Development of internal damage within resin composites was evaluated under compressive loading in order to predict crack initiation and fracture. Moreover, three-point bending tests were also carried out in order to clarify mechanical behavior and fracture under tensile stress state in comparison with those under compressive stress state. Both of them were conducted for the purpose of obtaining data to formulate constitutive equations for resin composites and to implement precise numerical simulation. Columnar specimens for compression tests and square pole specimens for three-point bending tests were prepared by using clinical resin composites. In compression tests, loading–unloading (or –reloading) was given to columnar specimens and Young’s modulus was evaluated by the gradient of stress–strain curves under unloading. Internal damage was evaluated from the variation of Young’s modulus as a scalar damage variable based on the continuum damage mechanics. The variation of apparent density and residual strain at vanished stress were also discussed in association with the development of internal damage. Accumulation of internal damage was found on the stress–strain curve under loading–unloading–reloading in comparison with the curve under monotonic loading. On the other hand, in three-point bending tests, dependence of stress–strain curves on light curing time and strain rate was clarified. Since compression tests have been carried out under similar experimental conditions by authors so far, mechanical behaviors of resin composites under tensile stress state were discussed in comparison with those under compressive stress state. Brittleness under tensile stress state was indicated in comparison with compressive stress state.
