The possibility of applying a bi-axial flexure strength test on composite resin was examined using three point and bi-axial flexure strength tests to measure the strength of the light-cured resin and to compare the relative reliability using the Weibull modulus.

The materials used in this study were light-curing restorative materials, MICRONEW™, RENEW® (Bisco, Schaumburg, USA). The bi-axial flexure strength measurements used the piston-on-3-ball test according to the regulations of the International Organization for Standardization (ISO) 6872 and were divided into 6 groups, where the radius of the specimens were 12 mm (radius connecting the 3-balls: 3.75 mm), 16 mm (radius connecting the 3-balls: 5 mm), and the thickness were 0.5 mm, 1 mm, 2 mm for each radius.

The bi-axial flexure strength of the MICRONEW™ and RENEW® were higher than the three point flexure strength and the Weibull modulus value were also higher in all of the bi-axial flexure strength groups, indicating that the bi-axial strength test is relatively less affected by experimental error.

In addition, the 2 mm thick specimens had the highest Weibull modulus values in the bi-axial flexure strength test, and the MICRONEW™ group showed no significant statistical difference (p > 0.05). Besides the 2 mm MICRONEW™ group, each group showed significant statistical differences (p < 0.05) according to the thickness of the specimen and the radius connecting the 3-balls.

The results indicate that for the 2 mm group, the bi-axial flexure strength test is a more reliable testing method than the three point flexure strength test.