Corner stress associated with bond strength testing of endodontic sealer

Objectives: A characteristic stress singularity or ‘corner stress' can exist at an interface corner of butt tensile joint design, which can be used to predict failure of bonded materials. This study observed the interface corner stress in a methacrylate resin-based root canal sealer cement and calculated its failure estimation validity based on layer thickness and bond strength. Methods: Different thicknesses (0.03 to 1.3 mm) of methacrylate resin-based sealer cements, EndoREZ® were prepared between metal-to-metal joints of aligned rods. The exact thickness was measured and tensile strength was tested after 48 hour using a universal testing machine. Fracture surface images were evaluated with SEM. The interface corner fracture toughness (Kfc) was calculated based on Reedy and Guess (1993). Results: Tensile bond strength was found to be inversely related to adhesive layer thickness. SEM images demonstrated the corner stress failure pattern along the periphery of the joint. The average Kfc was 4.3 MPa mm. The Kfc was not influenced by thickness and is a material property of the sealer. The combined Kfc value estimation and the measured tensile strength were in good agreement. Conclusions: Corner stress in a bonded butt tensile joint was characteristic of EndoREZ® interface failure. The tensile bond strength results indicated that the interface corner fracture toughness criterion is valid and can provide predictions of bonding failure.