Study on Failure Mechanism and Lifetime for Dental Porcelain

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This study has been made to predict a fatigue life and to clarify a mechanism of fatigue failure on porcelain -fused-to-metal (PFM). The dynamic fatigue test in the four point bending mode was performed at five different strain rates, 8.33x10^<-x> (x=2, 3, 4, 5, 6) sec^<-1> in distilled water at 37ﾟC. To confirm a subcritical crack growth in water, the four point bending strength and critical stress intensity factor in Ar gas (dry condition) at 37ﾟC were measured. Furthermore, the elastic costants measurement was carried out to get the fracture energy.The weibull parameters resulted from dynamic fatigue test at five strain rates were in the range of 15 to 18. Because of their small standard deviations, the fracture mode was regarded to be expressed by a single weibull mode. The fatigue parameter given from this test was estimated at n=35.3<plus-minus>0.8. Although this value is not so small compared with other ceramics, the present PFM indicates to take place the subcritical crack growth by stress corrosion in the presence of wateBy using the fatigue parameter, fracture stress and time-to-failure data, the SPT (Strength-Probability-Time) diagram was obtained and it gave- a conclusion, as an example, that the stress less than 21 MPa would assure 20 years in lifetime with the survival probability of 99.9 %.The average strength in Ar gas was 23 % larger than that in water, but the weibull parameter was small. Using both these strength data and critical stress intensity factor, th ratio crack length in water and Ar gas just before failure was found to be 1.3 to 2.3. This result indicates that PFM takes place the subcritical crack growth in water.The fracture energy calculated was 6600 erg/cm^2. This value is considered to be small in water because this calculated energy is regarded to be critical fracture energy. The further interesting finding by the scanning electron micrograph observation is that PFM produced the striation pattern like a metal.