-polyamide PA10T/X (C2000) was investigated. The chemical structure of PA10T/X reveals large aliphatic re-gions, segmented by terephthalic acid which is why a sufficient adhesion to CF-TP fibers is expected. Parts of the following experimental work have previously been published in [73].
In Figure 2-12, the transverse flexural stress is plotted against the strain of the outer fiber for CF-EPY/C2000 and CF-TP/C2000. In contrast to aliphatic polyamides such as B3S, the epoxy-based sizing in combination with C2000 leads to higher stresses at higher strains compared to the polyamide sizing. The flexural stiffness
of the specimens is comparable for both carbon fiber sizings.
Figure 2-12 Stress-strain curves for a) CF-EPY/C2000 and b) CF-TP/C2000.
The R curves of the C2000 with EPY and TP sizing are presented in Figure 2-13.
Lower scatter but also reduced GIc is found for CF-EPY/C2000 specimens
com-40 50 60 70 80 90 100 110
Figure 2-13 a) R curves of CF-EPY/C2000 and b) CF-TP/C2000.
pared to CF-TP/C2000 specimens. After crack initiation, the propagation values GI of TP/C2000 specimens further increase considerably, compared to CF-EPY/C2000 specimens. This is assumed to arise from fiber bridging noticed during testing.
Figure 2-14 compares the mean transverse flexural strengthσf2 as well as the mean opening mode I interlaminar fracture toughness GIc of specimens with C2000 rein-forced by CF-EPY and CF-TP fibers. In contrast to the aliphatic B3S, the trans-verse flexural strength of specimens with CF-TP fibers was found to be decreased by 23 % in comparison to CF-EPY/C2000. However, theGIcwas increased by 44 % for C2000 reinforced by polyamide-sized fibers.
CF-EPY/
C2000
CF-TP/
C2000 0
10 20 30 40
50
b
Meanσ f2[MPa]
a
CF-EPY/
C2000
CF-TP/
C2000 0
50 100 150 200 250 300 350
MeanG IC[J/m2 ]
Figure 2-14 a) Mean transverse flexural strengthσf2and b) mean mode I interlaminar fracture toughnessGIc of CF-EPY/C2000 in comparison to CF-TP/C2000.
In Figure 2-15, the fractured surfaces of flexural test specimens with C2000 as ma-trix are compared with respect to carbon fibers with epoxy- and polyamide-based sizing.
a) CF-EPY/C2000 b) CF-TP/C2000
Figure 2-15 Fracture surface analysis of tested flexural specimens made of a) CF-EPY/C2000 compared to b) CF-TP/C2000.
The fractured surface of representative CF-TP/C2000 specimens show a pattern of several broken fiber bundles that may cause the drop to residual stress levels as observed during testing (see Figure 2-12). When several fiber bundles break at maximum stress non-broken fiber bundles remain to withstand the bending load.
In contrast, a more homogeneous fracture surface is found for CF-EPY/C2000.
The epoxy-based sized fibers were spread to a larger width (increased by 14 %) than CF-TP fibers. Cross-sections of the four-point bend test panels presented in Figure 2-16 show insufficiently spread tows leading to fiber agglomerations in case of polyamide-sized fibers. These areas with high fiber agglomerations are assumed to induce premature failure of the specimens during four-point bend testing. In this case, the four-point bend test gives more information about the laminate quality of
a) CF-EPY/C2000 b) CF-TP/C2000
Fiber agglomerations Homogeneous ply distribution
Figure 2-16 Micrographs of four-point bend test panels made of a) CF-EPY/C2000 and to b) CF-TP/C2000.
CF-TP/C2000 than about the level of adhesion. Thus, the adhesion of differently sized carbon fibers was further analyzed by comparing the fracture surface of rep-resentative DCB test specimens presented in Figure 2-17.
a) CF-EPY/C2000 b) CF-TP/C2000
Figure 2-17 Fracture surface analysis of tested DCB specimens made of a) CF-EPY/C2000 compared to b) CF-TP/C2000.
Improved adhesion between polyamide-sized fibers and C2000 is observed which is indicated by residual amount of matrix on the surface while epoxy-sized fibers reveal a comparatively clean surface.