[Estimation of fatigue parameter and life-time prediction using SPT diagram for hydroxyapatite coating layer in 37 degrees C distilled water].

Lifetime and survival probability of brittle materials under given conditions of stress distribution, environment and component size can be predicted using an SPT (Strenght-Probability-Time) diagram. The SPT diagram for sintered hydroxyapatite (HAP) coated on the metal substrate via the fused glass was drawn and the safe working stress in 37 degrees C distilled water was estimated. The dynamic fatigue test carried out in 37 degrees C distilled water gave the fatigue parameter n = 19 for sintered HAP. This parameter is constant except that it depends on material and environment, and the greater the value of n, the greater the resistance to fatigue failure. For sintered HAP, therefore, the resistance to fatigue failure under this wet condition is low. To draw the SPT diagram, anchor points were taken from the fracture stress and time-to-failure data measured at 1 mm/min. The SPT diagram obtained suggested that a lifetime of 20 years with survival probability of 99.9% required the safe working stress of less than 3.2 MPa. The effects of accuracy of fatigue parameter on the uncertainty in SPT diagram were estimated.