Dentin bonding system. Part II: Effect of crosshead speed.
Dentin bond strengths are normally evaluated by conducting mechanical tests in tensile and/or shear mode at a certain level of crosshead speed. According to the literature review, crosshead speed is varied in a relatively wide range from 0.1 mm/min to 10.0 mm/min (with the exception of a few studies conducted above 10.0 mm/min). It was also found that crosshead speeds of 0.5, 1.0, 2.0 and 5.0 mm/min were commonly employed to evaluate dentin bond strengths for both tensile and shear modes, although no rationale for selecting the crosshead speed was described in the articles. For testing dentin bond strengths, no plastic deformation should be involved, and fracture only should take place during the fracturing process. With the Scotchbond Multiple-Purpose/Z100 bonding system, tensile bond strengths were evaluated at crosshead speeds of 0.1, 0.5, 1.0, 5.0 and 10.0 mm/min. It was found that (i) there was no strain rate sensitivity when the test was conducted below the 1.0 mm/min crosshead speed, and (ii) if the crosshead speed exceeded 1.0 mm/min, a strain rate dependency of the tensile bond strengths was observed. Knowledge that the composite resin (Z100) did not exhibit strain rate dependency influenced the conclusion that (iii) the above observed strain rate dependency was solely due to the mechanical response from the bonding interface region.