Study on Toughening of composite resins using fatigue crack propagation test

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The equation, da/dN=C (DELTAK) ^m, could be applied to the fatigue crack propagation of the composites both in the atmosphere and in the water environment. The DELTAK_<th> for composite resins in the atmosphere was in the range of 0.43-0.82MPa・m^<1/2>, and that in the water increased to the range of 0.52-1.12Mpa・m^<1/2.> The expansion of matrix due to the water absorption enhanced the fatigue resistance of composites. The residual compression stress in the matrix might arrest the fatigue crack.The fatigue crack of dental composites propagated mostly along the interface between inorganic filler and matrix. In the contrast to inorganic filler, some organic fillers were penetrated with fatigue cracks. The crack was bridged, bowed or deflected by the inoganic filler, and consequently the energy release rate for crack propagation was increased. These showed that crack deflection and crack bridging by inorganic filler increased the fatigue resistance of composites. The inorganic filler larger than 10mum was effective in arresting the fatigue crack propagation. The aspect ratio and the size of inorganic filler in composites might be influential factors for their fatigue resistance.There was no significant reduction of bending strength after 3m aging in water. On the contrary, the fatigue resistance measured as DELTAK_<th> was decreased by aging for only 1m. Fatigue crack growth rate was more sensitive to degradation of composites by aging in water than bending strength.Thus, it can be concluded that the crack deflection and the crack bridging by inorganic filler increase the fatigue resistance of composites, and the expansions of matrix due to the water absorption enhance the fatigue resistance of composites. From the present study, it was shown that the fatigue crack propagation test was an effective method to investigate toughening and degradation of dental composites.