The use of bioceramics in dentistry has grown significantly in recent years, mainly for the production of fixed partial dentures (FPDs), single crowns, inlays and onlays. This growth has led to a decrease in the use of traditional metallic substructures [1]. Yttria-stabilized polycrystalline zirconia (Y-TZP) is one of the most recently introduced dental ceramics for the fabrication of FPD frameworks. Y-TZP was initially used in the medical field as a material option for femoral head replacement prostheses. It was only in the 1990 s that the use of Y-TZP started in dentistry, with applications for both anterior and posterior restorations. The growing demand for ceramic materials can be attributed to their high biocompatibility, high wear resistance and better esthetic quality compared to metal-ceramic restorations [2], [3].

The improved mechanical behavior of dental ceramics allows their use in locations subject to relatively high stresses and provides clinicians with the possibility of making more extensive prostheses in the posterior region of the oral cavity. However, these materials still have limitations and may fracture after a few years of clinical use. The fracture rate of Yttria-stabilized polycrystalline zirconia (Y-TZP) dental ceramics, used for fixed partial dentures (FPDs) and crowns, has been a focal point of various studies. Anterior single crowns showed fracture rates of 0.97% at 5 years, while posterior single crowns had an incidence of 0.69%. Notably, FPDs fractured at a higher rate: 3.26% for anterior and 2.42% for posterior [4]. For monolithic zirconia crowns, the fracture rate was significantly lower at 0.54% after 7.5 years, compared to layered zirconia crowns and lithium disilicate crowns which had rates of 2.83% and 1.26% respectively [5]. Y-TZP FPD frameworks of 3 to 5 elements reported an 8% fracture rate after 5 years [6], while 3-element PPFs had a 5% failure rate after 3 years. [7] Y-TZP frameworks used in FPPs containing 4 to 7 elements fractured at a rate of 3% after 2 years [8], and Y-TZP single crowns reported a 7% fracture rate after 2 years of use [9]. Despite these rates, a 98% survival rate was reported for monolithic zirconia crowns in a 5-year study, highlighting the durability of the material [10]. An important aspect to be considered when studying the phenomenon of fracture in dental ceramics is the fact that intra-oral failures are generally related to a fatigue process. Fatigue failure occurs after the material has been subjected to a sequence of stresses with a magnitude lower than that related to the strength of the material. Normally, the fatigue failure process occurs by the initiation and slow propagation of defects [11]. This mechanical challenge can be found in the chewing process in the oral environment. The fatigue behavior of dental materials can be used to determine how resistant a material is to cyclic loading prior to fracture [12].

There are different modes of load application in fatigue processes, which can be static (remaining constant over time and leading to subcritical crack growth), dynamic (generated with a constant loading rate, which leads to the growth of defects until the specimens fracture), [13], [14] or cyclic (load applied cyclically or in an oscillating manner, which induces crack propagation) [3], [15], [16], [17].

A relevant phenomenon that is observed in ceramic materials subjected to fatigue is the subcritical or slow growth of cracks (Slow Crack Growth, SCG). This phenomenon is caused by the interaction between water and the ceramic material at the crack tip under stress. In the presence of stresses below the critical level, water causes the hydrolysis of metallic oxides in the ceramic material, resulting in the stable growth of these cracks [18], [19], [20]. The oral environment has many elements favorable to SCG in ceramic restorations, such as the water present in saliva, the masticatory forces that generate subcritical stresses, the temperature of 37 °C and pH variations [19], [21], [22], [23].

SCG is characterized by the coefficient of susceptibility to subcritical growth (n), which is dimensionless and indicates the susceptibility of the material to the growth of defects that will lead to failure. Once the value of n is known, it is possible to predict the degradation in the strength of materials subjected to SCG [13], [24]. Some studies evaluated the SCG for InCeram Alumina (ICA) and found n values of 31 through dynamic fatigue and 21 through cyclic fatigue. As for the material InCeram Zirconia (ICZ), values of 54 and 21 were found, respectively, for the dynamic and cyclic fatigue methods. For Y-TZP, the n values found were 76 for dynamic fatigue, 29 by means of mathematical calculations from the Weibull distribution obtained by cyclic fatigue, and 25 using SCG curves, also obtained by cyclic fatigue [15], [24], [25], [26], [27]. The literature is controversial with regard to the values of n, which vary widely in different works because the test setups are generally different; therefore, it is difficult to compare results.

A widely used method to determine the crack growth parameters in ceramic materials, without having to directly measure the crack growth rate, is the dynamic fatigue test. This method is preferred over tests that analyze fracture mechanisms based on crack growth, as it allows for a less subjective lifetime estimate than those obtained by the crack measurement method, and the defects that cause fractures are simulated in a way that is closer to reality [13]. In addition, the cyclic fatigue test requires more complex testing machines such as servo-hydraulic or electromagnetic ones, while dynamic fatigue can be performed with a simpler and lower cost universal testing machine.

The main motivation of the present study is to find out if it is possible to compare the values obtained for the Y-TZP through two different fatigue tests (cyclic and dynamic). This comparison is relevant because, in addition to not having been presented in the dental literature, these two types of tests are very different in terms of the time taken to perform them – cyclic tests consume significantly more time than dynamic ones. Therefore, this study intends to generate results that can be used by researchers in the area of dental biomaterials to choose the most appropriate test for the dental ceramics of interest. Another intended scientific contribution is the verification of the effect of the frequency used in the cyclic fatigue test, as the use of higher frequencies could accelerate the test. Some variables, such as the loading frequency, must be determined for cyclic fatigue tests (number of loading cycles per second). Many cyclic fatigue studies have used frequencies between 1 Hz and 2 Hz, based on the work of Wiskott et al. [28], and on the fact that chewing activity occurs primarily between 0.94 and 2.17 Hz [29]. However, at low frequencies, the data collection becomes very time consuming.

This study aims to understand the fatigue behavior of Yttria-stabilized polycrystalline zirconia (Y-TZP), a preferred dental ceramic due to its superior properties, but with reported long-term fracture concerns. The focus is on the comparison of susceptibility to subcritical growth from two fatigue tests: cyclic and dynamic, and the impact of loading frequency in cyclic fatigue tests on Y-TZP lifetime. It is hypothesized that similar fatigue parameters can be obtained from both tests, and that the loading frequency does not significantly influence Y-TZP lifetime. These findings could serve as a guide for future research in selecting appropriate testing methods for dental ceramics.