Fatigue failure of lithium disilicate and translucent zirconia crowns with different occlusal thickness

Step-stress accelerated life testing (SSALT) (36), which comprised 18 specimens distributed into three profiles: mild (n=9), moderate (n=6), and aggressive (n=3). Each profile started at 300 N and finished at 3,000 N. The mild profile was designed with 200 N load increase at each 20,000 cycles up to 320,000 cycles; the moderate profile was designed with a 250 N load increase each 15,000 cycles up to 195,000 cycles; the aggressive profile had 300 N load increase at each 10,000 cycles up to 110,000 cycles. The data were analyzed using an underlying life distribution to describe the life data collected at different stress levels and a life-stress relationship to quantify how life distribution changed across different stress levels(36)(37). Thus, the Weibull Distribution was chosen to fit the life data collected in SSALT. Considering the time-varying stress model of SSALT, the inverse power law relationship was selected to extrapolate a use level condition considering the cumulative effect of the applied stresses, commonly referred as the cumulative damage model. From the extrapolated use level condition, a variety of functions could be derived. Hence, the use level probability Weibull curves (probability of failure versus number of cycles) with a set load of 300 N at 90% two-sided confidence interval were calculated and plotted (CI: 90%) (Synthesis 9, Alta Pro, Reliasoft, Tucson, AZ, USA).The reliability was calculated for completion of a mission of 100,000 cycles at 300, 600, 900, and 1200 N and the differences between groups were identified based on the non-overlap of the CI. Parameters estimation for all analyses was calculated via maximum likelihood estimate (MLE) method, and 90% two-sided confidence interval (90% CI) was approximated using the Fisher matrix approach..

步进应力加速寿命测试（SSALT）（36），包含18个样本，分为三个剖面：轻微（n=9）、中等（n=6）和激进（n=3）。每个剖面从300 N开始，至3,000 N结束。轻微剖面设计为每20,000周期增加200 N的负荷，直至320,000周期；中等剖面设计为每15,000周期增加250 N的负荷，直至195,000周期；激进剖面每10,000周期增加300 N的负荷，直至110,000周期。数据采用潜在寿命分布进行分析，以描述在不同应力水平收集的寿命数据，并利用寿命-应力关系来量化寿命分布随不同应力水平的变化（36）（37）。因此，威布尔分布被选用来拟合SSALT中收集的寿命数据。考虑到SSALT的时间变化应力模型，选择逆幂律关系来外推使用水平条件，考虑施加应力的累积效应，通常称为累积损伤模型。从外推的使用水平条件中，可以导出多种函数。因此，计算并绘制了使用水平概率威布尔曲线（失效概率与循环次数之比），设定300 N的负荷，在90%的双侧置信区间内（置信区间：90%）（综合9，Alta Pro，Reliasoft，图森，亚利桑那州，美国）。计算了在300 N、600 N、900 N和1200 N下完成10万次循环任务的可靠性，并根据置信区间的非重叠识别了各组之间的差异。所有分析参数的估计均通过最大似然估计（MLE）方法计算，90%双侧置信区间（90% CI）使用Fisher矩阵方法近似。