Fiber Push-Out Test

The shear strength of the fiber/matrix interface was determined by means of single fiber push-out tests. Three kinds of fibers were tested, namely as-received CF, desized CF (CF without polymer sizing) and CNT-CF. Fibers were put into a small cylinder mold; resin was introduced and cured. Thin slices transversal to the fibers were cut from the composite rod and thinned down to the desirable thickness (15-40 µm) by polishing. The push-out test was performed using an indenter with the diameter of 5 µm on selected fibers. During the movement of the indenter in the fiber direction, the applied force against the displacement for each tested fiber was recorded. The IFSS was calculated using the following equation:

where Fmax is the peak push-out force, df is the fiber diameter and l is the specimen thickness. At least 20 fibers were pushed out from each sample.
Beside the in-situ experiments, the crack morphology and fracture surface of the post mortem composite beams were examined by SEM (Philips XL30 FEG) operating at an acceleration voltage of 10 kV.
1. Results and discussion
3.1 Fracture tests and fractographic analysis
The force-elongation curves of three-point bend tests on notched beams of unidirectional CF/epoxy and CF-CNT/epoxy composites are plotted in Fig. 2. The force increases linearly up to the maximum value and then suddenly drops down as the crack propagates very fast throughout the beam. The CNT-CF/epoxy composite shows fracture at a larger elongation (1.22 m) compared to the CF/epoxy composite (0.83 m), which reveals a delayed global failure. After fracture, the CNT-CF/epoxy composite can resist higher load

6: Single fiber push-out test results showing the IFSS of various CFs reinforced epoxy composites. Fig. 7: SEM image of the fiber push-out hole: (a) smooth surface in CF/epoxy composite, (b) rough hole in nFRC, and (c) a push-out fiber with a layer of CNTs embedded in epoxy in CNT-CF/epoxy composite. Scale bars equal 1 μm.

2.