CRACK-PROPAGATION DIRECTIONS IN UNFILLED RESINS

Posterior composite restorative materials undergo accelerated wear in the occlusal contact area, primarily through a fatigue mechanism. To f acilitate the timely development of new and improved materials, a pred ictive wear model is desirable. The objective of this study was to dev elop a finite element model enabling investigators to predict crack pr opagation directions in resins used as the matrix material in composit es, and to verify these predictions by observing cracks formed during the pin-on-disc wear of a 60:40 BISGMA:TEGDMA resin and an EBPADMA res in. Laser confocal scanning microscopy was used to measure crack locat ions. Finite element studies were done by means of ABAQUS software, mo deling a cylinder sliding on a material with pre-existing surface-brea king cracks. Variables included modulus, cylinder/material friction co efficient, crack face friction, and yield behavior. Experimental resul ts were surprising, since most crack directions were opposite previous ly published observations. The majority of surface cracks, though init ially orthogonal to the surface, changed direction to run 20 to 30 deg rees from the horizontal in the direction of indenter movement. Finite element modeling established the importance of subsurface sheer stres ses, since calculations provided evidence that cracks propagate in the direction of maximum K-II(theta), in the same direction as the motion of the indenter, and at an angle of approximately 20 degrees. These f indings provide the foundation for a predictive model of sliding wear in unfilled glassy resins.