THE EFFECT OF IRRADIATION TIME ON THE SHEER STRENGTH OF COMPOSITES

Objectives. The purpose of this study was to determine the shear stren gth of composites at a specific depth by a double shear test and to re late it to irradiation time. Methods. Aluminum molds fled with three d ifferent composites were irradiated for the manufacturer's recommended time, as well as for three longer and two shorter times. Fifteen cyli ndrical specimens were prepared for each combination of material and e xposure time and subjected to a double shear test based on the princip le of rivets connecting various structural members. The shear planes w ere symmetrically located 1.5 mm from both outer surfaces. Results. Sh ear 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 valu es obtained were 42.5 MPa for Heliomolar (Vivadent) and Durafill (Kulz er) and 66.9 MPa for P-50 (3M). Only the shear strengths using the sho rtest light activation time were significantly different for each prod uct. Significance. Shear strength of composites at a specific depth is a function of the combination of light energy and the attenuating pow er of the specific material. A double shear test might serve as an adj unct to microhardness in determining the extent of cure of composites at a specific depth. The advantage of this testing is the extra inform ation to be gathered regarding the ability of composites to withstand internal stresses at a predetermined distance.