Miniature specimens for fracture toughness evaluation of dental resin composites

Millimeter-scale ("miniature") specimens enable in-situ evaluation of mechanical properties of engineering materials at reduced cost. Here three such specimens for measuring fracture toughness (K_C) are developed and implemented to new dental materials. The latter include concurrent methacrylate-based and new ether-based resin composites designed to reduce polymerization stress and enhance service life in restored teeth.

Fracture toughness of four experimental and one commercial dental resin composites are evaluated using three-point bending (3PB), wedge double-cantilever-beam (WDCD) and edge chipping miniature test specimens. The values of K_C were compared with those obtained following ISO standard method ISO6872: 2014. The stress intensity factor (K) for the 3PB and WDCB specimens was determined using linear fracture mechanics analyses made in conjunction with the Finite Element technique, with due consideration given to the finite width of pre-crack.

Analytic expressions for predicting K_C were developed for all three miniature specimens. The width of pre-crack, generally neglected for conventional specimens, significantly affect K. Measured K_C conclusively agree with those of commercial or well-studied materials as obtained using conventional specimens, with error bounded by 5-10 percent.

The edge chipping test was successfully applied for the first time to non-brittle materials like dental resin composites. The miniature specimens developed will expedite the evaluation of fracture toughness of dental resin composites by saving materials and provide needed in-situ assessment capability. The chipping test which requires no introduction of initial crack and involves no use of elastic constants is especially suitable to functionally graded materials and in-situ study of restored teeth. The WDCB specimen enables stable crack growth, a useful trait in fatigue studies.