Department of Conservative Dentistry, Federal University of Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil.
Post Graduate Program in Dental Science (Prosthodontics Unit), Federal University of Santa Maria, Santa Maria, Rio Grande do Sul, Brazil.
J Mech Behav Biomed Mater. 2020 Jul;107:103769. doi: 10.1016/j.jmbbm.2020.103769. Epub 2020 Apr 3.
Considering the long time spent in low frequency cyclic fatigue tests, this study aimed to evaluate the influence of loading frequency (2 Hz and 20 Hz) on the flexural fatigue strength (FFS) and on the time and number of cycles to failure of a leucite-reinforced glass-ceramic. Disc-shaped specimens were produced using leucite-reinforced glass-ceramic CAD/CAM blocks (IPS Empress CAD), according to ISO 6872/2015. Two fatigue tests were performed. The FFS (n = 17) was determined by staircase approach at a lifetime of 500,000 cycles, for 2 Hz (control - chewing frequency estimative) and 20 Hz (accelerated approach). To determine the time and the number of cycles to failure in flexural fatigue, discs (n = 20) were submitted to a cyclic loading ranging from 10 MPa to 99 MPa (60% of the monotonic strength), until a maximum of 500,000 cycles. Means, standard deviation and confidence intervals (CI) at 95% for FFS were calculated, whereas statistical differences were detected based on maximum likelihood estimations and overlapping of 95% CIs. Kaplan Meier (α = 0.05) and log rank post-hoc tests were used to analyze the time (in minutes) and the number of cycles to failure in the lifetime test. FFS did not differ significantly between 2 Hz (mean: 78 MPa; 95% CI: 69-88 MPa) and 20 Hz (mean: 84 MPa; 95% CI: 78-90 MPa). Regarding the lifetime test, there was no difference (p = 0.3) in the time to failure for 2 Hz (mean: 13 min; 95% CI: 6-20 min) and 20 Hz (mean: 69 min; 95% CI: 9-128 min). However, the group tested with 20 Hz survived a significantly (p < 0.01) higher number of cycles (mean: 82,247 cycles; 95% CI: 11,450-153,044) than the group tested with 2 Hz (mean: 1588 cycles; 95% CI: 779-2397). Therefore, in leucite-reinforced glass-ceramic fatigue strength tests, limited to a lifetime of 500,000 cycles, the use of loading frequencies up to 20 Hz did not influence the FFS estimations when compared to 2 Hz (chewing frequency estimative), and may be an alternative to accelerate data collection in this type of mechanical test. However, in lifetime tests, the use of higher loading frequencies, as 20 Hz, did not save time, since a higher number of cycles was necessary to promote the failure, when compared to 2 Hz.
考虑到在低频循环疲劳试验中花费的时间较长，本研究旨在评估加载频率（2 Hz 和 20 Hz）对玻璃陶瓷弯曲疲劳强度（FFS）以及失效时间和失效循环次数的影响。根据 ISO 6872/2015，使用透锂长石增强玻璃陶瓷 CAD/CAM 块（IPS Empress CAD）制作圆盘形试件。进行了两项疲劳试验。FFS（n=17）通过楼梯法在 500,000 次循环的寿命时确定，用于 2 Hz（咀嚼频率估计值）和 20 Hz（加速方法）。为了确定弯曲疲劳失效时的时间和失效循环次数，将圆盘（n=20）在 10 MPa 至 99 MPa 的循环载荷下进行加载（最大强度的 60%），直到达到 500,000 次循环。计算了 FFS 的平均值、标准差和置信区间（CI）为 95%，并基于最大似然估计和 95%CI 的重叠检测到了统计差异。采用 Kaplan-Meier（α=0.05）和对数秩事后检验分析寿命试验中的失效时间（以分钟为单位）和失效循环次数。2 Hz（平均值：78 MPa；95%CI：69-88 MPa）和 20 Hz（平均值：84 MPa；95%CI：78-90 MPa）之间的 FFS 无显著差异。关于寿命试验，2 Hz（平均值：13 分钟；95%CI：6-20 分钟）和 20 Hz（平均值：69 分钟；95%CI：9-128 分钟）之间的失效时间无差异（p=0.3）。然而，20 Hz 组的存活循环数明显更高（p<0.01）（平均值：82,247 次；95%CI：11,450-153,044），而 2 Hz 组的存活循环数（平均值：1588 次；95%CI：779-2397）。因此，在透锂长石增强玻璃陶瓷疲劳强度试验中，在 500,000 次循环的寿命限制内，与 2 Hz（咀嚼频率估计值）相比，使用高达 20 Hz 的加载频率不会影响 FFS 估计值，并且可以作为加速此类机械试验数据采集的一种替代方法。然而，在寿命试验中，使用较高的加载频率（如 20 Hz）并不能节省时间，因为与 2 Hz 相比，需要更多的循环才能导致失效。
