Fatigue failure load of prefabricated fiber reinforced post: The influence of the post diameter and fatigue test method

The present study analyzed the fatigue performance of prefabricated fiber reinforced composite posts (FRC post) with different diameters by means of two different fatigue testing methods. A total of 69 FRC double-tapered posts, presenting 20 mm in length associated with different coronal diameters (1.4, 1.6, 2.0 mm) were embedded with acrylic resin in PVC cylinders, maintaining 6 mm free from the coronal part of the post. The posts of each diameter were submitted to step stress testing (inclination: 45°; initial load: 20 N; step-size: 10 N; cycles per step: 18,000; frequency: 5 Hz) (n = 5) or staircase testing (inclination: 45°; initial load: 60% of the monotonic load-to-fracture reported on existing literature; increment: 10% of the monotonic load-to-fracture reported on existing literature; cycles: 18,000; frequency: 5 Hz) (n = 18). The statistical differences were determined for Kaplan-Meier and Mantel-Cox (Log-Rank) tests for step stress testing and based on confidence interval overlapping for staircase testing. All the failures were analyzed in a stereomicroscope (10 × magnification) after the tests, and representative samples were analyzed by scanning electron microscope (500 × and 1000 × magnification). The FRC post with the wider diameter had greater fatigue failure load in step stress testing (DT1.4–56 N < DT1.6–60 N < DT2–100 N) however, a significant difference for the staircase testing was only found for DT1.4 and DT2 (DT1.4–57.4 N ≤ DT1.6–62.2 N ≤ DT2–74.5 N). Additionally, the step stress test revealed that the wider the FRC post diameter, the greater the number of cycles required for failure (DT1.4–73,319 < DT1.6–99,305 < DT2–154,743). A central longitudinal crack perpendicular to the applied load direction was the failure pattern observed for all conditions tested. Thus, the fatigue performance of FRC posts is influenced by its coronal diameter (wider diameter means higher failure loads and number of cycles for failure), but the fatigue test used (staircase or step stress) had no influence on the outcome, expressing similar results.