2.5. Measurement of KIC and self-healing efficiency
The notched specimens were used to measure the KIC values.
Specimens were tested in three-point fixture with the notch
on the tensile side, and the middle loading pin was aligned
with the notch via the use of a magnifier (3). KIC was
calculated following an established SEVNB method.33 This
yielded the original virgin KIC of the specimen, KIC-virgin.
In order to test the self-healing efficacy, immediately
following specimen fracture, the two halves of the specimen
were placed back into the mould to ensure a good contact of
the two fractured planes. This would be similar to the case of a
resin-based restoration in a tooth cavity, where the cracked
restoration would stay in the tooth cavity to allow the released
healing liquid to heal the crack. The healing of a completed
cracked specimen would be a more rigorous test than the
healing of a partial or small crack in the specimen. Because the
fracture ruptured the microcapsules in the composite, the
released TEGDMA-DHEPT from the microcapsules would react
with the BPO in the resin matrix. This would cause the
polymerization of the released liquid to heal and bond the two
cracked planes into one cohesive specimen. Specimens were
incubated in a humidor at 37 8C for 24 h. The notch length of
the healed specimen was again measured, and made sure that
it was the same as the virgin notch length of the same
specimen. Then the healed bar was tested again using the
same flexural method to measure the KIC. This yielded the
healed KIC value, KIC-healed. The self-healing efficiency (h) was
assessed following a previous study as:34
h ¼ KIC-healed
KIC-virgin