Effect of Loading Rate on Strenght of Lithium Disilicate Glass-Ceramics

Methods: A suite of IPS e.max rectangular bars, n=50 CAD and n=50 Press, in the dimension of 2x3x25mm were fabricated according to the manufacturer’s specifications.  The surfaces and the edges of these rectangular bars were polished and beveled, respectively, with 15μm diamond grit (ISO 6872).  SCG parameters of LiDis were determined using a constant stress-rate flexural testing method (dynamic fatigue, ASTM C 1368–06).  A four-point flexure (inner span 10mm, outer span 20mm) rig was mounted onto the crosshead of a screw-driven universal tester.  Five displacement rates covering 4 orders of magnitude (1, 0.1, 0.01, 0.001, and 0.0001mm/min) were employed.  Critical loads for the flexural fracture of the LiDi bars were determined.  Stress rates were derived from the load–time plot for each test. 

Results: The dynamic fatigue data for LiDi CAD and Press showed a similar trend; flexural strength decreased as the loading rate decreased and are tabulated below.  The fast-load strength was noticeably higher for LiDi CAD than Press.  However, LiDi Press experienced less degradation in strength as the loading rate decreased with equal fracture strengths at the lowest load rate.  The reduced degradation rate is likely the result of the higher fracture toughness of LiDi Press relative to CAD.  

Conclusions: The LiDi IPS e.max CAD exhibited higher fast-load strength, while IPS e.max Press was less susceptible to SCG.  Supported by NIH/NIDCR-2R01DE17925, NSF/CMMI-0758530 and Ivoclar Vivadent USA.