2.2. Testing and characterisation
A microbond and single-fibre tensile tests were carried out using an
in-house developed tester equipped with a 250 gram-force (gf) load cell
[17]. In a microbond test, a fibre filament was first mounted onto a
paper holder. Next, epoxy resin droplets were applied on the fibre and
cured at room temperature for 24 h followed by post-curing at 60 °C
for 15 h. As shown in Fig. 1, in the microbond test, the fibre was pulled
at a speed of 1 μm/s while the epoxy droplet was constrained from
translating along the loading directions by a microvise jaw.
All specimens were checked under the microscope to observe geometry
and embedded length of the droplet. Samples with defects (such as
kink bands on the fibre or obvious lack of symmetry of the droplet) were
rejected. Ideal epoxy droplet length was around 80–200 μm. This ensures
that the required pullout forces will not exceeds the breaking
strength of the fibre. The force needed to pull the fibre out of the resin
was then determined. Assuming that the measured force is equal to a
shearing force that is applied to the entire interface and distributed uniformly,
the apparent interfacial shear strength (IFSSapp) of carbon/epoxy
interface is determined as
I FSSapp ¼ Fd
πDl ; ð1Þ
where Fd is the peak pull-out force, D is the fibre diameter and l is the
embedded length of the fibre. During the experiment, as the fibre was
pulled, the epoxy droplet on the fibre first contacted the microvise,
which transferred the load from the fibre to the fibre/epoxy interface.
Post-engagement, the initial load–displacement behaviour was nearly