Study on Failure Mechanism and Lifetime for Dental Porcelain

本研究旨在预测瓷熔金属（PFM）的疲劳寿命，并阐明其疲劳失效机理。在37ºC蒸馏水条件下，以8.33 × 10^<-x> (x= 2,3,4,5,6) sec^<-1> 5种应变速率进行四点弯曲模式下的动态疲劳试验。为了证实水中存在亚临界裂纹扩展，在37℃的氩气（干燥条件）中测量了四点弯曲强度和临界应力强度因子。此外，还进行了弹性常数测量，得到了断裂能。5种应变速率下动态疲劳试验得到的威布尔参数在15 ~ 18之间。由于它们的标准差较小，断裂模式被认为是由单一的威布尔模式表示的。该试验给出的疲劳参数估计为n=35.3<正负>0.8。虽然与其他陶瓷相比，该数值并不小，但PFM表明，在有水存在的情况下，应力腐蚀会导致亚临界裂纹扩展。利用疲劳参数、断裂应力和失效时间数据，得到了SPT（强度-概率-时间）图，并给出了一个结论，以应力小于21 MPa为例，可以保证20年的使用寿命，存活概率为99.9%。氩气中的平均强度比水中高23%，但威布尔参数较小。结合这些强度数据和临界应力强度因子，发现破坏前水与氩气中的裂纹长度之比为1.3比2.3。结果表明，PFM在水中发生亚临界裂纹扩展。裂缝能计算值为6600 erg/cm^2。这个值在水中被认为很小，因为这个计算的能量被认为是临界断裂能。扫描电镜观察的另一个有趣的发现是，PFM产生了像金属一样的条纹图案。

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.