SAMPE Journal - January/February 2021 - 42

FEATURE / SYNTHETIC EPOXY FOAM
sion is 24 hours. To evaluate the damage evolution
during the cyclic tests, damage variable D is employed in this study, and can be written as
D=1-Eun/Ein
where is the elastic modulus of intact material.
EXPERIMENT RESULTS
Quasi-static Compressive Response of
Capsules
The measured quasi-static compressive response
of the capsules are plotted in Figure 6: (a) for hardener capsules, (b) for resin capsules and (c) for
double layer capsules. To make sure the accurate
measurements, parallel LVDTs are installed in each
test, as shown in Figure 6 (a). The compressive
responses of resin capsules, as shown in Figure 6
(b), have the plateau region after yielding, and are
stronger than those of hardener capsules, as shown
in Figure 6 (a). Besides, the compressive responses
of double layer capsules shown in Figure 6 (c) and
resin capsules have similar yield force and plateau
region.
Cyclic quasi-static Compressive Response of
Epoxy Foams
Four groups of epoxy foams are implemented under cyclic quasi-static compressive tests. The results of seven successive compression cycles for
each group are shown in Figure 7 (a). Groups of
foams filled with double layer capsules and foams
filled with resin & hardener capsules have the
self-healing capability, while groups of foams filled
with resin capsules and pure foams do not have the
self-healing capability. Two healing effects groups
are used to compare whether the proposed double
layer capsules-based self-healing system has better
self-healing effect than the conventional one, and
the group of foams filled with resin capsules is used
to check whether the inserted capsules could influence the damage evolution compared with pure
foams. The stress-strain curves shown in Figure7
(a) indicate that inserting capsules have the capability to enhance the compressive strength from 5.5
MPa (pure foam) to over 7 MPa and elastic modulus
from 260 MPa to over 300 MPa. The averaged elastic
modulus and strength for each group of foams are
listed in Table 1.

Figure 6. Compressive results of capsules: (a) hardener capsules,
(b) resin capsules and (c) double layer capsules.

To evaluate the healing effects on each cycle of
loading and unloading, normalised elastic modulus
El = El / Ein for loading or Eun = Eun / Ein unloading
are calculated. Figure 7 (b) shows the normalised
elastic modulus for each group of foams. The solid
line and dashed line represent the normalised elastic modulus of loading and unloading, respectively.
For the foams filled with double layer capsules, the
initial El is 1. Then, after 1st loading, the Eun of 1st
unloading dropped to 0.7. After 24 hours, the 2nd
loading is implemented, and El increased to 0.967.
Self-healing effect of proposed double layer capsule-based self-healing system has been achieved
in the 1st cyclic loading - unloading process. The
healing effect stopped until the 4th cyclic loading
- unloading process, as the Eun of 4th unloading is
nearly the same as the El of 5th loading. Similarly,
the multiple self-healing process is achieved in the
foams filled with resin and hardener capsules. However, the recovered El of 2nd loading is 0.898 which is
smaller than that of foams filled with double layer
capsules. As for the groups without self-healing capability, the Eun of 1st time unloading is similar as El
of 2nd loading, which means the healing effect is not
triggered after 24 hours.

Table 1.

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SAMPE JOURNAL

Double layer capsules
filled foam

Resin and hardener
capsules filled foam

Resin capsules
filled foam

Pure foam

Elastic Modulus

325.06 MPa

293.41 MPa

311.72 MPa

269.33 MPa

Strength

7.67 MPa

7.25 MPa

7.12 MPa

5.63 MPa

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SAMPE Journal - January/February 2021

Table of Contents for the Digital Edition of SAMPE Journal - January/February 2021
