Ags h

Three-point bending tests were performed using rectangular samples (4 mm × 14 mm × 1.2 mm) using a universal testing machine (AGS-H, Shimadzu Corp., Kyoto, Japan) with a crosshead speed of 1 mm/s and a support span of 12 mm. The flexural strength (σ) was calculated using Equation (1):
where F is the maximum load by the fracture, L is the support span, and b and h are width and thickness of the sample, respectively. Ten measurements were taken in each group (n = 10).

To analyze the interface of core resin and root canal sealer at the deep area of the root canal cavity, the shear bond test and microscopic analysis after the test were carried out. Figure 4 shows the schema of the experiment. The root canal sealer was hardened in the disk-shaped mold fabricated by acrylic resin (diameter: 10 mm; height: 2 mm). After one day, an artificial light blocking the root canal cavity model fabricated by the Teflon tube (inner diameter: 4 mm; height: 15 mm) was placed on the hardened root canal sealer and filled with core resin. After the injection of core resin into the tube, a silicone cover was used to block the light. A fiber post by the same manufacturer was inserted into the cavity and irradiated for 30 s and stored for 1 week. The shear bond strength was measured at a crosshead speed of 1.0 mm/min using a universal testing machine (AGS-H; Shimadzu, Kyoto, Japan). The prepared specimen was mounted along the horizontal axis adding shear strength along the vertical axis with 1.0 mm crosshead speed [31 (link)].
After the shear bond test, specimens were embedded with acrylic resin and cut vertically. The cut surface was polished to #8000 and observed under a scanning electron microscope (SEM) (JCM-7000, JEOL, Tokyo, Japan).