The effect of irradiation time on the shear strength of composites

The purpose of this study was to determine the shear strength of composites at a specific depth by a double shear test and to relate it to irradiation time.

Aluminum molds filled with three different composites were irradiated for the manufacturer's recommended time, as well as for three longer and two shorter times. Fifteen cylindrical specimens were prepared for each combination of material and exposure time and subjected to a double shear test based on the principle of rivets connecting various structural members. The shear planes were symmetrically located 1.5 mm from both outer surfaces.

Shear strength vs. irradiation time was directly but nonlinearly related. All test materials attained a maximum shear strength value which was not exceeded by a further increase of the exposure time. Maximum values obtained were 42.5 MPa for Heliomolar (Vivadent) and Durafill (Kulzer) and 66.9 MPa for P-50 (3M). Only the shear strengths using the shortest light activation time were significantly different for each product.

Shear strength of composites at a specific depth is a function of the combination of light energy and the attenuating power of the specific material. A double shear test might serve as an adjunct to microhardness in determining the extent of cure of composites at a specific depth. The advantage of this testing is the extra information to be gathered regarding the ability of composites to withstand internal stresses at a predetermined distance.