Fracture toughness resistance curves describe a material’s resistance against crack propagation. These curves are often used to characterize biomaterials like bone, nacre or dentin as these materials commonly exhibit a pronounced increase in fracture toughness with crack extension due to co-acting mechanisms as crack bridging, crack deflection and microcracking. The knowledge of appropriate stress intensity factors which depend on sample and crack geometry is essential for determining these curves. For the dental biomaterials enamel and dentin it was observed that under bending and tensile loading crack propagation occurs under certain constant angles to the initial notch direction during testing procedures used for fracture resistance curves determination. For this special crack geometry (kink crack of finite length in a finite body) appropriate geometric function solutions are missing. Hence, we present in this study new mixed-mode stress intensity factors for kink cracks with finite kink length within samples of finite dimensions for two loading cases (tension and bending)which were derived from a combination of mixed-mode stress intensity factors of kink cracks with infinitely small kinks and of slant cracks. These results were further applied to determine fracture resistance curves of enamel and dentin by testing single edge notched bending(SENB) specimens. It was found that kink cracks with finite kink length exhibit identical stress fields as slant cracks as soon as the kink length exceeds 0,15 times the initial straight crack or notch length. The usage of stress intensity factor solutions for infinitely small kink cracks for the determination of dentin fracture resistance curves (as was done by other researchers) leads to an overestimation of dentin’s fracture resistance of up to 30%.
