The invention of dental resin based-composites (RBCs) has provided a broad range of materials for the restoration of load-bearing posterior teeth with excellent clinical results and adequate longevity. Currently, a lack of consensus exists among researchers regarding classification of RBCs as a result of slight variations in filler size and associated interchangeable mechanical properties of “microhybrid”, “nanohybrid” and “nanofilled” RBCs. Also, the inconsistency in mechanical property testing of RBCs is evident amongst researchers. This research explored the variability in experimental and statistical testing methodologies of RBCs. The current study identified a wide variation in the bi-axial flexure strength (BFS) of commercial and experimental RBCs with respect to deformation rate with a complex relationship between resin constituents and filler morphology. Experimental unfilled resins revealed deformation rate dependence in BFS following 1 week dry, 1 and 13 weeks wet storage regimes, whereas the addition of fillers modified the deformation rate dependence following 13 weeks wet storage and resulted in the BFS of filled resin composites being independent of testing speed. These findings suggested the need for the development of RBCs with appropriate formulations for clinical situations where variable strain rates may occur, for example, patients with parafunctional habits. To date, the alignment of specimens during storage regimes prior to mechanical property testing has rarely been reported. The effect of specimen alignment on the BFS and surface hardness of RBCs was evaluated and a greater decrease in the both properties were found following wet upright compared with stacked and upper surface exposed alignments. These observations were attributed to a variation in diffusion of water as the result of difference in exposed surface areas of specimens, which may lead to different findings and associated interpretation between investigators. Weibull statistics are used for the analysis of strength data of RBCs, however their applicability to RBCs might be questioned due to some viscous deformation prior to brittle failure. The findings of current study supported the applicability of Weibull statistics for the microhybrid and nanofilled RBCs but not a flowable RBC, which suggested that Weibull statistics may not necessarily be applicable for all RBC types. It was demonstrated that variability and irrelevance in testing methods may cause incorrect interpretation of data among researchers and consequently affect the future research and development of RBCs. Therefore, further standardisation of testing methods is required
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