Comparative evaluation of different mechanical test methods in additively manufactured permanent resin materials

Purpose: The purpose of this study is to compare the results obtained from different mechanical tests applied to additively manufactured permanent resin materials and to evaluate whether there is a meaningful relationship or correlation between these methods. Materials and Methods: A total of 360 permanent restorative resin specimens were fabricated using a 3D printer (Form 3, Formlabs,USA) based on stereolithography (SLA) technology. To compare methods and evaluate correlations, a power analysis was conducted using software(G*Power 3.1.9.7), assuming a moderate correlation (r = 0.30), 80% power, and a 5% significance level, resulting in 90 samples per test. Biaxial and three-point flexural strength tests were conducted using a universal testing machine (Testometric, England) in accordance with ISO 6872 standards. For the bi-axial test, 90 disc-shaped specimens (14 mm in diameter and 1.5 mm in thickness) were subjected to continuous loading at a crosshead speed of 1 mm/min until fracture occurred. Additionally, 90 rectangular bar-shaped specimens (25 × 2 × 2 mm) were tested under the same loading speed in the three-point bending test. Vickers and Knoop microhardness measurements were performed using a microhardness tester (Wilson, Buehler) in the same laboratory under a standard load of 100 gf. Statistical analyses included normality testing with the Kolmogorov-Smirnov test, group comparisons using independent and paired samples t-tests, and consistency and agreement evaluation of repeated hardness measurements using Intraclass Correlation Coefficients (ICC) (α=.05). Results: Flexural strength values obtained from biaxial tests were significantly higher than those from three-point tests (p < 0.001). A significant difference was also found between Knoop and Vickers microhardness values on the same specimens (p < 0.001), with Knoop values averaging 6 units higher. Conclusions: Different mechanical tests applied to the same material group have yielded significantly different results. This shows that the test method used on resin materials has a decisive effect on the results.